Session 1: Introduction: Which Cells Are the Players?

00:00:07.12 The immune system is responsible
00:00:09.06 for fighting infection and disease.
00:00:10.29 It is comprised of many specialized cell types,
00:00:13.16 all which work together to keep your body healthy.
00:00:16.24 In this short video,
00:00:18.11 you will be introduced to the major cellular players of the immune system.
00:00:23.10 Let’s start by introducing the two major arms of the immune system:
00:00:26.00 innate and adaptive immunity.
00:00:28.25 Innate immunity serves as the first line of defense
00:00:31.14 and is a more general immune response.
00:00:33.27 You might recognize some of the classic symptoms of innate immunity,
00:00:36.24 such as fever and inflammation.
00:00:39.09 Adaptive immunity allows for very specific detection
00:00:42.16 and elimination of pathogens.
00:00:45.04 However, it takes a longer time to ramp up than innate immunity.
00:00:48.14 Importantly, the adaptive immune system provides immunological memory --
00:00:52.23 the ability of our immune cells to remember previous infections and clear them more quickly in the future.
00:01:00.17 All immune cells develop from a single pluripotent cell
00:01:03.07 in the bone marrow,
00:01:04.20 the hematopoietic stem cell.
00:01:06.07 Hematopoietic stem cells give rise to
00:01:08.09 lymphoid and myeloid progenitors,
00:01:09.24 each of which differentiate into a range of immune cell types.
00:01:13.27 We refer to the cells on the top as the lymphoid lineage,
00:01:16.18 whereas the cells on the bottom are classified as the myeloid lineage.
00:01:20.27 The myeloid progenitors also give rise to red blood cells and platelets,
00:01:24.09 although we won’t focus on those here.
00:01:27.25 Let’s now look closely at the lymphoid lineage.
00:01:31.08 The lymphoid progenitor differentiates into three cell classes.
00:01:35.09 We’ll start by introducing B cells.
00:01:38.02 Upon activation, mature B cells differentiate into memory cells or plasma cells.
00:01:43.09 Plasma cells are the immune cells that are responsible for secreting antibodies,
00:01:47.10 an important component of adaptive immunity.
00:01:52.05 Natural killer cells are cytotoxic cells of the innate immune system.
00:01:56.10 They detect virus-infected cells and kill them.
00:02:00.29 T cells arise from a common progenitor.
00:02:03.22 There are many types of mature T cells,
00:02:05.25 but the best known ones -- those which we’ll cover here -- include:
00:02:10.08 memory T cells, cytotoxic T cells,
00:02:12.11 and helper T cells.
00:02:14.22 Like memory B cells,
00:02:16.15 a fraction of mature T cells remain in the body as memory T cells
00:02:19.09 to help the body mount a faster immune response in future infections.
00:02:24.01 Cytotoxic T cells recognize antigen,
00:02:26.19 or small pathogen-derived particles,
00:02:28.28 on infected cells and kill them in a pathogen-specific manner.
00:02:32.25 Lastly, upon activation by specialized cells in the body,
00:02:37.18 helper T cells secrete cytokines that boost the adaptive immune response.
00:02:41.18 For example, helper T cells play an important role in B cell activation.
00:02:47.28 Now let’s take a look at the myeloid lineage.
00:02:52.01 The myeloid lineage produces most cells of the innate immune system
00:02:54.29 as well as important antigen-presenting cells that prime the adaptive immune response.
00:03:00.25 The myeloid progenitor gives rise to neutrophils,
00:03:03.22 which are innate immune cells that specialize in the capture and killing of microorganisms
00:03:07.27 throughout the body;
00:03:09.20 eosinophils, which are a type of granulocyte
00:03:12.10 that releases cytokines to defend against parasites;
00:03:15.24 and monocytes, which further differentiate into dendritic cells and macrophages.
00:03:21.02 Dendritic cells are a specialized type of phagocytic cell
00:03:24.10 that bridges innate and adaptive immunity.
00:03:27.14 Macrophages are tissue resident phagocytic cells.
00:03:30.21 They patrol the body and assist in cleaning up infection
00:03:33.12 and activating other immune cells.
00:03:37.08 Other cells that arise from the myeloid progenitor
00:03:40.06 include mast cells, another type of granulocyte that are implicated in allergy;
00:03:45.02 as well as basophils,
00:03:47.11 which are a less well-understood cell type that is involved in the immune response to parasites.
00:03:54.06 So how are immune cells activated?
00:03:56.28 Here, we’ll briefly cover the molecular paradigms
00:03:59.29 for innate and adaptive immune activation.
00:04:02.16 Cells of the innate immune system
00:04:05.13 express molecules known as pattern recognition receptors at their surface.
00:04:09.07 These receptors bind pathogens or parts of pathogens,
00:04:12.04 which induce intracellular signals
00:04:14.29 that activate an innate immune response.
00:04:18.17 The particles recognized by these receptors
00:04:20.20 are common amongst pathogens.
00:04:23.14 To activate adaptive immunity,
00:04:25.12 cells present antigen
00:04:28.06 -- small peptide fragments of pathogens --
00:04:30.01 to T cells to inform a specific immune response.
00:04:33.01 Antigen is presented by two types of surface molecules:
00:04:36.22 MHC class I and MHC class II.
00:04:39.27 MHC class I molecules are expressed by all cells in the body
00:04:43.24 and are used in defense against intracellular pathogens such as viruses.
00:04:49.00 They do this by presenting endogenous, or intracellular, antigens
00:04:53.18 to cytotoxic T cells.
00:04:56.02 MHC class II molecules present exogenous antigen,
00:04:59.02 which is antigen found on pathogens outside of cells,
00:05:02.09 and activate helper T cells.
00:05:05.21 MHC class II molecules are expressed
00:05:07.14 by what are known as professional antigen-presenting cells,
00:05:11.27 which include dendritic cells, macrophages, and B cells.
00:05:15.02 There are safeguards in place to ensure that the immune system
00:05:18.01 isn’t activated against self antigens.
00:05:23.15 The mammalian immune system
00:05:25.09 is comprised of a complex set of cells
00:05:27.04 that contribute to innate and adaptive immunity.
00:05:30.16 The foundational understanding you’ve gained from this video
00:05:32.26 serves as a starting point for a deeper look at this topic.

00:00:06.19 Hi, my name is Ira Mellman. I'm a scientist at Genentech,
00:00:10.02 which is a biotechnology company here in San Francisco.
00:00:12.24 I study cancer, but I also study the immune system, so I'm happy to be here today
00:00:17.00 to tell you something about the immune system, what it is, and how it works.
00:00:20.11 It is really not as complicated as a lot of us fear.
00:00:23.16 Now, what is the function of the immune system?
00:00:26.05 Now, it turns out that in our daily lives we are surrounded by a wide variety of pathogens.
00:00:32.00 Bacteria, viruses, other little organisms that cause an almost limitless number of infections in us,
00:00:38.26 in humans, and we need to be protected against these environmental pathogens,
00:00:42.16 and toxins that they make, on an almost continuous basis. Every time we breath, we take in bacteria or viruses,
00:00:49.15 every time we drink or eat something,
00:00:51.29 also, we have the potential of taking in viruses and bacteria
00:00:56.01 and obviously we don't get infected and sick as a consequence of just daily life on a routine basis.
00:01:01.11 And the reason that happens is because we have immune systems to protect us
00:01:04.22 against all of these pathogens and toxins.
00:01:07.27 Now, the immune system has to work in a very special way though.
00:01:10.24 Although it's very powerful at being able to rid us of all sorts of noxious organisms,
00:01:15.16 it has to be able to understand who is foreign, in other words, who are the pathogens
00:01:20.13 that are trying to infect us, and distinguish those pathogens from our own cells.
00:01:24.15 So, all of this has to occur, without avoiding injury to the host, which is us.
00:01:29.04 To do that, what the immune system has to do, and this is the complicated part,
00:01:34.13 is to distinguish self from non-self, in other words, distinguish the invaders from that which it is trying to protect,
00:01:42.15 which, again, is us, so this is a very, very important feature of the immune system,
00:01:47.01 and means that it's actually really highly specialized, and in many ways, really smart
00:01:51.24 to be able to tell the differences, and I'll touch on some of the ways in which the immune system does this.
00:01:56.28 Now, it turns out that all multicellular organisms have immune systems,
00:02:00.16 whether they are animals, humans, fish, insects or plants.
00:02:05.16 The degree of complexity of the immune system, as you go through different types of organisms
00:02:10.29 that you find on earth, is entirely different, where, you know, humans and dogs and cats
00:02:17.07 have much more complicated ways protecting themselves than do plants and insects,
00:02:23.00 but nevertheless, the same basic principle is conserved throughout evolution,
00:02:27.12 and in fact, you can find even the most primitive types of immune responses, and immune systems,
00:02:32.19 that one can see in even organisms as simple as insects,
00:02:37.04 and also playing a very, very important role in protecting us against the very same pathogens that insects have to be protected against.
00:02:43.19 Now, as I said, immune response is complicated,
00:02:47.18 yep, that's true, but it's really not that much more complicated than anything else in biology
00:02:51.25 and if you break it down in terms of the cells that are involved, that actually make it work,
00:02:57.13 it's a lot easier to understand.
