Mucosal Immunity PDF

MUCOSAL IMMUNITY Raymond F Adebiyi Professor Immunology and Medical Microbiology Learning Objectives 1. Describe the major components of the respiratory, gastrointestinal, and genitourinary system and discuss key factors that each component employs in physical, chemical or biological defense. 2. Des...

MUCOSAL IMMUNITY Raymond F Adebiyi Professor Immunology and Medical Microbiology Learning Objectives 1. Describe the major components of the respiratory, gastrointestinal, and genitourinary system and discuss key factors that each component employs in physical, chemical or biological defense. 2. Describe disorders that may arise due to a failure of effective innate or adaptive mucosal defense due to a defect of any of the factors listed above. Examples: Sjogren’s Syndrome, Zollinger-Ellison, Crohn’s disease. 3. Discuss the nature and properties of structures, cells and molecules that contribute to the innate immune defense of the m ucosal system. 4. Explain the role of commensal microorganisms in the development and maintenance of normal immune responses. 5. Discuss the relationships between the immune inductive sites and the immune effector sites of the mucosal system. 6. Discuss the pathways of antigen uptake from the intestinal lumen. Indicate which pathway(s) may contribute to tolerance i nduction and which pathway(s) may contribute to adaptive immunity. 7. Outline the factors that may contribute to the induction of tolerance in the mucosal system using the gastrointestinal tra ct as a model and explain how they function. 8. Describe the characteristics of intra-epithelial T lymphocytes and outline their function in the mucosal system. 9. Discuss the role of Tamm-Horsfall protein in genitourinary tract immunity. 10. Explain how immune dysregulation in the gastrointestinal tract may lead to inflammatory bowel disease SUGGESTED READINGS: Andrew E Williams (2012) Immunology: Mucosal and Body Surface Defences, Wiley-Blackwell. Chapter 6 Peter Parham (2015) The Immune System, 4 th Ed. Garland Science. Chapter 10 Immunology in the Gut Mucosa https://youtu.be/CXz6FVqPqHw OFFICE HOURS Please check Canvas calendar email: [email protected] https://atge.webex.com/meet/radebiyi Characteristics of Mucosal Defense 1. The epithelium is the main physical barrier; secretions provide chemical defense while resident and recruited cells provide biological defense. 2. Effector cells induced at one mucosal site are imprinted with receptors that enable them to respond to homing signals back to the mucosal system. 3. Inflammation at mucosal surfaces is normally discouraged through the activity of T regulatory cells and the production of anti-inflammatory cytokines. Mucosal Immunity mucosa – at main portals of entry of microorganisms - respiratory tract - gastrointestinal tract - genitourinary tract covered by mucus mucins (glycoproteins) sticky, anti-microbial layer serves to (1) protect the epithelium, and (2) trap and destroy microorganisms Mucosal Immunity Mucus secretions: Produced by goblet cells Provide physical and chemical barriers Composed of mucins (glycoproteins) Contain antibody (sIgA, IgM) Contain salt Trap microorganisms Mucosal Immunity Mucosal Immunity The Respiratory Tract most exposed of all mucosal surfaces Surface area  160 m2 20,000 breaths / day 10,000 liters of air Respiratory Tract Infection  Most common causes of disease in humans  85.6 infections per 100 persons per year  54% of all acute conditions, excluding injuries The Respiratory Tract The Respiratory Tract The Respiratory Tract Epithelial cells Barrier function: – Tight junctions – Desmosomes – Adherence junctions Interface between the environment & the sterile compartments of the body The Respiratory Tract: Epithelial Cell Secretions β-defensins promote bacterial lysis via pore formation, are chemotactic for many leukocytes and promote phagocytosis. Cathelicidins possess broad spectrum antimicrobial activity against many organisms including Gram-positive and Gram-negative bacteria by the disruption of membrane integrity. Surfactant Proteins A & D are collectins that are produced by type II alveolar cells. They promote phagocytosis (opsonins) and are directly bactericidal by destroying bacterial membranes. Lysozyme (Muramidase) is found in most secretions and is able to cleave the cell wall of Grampositive bacteria. The Respiratory Tract Long-lived Macrophages: - First line of defense against daily pathogen challenges of the epithelium - Avidly phagocytic - Able to kill ingested pathogens - Orchestrate inflammatory and adaptive immune responses - Act as antigen presenting cells - Secrete IL-10 to avoid unnecessary inflammation The Respiratory Tract Macrophage Function: Mechanisms of Action - innate phagocytosis of surfactant protein A (SP-A) and surfactant protein D (SP-D) opsonized pathogens - adaptive phagocytosis of IgG opsonized pathogens - killing by reactive oxygen species (nitric oxide, superoxide anion, hydroxyl radicals, hydrogen peroxide, hypochlorous acid) as well as antimicrobial cationic peptides (beta-defensins, cathelicidins, lysozyme) Secretion of IL-8 to recruit neutrophils when the challenge is severe or persistent and cannot be handled by macrophages alone The Respiratory Tract Short-lived Neutrophils: - Recruited into alveolar spaces when pathogen challenge is severe - Killing mechanisms include: opsonin-enhanced phagocytosis reactive oxygen species antimicrobial cationic peptides The Respiratory Tract Lymphoid Tissues BALT, NALT, SALT, GALT, MALT -act as immune inductive sites- Mucosal Immunity The gastrointestinal tract -probably the most complex organ in terms of immunological processes in operation - Mucosal Immunity Saliva in oral cavity mucus amylase bicarbonate lysozyme RNAse DNAse IgA etc……. Sjogren syndrome (dry eyes – xerophthalmia; dry mouth - xerostomia) Mucosal Immunity The Stomach  Essentially a pouch  Gastric mucosal barrier Mucus Pepsinogen Bicarbonate Hydrochloric acid Mucosal Immunity Stomach Secretions G cells – gastrin secretion Parietal cells hydrochloric acid (HCl) release Chief cells pepsinogen release Hyperchlorhydria Hypochlorhydria Achlorhydria  HCl (Zollinger-Ellison; Peptic Ulcer Disease)  HCl No HCl Mucosal Immunity: the Intestine (1) Intestinal mucus production - lubrication - entrapment of bacteria - expulsion of parasites Mucosal Immunity: The Intestine (2) Intestinal Epithelium - physical barrier - chemical barrier antimicrobial secretions mucosal antibody (IgA mainly; and IgM) IgA may lead to infection, e.g. by Giardia lamblia Mucosal Immunity: The Intestine (3) Peristalsis -expulsion of pathogens and waste products -required for normal functioning of system intestinal stasis disease Mucosal Immunity: The Intestine (4) Intestinal microbiota - competitive inhibition of pathogens - required for normal immune development - Vitamin synthesis - deconjugation of bile salts - production of antimicrobials Health Care Associated Infection excessive use of antibiotics destroys the normal flora may lead to disease due to growth of opportunistic organisms Clostridium difficile growth and toxin production leads to diarrhea and pseudomembranous enterocolitis Antigen Presentation In The Gastrointestinal Tract Mucosal Immunity: Immunologic Signaling Pathway -TLRs present on mucosal epithelium- PRR (TLR)/PAMP TOLLIP downregulates TLR  signal generation  signal transduction (protein kinase, etc.)  transcription factor (NF-B, etc.)  upregulation of genes in nucleus  cytokine production and release cell recruitment, activation and effector function Mucosal Immunity Mucosal Immunity Antigen sampling (1) Enterocyte pathway  no co-stimulatory signals (B7 negative)  no activation (2) M cell pathway  trancytosis  antigen delivery to APC  activation (3) Dendritic cell pathway  professional APC  activation Mucosal Immunity: cellular Response To Intestinal Microorganisms - Tolerance Induction - Mucosal Immunity The induction of tolerance to endogenous bacteria  Recruitment and activation of T regulatory cells  Increased production of anti-inflammatory cytokines (IL-10, TGF-β) Loss of regulation inflammatory bowel disease (e.g. Crohn’s)  Decreased expression of co-stimulatory molecules (B7: CD80/CD86)  Increased Tollip expression  Down-regulation of TLR/NLR expression  Decreased CD14 expression on intestinal macrophages Mucosal Immunity: Paneth cells – essential intestinal scavengers; produce antimicrobial defensins Genitourinary Tract - unique situation due to hormonal influences - great variability in structure and function at different locations and at different times Genitourinary Tract physical barrier defenses provided mainly by epithelium and mucus secretion as well as flushing action of urine epithelial cells express receptors for pathogens (PRRs) and do respond to antigenic challenge like similar cells at other locations Genitourinary Tract chemical defense is mainly due to epithelial cell secretions β- defensins lysozyme vaginal lactic acid secretory leukocyte peptidase inhibitor (SLPI) lactoferrin cathelicidin Tamm-Horsfall protein (THP) urine acidity nitric oxide Genitourinary Tract Immunty Genitourinary Tract biological defense is provided by resident macrophages and cytokine-recruited immune cells macrophages dendritic cells specialized NK cells neutrophils B cells (sIgA) T cells Mucosal Immunity: specialized lymphocytes intraepithelial lymphocytes mainly CD8+  found within gut associated lymphoid tissue  may express TCR  considered innate T cells (!!!)  