Fundamentals: 11-12:00 Scribe: Jennifer Grimes

Fundamentals: 11-12:00 Scribe: Jennifer Grimes

Thursday, October 1, 2009 Proof: Kosha Shah and Susan Whitt

Dr. Collawn Mucosal Immunology Page 1 of 6

I.  Introduction [S1]:

-What we’re going to talk about is the component of the immunity that you should care most about since it deals with the mucosal surfaces that you guys will be dealing with.

II.  Why do we need to understand how the mucosal immune system works? [S2]

a.  The mucosa (including oral, GI, eye mucosa) is the major site of contact in the body for foreign antigens. This is where tolerance or immunity to foreign antigens or infections is initiated. Understanding how this occurs and how vaccines work is critical as we go forward in designing treatment strategies.

b.  Gastrointestinal diseases kill more than 2 million people every year. |

c.  The number one infectious disease is dental caries, so understanding how the immune system reacts to the bacteria that causes them is crucial.

d.  We currently lack very many effective mucosal vaccines because we don’t really understand how the design for them works. For instance, the ones we have (polio, nasal flu vaccines) are ones that just happen to work (aren’t designed specifically to take advantage of what we know about the mucosal immune system).

III.  The mucosa is bombarded by foreign antigens [S3]

a.  One of the things to warn you about, the diagrams come from GI mucosa (primarily small intestine) since that is where most of the work has been done. We believe that the concepts from intestinal mucosa apply to the oral mucosa and in some circumstances the eye mucosa.

b.  When you think about the mucosa, you think about the mucosa barrier. All of your mucosal surfaces have an epithelial layer with tight junctions to protect and create a barrier to bacterial cells and other antigens.

c.  You have to think about the fact that it is exposed to different antigens (such as those from food). When you eat lunch, your body has to know not to react against antigens in food you eat. For example, we have a tolerance to antigens found on the proteins from the meat in this hamburger. Your body knows not to mount an immune response to that. In some circumstances, it doesn’t do that well so you end up with food allergies (ex: Celiac Disease where you react to gluten).

d.  Within your GI tract and on mucosal surfaces, you have a lot of bacteria. Most of those bacteria are not pathogenic bacteria, so your mucosal immunity must know to live in concert with it and not to react to it.

e.  “Not reacting to it” isn’t the best way of phrasing it; your immune knows that the bacteria are there, and it will respond to it a small amount, but it will not start a large immune system response to it.

f.  We’re also going to learn how your mucosal immune system will fire up the response if a pathogen is present to get rid of those organisms.

g.  We’ll talk a little bit about how your mucosal immune system does this, but in reality we don’t know how it works exactly.

IV.  Learning Objectives [S4]

a.  These are the things you need to take away from this lecture.

b.  We need to identify the major populations of lymphoid cells in the mucosa.

c.  Describe the basis of lymphocyte migration (homing) into the mucosal immune sites.

d.  Describe secretory immunoglobulins, how they are different from other immunoglobulins, and how they’re transported at mucosal surfaces.

e.  Understand the concept of mucosal immunity versus oral tolerance.

V.  The Human Gut Flora [S5]

a.  There are commensal bacteria in your GI tract, oral, eye areas. Humans and animals are colonized with these bacteria immediately after birth. Babies born vaginally colonize these faster than those born C-section, but all babies will colonize these Commensal bacteria from the surroundings, the mother handling them, etc.

b.  Eventually, there will be 1014 bacterial cells. You have more bacterial cells on mucosa than total body cells. Each individual has slightly different composition of bacteria in mucosa. It is influenced by mother’s composition. Must have a healthy flora in GI tract and mucosal cavities to have a healthy mucosal immune system. Probiotics help create the right balance of this flora (ex: yogurt).

c.  There have not been lots of good basic science studies to explain this, but there have been some studies that show that giving probiotics to premature babies actually helps reduce the complications of premature birth. After certain surgeries for inflammatory bowel disease can reduce the incidence of complications. Whether probiotics contribute to overall health, that is unknown (probably doesn’t hurt).

VI.  Bacteria of the Oral Commensal Flora [S6]

a.  As you look through the mucosal tube starting from the mouth and going all the way to the rectum, types of bacteria that are present change depending on the section of the tract.

b.  Oral flora is dominated by a-haemolytic streptococci. This data on this slide is known due to culturing cells from these areas (swab the mouth, take fecal samples, and culture cells from that sample). We now know that those cultures are actually a very small proportion of the total organisms in any of these cavities because most of the organisms that are there, we don’t know how to culture them (what medium to grow them one, what growth factors they need, etc.). Now we can actually do PCR (DNA) based tests to see the types of bacteria that are there, and that shows that there are many other types of bacteria in those areas.

c.  As you move down the tract, you start to see other organisms such as Streptococcus salivarius (on the tongue). Streptococcus mutans is hardly ever seen on the tongue, but it can be found on the teeth

d.  Don’t memorize the names; just know that there are different microcommunities of organisms along the mucosal tract.

VII.  Factors Controlling the Intestinal Microflora [S7]

a.  There are a lot of factors that occur throughout the mucosa that affect the different types of flora.

b.  You can imagine in the oral cavity, the organisms are tolerant to oxygen whereas those in the colon are not.

c.  All of these factors listed on the slide will affect the types of flora found throughout the oral cavity and along the GI tract (ex: presence of stomach acid or bile).