00:02:59.18 Now, it is important to understand, not only because, as I already told you,
00:03:04.02 on a day-to-day basis we have to protect ourselves against invading pathogens,
00:03:07.15 but also, many, many important diseases, such as infectious disease, as we've already been discussing,
00:03:13.07 or AIDS, or asthma or lupus, autoimmune disorders, or even cancer, and various allergies,
00:03:19.10 are caused by breakdowns, or at least are aided by breakdowns of the immune system
00:03:24.17 and as a result, in order to really understand the basis for these diseases
00:03:29.29 and understand their biology, we have to know something about how the immune system works.
00:03:34.22 So, what is the immune system anyway?
00:03:38.04 It's very, very simple, in very, very simple terms, it's a system of specialized cell types
00:03:43.16 that are mostly derived from the bone marrow. I'm sure all of you know what the bone marrow is,
00:03:47.21 especially in long bones, such as shown here,
00:03:49.26 you find a factory of cells that produce many, many types of cells that are found throughout the body.
00:03:57.11 These are called, in the first instance, the most popular and populous of them are called lymphocytes,
00:04:02.22 which come in two basic flavors, T cells and B cells.
00:04:06.20 There are also cells known as macrophages, and dendritic cells and monocytes
00:04:11.06 and K cells, granulocytes, each one of them has a very, very specific function
00:04:15.22 in helping us to protect ourselves against invading pathogens.
00:04:19.22 All of these cells together are called the immune system
00:04:24.00 and they're found all over the place.
00:04:26.10 So, they may form themselves in the bone marrow, but they then span out through the blood
00:04:31.15 and into virtually every tissue that we find in the body.
00:04:34.24 Here, what you're looking at is a picture of skin, which has one type of immune cell in it, called a dendritic cell,
00:04:41.18 which basically sits in the skin waiting for invading pathogens to come,
00:04:45.14 for example, after you cut your finger or something like that,
00:04:49.12 a bacteria will enter into the cut and that bacteria will be detected and recognized by the dendritic cells.
00:04:55.21 The dendritic cells, as I'll show you in a minute, will leave the skin
00:04:59.05 and then migrate elsewhere, through a system of little tubes and vessels, totally separate from your blood vessels,
00:05:06.09 but basically doing the same sort of thing.
00:05:08.14 And this is called the lymphatic system, or lymphatic vessels.
00:05:12.20 These are small tubes, as I said, that are found literally everywhere,
00:05:18.15 lymphoid cells enter into the lymphatics, and start migrating.
00:05:24.08 Where do they go? They migrate and then congregate in a series of small lymphoid organs,
00:05:30.07 well, some of them are not so small, like the spleen, which is found in your abdomen,
00:05:33.19 but the small ones, and in fact probably the most important ones,
00:05:37.11 are called lymph nodes. Now, lymph nodes are little aggregates, and I'll show you this in a minute,
00:05:42.11 of cells of the immune system that have very, very special functions
00:05:47.00 and very, very important activities to perform, but you can find them on yourself, throughout your body,
00:05:52.23 on your arms, legs, chest, everywhere else, but probably most typically,
00:05:57.25 if you just feel on your neck, particularly after you have any type of an infection,
00:06:02.29 such as a sore throat, you can find these little bumps that are there.
00:06:06.26 And I'm sure you probably, many of you know that these are lymph nodes,
00:06:10.11 but this is basically what they do, they're just not there sitting there, they're actually all connected together
00:06:15.13 in a very complex system that is again totally parallel to what's going on in the blood.
00:06:20.12 So, they really exist only to transport cells of the immune system from your peripheral tissues,
00:06:28.14 from your fingers, back to the lymph nodes where they can find other types of immune cells.
00:06:35.12 Now, this is a blow-up of what actually happens in a lymph node.
00:06:40.29 Precisely what happens here at the moment is not really important,
00:06:43.20 but I want you to look at is the fact that you see a great and high concentration
00:06:49.01 of all different types of cells of the immune system, particularly lymphocytes and dendritic cells,
00:06:54.01 that all are talking to each other and basically what they do
00:06:57.16 is they are communicating and trading information on what they encountered in the tissues:
00:07:04.07 what types of pathogens were there and decide among themselves essentially what to do about them.
00:07:10.22 So, these are centralized processing centers in which information about invading pathogens
00:07:18.05 is communicated to different cells that have different functions than actually mounting a protective response
00:07:24.28 against a particular pathogen type.
00:07:27.18 Now, the immune system system consists of two interconnected arms
00:07:31.22 that we call the innate immune system and the adaptive immune system.
00:07:36.01 Innate immunity is the most evolutionarily ancient and is found in insects,
00:07:42.27 in fact, it was first found in insects,
00:07:45.07 and is responsible for detecting components that are shared by virtually all pathogens.
00:07:51.06 So, although there are many, many different types of bacteria,
00:07:53.25 they do have a lot of very fundamental things in common,
00:07:56.24 and the innate immune system evolved to be able to recognize those things that are fundamentally the same
00:08:01.15 from one bacteria to another or from one virus to another or one protozoan parasite to another.
00:08:08.25 Regardless of what their species actually is.
00:08:12.13 The adaptive immune system, though, recognizes those things that are really very specific
00:08:17.08 and very special to an individual pathogen and requires a greatly more amplified
00:08:23.26 and complex series of events to take place at the cellular level in order to allow that type of specificity to take place.
00:08:31.08 These two systems work very, very closely with each other, hand in glove,
00:08:36.04 and how they connect one to the next has only recently been worked out
00:08:40.27 and it turns out to reveal yet another fundamental part of the immune system
00:08:45.12 which is the missing link between the adaptive and the innate immune responses
00:08:49.24 and we'll get to that just in a minute.
00:08:51.02 But just to give you a hint, that's what dendritic cells do.
00:08:55.07 Now, our understanding of how the immune system works really dates back into the 19th century,
00:09:02.16 really to the work of two scientists, the first of whom is shown here, this is Ellie Metchnikoff,
00:09:08.22 who really made a very, very important conceptual understanding,
00:09:12.11 which is that when you see an infection, particularly infection that would occur in the skin,
00:09:17.14 the swelling and the redness and the heat and the pain, and all of this stuff that occurs,
00:09:22.25 that we know is characteristic of infections, is not really an indication of tissue being destroyed,
00:09:29.05 but actually is an indication of the immune system, particularly the innate immune system,
00:09:34.08 trying to do its job, and trying to kill of the invading bacteria.
00:09:38.00 One of the cells, and perhaps the most important one, that Metchnikoff found is the cell called a macrophage
00:09:44.04 and this is one of his original drawings showing what a macrophage looks like.
00:09:48.02 You can see a cell with these long tentacles hanging off the ends of it,
00:09:53.03 plus a lot of structures inside the cell, some of which are colored red,
00:09:57.19 which are actually the intracellular destructive bodies
00:10:00.14 that are to a very large extent responsible for killing and then destroying invading bacteria.
00:10:07.19 And all of this Metchnikoff found out just with very, very crude microscopes
00:10:11.04 and very, very simple techniques, in fact, probably a lot simpler than what's available now
00:10:19.02 in even junior high schools and high schools everywhere,
00:10:22.23 he was still able to work this out.
00:10:24.20 As I said, microorganisms such as bacteria are recognized by macrophages
00:10:29.28 and they are killed, they are taken up, they're ingested by macrophages
00:10:33.14 and this process all enhances the protective immune aspect of what macrophages do
00:10:41.20 and starts the process of inflammation,
00:10:44.15 and inflammation is really the process whereby one cell, after detecting a pathogen,
00:10:49.23 signals to its neighbors that there's something going on, and as a consequence, more cells come in to help out.
00:10:56.25 Part of the reason that inflammation takes place
00:11:00.16 is because the act of killing bacteria involves a lot of agents that really serve as signals
00:11:07.10 that attract other cells of the immune system.
00:11:09.28 So, I've listed some of the more important ones here,
00:11:12.02 cytotoxic agents occur, so that macrophages can make large amounts of hydrogen peroxide
00:11:17.18 which actually is quite effective at killing most bacteria.
00:11:20.23 They secrete enzymes or expose the bacteria to enzymes that will degrade the bacteria themselves,
00:11:29.05 and then also, finally, they will release these inflammatory components,
00:11:33.19 which in the business we call cytokines, that are small proteins or hormones
00:11:39.06 that are released by the macrophage, rather, that attract other immune cells to the site.
00:11:44.29 Now, why is it that macrophages and in fact other similar immune cells are able to do all of this?
00:11:51.23 How can they even know when a bacterium is present,
00:11:54.28 so as not to turn on all of this inflammatory activity when only normal host cells are around?
00:12:01.29 Well, that's because they have a series of receptors
00:12:05.02 on the surface that are really very specialized for being able to understand when bacteria are present.
00:12:12.13 These are called Toll-like receptors which, rather interestingly,
00:12:17.10 were first discovered not because they had anything to do with bacteria,
00:12:20.18 but because they had something very important to do with the earliest stages of fruit fly development.
00:12:26.21 And so these are some images of just what a developing embryo of Drosophila looks like
00:12:32.09 that has mutations in Toll receptors and you can see a normal one in panel A, up on the left,
00:12:39.29 showing what a normal embryo should look like at this stage,
00:12:42.22 and here down in the lower right, you can see a mutant embryo that is not able to form
00:12:47.20 because it's absent of this Toll receptor.
