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Microbiology
09/03/2008
Mucosal Immunology
Transcriber: Sonia Nkashama
52:07
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.
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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
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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
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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.
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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.
In the intestine, the efferent lymphatics drain into the mesenteric lymph nodes (LN), and then
they circulate throughout the rest of the body. As you’ll see a little bit later in the talk, they essentially
have homing receptors on their surface that make them know that they should come back into the
mucosal surfaces. We already talked about the fact that they contain B lymphocytes, dendritic cells
(DC), macrophages, etc., whereas the dome and the parafollicular regions (the two side regions shown
in slide 13) contain T cells.
The other thing that’s unique about the mucosal immune system is that the B cells in the
mucosal immune system switch to a unique type of immunoglobulin known as IgA. So IgA is almost
entirely found in various mucosal surfaces, whereas you don’t see very much of any of the other
immunoglobulins (IgG, IgM, IgE, etc). Almost all of the immunoglobulin in a normal mucosal surface is
IgA. In the Peyer’s patch, you essentially don’t have the plasma cells to make the IgA. You have B cells
that are being activated; they migrate out of the PP, and then the plasma cells live in the lamina
propria.
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pg. 6
15. Remember I told you that PP don’t have afferent lymphatics. So how do they get their antigens?
They get their antigens because the epithelium that’s over a PP is a very specialized epithelium. It’s not
like the epithelium that’s over the rest of the villi in the intestine. This is true of the epithelium that lies
over the PP’s or the follicles that are in every single part of the mucosa. They have essentially what’s
called a Follicle-Associated Epithelium (FAE). Within that epithelium there are these rather unique cells
known as M cells. An M cell is a very critical cell in the initiation of the immune responses within the PP.
So M cells are called “M” because under electron microscopy, they look like they have little microfolds.
So they are named M because of microfolded cells; some reviews say their name came because they’re
membranous. They have a lot of membranes. You can see that it is defined either way.
16. The M cells are right up there in the epithelium. They actually have little pockets in them where
normal immune cells can actually live. So these pockets make it so that these immune cells are in very
close contact with this lumen where all the antigens are. So what happens is that these M cells take up
antigen from the lumen. These antigens are taken up by some type of APC, primarily dendritic cells.
Those dendritic cells then migrate down into the Peyer’s patch with their antigen and that’s what
stimulates the immune response. So the antigen essentially comes through this M cell instead of
coming in via some type of afferent lymphatic.
17. What this shows is the left side of the slide is the Peyer’s patch side. So there’s the antigen coming
in via the M cell, being taken up by the dendritic cell in the Peyer’s patch, which then will present that
antigen to the lymphocyte that will recognize whatever antigen it has. That lymphocyte will become
activated; it will go into the mesenteric LN & out into the thoracic duct. When it gets into the thoracic
duct, then it will migrate back into other mucosal surfaces (either in the intestine, the nose, the skin, the
GU tract, vagina, uterus, etc.—essentially any mucosal surface). You can see in cells in all of these places
that the antigen has initially been seen in other parts of the mucosa. So this is a way to send out cells
that have seen the antigens to all parts of the mucosa that might see that antigen again.
The
right side of the slide is the effector side after it’s been migrating back into the intestine. What you can
then see is that those T cells that have been stimulated will produce cytokines, which will then cause B
cells to switch into plasma cells & make antibodies. Those antibodies can then go out and protect the
surface and presumably protect you from whatever antigen, bacterial infection, virus, etc. that was
initially received through the M cell.
18. This is a picture of the nasal cavity from a mouse. If you look in a little newborn mouse, you don’t
actually see anything. There’s no little collection of dark blue cells, implying that there are lymphocytes
there. As you go along further and the mouse gets older, in this case the mouse is 8 weeks old, then you
can see this little tiny collection of lymphocytes. That’s what known as NALT (nasal-associated lymphoid
tissue). In the GI tract, it’s called GALT. This is actually one of the ways that we believe that antigens
that you breathe in through your nose actually then stimulate the immune response. In humans and in
mice, both the NALT and PP are actually developed after most other immune structures. So these are
structures that are developed very late in gestation. We believe that in some of the infants that are
born premature, this development may be messed up because they are born before these structures
actually start to develop.
