Lecture 28: Mucosal and Cutaneous Immunity PDF

Chapter 12: Mucosal and cutaneous immunity  Lecture outline Mucosal Immune Response Innate Response sIgA Th1, Th17 and CTL Common Mucosal Immune System Skin Associated Mucosal Tissue (SALT) Immune responses in GALT, NALT and BALT Most knowledge on muco...

Chapter 12: Mucosal and cutaneous immunity  Lecture outline Mucosal Immune Response Innate Response sIgA Th1, Th17 and CTL Common Mucosal Immune System Skin Associated Mucosal Tissue (SALT) Immune responses in GALT, NALT and BALT Most knowledge on mucosal immune responses comes from studies of GALT & similar principles apply in NALT & BALT. Contribution of the microbiota in GALT – Enterocytes express number of PRRs which interact with molecular patterns expressed by commensals & pathogens. – The results of this PRR engagement depend on whether the host is at rest or under attack. – Under steady conditions, the engagement of enterocytes’ TLRs by commensals induces the production of molecules that damp down inflammation & adaptive responses to innocuous Ags. Immune responses in GALT, NALT and BALT Contribution of the microbiota in GALT – Engagement of enterocytes’ TLR2 plays role in inducing Tregs & maintaining oral tolerance & preserving integrity of gut. – Similarly, TLR5, TLR9 and NOD2 play very important role in maintaining normal gut homeostasis. – Germ-free mice exhibit decreased GALT, abnormally high number of Th2 cells and low numbers of Th1 and Th17 cells.  Innate responses to pathogens If a host’s steady state is lost due to injury or aggressive pathogen or toxin, then engagement of enterocyte’s PRRs by PAMPs/DAMPs/PRMs activate these cells to produce pro-inflammatory cytokines and chemokines. Lectin pathway of complement activation may be activated. Nitric oxide produced by activated phagocytes is effective in inhibiting viral replication. Antimicrobial peptide (defensins & lactoferrin) secretion induced by binding of PAMPS/DAMPS to TLR9 or NOD2 of paneth cells is effective against enteric viruses & bacteria.  Innate responses to pathogens In response to inflammatory distress call, neutrophils and mast cells are recruited to the site of infection, and ab and gd T cells within or near gut epithelial layer are activated. NK and NKT cells tucked in epithelium are also stimulated by locally produced cytokines. Engagement of PRRs and/or Ag receptors of leukocytes by an attacking pathogen activates them, leading to all the effector actions of these cells described earlier.  Innate responses to pathogens Along with degranulation and phagocytic functions, neutrophils produce IL-18 that combines with DC-produced IL-12 to stimulate NK cells to release IFN-g. IFN-g activates DCs & macrophages & causes them to increase their production of inflammatory cytokines. Neutrophils also capture extracellular bacteria in neutrophils extracellular traps (NETs).  Adaptive responses to pathogens Most mucosae, particularly in gut & lungs, are inherently fragile structures, such that they can easily be injured by products (i.e. TNF & IFN-g) of cells activated during vigorous inflammatory responses. For this reason, mucosal immunity in these sites mostly depends on adaptive defense by secretory Abs such as sIgA sIgA Abs are key components of mucus and other body secretions such as saliva and tears and protect against pathogens or toxins without inducing severe inflammation.  Rational for mucosal sIgA Many features & functions of secretory IgA (sIgA) makes it ideal for mucosal defense. 1. sIgA is constitutively localized in mucus, ensuring that this Ab is ready to neutralize almost any pathogen or toxin trying to make contact with epithelial cells. 2. Independent of antigenic specificity, the carbohydrate moieties of sIgA molecules can bind to adhesion molecules expressed by many pathogens, trapping the invaders on the luminal surface. 3. At least in the gut, about half of all sIgA Abs are usually cross- reactive and thus a broader range of threats can be countered with fewer Abs. 4. sIgA is not an efficient activator of complement so there is less chances of inducing damaging inflammation. 5. sIgA is highly resistant to a wide variety of host and microbial proteases.  Production of mucosal sIgA The body’s production of sIgA far exceeds that of any other isotype with about 2-3gms sIgA synthesized every day. sIgA-producing plasma cells are present at mucosal surfaces even under steady state conditions due to the influence of microbiota on mucosal B cells. Constant production of sIgA directed against commensals keeps their numbers under control & maintain homeostasis under steady state conditions.  