00:12:50.26 But by performing genetic tricks, you can actually get these embryos to form real flies
00:12:56.26 and these real flies are defective in Toll receptors and what you see when those real flies grow up
00:13:02.07 is something like this.
00:13:03.13 So, here, you have a Drosophila blown up at a very high magnification using something called a scanning electron microscope,
00:13:12.25 and Drosophila don't normally have hair, or beards, in quite this way,
00:13:18.21 what you're looking at here is a very serious fungal infection that this fruit fly could not fend off against
00:13:29.29 because the fly is defective in one of these Toll receptors.
00:13:34.16 This is one of the earliest indications that Toll receptors are important in fly
00:13:41.10 for being able to protect them against fungal infections, and indeed, as it turns out, other infections as well.
00:13:47.02 Turns out that Toll receptors, this is just a molecular diagram of what they look like,
00:13:51.20 the details are not important, these are the ones that are found in fly,
00:13:55.14 which is D. melanogaster is Drosophila melanogaster;
00:14:00.02 these are the similar ones that are found in human.
00:14:01.27 And as soon as they were found, just by scanning the genome,
00:14:05.24 it became very, very clear that maybe they were performing a very similar type of function,
00:14:13.20 and in fact, a series of laboratory studies over the years, all fairly recent, by the way,
00:14:18.17 has really illustrated that these Toll-like receptors that one finds in humans and mice and dogs and cats
00:14:26.04 and everything else, really perform not so much of an important function in development,
00:14:32.12 of the human embryo or fetus, but rather in protecting the human adult,
00:14:39.10 and in fact all animals, against bacteria and indeed, all sorts of pathogens.
00:14:44.09 How, again, by serving as the sensor that macrophages and other similar cells in the immune system use
00:14:50.29 in order to detect when a bacterium is present and in order to allow those cells to know when to activate the immune response,
00:14:59.13 the innate immune response, and when to activate inflammation.
00:15:04.13 Now, again, the way these work is that there are many, many different types of Toll receptors,
00:15:09.13 maybe as many as 14 of them now, and they as a consequence of being so many,
00:15:14.22 they really can bracket the entire universe of different types of pathogens and bacteria.
00:15:19.17 I'm showing here a bacterium is releasing a portion of its cell wall,
00:15:24.16 which it can't avoid doing, and there's specific Toll-like receptors that can actually detect those components,
00:15:30.16 things with names like LPS, or lipopolysaccharide.
00:15:34.03 So, these activate the receptors, which then turn on a typical signaling cascade,
00:15:39.14 again, the details of this are not important, but I just wanted to indicate that something that happens outside the cell
00:15:46.00 stimulates a Toll-like receptor, that then generates an event that occurs inside the cell
00:15:51.20 that basically then licenses the cell, in this case a macrophage, to start the inflammatory process
00:15:57.12 and start protection.
00:15:59.06 Now, here what you're looking at is macrophages in action.
00:16:02.07 These are cells that detect bacteria; actually, what they're detecting is a small yeast particle,
00:16:08.17 such as there, and what you can see very quickly, that macrophage, as soon as I pointed at it,
00:16:12.25 came and ate it. The reason it was able to do that was because there are molecules that yeast make,
00:16:19.25 like bacteria, that can bind to specific Toll-like receptors present on the macrophage
00:16:24.25 and track the macrophage towards, in this case, the yeast particle,
00:16:28.21 and then the macrophage both eats and kills the yeast particle at the same time.
00:16:34.02 Now, what you're not seeing in that movie is that the macrophage also was releasing all sorts of cytokines
00:16:40.20 that would stimulate the surrounding macrophages,
00:16:43.18 but nevertheless, it's a pretty clear indication of exactly what happens.
00:16:47.08 I'd like to show you one more video of just what this process looks like in a bit more resolution.
00:16:52.17 So, here you're looking at 3 macrophages that have been stained with fluorescent dyes,
00:16:57.09 the green dye stains the surface of the plasma membrane with the macrophage
00:17:01.05 and the red dye stains these intracellular digestive elements,
00:17:04.23 exactly the same ones the Metchnikoff colored red in his early diagrams,
00:17:08.06 a hundred or so years earlier.
00:17:10.02 These are the lysozomes, which are responsible for both killing and digesting the bacteria that are being eaten.
00:17:17.09 So, the cell on your left, you can see this little crescent shaped thing there,
00:17:21.21 that's where a particle has bound and I just want you to watch what happens
00:17:27.11 when that particle is internalized, you'll see it first surrounded by green,
00:17:31.28 then rapidly, the green turns red.
00:17:34.08 So, what that means is the bacterium is eaten in a piece of membrane that is derived from the cell surface of the macrophage
00:17:42.12 and that intracellular vacuole then fuses, physically, coalesces with these lysozomes,
00:17:49.23 exposing the internalized bacterium to the cytotoxic and digestive enzymes that are found within the lysozomes
00:17:56.16 that are responsible for killing the internalized bacteria.
00:17:59.06 Ok, just to summarize: innate immunity, discovered by Ellie Metchnikoff, he got a Nobel prize for this work in 1908,
00:18:06.18 and what innate immunity does, is to, remember, to recognize shared pathogen derived components
00:18:13.00 that are recognized by Toll-like receptors, and related receptors, but the most important ones for today are Toll-like receptors.
00:18:19.15 The main cell of the innate immune system are phagocytes,
00:18:24.08 are the cells that eat the bacteria that are recognized.
00:18:27.24 These are cells such as macrophages, and another cell type closely related that we didn't talk about, called neutrophils.
00:18:33.18 These are the cells that are the main effectors, as we say, of the innate immune response
00:18:38.17 by protecting us against bacteria and other types of organisms by eating them and killing them.
00:18:45.11 Ok, so let's go on to the next system.
00:18:48.25 This is adaptive immunity, or the adaptive immune system, which was really discovered about the same time
00:18:55.16 as Metchnikoff discovered the innate immune system.
00:18:58.11 The individual really responsible for this was Paul Ehrlich, who also won a Nobel prize for his work, together with Metchnikoff, in 1908.
00:19:06.15 Here he is in his office. And what Ehrlich found was that when you immunize people
00:19:13.25 with foreign proteins, such as a bacterial toxin, or a protein from a cow or a sheep,
00:19:19.06 those individuals make what Ehrlich called protective antibodies in the blood.
00:19:25.01 And you could actually confer protection from individual to the next.
00:19:29.12 These antibodies were very specific to the individual pathogen or the individual protein that was added
00:19:36.22 and, unlike the innate immune system, you just, any protein wouldn't do, but had to be a very specific protein
00:19:46.00 that would be recognized by a different antibody
00:19:48.23 and finally, also, in this realm, although this really was beyond what Ehrlich himself did,
00:19:55.04 it turns out that the antibodies are not made by the macrophages,
00:20:00.05 but they're rather made by antigen specific lymphocytes,
00:20:03.19 which were the other major cell type that I told you about at the very beginning that really comprise the immune system.
00:20:08.17 So, lymphocytes figure out how to make antibodies against these individual proteins,
00:20:14.01 against these individual pathogens, and use those antibodies to help kill the infected cells.
00:20:20.02 Now this is what an antibody molecule looks like in a 3-dimensional structure.
00:20:24.18 It really consists of two major parts:
00:20:26.07 on top, you can see the so called Fab region and on the bottom you can see the Fc region.
00:20:32.00 Now, both have different functions, the Fab region, which actually consists of two arms,
00:20:37.06 is really where the specificity lies in antibody molecules, so it's this portion of the antibody
00:20:44.21 that is capable of recognizing and binding to any one of the thousands, if not millions,
00:20:50.29 of different proteins that are pathogen derived that can come into our bloodstream at a moment's notice
00:20:58.08 as a consequence of breathing in the wrong stuff or cutting ourselves in the presence of the wrong bacteria.
00:21:04.02 Now, this is just a diagram of what this looks like,
00:21:07.26 again, on the top, you can see these Fab domains, or Fab regions,
00:21:11.29 which are responsible for the specificity of the antibodies
00:21:16.03 and the Fc regions, which have a different function that we'll come to just in a moment.
00:21:21.04 And I think it's important to understand this a bit.
00:21:23.29 This illustrates what the different functions of the Fab and Fc regions are.
00:21:28.16 So, here, in green, is a bacterium. It's being recognized by an antibody that's specific to a protein on that bacterium
00:21:36.05 and you can see here we've drawn that the Fab regions are up, attached to the bacteria,
00:21:42.00 because it's the Fab regions that are responsible for understanding and decoding the specificity in the process.
00:21:48.14 The Fc regions are waving off in the breeze, but they have a very specific function.
00:21:52.15 The first instance they'll recruit another protein that one finds in the blood,
00:21:55.27 called complement. What complement does is basically bind to the Fc portion of an antibody molecule
00:22:02.01 and stick a hole in the surface of the bacteria.
00:22:06.08 It's very hard to live, if you're a cell, if you've got holes stuck in you,
00:22:10.13 so as a consequence, this is one way in which antibody molecules all by themselves can help kill bacteria.
00:22:17.07 Another thing that can happen though is that these antibodies can work in conjunction with Metchnikoff's macrophages
00:22:25.02 so that the Fc regions, that fixed complement to put holes in bacteria
00:22:31.03 also will bind to specific receptors that are present on the surface of macrophages and related cells, called Fc receptors,
00:22:38.22 and these Fc receptors have two functions.