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19. This is just a diagram that shows you in a human where you would see this NALT. So you can see
the structure within the sinuses. That’s where, in humans, you actually see the NALT and a lot of
dendritic cells that can take up antigen migrate out and stimulate the mucosal immune response.
Question—what is Waldeyer’s ring? It’s called a ring b/c if you section thru the tissue in exactly the right
plane, you can kind of see it as a ringlike structure. It’s a set of lymphoid tissues that have these M cells
in it. It’s sort of a ring of protection, if you want to think about it that way. As the antigens come in, the
M cells should see them in the nasal cavity& then transport those antigens into the underlying lymphoid
tissue and initiate an immune response.
The other things that are shown on here are essentially the sites of the adenoids and the tonsils.
Adenoids and tonsils don’t have the M cell structures over them, so they’re more like a traditional lymph
node than the NALT or the Waldeyer’s Ring.
20. This is another diagram of what I just showed you, only in this case it’s for the nose. Again, you
have antigen coming in via the M cell. That antigen would be taken up by APC’s, which would then
interact with T cells, causing activation of that T cell. And then in the nose, that would then drain in the
posterior cervical LNs instead of the mesenteric LNs. If you just have soluble antigen coming in across
the epithelium itself, again it can be taken up by APCs & drained into the cervical LNs. Those can either
go out into the systemic circulation or go through the lymphatics and migrate back up into the nasal
cavity. So this cycle of mucosal immune cells occurs in all mucosal tissues. So that’s PP and the
organized lymphoid structure. They function essentially identically to LNs; the only real difference is
how they get the antigen itself.
21. This is a drawing of what one of those villous structures looks like. What you can see within it here
is connective tissue that has a lot of lymphoid cells in it; that’s the lamina propria. Above the basement
membrane, you can see these little tiny cells that are the IELs.
22. So we’re gonna talk about the IELs first. The key is with IELs, just think about what the name is &
that will tell you where it is. So it’s a pretty simple cell to remember. What we do know about IELs is
that they come in very close contact with the epithelial cells throughout the mucosa. They do that by a
very unique interaction between an integrin & a cadherin. The description of this interaction was the
first description ever that an integrin could bind a cadherin.
So what happens is that epithelial cells have a cadherin known as E-cadherin (epithelial
cadherin) and the IELs have an integrin known as αEβ7 (αE for alpha-epithelial). This interaction is a
specific interaction between E-cadherin and what is now known as CD103 (the αEβ7 integrin). That is
what holds these IELs within the epithelium. It’s also thought that CD103 also has something to do with
migrating the cells into the epithelium; but that’s not entirely proven yet.
23. Why do we care about IELs? That’s actually probably a good question b/c we don’t really know
entirely what they do. Within the epithelial cells, they are almost all T cells. So if you’re looking at what
an IEL does, essentially you are looking at a T cell. Almost all of these are your classical T cells known as
αβ T-cells; they have the αβ TCR. These are almost all αβ TCRs, but this is one of the few places in the
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body that has a normal population of T cells that expresses this alternative T-cell conformation, this γδ
TCR. The other place where this is predominately is the skin. So it’s almost always mucosal surfaces.
The other interesting thing about IELs is that they are almost all CD8+. There are very few CD4 T-cells in
IEL population.
And the other thing (this is sometimes confusing): the TCR on these is an αβ heterodimer. The
CD8 molecule on these cells is different than the normal CD8 molecules that you see on all other T-cells
throughout the body. The CD8 molecule on this is actually an αα homodimer. So these cells express a
TCR that has the αβ heterodimer and a CD8 molecule that has an αα homodimer. Sometimes that is
confusing b/c they both have alpha and beta subunits. It’s thought that that has something to do with
their unique responses to mucosal antigens. That’s something that’s under intense study right now. We
talked about the fact that they adhere through their CD103, which is the αEβ7. And we know that they
can produce cytokines as well as cytolytic T cells. We don’t know whether either one of those functions
is critical to any type of human health. That still remains to be figured out.