Production of mucosal sIgA Enterocytes play a key role in sIgA production. When enterocytes PRRs are engaged by commensals, they release BAFF, APRIL & IL-10 which cause 80% of activated B cells in lamina propria to undergo IgA isotype switching. DCs upon TLR5 engagement by flagellin-expressing bacteria express an enzyme that converts vitamin A into retinoic acid which promotes the differentiation of mucosal B cells into IgA-producing plasma cells.  Production of mucosal sIgA Activated B cells from lamina propria circulate in the lymph & blood, home to mucosal effector sites & extravasate through local HEVs. In these locations, activated B cells complete their differentiation into mature IgA-producing plasma cells. Plasma cell secrete polymeric IgA which binds to pIgR expressed on the epithelial cells present in mucosal effector sites. Upon exocytosis, pIgR is cleaved which results in Fig 12.4: From antigen uptake to release of sIgA into mucus. secretory IgA production.  Other mucosal antibodies If need arise, some mucosal B cells may switch to produce anti-pathogen Abs of IgG isotype. These IgG Abs gain access to body secretions through Ab transporter proteins. Unlike IgA Abs, IgG Abs induce a robust inflammatory response & activate complement & so surrounding host tissue may suffer collateral damage, but invader is killed.  Other mucosal antibodies IgD isotype Abs are found in tears, salivary, nasal & lung secretions, but rarely in other body secretions. This mucosal IgD is produced by B cell lineage cells that have surface phenotype of IgD+IgM+ plasmablasts and are capable of J chain synthesis. Under steady state, mucosal IgD binds to commensals & ensures that they do not penetrate too far into tissue. In case of infection, IgD both bind to pathogen & activates basophils which release antimicrobial peptides & cytokines.  Th1 and Th17 responses When an aggressive pathogen attacks a mucosal surface, a Th1 and/or Th17 response may be needed to clear invader. Interfollicular DCs that acquire Ag in the presence of PAMPs/DAMPS preferentially produce IL-12 which induces locally activated T cells be differentiate into Th1 effectors. Some interfollicular DCs may also migrate to draining lymph node & activate naïve T cells & induce Th1 effectors.  Th1 and Th17 responses Presence of microbiota induces constitutively high numbers of Th17 cells in gut lamina propria & these cells are critical for mucosal defense against bacterial & fungal infections. Activated intestinal Th17 cells produce IL-17 & IL-21 which cause nearby cells to release inflammatory molecules. Like in the gut, IL-17 signaling is very important in the lung for defense against bacteria. In NALT/BALT, pulmonary Th17 and gd T cells are important source of IL-17.  CTL responses If a pathogen avoids mucus & secretory Abs and penetrate the mucosa, it may be captured by phagocytic APCs which activate naïve Tc cells resident in mucosal inductive sites. In PPs, Tc cells are found in the interfollicular areas surrounding the intestinal follicles. Nearby mucosal DCs in the dome of the EAE can acquire pathogen Ags from M cells, or from infected epithelial cells that have become necrotic or apoptotic. These DCs then can cross-present pMHCs that activate Ag- specific Tc cells. The activated Tc then generate Ag-specific CTLs that can migrate to multiple effector sites. Common mucosal immune system Lymphatic Duct Lacrimal Reproductive Tract Glands Lactating Mammary Glands Middle Ear Lymph Node Blood Vessel Respiratory Tract Nasal Cavity Spleen Salivary Glands Tonsils Gastrointestinal Tract  A common mucosal immune system A pathogen invasion at one location in the intestine can lead to appearance of sIgA not only in entire gut but also in respiratory tract, salivary glands, lacrimal glands, ocular tissue, middle ear & even in lactating mammary glands. Similarly, Ag introduction intranasally result in detectable Ag-specific sIgA in the saliva, tonsils, trachea, lung and gut. This disseminated protection has given rise to a concept called the ‘common mucosal immune system’ (CMIS). In CMIS, mucosal T & B cells from an inductive site migrate through blood & lymphatics to several effector sites & it is governed by shared expression of mucosal homing receptors.  