00:22:42.24 One is that they will help mediate the uptake of the bacteria by this process of phagocytosis
00:22:48.13 that I've already shown you in the two videos that we looked at,
00:22:51.09 but also, like Toll receptors, binding of bacteria that are coated with antibody to these Fc receptors
00:22:58.18 will also help aid the process of inflammation by enabling the macrophage and related cells
00:23:04.15 to secrete the hormones and the inflammatory cytokines and other components
00:23:10.00 that will basically indicate to the rest of the immune system that an infection is taking place
00:23:14.24 and help is needed at the site where the macrophages detected these bacteria.
00:23:19.28 Where do antibodies come from? As I already mentioned, they come from lymphocytes,
00:23:24.12 but they come from a very specific lymphocyte, which is called the B-lymphocyte,
00:23:28.22 or more simply, B-cell.
00:23:30.13 These are cells that have the capacity to be able to actually molecularly generate
00:23:37.11 this incredibly broad specificity array of antibodies that one finds,
00:23:44.12 and in fact, that one needs, in order to maintain proper immunity in the blood stream.
00:23:48.29 B-cells will continuously mutate the genes that encode for antibodies
00:23:55.15 and these are called immunoglobulin genes and as a consequence of this continuous process of mutation
00:24:00.17 which is very, very carefully controlled, B-cells can generate the type of diversity that they need,
00:24:06.25 in terms of recognition, to be able to provide and secrete antibodies, or release antibodies,
00:24:13.05 that can interact with virtually any type of pathogen that we are exposed to in life.
00:24:19.16 Now, although B-cells can make antibodies on their own, it turns out, of course, the system is more complicated than that
00:24:26.26 because the best antibodies that B-cells can make
00:24:29.06 are only made are only made when they are, as we say, "helped"
00:24:32.12 by a second major type of lymphocyte and these are called the T-cell.
00:24:36.15 So, T-cells interact with B-cells, while B-cells are interacting with the specific proteins derived from a given pathogen
00:24:45.07 and help the B-cells do a better job making even better antibodies than they could possibly have done on their own.
00:24:52.02 Now, how do they do this?
00:24:54.10 Well, turns out that T-cells have their own receptor, they don't make antibodies,
00:24:58.24 in fact, they make nothing that will go into the bloodstream and directly kill a pathogen like this,
00:25:04.27 under normal circumstances, but they will see another, a little bit of the pathogen,
00:25:10.22 or the pathogenic protein that's come in and use another receptor,
00:25:14.03 which not surprisingly is called the T-cell receptor to recognize that small bit,
00:25:19.14 and then, as a consequence of that, release its own hormones that then help the B-cell do its job
00:25:25.03 at making antibodies. This just shows this process in a little bit more clear fashion, I think,
00:25:30.27 so you see over here, the antigen, or the bacterium coming in,
00:25:34.25 it binding to a receptor on B-cells, which is actually antibody molecule that's embedded in the B-cell.
00:25:40.28 A small piece of that antigen is broken off and put on another receptor
00:25:46.00 on the surface of the B-cell, which interacts with this so-called T-cell receptor
00:25:50.03 that then turns on the T-cells, allowing these hormones, or cytokines, to be secreted by the T-cell,
00:25:56.03 basically telling the B-cell what to do, or at least to do its job better.
00:25:59.16 Finally, I'd just like to address the problem of where do T-cells come from?
00:26:03.19 And how do they know what to do?
00:26:05.19 Turns out, of course, that T-cells come in multiple components, or multiple types.
00:26:11.00 The two basic flavors of T-cell are called CD4 and CD8.
00:26:16.21 It's not really important to be able to distinguish between the two.
00:26:20.04 One of them, the CD4 T-cell, is actually the one that's actually responsible for helping B-cells,
00:26:25.18 CD8 T-cells, which we won't talk about today, have an additional property to that,
00:26:30.15 which is that they can actually kill stuff on their own
00:26:33.00 and form a very important component of anti-viral immunity.
00:26:37.18 But we can take that up another time.
00:26:39.14 Now, the way that T-cells develop their own ability to recognize these small pieces
00:26:46.05 of antigen that are derived from various bacteria or other pathogens,
00:26:51.00 is not because they interact with the B-cells, necessarily,
00:26:54.25 but because they interact with another cell type which I mentioned early on called the dendritic cell.
00:27:00.25 Dendritic cells have the special property of being able to also take up bacteria
00:27:06.12 and they don't really kill the bacteria, they analyze the bacteria.
00:27:10.20 And they ask, what type of bacterium it is, and then display small bits of that bacterium on their surfaces,
00:27:19.02 on molecules here that are called MHC class I or class II molecules
00:27:23.20 that have the very, very special property of being able to detect the appropriate recognition sequences on T-cells
00:27:33.24 such that the right T-cells are generated for the right type of bacterial infection that's taking place.
00:27:41.05 Now, the way this actually works is shown in this little cartoon,
00:27:44.12 and so, once again, you maybe can understand it a little bit better.
00:27:48.27 In the left, you see a happy dendritic cell taking up a not-so happy bacteria.
00:27:55.20 Parts of that bacteria are then generated as a consequence of the dendritic cell
00:28:00.27 having the ability to break it down, placing some parts of, small little bits of the bacterium
00:28:06.09 on the surface, small peptides derived from bacterial proteins, for those of you who know what a peptide is,
00:28:11.15 bound to these MHC class I or class II molecules.
00:28:15.11 T-cells will recognize this, the dendritic cell will make additional cytokines
00:28:21.14 and that T-cell, if it sees its right little bit, it becomes activated,
00:28:26.17 very happy, as you can see here and then runs off to find B-cells that it can help in the antibody generation process.
00:28:35.01 So, it's all a nicely closed loop.
00:28:37.13 I would like to show you a video that tries to put all of this together.
00:28:40.21 So, here you're looking at an animation of someone who's just gotten a sore throat,
00:28:44.18 so you can see the sore throat.
00:28:46.12 What happens in a sore throat, of course, is you have, in most cases, a bacterial infection.
00:28:52.21 So, here, you see a bunch of green bacteria that are colonizing, that have infected the throat
00:28:57.26 and are colonizing the surface of it.
00:29:00.04 Now, those bacteria are covered with specific proteins and that's what's shown in green
00:29:06.02 and the proteins then are sloughed off, or released from the bacteria,
00:29:11.03 and enter the circulation and then also enter into the lymphatics.
00:29:18.20 So, these, remember, are these small conduits that cells in the immune system travel through
00:29:24.18 in order to go from the peripheral tissues into lymph nodes.
00:29:27.27 So, cells of the immune system, such as dendritic cells,
00:29:31.02 have taken up these proteins, B-cells have taken up these proteins,
00:29:35.27 and then come back to these central congregating sites that are connected to all of these lymphatic vessels
00:29:42.21 called lymph nodes. We talked about these in the very beginning.
00:29:46.17 So, here, you see, in this particular video, not the cells, but the bacterial protein entering into the lymph nodes,
00:29:52.23 where it encounters all of these lymphocytes and dendritic cells
00:29:56.17 that begin to take up the bacterial protein
00:29:59.09 and whatever bacteria that come in and begin to interrogate what has happened,
00:30:04.23 respond via Toll-like receptors to these proteins and other components and start to be activated.
00:30:12.09 So here you see, now, a blow-up of the surface of the B-cell,
00:30:15.21 these are the receptors on the surface of the B-cell, this one seems to be specific for this particular bacterial protein
00:30:22.07 because you can see as these proteins come down,
00:30:26.00 they bind to these receptors, one and then a second and a third and a fourth.
00:30:31.12 And as a consequence of all of these receptors accumulating together, particularly if a T-cell is around,
00:30:37.12 these B-cells will then generate a signal, as you can see here,
00:30:43.01 that then tells the B-cell it has recognized the right antigen, the one that it was born to recognize,
00:30:50.05 and it basically gets activated and as a consequence of getting activated,
00:30:56.11 it not only starts moving, but as you'll see in a second, it starts to grow.
00:31:01.14 There, it's dividing two and four and eight, 16, et cetera, et cetera,
00:31:06.13 you wind up with a clone of B-cells, in other words, they're all identical to the first one,
00:31:11.17 they just expand, make more and more of themselves,
00:31:14.18 and the reason for that is so that the immune system can then generate a large amount of the antibody
00:31:21.01 that it already had that can then neutralize the bacteria.
00:31:26.20 So, here you see these B-cells after they've grown up, and replicated themselves,
00:31:30.23 now secreting large quantities of these antibodies that enter not only back into the lymphatics,
00:31:38.07 but now also permeate into the blood stream and can circulate throughout the body,
00:31:43.00 including going back to the original bacteria that had colonized the throat,
00:31:48.29 creating the sore throat in the first place, binding to it, complement will fix to it at that point,
00:31:55.14 and here, in this last image, a macrophage or something similar to that is coming in
00:32:01.02 and as a consequence of recognizing the antibody bound to the bacterium,
00:32:04.16 being attracted by the bacterium itself, you can see this macrophage eating these cells and kill them.
00:32:10.12 And eventually, we get better, as a consequence of all this.
00:32:15.17 Ok, just one last word just to make sure that you at least get the basic concept
00:32:22.01 of how the immune system works, what its logic is, what its function is.