require no priming respond spontaneously to challenge and release cytokines kill infected cells using perforin/granzyme and FasL Intraepithelial Lymphocytes Characteristics of Mucosal Defense 1. The epithelium is the main physical barrier, secretions provide chemical defense while resident and recruited cells provide biological defense. 2. Effector cells induced at one mucosal site are imprinted with receptors that enable them to respond to homing signals back to the mucosal system. 3. Inflammation at mucosal surfaces is normally discouraged through the activity of T regulatory cells and the production of anti-inflammatory cytokines. Mucosal Immunity THE END Intestinal Immune Response Mucosal Immunity Dr. Raymond F Adebiyi Mucosal Immunity Learning Objectives: 1. Describe the major components of the respiratory, gastrointestinal, and genitourinary system and discuss key factors that each component employs in physical, chemical or biological defense. 2. Describe 3 disorders that may arise due to a failure of effective innate or adaptive mucosal defense due to a defect of each of the factors listed above. 3. Discuss the functions of the cells and molecules that contribute to the innate immune defense of the mucosal system. 4. Explain the role of commensal microorganisms in the development and maintenance of normal immune response. 5. Discuss the contribution of the cells of the adaptive immune response (T cells, B cells) to immunity in the mucosal system. 6. Discuss the pathways of antigen uptake from the intestinal lumen. Indicate which pathway(s) may contribute to tolerance induction and which pathway(s) may contribute to adaptive immunity. 7. Outline the factors that may contribute to the induction of tolerance in the mucosal system using the gastrointestinal tract as a model and explain how they function. 8. Describe the characteristics of T lymphocytes and outline their function in mucosal immunity. 9. Discuss the role of Tamm-Horsfall protein in genitourinary tract immunity. 10. Explain how immune dysregulation in the gastrointestinal tract may lead to inflammatory bowel disease. SUGGESTED READINGS: Andrew E Williams (2012) Immunology: Mucosal and Body Surface Defences, Wiley-Blackwell. Chapter 6 Peter Parham (2015) The Immune System, 4th Ed. Garland Science. Chapter 10 http://www.nature.com/ni/multimedia/mucosal/index.html 1 Mucosal Immunity Dr. Raymond F Adebiyi Introduction Mucosal immunity is not usually presented as a distinct topic in undergraduate medical education but integrated into discussions of the functions of various organ systems. The reality is that this should not be the case because infections most usually occur initially at mucosal surfaces and that is where our study of immunity to infection should rightly begin. The sites that are most exposed to the environment and where the greatest numbers of microorganisms may be encountered are the mucosal surfaces of the integumentary, respiratory, gastrointestinal and genitourinary systems. These are the gateways to the body and that is where the action starts. The mucosal surfaces earned that name because they contain mucus which forms a protective layer over the epithelia. Mucus is composed of glycoproteins called mucins that form a sticky layer to trap microorganisms and prevent their attachment to the epithelial cell surface. Our discussions naturally begin with the respiratory tract as that is the portal that is most frequently employed by pathogens to gain entrance into the body. The air we breathe is packed with microorganism, pollutants and irritants that the body must deal with. The mechanisms by which the respiratory system handles these challenges will be presented. The next portal of great importance is the gastrointestinal tract. This system is called home by a very large number of microorganisms (the normal microbiota or normal flora or microbiome) and has of necessity developed a complex immunological mechanism that enables it to tolerate the vast number of resident microorganisms while retaining the capacity to detect and eliminate pathogenic organisms. This system will be discussed most extensively as it is by far the largest and the most complex. The final portal is the genitourinary tract. This includes both reproductive and excretory organs and thus presents a unique situation, especially in the female where the immune system must not only protect the mother but also prevent infection of the fetus during pregnancy. 