24-25. Now we’re gonna talk about lamina propria lymphocytes, which is the area within this connective
tissue space. The area underneath the villous is what is known as the lamina propria; that’s where the
name comes from. Within this area, there actually is a mixture of a lot of different types of
lymphocytes. There’s T cells, B cells, a lot of plasma cells making IgA, macrophages, dendritic cells, etc.
Just about any type of immune cell that you could come up with, you could find in the lamina propria.
The interesting thing about the lamina propria is that if you look at its T cell population, almost all of
them are αβ T cells. And in the lamina propria you usually see almost no γδ T cells. That’s a fairly rare
population in the lamina propria. But the other thing to differentiate it from the IEL population is almost
all of these cells are CD4+. The CD8’s are in the IEL population, and the CD4’s are in the lamina propria
population. We believe that these lamina propria T cells are critical to maintaining this tolerant
phenotype that you would normally see within your intestine, where you do not respond to food
antigens or your commensal microbiota. We already talked about the fact that there are a lot of plasma
cells there; almost all of them produce IgA. The few that do not produce IgA usually produce IgM. That’s
in a normal situation. If you have a patient that has an inflammation anywhere within the mucosa, then
you can see IgG-producing plasma cells. And that’s actually one of the hallmarks of inflammation within
the mucosa; you get more IgG as a ratio to IgA during inflammation.
26. This is just a picture to show distinct separation in the types of T cells. So if you stain for CD8 T cells
and CD4 T cells, almost all of the CD8 staining is up here within the epithelium. There are very few cells
with CD8 in the lamina propria. And you see the opposite when you stain for CD4 T cells; almost all of
those cells are within the lamina propria and not within the epithelium.
27. This is also just a picture to show many plasma cells there are within the lamina propria that make
IgA. These are just the low-power and the high-power views of the villi here. This lamina propria is just
jam-packed with cells that are producing IgA. The amount of IgA produced is essentially 3 grams/day.
You don’t usually measure that when you measure immunoglobulin levels in a patient b/c you’re
measuring serum immunoglobulin levels. IgA is a secreted immunoglobulin, so all of that is secreted in
secretions in the oral cavity or within the GI tract.
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28-29. We are going to talk very briefly about lymphoid migration. There’s been a lot of research that’s
come out within the last 18 months or so about the fact that there are specific adhesion molecules that
help tell these cells whether they’re supposed to migrate into the colon or into the skin or into the small
intestine. They haven’t yet found one for the oral cavity, but I suspect that’s coming. We’re not going
to go into all of that detail. I just want you to get the general concept that lymphocytes that are
stimulated in the mucosa home back into mucosal surfaces, or what’s known as the common mucosal
immune system.
30. This is the primary molecule that is thought to be involved in that type of migration. And that’s
known as MadCAM-1 (mucosal addressin cellular adhesion molecule). What happens is the endothelial
cells in the high endothelial venules within the mucosa express this MadCAM molecule, and a
lymphocyte that has been stimulated within the mucosa turns on an integrin known as α4β7. This α4β7
integrin binds to MadCAM-1, and this is what directs that cell back into the mucosal surfaces. The
additional molecules that get directed into the colon versus the skin, etc. are on top of this MadCAMα4β7 interaction. This interaction is very critical for cells to migrate back into the mucosa. There are
biologics out right now that try to block this interaction to block various types of chronic mucosal
inflammatory diseases.
31. We are going to finish up with what are the two special functions of the mucosal immune system.
One is the fact that there is a predominance of IgA that is made in the mucosal surface versus the rest of
the immune system where the predominant immunoglobulin is IgG. And there is selective
nonresponsiveness or tolerance towards antigens that you normally see via the oral route.