00:32:26.12 I want you to remember that the immune system consists of two basic components:
00:32:30.25 the innate immune system, discovered by Metchnikoff, and the adaptive immune system, discovered by Ehrlich.
00:32:36.02 The innate immune system exists and it’s a very primitive form of the immune response.
00:32:41.04 It exists to recognize components that are found on virtually all pathogens,
00:32:46.21 without really distinguishing so well one pathogen from the next.
00:32:49.27 The adaptive immune system, on the other hand, is highly specific
00:32:53.07 and makes antibodies through the activity of T-cells and B-cells that can specifically identify very, very highly individual pathogens
00:33:02.01 and very highly individual viruses and help, working together with the innate immune system,
00:33:09.00 killing them off.
00:33:10.08 Now, as I mentioned, one of the big problems that we've had until fairly recently,
00:33:16.22 just really within the last 15 years or so is really understanding
00:33:20.17 how the innate and the adaptive immune system work together.
00:33:23.28 In fact, Metchnikoff or Ehrlich didn't really understand this either
00:33:27.13 but this is a realization that was achieved by another scientist, just in 2011,
00:33:36.23 received a Nobel prize, tragically died just days before learning of the award,
00:33:42.10 this is Ralph Steinman, who worked at the Rockefeller University in New York,
00:33:47.23 and Ralph was really the person who is responsible for our understanding that dendritic cells exist
00:33:54.01 and that dendritic cells provide this missing link.
00:33:56.27 They are like macrophages in the sense that they have all of the Toll-like receptors macrophages have,
00:34:01.08 they have the capacity of taking up bacteria and other pathogens,
00:34:04.10 and to some extent killing them.
00:34:06.29 But that's not really what their major role in the immune system is.
00:34:10.11 What their role is is to carry the information, small pieces of the bacteria that are preserved on the surface of the dendritic cell,
00:34:18.03 they take that information from the periphery, where the dendritic cell first encountered the bacterium,
00:34:25.21 into lymph nodes and into the lymphoid organs,
00:34:28.14 basically instructing the cells of the adaptive immune response, the B-cells and the T-cells,
00:34:34.01 as to exactly what type of bacterium is present,
00:34:37.12 stimulating their activities, stimulating their growth, and ultimately leading to the protective antibody responses
00:34:44.22 that you saw in the video.
00:34:46.18 Ok, I hope you enjoy understanding something about the immune system
00:34:52.21 because it really is very important and there are a lot of resources,
00:34:56.12 both online and in print, that you can go to in order to be able to learn more
00:35:00.21 and test your own knowledge of this very, very important and very elemental part of biology.
00:35:06.09 Thank you.

00:00:04.01 Hi, my name is Ira Mellman. I'm Vice President of Research on Oncology at Genentech,
00:00:09.09 a large biotechnology company here in San Francisco. Prior to that though,
00:00:13.12 in fact up to 2007 I was a professor at the Yale University School of Medicine,
00:00:17.29 chairman of the Department of Cell Biology and a member of the Ludwig Institute for Cancer Research.
00:00:22.01 During that time and in fact up until today, my laboratory has been
00:00:26.15 very interested in understanding the fundamental mechanisms of membrane traffic.
00:00:30.02 In other words, understanding the movement of cells and membranes
00:00:33.27 and the proteins and lipids that go to comprise those membranes
00:00:37.00 allowing them to work together to make cell shapes and cell functions of various sorts.
00:00:43.00 The last, more recent period of time though, we've become increasingly interested
00:00:47.27 in trying to understand how membrane traffic acts in highly specialized cells to allow them
00:00:53.16 to carry out the variety of important and specialized functions that are associated with specific
00:00:59.05 and more complex systems. The system that we have chosen to work with
00:01:03.02 over the last several years is the immune system, trying to understand how membrane traffic conspires
00:01:09.06 to get cells of the immune system to work and work together to generate an immune response.
00:01:14.02 It's a terrific problem of cell biology which almost inexplicably to me, at least,
00:01:18.05 has been relatively inaccessible to most cell biologists almost as inaccessible
00:01:22.17 as cell biology has been to immunologists. But nevertheless, there is an enormous amount
00:01:26.29 of very important cell biology to be learned by looking at cells of the immune system,
00:01:30.20 but it also provides a lot more. It provides you with the opportunity to actually understand
00:01:35.27 how events that occur at the molecular level and at the cellular level
00:01:39.22 can integrate with each other at the systems level to understand a very complex
00:01:44.09 and important phenomenon, which has a great great deal of relevance with respect to
00:01:49.13 understanding important and very devastating human diseases.
00:01:53.02 Now in order to introduce you to the topic, and let you know what it is you can indeed find out,
00:02:00.00 I have to first review with you some of the basic features of what the immune response is,
00:02:06.05 and how it's organized in its cellular level of organization.
00:02:10.03 So, what is the basic function of the immune system? It seems to be to be able to provide us with
00:02:15.14 protection against a limitless number of pathogens and toxins
00:02:19.07 that one finds in the environment. It's basically conserved in it's most elemental forms across evolution.
00:02:25.16 Invertebrates have a type of immune system and certainly we vertebrates
00:02:29.23 have a type of immune system, which provides us with a terrific amount of protection
00:02:34.16 under normal circumstances to viruses, bacteria, protozoa, and other environmental poisons and allergens.
00:02:42.00 Now, the immune system is extremely good at this and it's also extremely clever.
00:02:46.02 It can kill incoming viruses or incoming bacteria, but it does so in a fashion which avoids the injury
00:02:53.13 to the host, in other words, us, which provides a really important conundrum in
00:02:59.09 understanding how the immune system works. How is it that cells in the immune system
00:03:03.15 can distinguish self from non-self? How can they distinguish
00:03:07.01 the normal cells and tissues of our bodies from the proteins and components
00:03:13.02 that are associated with the protozoa and bacteria and other microbial invaders
00:03:17.07 that are coming in from the outside. This is probably the major fundamental
00:03:21.29 and conceptual problem in understanding the immune system
00:03:24.25 and it's one that undoubtedly will have the cell biological solution associated with it.
00:03:30.02 Now is this complicated? It's absolutely very complicated, but in my view it’s not
00:03:35.03 really any more complicated than anything else,
00:03:37.00 and if you try and understand how the immune system works in cellular terms,
00:03:40.29 through the eyes of the cell biologists, in fact I believe it does become quite accessible.
00:03:45.02 Now, it's worth the effort, as I've said already, because it's not only interesting,
00:03:50.28 but it's of profound importance to understanding and controlling a wealth of
00:03:55.03 important human disorders, infectious disease, is obvious, arthritis,
00:03:59.10 Crohn's disease, asthma, autoimmunity, and quite possibly even cancer.
00:04:04.01 Now the immune system, or immune response rather, consists of two interconnected
00:04:08.08 arms, the innate immune response, or innate immunity, and adaptive immunity.
00:04:14.02 Well innate immunity refers to that portion of the immune response,
00:04:18.15 which is responsible for detecting components shared by all pathogens
00:04:24.27 that we have to deal with as invaders.
00:04:28.00 An interesting concept, in terms of how this might work, and not immediately
00:04:32.15 obvious how it works, but indeed it turns out to be the case.
00:04:35.01 Adaptive immunity is something different. Adaptive immunity
00:04:39.24 refers to the type of immune response that is molecularly crafted to mold and detect
00:04:45.21 individual antigens that are specific to individual pathogens and individual types of pathogens.
00:04:50.21 These two work together in some rather interesting and mysterious ways that we'll
00:04:55.14 go into later, but nevertheless it is important to keep it in mind that there are
00:04:59.10 these two equal and equally important aspects of the immune response:
00:05:03.04 innate and adaptive immunity. Innate immunity was really first discovered
00:05:07.01 by this great scientists of the past, Elle Metchnikoff, shown here working in his
00:05:12.20 laboratory about 100 years ago at the Institut Pasteur in Paris.
00:05:16.00 Metchnikoff's great conceptual advance was the understanding that inflammation,
00:05:22.27 which is what happens after you become infected by the introduction of a bacterium or a virus,
00:05:30.02 this is why your cuts become red and inflamed and hot and painful if they go untreated,
00:05:37.09 this process of inflammation is a protective process of immunity
00:05:41.26 and not a manifestation of the tissues destruction. In other words, as bad as it looks,
00:05:46.01 this is actually a representation of how the body is responding to invading microorganisms,
00:05:52.05 to rid the body of those microorganisms, rather than a reflection
00:05:57.00 of the microorganisms capacity to meet out damage on the hosts tissue.
00:06:03.01 Prior to this point, this idea was completely foreign to scientists.
00:06:10.01 Now the story goes that one of the most important observations that Metchnikoff
00:06:17.09 made was actually made while on vacation on the beach with his family,
00:06:22.03 and like many modern day scientists, who bring their computers with them on vacation,
00:06:27.03 Metchnikoff in the age before computers brought his microscope,
00:06:31.10 and studied the reaction of starfish larvae to the introduction of foreign materials
00:06:38.16 such as bacteria introduced experimentally into the larvae.
00:06:42.02 What Metchnikoff found was that a series of cells would come in
00:06:46.00 and surround the bacterium and in fact, quite visibly, engulf these microorganisms
00:06:52.00 shown here as these red oblong creatures, contained within this cell.