A common mucosal immune system Mucosal homing receptors differ from those expressed by conventional T & B cells activated in lymph nodes and bind to ‘addressin’ proteins expressed exclusively in mucosal effector sites. For example, a conventional B cell bearing a4b1 homing receptor circulate systemically and bind to addressin VCAM- 1 expression by endothelial cells at site of inflammation. In contrast, a mucosal B cell expresses a4b7 integrin & ignores sites in peripheral tissues, homing instead to mucosal effector sites where its a4b7integrin can bind to MAdCAM-1 expressed by endothelial cells in the mucosae.  A common mucosal immune system In case of mucosal T cells, the expression of a4b7 integrin and chemokine receptor CCR9, is induced specifically by interaction with mucosal DCs in inductive sites. CCR9 binds to the chemokine TECK secreted by mucosal epithelial cells in effector sites. The immune responses induced at different mucosal effector sites are not of uniform strength, being strongest at those sites closest to inductive site or in tissues sharing lymph drainage. For example, if the PPs in the GALT are inductive site, a strong Ab will be detected in the nearby small intestine mucosa (GALT), but only a weak response will be observed in more distant tonsils (NALT) and vice versa.  A common mucosal immune system Interestingly, a response to one pathogen in one inductive site can influence the response to a completely different pathogen in a different inductive site. For example: In individual infected with Mycobacterium tuberculosis, the immune response in the BALT mounted against this invader seems to be influenced by responses in the GALT against other pathogens. Variations in co-infection patterns and in the efficiency of the CIMS in different individuals may account for the fact that globally over 2 billion people are infected with M. tuberculosis, but only 20 million suffer from active TB.  MALT in the urogenital tract Different regions of female and male urogenital tracts feature different types of mucosae. In general, the composition of the microbiota associated with urogenital mucosae are quite different from the commensals protecting the gut and respiratory mucosae. Vagina has type II mucosae & generally lacks the organized lymphoid structures typically present in MALT inductive site  MALT in the urogenital tract Intraepithelial DCs & macrophages occur in cervical & vaginal epithelium & so Ag introduction into vagina induces only weak immune responses & no systemic responses. This lack of responsiveness in vagina is evolutionarily desirable because an immune response to incoming sperm could block reproduction and thus species survival. In addition, the mucus protecting cervix is much less acidic than in other locations, allowing sperm to penetrate. The composition of microbiota in vagina is also unique, involving species of Lactobacillus that provide protection against a wide variety of pathogens.  MALT in the urogenital tract sIgA are found in upper vaginal secretion, confirming that at least some vaginal regions are mucosal effector sites. The penile urethra is both an inductive and effector site. Epithelial cells lining penile urethra express pIgR & penile lamina propria contain many IgM- & Ig-A secreting plasma cells. IgG-producing plasma cells may also be present here. Mucosal DCs reside among urethral epithelial cells, whereas macrophages & large populations of CD4+ & CD8+ memory T cells are also found in the lamina propria.  MALT in the ear Middle ear cavity is lined with a thin layer of mucus which is continuously conveyed towards the Eustachian tube and nasopharynx by beating of the cilia on ciliated epithelium. This action keeps the middle ear cavity sterile and antimicrobials present is the mucus also provide protection. There are very few organized lymphoid structure or cells in a middle ear cavity, so it is not an inductive site. However, when infection occurs, cavity becomes a mucosal effector site with local production of sIgA.  MALT in the eye Eye is an immune-privileged site in which immune responses and inflammation are generally discouraged. Cells & macromolecules can not readily pass through the eye blood vessels & the eye is not connected to a draining lymph node. Ags that do access the eye are captured by intraocular APCs (specialized subsets of DCs) that have been influenced to promote Th2 development by the high concentration of TGF-b present in aqueous humor of the eye. The intraocular APCs migrate from the eye into the blood and then to spleen, where lymphocyte activation occurs. Effector Th2 cells home back to eye where they support non-inflammatory humoral responses.  