VIII. Cartoon [S8]

a.  skipped

IX.  Physiologic Functions of Intestinal Microflora [S9]

a.  Why are we learning about these bacteria at the beginning of the mucosal immune response? These types of bacteria have a very important role in the development of mucosal immunity.

b.  From animal models, we have found that intestinal microflora is not required for survival and health. You can live in a sterile environment with no bacteria. We have done studies on animals that live in sterile, bacteria-free environments (everything is autoclaved, live in sterile plastic boxes). They are healthy and are able to live, grow, reproduce. However, these animals have a very, very reduced level of mucosal immunity. We will talk about Peyer’s Patches (these germ-free animals don’t have those structures because the bacteria will induce the immune system to migrate in and form these structures). These animals live, but they will not respond normally to antigen exposure.

c.  Intestinal Microflora are also important for “colonization resistance”. The next slide shows how this works.

X.  Our Mucosal Flora Helps Prevent Colonization by Pathogens [S10]

a.  Normally, in your GI tract or oral cavity, you have bacteria that live there all the time (huge number of them in their niche), and they live slightly above mucosal barrier. What has been observed for many years in hospital environments, when a patient comes in with an infection and you give them a broad-spectrum antibiotic. This causes the normal flora in the GI tract to die, and opportunistic pathogens (such as Clostridium difficile, which is very common in hospitals) can now come in and find a niche, divide, and cause disease. C.dif produces a toxin that can kill GI epithelial cells and cause abnormal secretions (results in bloody diarrhea). This is almost never seen in patients that haven’t had antibiotics. We know that some pathogens require that the normal microbiota need to be decreased before they can take hold and cause disease.

XI.  Organization of the Mucosal Immune System [S11]

a.  We now know that we need bacteria in order to have a healthy mucosal immune system.

b.  The components are primarily named based on their terminology in the GI passage (she will show us the equivalents in the nasal passage as we go along).

c.  First you have organized mucosal lymphoid tissue: tonsils, adenoids, Peyer’s patches, isolated lymphoid follicles, and the appendix. These are what you normally think about when you think of the immune system.

d.  Then there is the diffuse mucosal immune system, which includes the intraepithelial lymphocytes and the lamina propria lymphocytes.

XII.  Three major lymphoid populations in the intestinal tract [S12]

a.  This shows approximately where those different structures are in the intestine. You know where the tonsils and the adenoids are.

b.  You can think of a peyer’s patch of a lymph node in the intestine (almost identical, very similar functions, such as recognizing antigens, activating lymphocytes and they hone in on other structures in order to protect the host from various pathogens). Those other structures that are activated by the lymphocytes are located in these structures here (the 2 black projections on either side of the Peyer’s Patch), which are villi, which stick up into the lumen of the intestine and serve to increase surface area for food reabsorption. Inside the structures (below the basement membrane) are a whole population of cells known as lamina propria lymphocytes. When you get inflammation in the intestine, it is primarily in this space.

c.  The cells above the basement membrane inside the epithelial layer are the intraepithelial lymphocytes (easy tp remember)

XIII. Peyer’s patch Structure [S13]

a.  This is what a Peyer’s patch looks like.

b.  You have T-cell areas and B-cell areas (very similar to lymph nodes). There is a B-cell follicle area, a germinal center where activated B-cells begin switching to various types of B-cells. Around the germinal center you have T-cell areas.

XIV.  Peyer’s patches [S14]

a.  There are several differences between lymph nodes and peyer’s patch.

b.  Key difference: how they get antigens.

c.  In a lymph node, you have lymph that is brought into the node that brings in antigens. In Peyer’s patch, you don’t have that because there are no afferent lymphatic structures. Antigens are transported across the epithelial layer and into the Peyer’s patch. (she repeated this a couple times, so this is important).

XV. Peyer’s patch: Follicle-associated epithelium (FAE) [S15]

a.  In the intestine, Peyer’s Patch drain into the mesenteric lymph nodes

b.  We talked about the fact that they have B cells, T cells, dendritic cells, and all your classical immune cell types. In Peyer’s patches, B cells will be primarily switching to IgA (mucosal immunoglobin); this is very different from lymph nodes (most will be IgG, also IgE).

c.  In most Peyer’s patches and other organized structures in the mucosal immune system, you have this germinal center activated because you continuously have exposure to bacteria in the lumen and antigens from food. In lymph nodes, that is not true; they are quiescent unless you have an infection (lymph nodes like in your neck are only swollen and active when you get sick). This continually happens in Peyer’s Patches.

XVI.  Initiation of Gut Responses [S16]

a.  This diagram shows how antigens are transferred into the peyer’s patches through M cells.

b.  M cells are called that because under EM (electron microscopy), they have microfolds (don’t have microvilli like intestinal epithelial cells). They are very unique because they have cavities on basolateral surface that encompass immune cells (T cells, macrophages, and dendritic cells) so that it can take up the antigen, take it across the cell, and give it to dendritic cells to take up in the Peyer’s patch.

c.  That’s how the immune response in a Peyer’s patch or any isolated lymphoid tissue is initiated.

XVII.  Inductive sites & Effector Sites [S17]

a.  Once you get an immune response started in the Peyer’s patch, what happens is that those lymphocytes are activated in the Peyer’s patch and go back out of the Peyer’s patch through the efferent lymph and into the draining mesenteric lymph nodes, go back out into the lymphatic system into the body. Cells that are activated in the mucosa know that they are supposed to go back to the mucosa. We will talk about their honing skills later. There have been studies that show that if the antigen is put on the skin and the cells are activated, they will eventually hone back to the skin after their response. If the antigen is via the nasal cavities or the intestinal tract, those cells will hone back to those areas as well, respectively. That’s why when you are immunizing against an organism that might infect a certain mucosal surface (nasal cavity), you immunize at that same surface.