00:06:56.02 He named these cells macrophages, because they were large cells that ate bacteria,
00:07:02.08 macrophage: “large eater". The bacteria were taken into the interior of the cell,
00:07:08.00 and Metchnikoff further found that if he stained these cells with a dye
00:07:10.02 that partitioned into acidic elements within cells and other things,
00:07:17.00 the bacteria stained red. Further indicating to him, and in fact correctly to us,
00:07:22.26 that the bacteria after they entered into the cell would be delivered to
00:07:26.28 an acidic intracellular compartment, which we now understand and know as lysosomes.
00:07:32.01 Now bear in mind, this was well over 100 years ago,
00:07:35.24 and long before even membranes were truly discovered. Now these bacteria eventually stopped growing,
00:07:42.18 they died after being taken up by the cells, and eventually disappeared
00:07:49.00 and Metchnikoff further understood that as a process of degradation.
00:07:51.02 So what actually happens when a macrophage eats a bacteria or encounters a bacteria is shown here.
00:07:58.01 The response is not only limited to the uptake and intracellular killing of the bacteria, but also
00:08:05.01 these cells secrete a variety of important cytotoxic agents,
00:08:09.03 degradative enzymes as well as protein hormones referred to as cytokines.
00:08:14.01 Cytotoxic agents include important mediators, such as hydrogen peroxide,
00:08:18.28 which is a common disinfectant that is used, but macrophages do this on their own,
00:08:25.05 as well as other types of active forms of oxygen. They secrete an enzyme called
00:08:30.06 lysozyme, which is quite adept at degrading the cell walls of incoming bacteria,
00:08:34.17 and a series of lysosomal enzymes contained within the lysosomal structures of
00:08:39.09 these cells that basically degrade all sorts of other things. As I mentioned,
00:08:43.11 the cytokines that are released by cells, by macrophages after encountering
00:08:48.17 bacteria, some of which you may already know, such as the molecule interferon,
00:08:54.03 macrophages are a prime source of interferon have the effect of recruiting more
00:08:59.03 macrophages and other cells of the immune system to the site of bacterial
00:09:03.01 invasion, increasing the overall level of inflammation, but again,
00:09:06.24 not for the purpose of tissue destruction, but rather to recruit
00:09:10.24 more and more cells of greater complexity and of increased levels of sophistication
00:09:17.00 that help deal with the infection and hopefully eradicate it before it
00:09:20.27 gets out of hand. Now why is it that macrophages and other cells
00:09:26.07 of the innate immune system, such as neutrophils,
00:09:28.17 know to respond to microorganisms? What is it that they are recognizing?
00:09:33.18 So here the story turns to a relatively recent observation
00:09:38.27 having to do with a class of membrane protein receptors found on macrophages
00:09:43.02 as well as other cells, called Toll-like receptors. What are these?
00:09:47.18 The story actually starts a little bit earlier than that, in the work of
00:09:52.07 Catherine Anderson and Carl Hashimoto, who identified the activities associated
00:09:56.25 with a well known gene in Drosophila called Toll. Now Toll was originally identified
00:10:05.26 as acting in early embryonic development in Drosophila, specifying the dorsal
00:10:11.13 from the ventral pole of the early Drosophila embryo. Toll was known to bind to a ligand, called Spaetzle,
00:10:19.07 which activated the Toll-receptor, sending a signal to the embryo
00:10:23.01 allowing this dorsal ventral polarity axis to be established. Here you can see some embryos
00:10:29.16 that either do or do not express wild-type Toll, or members of the pathway.
00:10:35.01 In panel A, all the way on your right, you can see what the normal embryo looks like,
00:10:39.26 and in the absence of functional Toll-receptor or an active Toll-like pathway
00:10:43.03 you can see that these embryos completely disorganized and they don't develop
00:10:49.01 and it is known as a lethal defect early in embryogenesis. Now Toll it turns out also
00:10:57.17 has a role in the adult fly, a quite different role, but one that is no less important
00:11:05.06 in so far as the fly is concerned. Here a different group of investigators,
00:11:10.23 lead by Bruno Lemaitre and Jules A. Hoffmann found that Toll was absolutely
00:11:16.18 required in order to protect adult Drosophila from their pathogens,
00:11:21.20 in this case what you are looking at is a poor dead fly that was unable to combat
00:11:26.00 the fungal infection that you can see here, growing almost as a beard, or a beard of death
00:11:34.01 around this fly's torso because of the absence of a functional Toll-receptor
00:11:41.05 in the fly as the adult. This is really one of the very very first indications
00:11:46.19 that Toll has a role outside of early morphogenesis and also plays an important role in host defense.
00:11:55.01 Now how does it do this? Here is a diagram of what the Toll-receptor looks like in flies,
00:12:01.24 it has a nice extracellular domain with some repeats,
00:12:04.22 it has an intracellular domain that one can presume has signaling molecules
00:12:09.01 or signaling features associated with it, but one of the most interesting features
00:12:14.06 that fell out of the analysis of what this receptor looked like,
00:12:17.18 was that it was very very similar to a receptor for cytokine, called IL-1.
00:12:22.02 Again, another product of the macrophage. And specifically where it was similar
00:12:27.17 was in the cytoplasmic domain, here in the case of Toll, and here in the case of IL-1 receptor.
00:12:34.03 Since it was known that stimulation of IL-1 receptor turned on an inflammatory
00:12:38.21 response from other sorts of studies, it was surmised that
00:12:42.21 indeed the Toll-receptor did much the same thing. So in subsequent years,
00:12:48.11 when Charlie Janeway and his colleagues started looking for the expression of these receptors
00:12:54.21 in vertebrates, they found indeed that there is a whole family of Toll-receptors,
00:12:59.08 and indeed well over a dozen of them are now known that have the same basic organization as
00:13:03.26 Toll does in Drosophila with an extracellular domain that is similar in many ways,
00:13:09.07 but most importantly the cytoplasmic domains that have a large degree of homology
00:13:15.14 to the Toll-receptor in Drosophila as well as to the IL-1 receptor.
00:13:20.01 Now what these receptors do, and the reason they are so many of them
00:13:24.17 is that each one has a specificity for a different microbial component.
00:13:29.01 In each case, the microbe itself needs those components to survive.
00:13:34.00 So here Toll-like receptors are expressed on the plasma membrane,
00:13:40.05 such as Toll-receptor four, or TRL-4 and TRL-5, see various important
00:13:46.00 components associated with bacteria. So TRL-4 sees components associated
00:13:52.15 with the peptidoglycan, a lipopolysaccharide component of bacterial coats.
00:13:58.01 TRL-5 sees a major protein associated with bacterial flagella,
00:14:02.01 both of which are components that are absolutely needed by these bugs in order to survive.
00:14:07.03 They are shared by great many types of bacteria, hence these Toll-receptors
00:14:12.01 recognize these conserved molecular patterns and act as the innate immune system sensor.
00:14:18.03 Another set of Toll-receptors are actually found in intracellular vesicles, endosomes
00:14:23.09 and lysosomes found within macrophages and other cells, examples here are TLR-7 and TLR-8, and -9.
00:14:30.01 And they have a tendency to recognize nucleic acid components
00:14:34.09 such as double-stranded RNA, DNA, and single-stranded RNA,
00:14:38.29 which are associated with other types of incoming pathogens
00:14:41.03 such as both enveloped and non-enveloped viruses. So here you have a complete pattern
00:14:48.20 of conserved receptors that have this remarkable capacity to be able to detect,
00:14:54.19 again, the shared components that can be found with a wide variety
00:14:58.13 of different types of incoming and invading microbes, turning on the macrophages
00:15:03.26 and in fact any other cell that happens to be expressing them.
00:15:06.26 Now the way that this turn-on takes place, or the way that this signaling occurs,
00:15:13.06 perhaps not surprisingly, is similar to the way that stimulation of IL-1 receptor,
00:15:19.26 the cytokine receptor, does it, turning on a series of signaling intermediates leading into the nucleus
00:15:27.03 activating I think the most important and simplest element, the so-called Nf-kappa-B pathway,
00:15:33.01 which leads to in turn a great number of events that prime
00:15:38.27 almost everything important that happens in the context of an innate immune response.
00:15:44.02 So intracellular signaling pathways are all activated by TLR receptors,
00:15:49.21 the pathways themselves can be different from TLR to TLR as they can from cytokine to cytokine,
00:15:56.13 but this really summarizes the basic feature of how the system works.
00:15:59.02 Now the consequences of it are shown here, here what you are looking at is a video
00:16:04.24 of macrophages that have been given yeast to play with, and what you can see
00:16:11.11 is that the macrophages move towards the yeast and then engulf them.
00:16:16.00 So here is one that is moving directly towards it, and engulfs it. Here is another yeast
00:16:20.19 that is here, which will eventually be attacked and eaten by another macrophage,
00:16:24.10 shown right there, so in much the same way that Metchnikoff found
00:16:28.03 these invading microorganisms activated the macrophage, turned it on,
00:16:35.28 increased its capacity for migration, its capacity for cytotoxic killing,
00:16:41.14 and as shown here its capacity for actually killing and eating the invading microorganisms.