32. We’re going to talk about IgA first. The synthesis of IgA is much higher than all other
immunoglobulins in the body. So you can see the comparison between IgA and IgG; the amount made is
almost 2-3 times as much.
33. Now when you think about the development of IgA-producing cells in the intestine, it’s very clear
that what you need in order to produce IgA is some sort of stimulus within the mucosa. This case is
humans. So if you look at the amount of IgA that’s in the small intestine, essentially the stimulation of
IgA parallels the colonization of the GI tract with the various types of microbiota. And if you look in the
animals that have been made germ-free (or gnotobiotic), they don’t make any IgA. So you need this
stimulus, something within the lumen of the GI tract, in order to trigger IgA production.
34. IgA is a slightly different immunoglobulin than your normal IgG molecule. That’s because in the
secretions, IgA is a dimer. So what you see are two IgA monomers with heavy chains and light chains.
What makes the IgA unique is two things; one is that the dimers are held together by the J chain
(“joining chain”). Who knows the other antibody with a J chain? IgM. So these two polymeric
immunoglobulins (IgM and IgA) are held together by the J chain. That is made by the B cell. The J chain,
heavy chain, and light chain are all made by the B cell, the plasma cell.
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The other thing that is unique is the secretory component (SC). The secretory component is
always a part of secretory IgA. If you look in the secretions of saliva, gastric washes, intestinal washes,
etc. the IgA is predominately this dimer with the SC. SC protects the IgA from being degraded by the
various proteases that are there within your GI tract. If you remember, the function of the GI tract is to
degrade the food that you have eaten so that you can get the nutrients out of it. And those are
proteins, so there are proteases. And you don’t want those proteases to degrade your IgA, so the SC
can partially protect the IgA from protein digestion.
35. We talked about that IgA is a dimer & what it’s composed of. IgA and the J chain are made by the B
cell or plasma cell that secreting the antibody. The very unique thing about secretory IgA is that the SC
comes from an entirely different cell, the epithelial cell. SC is essentially part of the polymeric IgA
receptor. We are going to show how it gets attached to the IgA. The SC is just a cleaved off piece of the
polymeric Ig receptor.
36. When you think about the mucosa and how does the secretory IgA get out into the secretions, this is
what is known to happen. You have your IgA-secreting plasma cells; it secretes out the dimeric IgA. So
it has two IgA subunits and the J chain. Then that is bound by the polymeric IgA receptor; that binding
triggers the receptor to be internalized in the epithelial cell. Then it is transported from the basolateral
side up to the apical side of the epithelium. So it comes from the lamina propria where the plasma cells
are, they bind, and they get transported to the apical side. When they get to the apical side, then the SC
is cleaved from the polymeric Ig receptor. By that cleavage, it then releases that entire secretory IgA,
which has the SC with it, the two IgA molecules, and the J chain.
This has been very extensively studied because this was the first example of a protein that was
transported basolaterally to apically instead of the other way around, which is how most proteins are
studied. So all of the signals for how this happens actually are known. Suffice it that you need to know
that it transports basolaterally to apically, and during that transport, it essentially gains that secretory
component that protects it in the lumen.
37. So if you look at IgA in the secretions, almost all of the IgA in secretions is a dimer. If you look in the
serum, a very tiny component of the IgA in the serum is a dimer. Almost all of the IgA in serum is a
monomer. And the other difference is that the dimeric IgA serum does not have secretory component.
So it has the two IgA molecules and the J chain, but not the secretory component. The other difference
is in humans: we actually have two IgA molecules that could be produced, known as IgA-1 and IgA-2.
The actual amounts of these are very different, depending on whether you’re looking in the secretions
(where it’s almost 1:1) versus in the serum (where almost all of the IgA is IgA-1).
This is actually more than just factoid significance. There are a number of people that turn out
to be IgA-“deficient.” The most common immunodeficiency that is seen in the world is IgA deficiency.