Cutaneous immunity Cutaneous immunity defends the skin against damage caused by infection or injury. Immune system elements that underlie the skin are collectively called skin-associated lymphoid tissue (SALT ). Skin is composed of epidermis, dermis and hypodermis. Epidermis is not vascularized & is separated from the dermis by the basement membrane. The dermis contains both lymphatic and blood vessels. Hypodermis, a fatty layer that provides passive barrier defense and support for the lymphatics and blood vessels. It functions chiefly as an energy source. Plate 12.3: The skin.  Epidermis Keratin layer: Tough outer layer of the skin that resists penetration of inert stimuli and microbes is made up of filaments of resilient, fibrous protein called keratin. Keratin is produced by squamous epithelial cells called keratinocytes and they are connected by desmosomes. The constant turn over of keratinocytes prevents microbes from becoming entrenched. Fig 12.5: SALT components  Cutaneous immunity In addition to being physical barrier, in keratin trillions of commensals compete with pathogens for space & nutrients. The skin microbiota also secrete antimicrobial substances such as lipases which degrade fats and reduce the pH of the skin surface and discourage pathogen replication. The acidity of skin is also maintained by sebum produced by sebaceous gland originating in the dermis.  Cutaneous immunity Lower epidermis: – In lower epidermis, small numbers of ab & gd epidermal T cells & immature skin DCs called Langerhans cells (LCs) are present. – B cells are not generally found in skin. – The LCs acquire Ags that have penetrated the keratin layer. – Survival & activation of LCs & epidermal T cells depend on growth factors & cytokine routinely secreted by keratinocytes.  Cutaneous immunity Lower epidermis: Cont’d – Keratinocytes also constitutively express several TLRs. – Keratinocytes rapidly release inflammatory cytokines and chemokines in response to TLR engagement. – Innate leukocytes responding to these cytokines produce IFN- g, which promotes LC maturation. – As LCs express CD1 proteins as well as MHC class I and II, they are ideal presenters for peptide Ags to ab epidermal T cells and glycolipid Ags to gd epidermal T cells.  Cutaneous immunity Basement membrane: – Separates epidermis from dermis. – Epidermis leukocytes secrete enzymes that dissolve small regions of basement membrane & allow passage of leukocytes. Dermis: – Dermis contain structural fibers, neurons, blood vessels, lymphatics, hair follicles, fibroblasts & number of immune cells. – Most abundant leukocytes in dermis include macrophages, mast cells, dermal DCs (distinct from LCs) & ab memory T cells.  Immune responses in the SALT Innate responses to pathogens: A pathogen when penetrates into epidermis provides PAMPs & also causes damage to keratinocytes & generate DAMPs. gd T cells in the epidermis immediately recognize these skin stress Ags & are activated upon engagement of their TCRs. Damage to a keratinocyte also trigger that cell to produce IL-1 and TNF. These inflammatory cytokines induce other keratinocytes to produce chemokines, growth factors & additional cytokines. Innate responses to pathogens: cont’d A diffusion gradient is established that penetrates through the basement membrane into the dermis. Dermal fibroblasts & macrophages respond to these molecules with the synthesis of additional inflammatory cytokines and chemokines. Some of these cytokines reach endothelial cells of dermal blood vessels & promote local vasodilatation & selectin expression. This results in extravasation of additional leukocytes especially neutrophils & other granulocytes. Neutrophils produce hydrolyses that degrade basement membrane, allowing other leukocytes to enter epidermis. Innate responses to pathogens: cont’d Neutrophils & macrophages at the site of assault/infection, in the presence of inflammatory cytokines, upregulate their PRRs & phagocytic receptors & internalize & kill pathogens. Complement may be activated & phagocytosis is enhanced if pathogens are opsonized by complement components. Complement activation triggers degranulation of mast cells. NK cells & plasmacytoid DCs (pDCs) can be recruited to sites of injury or infection. pDCs have strong anti-viral activity (IFN-a & b production) & promote wound healing.  Immune responses in the SALT Adaptive responses to pathogens: Adaptive response in SALT is initiated when Ags from dying keratinocytes & attacking microbes are taken up by LCs. If cytokine milieu is rich enough, LCs mature within epidermis & present Ag to epidermal ab Th & Tc cells. As these ab T cells are primarily memory cells, their response is as rapid as that of gd T cells & within 24 hrs, they commence differentiation into CTLs & Th effectors. