Microbiology: Mucosal Immunologypg. 1
Sonia Nkashama
1. I am going to talk to you guys over the next hour about a component of immunology that sometimes seems like the dark side of everything else that you learn in this course. We are going to talk about mucosal immunology, and it has some very unique features that make it different than the systemic immunity that you’re hearing about in a lot of the other lectures. This does, however, apply to the things that you all should be thinking about because essentially the mucosa that we talk about is the entire gastrointestinal tract (starting with the mouth) and includes some of the special functions of any other surface that is exposed to pathogens and bacteria, which would include the eye. If you have any questions, feel free to email. After we are done, I am happy to answer any questions. And I am happy to answer during the talk too.
2. Why do we need to know about the mucosal immune system? As I just indicated, the mucosa is essentially the most important area of contact in the body for any type of foreign antigen. This includes any type of bacterial or viral challenge; it also includes any kind of contact with food products. Anything that you potentially put in your mouth or in your eye, your mucosal immune system has to deal with. In addition, gastrointestinal diseases kill more than 2 million people every year. So it’s actually important that we try to figure out why it is that your mucosal immunity doesn’t protect you against a number of these different diseases. Finally, we have almost no effective mucosal vaccines. The polio virus vaccine is essentially the only real example of a good mucosal vaccine. We are now starting to get some of the nasal flu vaccines that are starting to work, and the reason we don’t have good effective mucosal vaccines is that we don’t really understand the mucosal immune system very well. Since vaccines administered mucosally are essentially the easiest way for us to do vaccines in Third World countries, we really would like to develop more of these types of vaccines.
3. When we think about what the mucosa is exposed to, it’s bombarded by a lot of foreign antigens. For instance, whatever you ate at lunch, unless you have eaten that exact thing before, it probably had some type of antigen in it that your body has never been exposed to. Your mucosal immune system knows that it’s supposed to ignore those things. So unless you have a peanut allergy or an allergy to a known food product, your body ignores or is tolerant to food proteins. That’s because of your mucosal immune system. Other things that your mucosa is exposed to are various types of bacteria. Some of those bacteria are what we think of as commensal bacteria or bacteria that normally live within our body. Your mucosal immune system has to be able to realize that those are bacteria that should be there, that they’re beneficial for bodily functions, and that it should not respond with some type of inflammatory response to those organisms. However there are other organisms, like Salmonella or some type of virus, that your body does need to respond to and needs to get rid of. The mucosal immune system has to be able to tell the difference between these types of organisms and respond. What your mucosal immune system needs to do is: if it’s a pathogen, it needs to eradicate it. Or at least contain it within the GI tract and not allow it to get across into the systemic immune system. If it doesn’t do those two things, then some type of disease will occur.
4. These are the things I want you to get out of this presentation. They are fairly straightforward, and we will go over every single one of these. You need to be able to identify the major populations of lymphoid cells that are in the intestinal tract. Almost all of the examples of what I will be talking to you about will come from the intestine. That’s because that is the mucosal surface that has been studied the most. We believe that the principles that are found in the intestine apply to the entire mucosal surfaces, including things like the lung and the skin. But most of the examples that have been done to date come from the intestinal tract. You need to be able to describe how lymphocytes migrate or home into the intestine. Know what secretory immunoglobulins are and how they get transported through the mucosa. And understand the concept of mucosal immunity versus mucosal tolerance.
5. When we think about tolerance to either food products or through the commensal bacteria, if you think about how many microorganisms there are within the human GI tract, it actually seems like it’s a daunting task. We actually have more than 1014 different bacterial organisms that colonize throughout our GI tract. These come very rapidly after birth. Especially in normal vaginal deliveries, that is where the human infant gets colonized initially with these GI microbiota. Over the period of about the first year of life, those bacteria get to the level that they will be for the entire adult lifetime of that organism. There are at least 400 different species of bacteria that are known to be in the commensal microbiota. And each individual has a different pattern. Even individual twins, because they’ve been exposed to slightly different environments (different foods, maybe one puts their hand in their mouth, maybe one doesn’t), they each have different, immunologically distinct microbiota. This actually makes it very challenging when you think about how tolerance exists because each person is tolerant to all of these commensal microorganisms. This may make their immune response to other organisms altered, depending on what their commensal flora is.
These organisms are known to have a symbiotic relationship with the host. They actually help to provide nutrients, help digest various food products, etc. They therefore help the host. And one of the things that you see a lot about in advertising today or in the grocery store is probiotics. Probiotics are microbiota, or bacteria, that are thought to have a positive effect on human health. Therefore, essentially, the thought is that you can slightly alter an individual’s commensal microbiota by taking probiotics, therefore improving their health.
Almost all of the claims that are made about probiotics are not backed up by scientific experiments. The only ones that actually are done in a double-blinded fashion are the effects of probiotics on pouchitis, which occurs after surgery for ulcerative colitis. None of the other claims are really backed up strongly. Now, I fully believe that probiotics do something; we have a grant based on probiotic effects on diabetes. So I think that there will be proof that probiotics will actually alter the mucosal immune system and cure systemic diseases, but the data isn’t entirely out there yet.
Question—What’s an example of a common probiotic? Yogurt that has active cultures in it, like Lactobacillus acidophilus. There’s now cheese & Activia yogurt. There are a number of examples out there. A lot of them are a lactobacillus type of organism because that is known to not cause problems in the majority of the human population.
6. If we think about commensal microbiota, and we look in the oral cavity, what you know is that the oral flora (or the oral bacteria) is dominated by an alpha-hemolytic strep. But you have to take all of these types of comments with a grain of salt because almost everything that’s been done, as far as what microbiota are in various components of the mucosa, are based on culture. We now know that a lot of bacteria we don’t know how to culture. So this (alpha-hemolytic strep) is essentially the predominant bacteria that we know how to culture.
One of the interesting things about the oral flora is that it really demonstrates how there’s different microcommunities of bacteria that are there within the microbiota, within a very small area of the body. Take for example the soft tissue of the tongue versus the teeth. In the soft tissue of the tongue, the predominant organism is Streptococcus salivarius; there’s not much Strep mutans. But if you look at the teeth, then essentially it’s the opposite. So Strep mutans is found in large numbers on the teeth, but Strep salivarius is not. Each of these organisms has their own niche within the various areas of the body in which they live.
7. As you go on down the GI tract, the types of bacteria that you see that essentially affect the mucosal immune system are controlled by a number of different factors. For instance, in the stomach, there are actually not very many bacteria at all because of the very low gastric pH. The pH of the stomach is somewhere between 2 & 3 in the normal individual, and there’s not very many bacteria that can live there. If you have ever heard of an infection with a microorganism called Helicobacter pylori, this is actually why H. pylori can easily infect the human population. It’s one of the only organisms that can live in a pH of 2-3, and there’s not much else there to compete for it. As you go on down, you can see that peristalsis, the movement of the bacteria through the GI tract, will affect which bacteria can live there and adhere. The type of mucus and the different types of antimicrobial products that are secreted by various parts of the intestine also affect what types of bacteria are there. So all of these come together to sort of shape the commensal microbiota.
8. The gastroenterologist and the dentist can fight.
9-10. Why am I telling you, in an immunology lecture, about the microbiota that are in the GI tract or are adherent to the mucosa? What we know is that these bacteria are actually not required for survival of the human species or any animal species that have been generated because this experiment has not been done in humans. You can make mice and rats germ-free, which means that they are essentially in a sterile environment. And if you do that, those animals actually do live and can reproduce. However, interestingly enough, those animals grow at a very different rate than normal litter-made animals that would have that bacteria or normal commensal microbiota. So it’s very clear that having those microbiota there affect the nutrition of the animals.
Now the other thing that we know is that having those microbiota there actually forms a part of the mucosal immunity; in this case it would be an innate-type of immunity by something called “colonization resistance.” Colonization resistance is essentially this concept that is shown here in this diagram (slide 10). So what we see here is the intestine, but this would be true in any mucosal surface. In the normal mucosal surface, there actually are these commensal microbiota that form essentially a layer on top of the normal mucosa. Now if you get rid of those microbiota, let’s say by a patient that you’ve given a lot of antibiotics t. Then what happens is that the normal, protective layer is not there. And then the epithelium of the mucosa is actually now exposed, and other pathogenic bacteria can come in and adhere and have a much easier time setting up the pathogenic infection. This has most clearly been shown with infection with Clostridium difficile in patients in the hospital that have gotten broad-spectrum antibiotics for various types of infections. Those patients very commonly will come up with a severe diarrhea that is secondary to C. difficile infection. So now when we treat patients with broad-spectrum antibiotics, we actually try to get them to eat yogurt and stuff that has live active cultures in them so that they can keep this from happening.
The other thing that we know is this intestinal microbiota or microflora affects a number of immunologic functions. In these animals that are germ-free, their immune system (both their mucosal & systemic immune system) doesn’t develop normally. We now know that you need exposure to these organisms in order to get development of a normally functioning immune system.
11. We’re going to talk over the next few slides essentially about the organization of the mucosal immune system. It has several unique compartments that in some cases have correlates in the systemic immune system; and in some cases, they do not. So first we will talk about the organized mucosal immune system. By organized, we mean that there are actual structures that you can see throughout the mucosa. The most obvious one that everybody’s seen in their bathroom mirror are the tonsils. So tonsils are essentially like lymph nodes that are found in the back of your throat. Adenoids are essentially the same. Peyer’s patches (PP), isolated lymphoid follicles, and the appendix are also all essentially lymph node-like structures that are found within the mucosa. We are going to talk about Peyer’s patches in a little more detail and talk about what the differences are between Peyer’s patches & a true lymph node.
Then we are also going to talk about what some people call the diffuse mucosal immune system. That is cells in the immune system that we can identify by cell-surface markers. If you actually just look at a histological slide, you can’t really ID them as a structure in the same way that you can with the organized mucosal system.
12. This is just a diagram that shows you overall where these three types of structures are. So here in the mucosal immune system, what we are going to show you is the Peyer’s patch. So this is the organized lymphoid tissue that functions similar to a lymph node. If you look at sort of the loose connective tissue that are underneath the villi of the intestine (in this case), and if you think about the intestine, its function is actually to absorb nutrients. So it does that by having all of these little fingerlike structures called villi so that there is a huge surface area in your intestine. It’s estimated that the surface area of a normal adult human intestine is essentially the size of a basketball court. So there’s a lot of cells there and a lot of space underneath these villi that can be filled with lymphocytes. And we’ll talk more about what type of lymphocytes are there.
That’s called the lamina propria [refer to the slide]. And then those villi in the entire GI tract are lined by a simple epithelium, and those cells are all involved in absorption functions of the intestine. And there’s a basement membrane underneath them that separates those cells from the rest of the lamina propria, Peyer’s patches, etc. Above that basement membrane, up in the epithelium, by the little green cells, are cells known as the intraepithelial lymphocytes (IEL). Those are lymphocytes, and we’ll talk about the kind of lymphocyte they are. They essentially migrate through that basement membrane and are found up within, in close contact with, the actual epithelial cells.
13. Here is a drawing of what a Peyer’s patch sort of looks like. And then to the right is a histologic picture of a real Peyer’s patch. So what you can see up here is these are the villi of the intestine. And what you see down here is what would look like a structure that you can see in any type of lymph node. What you have is a very large follicle, and in that follicle is primarily a B cell follicle. Within the follicle, if it’s activated/if it’s seeing antigen, then it would have a germinal center. Next to that germinal center, you essentially have T cell areas. So this type of structure is essentially the exact same type of structure that you see within a lymph node. Now the one thing that is really different about a Peyer’s patch than a normal lymph node is that most normal lymph nodes don’t have at their resting state a germinal center. Lymph nodes throughout your body are not normally exposed to antigens, unless you’ve been infected by some type of virus, etc. And you’ve all experienced that if you get some kind of tonsillitis or something, your lymph nodes in your neck grow because you’re getting activation of these cells within these follicles. And the germinal center, which is the proliferating cells, is essentially expanding. That’s the one thing that is kind of different about the Peyer’s patch. Because you’re constantly eating things and you constantly have bacteria coming through your GI tract, it’s always seeing antigen. So almost all Peyer’s patches have germinal centers in them at all times because they’re constantly being activated. And that constant activation is something that you will see throughout all the different aspects of the mucosal immune system b/c it is in constant exposure to antigens, unlike the rest of the immune system.
14. So we believe that PP’s play a very key role in the induction of the immune response to antigens that are seen via the GI tract or the oral cavity. The thing that differs about Peyer’s patches [“you should note”--hint**] is that they are different from other lymph nodes in that they do not have afferent lymphatics. So normal lymph nodes have lymphatics that drain some part of the body. The antigen comes through those lymphatics, into the lymph node, and then the cells are stimulated within the lymph node. Then there’s an efferent lymphatic that takes the cells out. Peyer’s patches do not have these afferent lymphatics; instead they get stimulated essentially through the epithelium b/c they are right up underneath the epithelium of the intestine or, as you will see, the nasal cavity. So the antigens essentially directly contact the PP. That’s one major difference about these from regular lymph nodes.