2 Mucosal Immunity Dr. Raymond F Adebiyi The components of mucosal defense are physical, chemical, and biological and the discussion will be organized accordingly. Each of these components is of equal importance and the failure or defect of any one would seriously impair the well-being of the host. The respiratory system The respiratory system is probably the most exposed of all mucosal surfaces. Breathing is a non-stop activity (20,000 times a day!) and results in the intake of some 10,000 liters of air, laden with microorganisms. The adult respiratory tract covers a very large area of more than 160m2 and forms an interphase between the environment and lungs (lower respiratory tract) that are essentially sterile. Therefore, there must be adequate defense mechanisms to ensure normal physiological function while eliminating dangerous materials. The importance of this defense system is obvious from the observation that acute respiratory tract infections are the most common causes of disease in humans. Non-specific (Innate) barriers There are many non-specific physical, chemical and biological factors at play within the respiratory tract. Nasal hairs filter microorganisms while nasal turbinates force particles into mucus for entrapment and destruction. A critical component of defense is the sneeze3 Mucosal Immunity Dr. Raymond F Adebiyi cough reflex. The trachea is lined with cilia and mucus-secreting goblet cells (the muco ciliary escalator), and this is responsible for arresting and expelling any potentially hazardous materials. Respiratory epithelial cells also provide a physical barrier via impenetrable tight junction, and chemical defense through the secretion of antimicrobial peptides such as β-defensins, cathelicidins, surfactants and lysozyme, among others. -defensins promote bacterial lysis via pore formation, are chemotactic for many leukocytes and promote phagocytosis. Cathelicidins possess broad spectrum antimicrobial activity against many organisms including Gram-positive and Gram-negative bacteria by the disruption of membrane integrity. Surfactant Proteins A & D are collectins that are produced by type II alveolar cells. They promote phagocytosis and are directly bactericidal by destroying bacterial membranes. Lysozyme (Muramidase) is found in most secretions and is able to cleave the cell wall of Gram-positive bacteria. Epithelial cells also secrete a variety of cytokines, including IL-1, IL-6, IL-8, TNF-α, IL-15, IL-10 and TGF-β. Phagocytic cells of the innate immune response (neutrophils, macrophages, NK cells) are attracted to the site of infection and these destroy any invading 4 Mucosal Immunity Dr. Raymond F Adebiyi microorganisms. In addition, epithelial cells are responsible for the transposition of secretory IgA as well as IgG and IgM that are abundant in the respiratory lumen. Adaptive immune responses are inducted in the parabronchial, hilar and paratracheal lymph nodes that comprise the bronchial associated lymphoid tissue, BALT. Alveolar macrophages and dendritic cells are very active at this site where antigen processing and presentation take place and effector cells are produced. Effector B cells become plasma cells that produce mainly IgA antibody but also IgM and IgG. Effector T cells are also generated both via classical presentation, and cross presentation, involving exogenous antigen presentation to CD8+ cells, particularly in the defense against viral pathogens. The Gastrointestinal System The gastrointestinal tract is often thought about in terms of food and food processing (digestion) but we do know that the system does a lot more than that. Anatomically, the 5 Mucosal Immunity Dr. Raymond F Adebiyi system is unique in that it provides openings in the integument for the intake of materials through the oral orifice and the output of waste products via the anal orifice. In this respect, it may be considered an extension of the integument but lacking the physical epidermal structures that characterize the integument. Instead, it has developed its own special sets of processes that ensure that it can play its role in the overall defense of the host. The gastrointestinal system comprises the following: Mouth or oral orifice Esophagus Stomach Intestines Appendix Rectum and anal orifice plus associated organs: Liver Gall bladder Pancreas Oral cavity The oral epithelium contains tight junctions and if these are intact, invasion is prevented. Second, the epithelium is bathed in large amounts of saliva, which contains many substances such as amylase, bicarbonate, lysozyme, RNAse, DNAse, mucus, EGF (epidermal growth factor) and IgA, which are microbicidal. An inability to produce saliva may result in serious buccal or dental disease. The oropharyngeal cavity is protected by a cluster of lymphatic tissue called the tonsils. These contain immune cells including macrophages and lymphocytes that provide both innate and adaptive protection against infection. 6 Mucosal Immunity Dr. Raymond F Adebiyi The esophagus that carries food from the mouth to the stomach is mainly a muscular tube that propels food by peristalsis. The epithelium is also composed of non-keratinized stratified squamous cells and peristaltic action is necessary not only to move food but also to prevent ingested microorganisms from colonizing the esophagus. The mouth, saliva and the tonsils make it very difficult for any unwanted materials to get into the esophagus, and should they get there, they are rapidly propelled to the stomach where the environment is very harsh indeed. Thus, peristalsis is an essential host defense mechanism. The stomach is essentially a pouch that holds food for the initiation of digestion. The stomach epithelium contains pits in which glands are located. These glands produce secretions that are very important for the control of the process of digestion. Mucus producing cells are abundant, as are G cells. The G cells secrete gastrin, a hormone that controls the release of hydrochloric acid by parietal cells and pepsinogen by chief cells. Stomach acidity is particularly important as it kills most microorganisms. The stomach is essentially sterile as only a handful of viable organisms may be found there and only under very special circumstances. Providing a supporting antimicrobial role for HCl are mucus and bicarbonate both of which are essential for the functioning of the gastric mucosal barrier. Problems in the stomach arise when the mucosal barrier is defective, or acid production is abnormal. Achlorhydria and hypochlorhydria indicate conditions in which stomach acid production is absent or diminished. This would result in a number of complications including bacterial overgrowth due to the loss of antimicrobial function. At the other end is hypersecretion of HCl (Zollinger-Ellison Syndrome) that is usually due to a gastrinoma. This may result in the formation of gastric ulcers. It is useful to note that there is strong evidence suggesting that a major causative factor of gastric and duodenal ulcers is chronic inflammation due to Helicobacter pylori that colonizes the antral mucosa. The bacterium can cause a chronic active gastritis, resulting in a defect in the regulation of gastrin production by that part of the stomach. 7 Mucosal Immunity Dr. Raymond F Adebiyi The intestine In immunological terms, the intestine is a major organ as lymphoid tissue at this location far outweighs lymphoid tissue at any other location. By the intestine, we mean both the small and large intestine and, even though they are structurally different, we will discuss them together as the protective mechanisms are quite similar. The Intestinal mucosa is covered by a large population of microorganisms that make up the endogenous microbiota (numbering up to 1012 per gram of intestinal content) in addition to food substances and potentially pathogenic organisms. The mucosal epithelium plays an essential role as a barrier, preventing colonization or invasion by microorganisms. This barrier function depends on specialized factors such as tight junctions, apical surface coats and mucus secretions. Also of importance is the maintenance of an environment that prohibits the establishment of pathogens through the production of antimicrobial substances such as bacteriocins, colicins (produced by resident intestinal flora), defensins, lactoferrin and lysozyme. It is known that intestinal epithelial cells are able to increase their barrier activity by up-regulating the production of defensins and other substances. When the primary barriers are broken, epithelial cells may also produce chemokines that attract monocytes, macrophages and dendritic cells for effective host defense. 8 Mucosal Immunity Dr. Raymond F Adebiyi An early step in the elicitation of an immune response is the recognition, sampling and processing of antigens that may be present in the intestinal lumen. Thus, intestinal epithelial cells express Toll-like receptors, TLRs, nucleotide oligomerization domain receptors, NOD1 and NOD2, and others, which recognize and bind to pathogen associated molecular patterns, PAMPs. This may lead to signal transduction and cytokine production and the activation of immunological reaction against the microorganisms. Strategically located throughout the intestinal tract are cells and structures that are critical for effective host defense. First are the Paneth cells which are the scavenger cells of the intestine. They are located at the base of intestinal crypt where they are able to mount a phagocytic response against a wide range of microorganisms. The intestinal crypts are not static. They contain stem cells that contribute to the constant replenishment of intestinal epithelial cells that die and are removed. 9 Mucosal Immunity Dr. Raymond F Adebiyi Thus, the intestinal barrier is under constant maintenance. When microorganisms enter the intestinal lumen and invade the crypt, Paneth cells become stimulated to produce alphadefensin. This is found in the Paneth cell granules as a precursor (proHD5, pro-humandefensin-5) that is secreted and then cleaved to the active substance by trypsin. Paneth cells also produce lysozyme, RNAse and phopsholipase-A2, which have potent antimicrobial activity. Second are the Peyer’s patches, large lymphoid follicles that are located in the mucosa and the submucosa of the small intestine. These follicles contain B cells and T cells and are a zone of intense immunologic activity. These are the inductive sites for intestinal immune response. They are closely linked to the mesenteric lymph nodes that serve as concentration sites for antigen from the intestinal lumen. Thus, it is clear that local antigen stimulation leads to activation of cells in the Peyer’s patches and draining mesenteric lymph nodes resulting in the induction of an adaptive immune response. The B cells differentiate into IgA antibody producing plasma cells while other activated (effector) lymphocytes now exit the node via efferent lymphatic channels and express unique integrins that enable them to respond to homing signals. They may now enter the systemic circulation and be guided back into the gastrointestinal tract to discharge their effector functions. 10 Mucosal Immunity Dr. Raymond F Adebiyi Also present are intraepithelial lymphocytes that are found within the epithelial layers of mucosal tissue. Many of these appear to be a special subset of T cells that may express TCR, unlike regular T cells that express αβTCR. Intraepithelial lymphocytes expressing TCR are considered to be innate cells because they do not need priming. Upon encountering antigens, they immediately release cytokines and cause killing of infected target cells, acting both as cytotoxic T cells and as NK cells. The sampling of antigen in the intestinal tract may occur via three pathways: (1) The first is the enterocyte pathway by which the enterocytes of the small and large intestines internalize antigen via “bulk uptake” and receptor-mediated endocytosis. The processed antigen, or peptide, is then presented to intra-epithelial lymphoid 11 Mucosal Immunity Dr. Raymond F Adebiyi cells. Enterocytes do not express CD80/CD86 and therefore do not generate a costimulatory signal, resulting in immunological unresponsiveness. (2) The second pathway is via the M cell, microfold cell, a specialized epithelial cell overlying the Peyer’s patches that is able to effect transepithelial antigen transportation to dendritic cells in the subepithelial dome (SED) in a process called trancytosis. (3) The third pathway involves the professional antigen presenting dendritic cell. The dendritic cell is able to send dendrites through the tight junctions to capture antigen within the intestinal lumen. The dendritic cell moves to the gut associated lymphoid tissue or migrates via the lymphatics to the draining mesenteric lymph nodes where the antigen is processed and presented to naïve T lymphocytes. However, because of the very high density of PAMPs / MAMPs in the intestinal lumen, there is a need for mechanisms to regulate the inflammatory response in the gut in the absence of pathogenic organisms. It has been established that epithelial cells of the colon, for example, express low levels of TLR4 and are therefore poorly responsive to Gram 12 Mucosal Immunity Dr. Raymond F Adebiyi negative bacterial lipopolysaccharide. The result is a muted immune response that is necessary to permit the gut to discriminate between commensal organisms and pathogens. Prolonged exposure to lipopolysaccharide or lipotechoic acid results in (1) the down-regulation of TLR expression (2) decreased kinase activity and (3) increased Toll-interacting protein (TOLLIP) expression. Increased TOLLIP expression results in the inhibition of TLR-mediated activation of immune responses and causes tolerance of commensal organisms. Pathogens produce factors that provide additional signals to drive an effective immune response. As shown in the above figure, various other immunosuppressive factors, such as the activation of T regulatory cells (CD4+, CD25+, Foxp3+ cells) and the expression of IL-10 and TGF-β, are induced that assure the survival of commensal microorganisms. Subsets of dendritic cells exist that possess distinct CD markers that are responsible for directing the response (1) towards Treg and IL-10 production, or (2) towards Th17 and an inflammatory response. 13 Mucosal Immunity Dr. Raymond F Adebiyi Some of the mechanisms of tolerance induction in the intestinal tract are listed below: (1) Activation of T regulatory cells and increased production of IL-10 and TGF-β (2) Decreased expression of co-stimulatory molecules (B7: CD80/CD86) (3) Down-regulation of TLR/NOD expression and activity through increased TOLLIP and other factors (4) Decreased CD14 expression on lamina propria macrophages thus limiting their response to bacterial lipopolysaccharide (LPS). The genitourinary system is considerably different from the respiratory or the gastrointestinal system, especially in the female. The fluctuating effects of various hormones during the ovulatory cycle create conditions that are very difficult to study due to their high variability. Furthermore, the mucosa associated lymphoid tissue of the genitourinary tract does not possess structures like Peyer’s patches or bronchial lymphoid aggregates that are the hall marks of inductive sites. Instead, immune cells may be found in loose associations throughout the system and immune defense induction is diffuse, and it has been suggested that this could be a reason for the difficulty in inducing local artificial immunity at this site. Physical defense is provided by epithelial cells, but due to the different functions of parts of the tract such as the vagina, cervix, endometrium, uterus, fallopian tube and the ovary, there is necessary epithelial specialization resulting in great variation within the system. However, these epithelial cells, regardless of their structure, do possess toll-like receptors and are thus able to recognize and bind pathogens and produce pro-inflammatory cytokines that attract and activate immune cells (neutrophils, macrophages, specialized NK cells, B and T cells, etc.). 14 Mucosal Immunity Dr. Raymond F Adebiyi Chemical defense is provided by antimicrobial peptides produced by the epithelial cells. These include β-defensins, secretory leukocyte peptidase inhibitor (SLPI), lysozyme, and lactoferrin. We must, of course, remember that there is abundant mucus secretion by goblet cells and that mucin contents are powerful antimicrobials. The urine contains large amounts of antimicrobial peptides including Tamm-Horsfall protein (THP) which is produced almost exclusively by urinary tract epithelial cells. Tamm-Horsfall protein binds to pathogenic bacteria, prevents attachment, and facilitates flushing during urination. Biological defense is provided by the many cells (e.g. neutrophils) that are attracted following an infection. As with other mucosal tissues, IgA transcytosis is very effectively achieved in the genitourinary tract, so that the lumen contains high levels of sIgA as well as IgG. Professional antigen presenting cells are present within the lamina propria and upon stimulation, they migrate to the inguinal lymph nodes for antigen presentation and the generation of effector B and T cells for host defense. 15 Mucosal Immunity Dr. Raymond F Adebiyi Inductive Sites and Effector Sites Following antigen sampling or uptake at the mucosal surface, antigen is delivered to antigen processing cells which then migrate to the inductive sites where antigen is processed and then presented along with the necessary co-stimulatory signals to T lymphocytes. Activated lymphocytes migrate from the inductive sites and traffic to the effector sites (sites of infection) to perform their function. B cells become plasma cells that produce antibody for pathogen neutralization, phagocytosis and complement medicated lysis. Effector lymphocytes, both T helper and cytotoxic T cells, may require additional signals for optimal functioning and may also be subject to homing signals that direct them to the appropriate effector sites. 16 Mucosal Immunity Dr. Raymond F Adebiyi Postscript (OPTIONAL READING. NOT TESTABLE) When we consider that the mucosal immune system must function in an environment that may include very large numbers of microorganisms that are not only welcome but are indeed desired because of their specialized roles in normal host physiology, we must appreciate the balancing act that is in play, between maintaining normal homeostasis and defending the body against potential invaders. Ingested food obviously contains foreign antigen as well as potentially pathogenic microorganisms. The immune cells must process the food antigens and induce a state of tolerance to them while ensuring that pathogens are destroyed. This is achieved through the generation of immunoregulatory cells that possess tightly controlled selective activity. Antigen processing and presentation typically results in the production of antibodies such as IgM, IgG and IgA that provide protection in the lumen. Overall, the immune response, both innate and adaptive, provides protection throughout the mucosal system and maintains a delicate balance between what contents 17 Mucosal Immunity Dr. Raymond F Adebiyi must be tolerated and what must be eliminated. In addition, the immune response often contributes to disease processes through over-activation, lack of regulation or an inappropriate inflammatory response to specific antigenic challenges. There is increasing evidence that microbial translocation, the movement of microbes to ‘abnormal’ sites, may contribute to the development of various inflammatory diseases. 18 Mucosal Immunity Epithelial Barrier Hypothesis 19 Dr. Raymond F Adebiyi