00:16:49.00 This is another movie showing that the process of phagocytosis of these yeast,
00:16:57.17 or in fact any other large particle that a macrophage encounters,
00:17:00.15 is intimately associated with the ability of the macrophage to polymerize actin
00:17:08.03 as a motile cytoskeletal component directly underneath where the particle binds
00:17:14.02 and turns on itself, growing out long pseudopods that engulf the particle,
00:17:20.12 capturing it and bringing it inside the cell, and eventually getting it to fuse with lysosomes,
00:17:25.09 again shown here. So what you are looking at in this particular video is a particle
00:17:31.02 that is entering a macrophage whose surface is stained green and shortly after internalization,
00:17:38.12 this green membrane turns red as a consequence of fusing with these
00:17:43.06 red endocytic vesicles that are found inside the macrophage.
00:17:47.02 Again, showing that very high tech components which Metchnikoff knew over 100 years
00:17:53.22 ago, which is that macrophages can detect, can bind, internalize,
00:17:58.24 and to deliver to lysosomes for purposes of degradation those incoming bacteria and
00:18:05.03 other sorts of pathogens that are found in the outside world against which we need protection.
00:18:09.02 So to summarize then, Metchnikoff really has to be credited with providing us
00:18:16.02 a great deal of not only conceptual understanding, but in many ways even
00:18:19.25 experimental detail that has defined for us the innate arm of the immune system.
00:18:25.03 The system that recognizes shared pathogen-derived components that are recognized
00:18:30.04 by Toll-like receptors as well as a variety of related receptors that
00:18:33.27 we won't have a chance to talk about. And it is powered to a very large extent by cells
00:18:40.16 which are referred to collectively as phagocytes and these include
00:18:44.19 the macrophage, which are the large cells that Metchnikoff first identified
00:18:48.02 as well as neutrophils, or granulocytes, polymorphonucleocytes,
00:18:53.15 which are actually smaller than macrophages, and indeed Metchnikoff had seen these too,
00:18:58.12 he referred to them as microphages. These mediated protection by
00:19:02.05 activating cytotoxic mechanisms, and as you have seen, quite graphically,
00:19:05.19 by actually physically eating, clearing, and degrading extracellular bacteria
00:19:10.24 and other types of microorganisms. Now, almost at exactly the same time
00:19:15.05 that Metchnikoff was working, elsewhere in Europe another
00:19:19.07 terrific scientist, Paul Ehrlich, was also hard at work
00:19:24.01 and you can tell from this image that Ehrlich was a terrific and great scientist
00:19:29.06 because he has one of the hallmarks of great scientists, which is an incredibly messy office.
00:19:34.00 So here he is sitting, reviewing some notes, and what Ehrlich's contribution was,
00:19:41.20 was to show for us how the other main arm of the immune system was organized and what it did.
00:19:49.03 This is called "adaptive immunity". What Ehrlich identified was that individuals,
00:19:56.10 whether it be the humans, it be the animals, that are immunized with various types of microorganisms
00:20:02.03 or bacterial derived toxins, make a response in the blood that is inherently protective,
00:20:10.06 and in fact Ehrlich coined the term "antibodies" to describe this response,
00:20:15.01 which has the ability, obviously, of protecting us against whatever it is that had entered
00:20:20.27 into our systems or into our bloodstreams.Antibodies see individual antigens
00:20:26.27 that are specific to individual pathogens types, so if you immunize with one type of bacteria,
00:20:33.02 you will not necessarily make antibodies that will recognize and protect a second individual
00:20:38.02 against another type of bacteria. So in this way, the system is fundamentally different
00:20:43.21 from the innate immune system. It's crafted in a molecular fashion
00:20:47.22 to the molecular entity that happens to be coming in. The way the cellular
00:20:54.00 mechanism, whereby the adaptive immune system works, is using not so much
00:20:58.11 macrophages, as we know now, but rather making use of antigen-specific
00:21:02.21 lymphocytes that indeed make these antibodies that recognize and kill the infected target cells
00:21:08.03 either separately from, or as you will see in a minute, in conjunction with the macrophages.
00:21:14.01 This is what an antibody molecule looks like in crystal structure. It consists of two major parts
00:21:21.04 a Fab fragment and a Fc fragment, both of which are linked together,
00:21:26.01 the molecular weight is about 150,000 Daltons and consists of four chains:
00:21:31.28 two light chains, shown in red, and two heavy chains shown in yellow and in blue.
00:21:37.01 The antigens themselves actually bind to the so called Fab regions of antibody
00:21:45.26 molecules, demonstrated in this diagram, out near the tips. So here you find a
00:21:50.26 great deal of sequence variation that allows the antibody to form complementary structures
00:21:56.02 allowing it to interact quite specifically with whatever antigen comes into an organism,
00:22:04.03 against which an antibody can be made. Now how antibodies work.
00:22:10.01 After recognizing their antigen, they are made, and as shown here they'll bind
00:22:16.03 to the antigen and this particular example that antigen is present on the surface of the bacterium,
00:22:21.20 and one of the simplest ways in which the antibody can work to kill
00:22:25.29 the bacterium is to recruit another set of proteins that are found in our plasma
00:22:31.00 called complement, which actually is a number of proteins, not just one. Complement is
00:22:37.00 recruited by antibodies, inserted into the bacterial membrane causing
00:22:41.23 the bacterium to lyse. Now in addition, these antibodies will work together
00:22:46.02 with element of the innate immune system, macrophages in particular,
00:22:49.26 but also neutrophils. Macrophages are probably more important
00:22:53.00 since macrophages actually have receptors for the Fc domains of these antibodies,
00:22:58.04 which have some of the same effects that Toll-receptors do,
00:23:02.01 in other words binding of antibody coated microorganisms to macrophages will trigger
00:23:06.24 the release of the same types of cytotoxic compounds that we've already discussed
00:23:11.01 and also trigger the ability of the macrophage to mediate phagocytosis
00:23:15.17 and intracellular killing and destruction of these antibody associated
00:23:21.09 or antibody coated microorganisms. So here you have this nice example of the innate system
00:23:26.12 working together with the adaptive immune system. Now antibodies are not made by
00:23:31.20 macrophages, although that was an idea that was originally thought of by
00:23:36.01 Metchnikoff and by Ehrlich who by the way shared the Nobel Prize
00:23:39.11 for their discoveries back in 1908, 100 years ago this year. Antibodies instead
00:23:45.16 are made by lymphocytes, a particular class of lymphocytes, as many of you know I'm sure,
00:23:49.27 called B lymphocytes. B lymphocytes exist in great number found throughout
00:23:57.22 a variety of lymphoid tissues in the body, and have a repertoire
00:24:04.03 of potential interactions, in other words, due to inherent and preexisting variability in
00:24:10.11 that small Fab region, or complementarity determining region
00:24:15.01 found in the Fab region of antibody molecules, incoming antigens can find or B cells
00:24:23.13 rather with the right specificity can find and actually bind to
00:24:27.00 antigens associated with bacteria, or antigens that are simply with soluble proteins,
00:24:32.09 and the simple act of binding of the antigen to the antibody,
00:24:37.03 which at this point in a B-cell's development is actually a membrane protein receptor
00:24:41.02 on the B-cell, is sufficient to simulate development of the B-cell and finally the
00:24:46.22 production of an amplification of more and more of these antibody molecules.
00:24:51.03 Now this is a video that shows you how the system works, when you get a sore throat,
00:24:57.27 which is indicated here in red. What that's again a manifestation of,
00:25:02.27 as Metchnikoff told us, a protective response, not a destructive response.
00:25:07.01 So here you see bacteria lining the surface of your throat following an infection
00:25:14.02 and these bacteria are putting off a variety of protein antigens that are associated with these bacteria,
00:25:22.03 so not only have you generated an innate immune response,
00:25:25.24 but now these bacterial specific antigens are being released and what happens to them is that they begin
00:25:33.13 to drain into the intercellular spaces, that one finds in all of your tissues
00:25:39.08 referred to as the lymph. The lymph drains into the lymphatics,
00:25:42.25 which then comes into these nodes, or specifically lymph nodes that monitor every 24 hours a day
00:25:52.04 what proteins, what antigens, what bacteria have entered into the lymph.
00:25:57.03 So here you see our bacterial protein entering into the lymph node, and what they
00:26:02.07 encounter here are the wide array of lymphocytes and other cells and here you can
00:26:08.01 see them in a higher magnification and as our bacterial derived proteins come in,
00:26:12.26 they filter through these millions or billions of cells as you can imagine,
00:26:20.22 and cells that are B-cells that may detect a component of that antigen will bind to it,
00:26:29.23 based on these antibody molecules or via these antibody molecules
00:26:33.07 that are intercalated as membrane proteins on the surface of these early B-cells shown here
00:26:38.19 clicking on, and this will then hit onto another B-cell receptor, and a third
00:26:45.05 and a fourth, until enough of them come together to actually generate a signal
00:26:50.00 to the B-cell, which says that I have now detected the antigen I was born to detect
00:26:57.10 and as a result, it is now time to start making more of myself. So these B-cells
00:27:03.23 become very active and begin to develop and divide.
00:27:10.29 The affinity that they have for the individual antigens, in essence, increases as B-cells with increasingly
00:27:20.24 large affinities, increasingly great affinities for antigens,
00:27:24.07 compete with each other to bind more and more. So here you see a clone of B-cells that gives rise
00:27:31.07 to a larger number of itself and then as you can see here, in this representation,
00:27:39.13 antibody molecules begin to be released into the extracellular space
00:27:44.19 and here is just an indication of the secretory event that takes place very similar
00:27:51.10 to that which is described in another of the iBioseminar science series given by Randy Schekman.
00:27:59.01 Now here the antibody molecules at this stage in development are actually pentamers
00:28:04.00 coming back homing to the bacteria, coating them with antibody molecules,
00:28:09.11 recruiting complement, and, as shown here also recruiting macrophages,
00:28:13.28 which again, just as Metchnikoff has already shown us, will come and eat those
00:28:18.08 bacteria that are coated with antibody taking them up by phagocytosis,
00:28:21.27 killing them and degrading them. Now it turns out that things are not that simple,
00:28:29.28 of course. What I've just shown you in that video isn't in every sense correct,
00:28:37.20 but it turns out that B lymphocytes make the best antibodies when they are helped
00:28:41.02 by another type of lymphocyte, referred to T, or thymus-derived, lymphocytes.
00:28:46.18 And the way that works is that here you can see an extracellular antigen coming,
00:28:50.28 binding to the surface antibody molecule of B-cells and then it turns out that
00:28:56.09 the antigen is internalized by endocytosis into the B-cell, and a small fragment
00:29:01.23 of this antigen is cleaved off and is bound to a molecule referred to as a major
00:29:07.02 histocompatability class II molecule, or MHC class II molecule. This complex of the
00:29:13.17 peptide plus the MHC class II molecule is then recognized by a preexisting T-cell,
00:29:19.26 which has a different type of antigen receptor referred to as the T-cell receptor,
00:29:24.11 again which has a great deal of diversity built into it as a consequence
00:29:30.24 of the development of T-cells in the early development of the immune system
00:29:37.09 and what the T-cell then does, is to secrete again our friends the cytokines,
00:29:41.06 in this case a class of cytokines, which helps the B-cell develop, helps them
00:29:47.28 divide even more, and helps them create antibodies with every increasing affinity.
00:29:54.22 Now the way the T-cell system works, is illustrated in a little bit more detail here.
00:30:00.08 T-cells come in two general flavors, and we'll come back to this later,
00:30:04.28 CD4 T-cells, as well as CD-8 T-cells. We've been talking about CD-4 T-cells,
00:30:09.27 which recognize bacterial or other derived peptides bound to these so-called MHC class II molecules,
00:30:16.23 and they do this by the virtue of having these T-cell receptors, which again have
00:30:22.14 their own specificities associated with them, such that by random chance,
00:30:28.08 the appropriate T-cell receptor is made for this particular complex of peptide and MHC class II.
00:30:36.00 Diversity in the T-cell receptors, rather than being so much generated by mutation,
00:30:41.00 which is what occurs in the antibody in the B-cell system, occurs by
00:30:45.11 rearrangement of the T-cell receptor gene, again rearrangements that cause a greater number of
00:30:53.15 potential combinatorial sequences that can recognize a wide array
00:30:57.27 of these peptide MHC complexes. So that said CD4 T-cell sees MHC class II molecules to a large
00:31:06.04 extent because it makes the second membrane protein called CD4,
00:31:09.17 which also recognizes the MHC class II molecules, but recognizes an invariable portion
00:31:16.01 of those molecules. CD8 T-cells express exactly the same type of T-cell receptors,
00:31:22.02 but they're targeted to a different type of MHC molecule, in this case a MHC class I molecule
00:31:28.01 and the reason this particular targeting takes place is because the CD8 membrane
00:31:33.01 protein found on the CD8 T-cell, recognizes the class I molecule and not the class II molecule.
00:31:41.01 So very complicated and this is obviously a simplistic view of how this works,
00:31:47.10 but at its most elemental form, this is how T-cells cooperate with B-cells
00:31:52.27 and in fact with a variety of other cells as well. Now CD8 restricted T-cells
00:32:00.28 or those T-cells that see antigens bound to class I in general don't see antigens
00:32:07.07 that are coming in from the outside. In much the same way,
00:32:11.11 or at least I should say, analogous to what happens in the case of Toll-like receptors
00:32:16.06 that are expressed either on the surface of a cell or inside, the CD4/CD8 system, or class I and class II
00:32:23.08 MHC systems are adapted to incoming pathogens that reveal themselves
00:32:28.04 either on the outside world or on the inside world. The CD8 system and the class I system
00:32:33.28 take care of those things that are going on in the inside world.
00:32:37.01 So when viruses infect our cells, those viruses almost invariably work by binding to
00:32:43.14 receptors on the surface of the cell and fusing, or entering, the cell
00:32:48.25 either at the level of the plasma membrane or following entry into endocytic vesicles,
00:32:53.22 and then breaking out into the cytoplasm. That type of endogenous pathogen,
00:33:00.16 because you can now refer to it as endogenous because it's found inside the cell,
00:33:06.15 that type of pathogen is treated by the immune system to generate peptides
00:33:11.29 that are then loaded onto class I as opposed to class II molecules. Now the way
00:33:17.13 these T-cells work, again, is somewhat analogous to what one finds in the
00:33:22.09 innate immune system, except that it is all very specific and very antigen driven.
00:33:26.01 So here you are looking at a cytotoxic CD8 positive (+) T-cell,
00:33:30.15 this is work of Julian Griffin at the University of Cambridge in England, and what Julian
00:33:36.18 has noticed as well as others over the years is that these cytotoxic CD8+ T-cells, when they recognize
00:33:44.07 a virus-infected target in fact polarize all of their lysosomes,
00:33:50.08 which actually now are differentiated to contain a wide variety of cytotoxic compounds,
00:33:55.27 towards the infected T-cell. These granules line up at the interface, or synapse,
00:34:02.19 between the T-cell and the target, and are eventually released, killing the target.
00:34:08.01 This is what you are looking at here in an electron micrograph also from Julian's work.
00:34:12.12 Here you can see these cytotoxic T-cell, or CD8 T-cell, or CTL granules
00:34:18.01 in the region of the cytoplasm that is very very close to the infected target cell.
00:34:24.02 These granules contain lysosomal enzymes, various lytic enzymes, ligands
00:34:30.04 such as Fas and receptors that will induce apoptosis or cell death in the target,
00:34:36.01 and also perforating molecules called perforins among others that will punch holes
00:34:43.10 in the target cell. All leading to its rapid death. So in other words, the virus has
00:34:48.15 infected this cell, think its safe by existing within the cell and then the cytotoxic
00:34:52.16 T-cell comes along and recognizes a peptide derived from the virus,
00:34:56.17 releasing the granules at the site of the infected cell, thereby killing the cell.
00:35:02.02 This is just the diagram showing how this process works. In fact over the last several
00:35:07.16 years, as a consequence of interrogating some important mutations
00:35:11.13 that lead to defective CD8 T-cell responses, which Julian and others have shown,
00:35:17.00 is that these granules actually have to physically polarize by moving on microtubule tracks
00:35:22.19 from diverse places within the cytoplasm to this interface, this synapse,
00:35:27.14 between the T-cell and its target. In the absence of genes that are required
00:35:33.10 for the translocation of these granules to this point in the cell, Rab proteins,
00:35:40.02 microtubule binding proteins, etc. these T-cells are incapable of docking
00:35:47.11 with or incapable of fusing with the plasma membrane of the T-cell close to the target.
00:35:54.02 Again, emphasizing one critical element of where membrane traffic
00:35:58.25 plays a major role in allowing the immune system to conduct its job.
00:36:03.00 Now how does this all work, we've discussed already a bit how TLR's activate
00:36:11.16 cells such as macrophages in the innate immune system. T-cells become activated
00:36:16.20 in two very important ways. The T-cell receptor itself is a signaling molecule,
00:36:23.01 recruiting tyrosine kinases to its cytoplasmic domain after it recognizes its ligand,
00:36:29.22 which as we've been discussing is a complex of peptides plus MHC molecules.
00:36:34.00 But also a variety of other interactions take place so-called co-stimulatory receptors
00:36:41.02 which are other receptors that are present on the surface of T-cells,
00:36:45.02 recognize a variety of other molecules that can be found on the surface of infected cells
00:36:49.28 or on B-cells or on other cells of the immune system, that send further signals
00:36:54.24 to the T-cell, enabling it to more effectively do its job and also amplify its own
00:37:01.03 numbers in a way that is selective and antigen driven,
00:37:05.12 quite analogous to what happens in the case of B-cells. Further, selected cells in the immune system
00:37:12.05 which are referred to as antigen-presenting cells or APCs,
00:37:15.22 which we'll come back to in a later lecture are capable of secreting their own sets of cytokines,
00:37:22.13 which have the effect of further accentuating and enhancing T-cell responses.
00:37:27.01 So here we have two aspects, two critical aspects of the immune system.
00:37:32.04 Innate immunity, which is characterized by direct cellular responses
00:37:37.09 to pathogens by detecting shared microbial patterns that is not antigen specific,
00:37:43.09 working together hand in glove with Ehrlich's observation of the adaptive immune response,
00:37:50.20 which generates specific antibodies to variable antigens,
00:37:54.01 and occurs as a consequence of an antigen stimulation of T-cells and B-cells. These two things,
00:38:01.09 as I've already intimated to you are connected, but they're connected in even a more intimate
00:38:06.01 fashion, which we'll turn to in the next lecture, provided by a recently identified
00:38:11.10 cell type referred to as the dendritic cell, which really we now understands
00:38:15.16 provides the long sought after missing link between the innate and adaptive immune system.