When we measure, looking for IgA deficiency, we only look in the serum, and that essentially means that
we are only looking for IgA-1 deficient people. Now the reason it might matter whether they are IgA-1
deficient or both is that if you are completely IgA deficient, and you actually get blood products, there
are enough IgA in that blood product that you actually can react to. And that’s one of the more
common reasons that people get transfusion reactions. So for people that are IgA deficient, you actually
Microbiology: Mucosal Immunology
Sonia Nkashama
pg. 11
have to give them washed red blood cell products or IgA-depleted products to keep them from reacting
against the IgA in blood product. However, it turns out that most people are probably IgA-1 deficient,
and if that’s true, then they probably won’t react with blood products. But we actually don’t test for
that, so we don’t know how common one deficiency versus the other is.
38. Why do you need secretory IgA? There are a number of functions that IgA can undertake. Probably
the most common one that’s quoted is the fact that it will actually bind various antigens, bacteria,
viruses, etc. within the mouth cavity, the lumen of the GI tract, etc. That essentially keeps any of those
pathogens from coming anywhere near contacting the epithelial cell. Therefore they keep your immune
system from seeing the pathogens at all. So that’s one of the more common functions of IgA.
It’s also known that IgA can protect newborns against various types of infections. And this IgA
essentially comes from the mother’s milk. So IgA that’s in the mother’s milk, because of this common
mucosal immune system, essentially has immunoglobulin against all of the various pathogens that the
mother has been exposed to in her GI tract. That protects the newborn against those infections. This is
why newborns that are nursed on mother’s milk usually have fewer infections because of this transfer of
secretory IgA. And that’s my plug for breastfeeding. IgA is relatively noninflammatory, going along with
this concept of tolerance in the mucosal immune system. It doesn’t fix complement by the classic
pathway.
39. What I’ve shown you is that essentially you get antigens out here in the GI tract that could be
neutralized by secretory IgA. If they are not, they could be taken up by M cells over Peyer’s patches.
Dendritic cells within those Peyer’s patches present them to T cells. Those essentially can migrate out
through draining lymph nodes and then back into the mucosa to protect us in the lamina propria or the
IELs.
40. I’m just gonna leave you with one extra thought. I want you to understand this concept, but it’s not
something that you need to know in great detail. Within the eye, there is something known as ocular
immune privilege. So there are a number of sites that are what we call immune-privileged sites. These
are sites that don’t normally get exposed to antigens. Therefore, whenever it does see an antigen,
rather unique immune responses occur.
So within the eye, if you get a foreign antigen within the anterior chamber, you don’t get a fullfledged immune response. You get more of a tolerant type of response like you see in the rest of the
mucosa. One potential mechanism for this is something known as Anterior Chamber Associated
Immune Deviation (ACAID). What happens in this case is that your normal inflammatory type of
immune response (such as delayed type hypersensitivity, antibodies that fix complement, etc.) don’t
occur. So they’re suppressed, or regulated, or whatever term you want to use. And other things that are
more tolerant, such as IgA that’s non-complement fixing and regulatory T cells, are dominant. The
bottom line is when the dendritic cell sees the antigen presented, it does so in a tolerant type of fashion.
And therefore, you don’t get a huge immune response within the eye. This exact thing occurs in the
intestinal mucosa also. So this is a very common theme throughout the mucosal immune system.
Microbiology: Mucosal Immunology
Sonia Nkashama
pg. 12
41. This is just a diagram that argues that. Actually you can’t really see it so it probably doesn’t really
matter. [her exact words]
42. What are some of the factors that could be involved in this type of regulation or tolerance? The one
that’s actually talked about the most is TGF-β2 (transforming growth factor beta). This is a cytokine that
is known to be regulatory throughout the immune system. So it’s really thought that that is the primary
cytokine that is involved in this type of tolerance. There have been studies that have talked about all of
these other factors, but it’s really not clear how important they are.
43. Just to finish it out, these are the other surfaces that are known to have immune privilege. So we
talked about the eye, the brain, essentially the pregnant uterus, the testes, etc. So there are a number
of sites like this within the body.
44. And that’s more reading if you want.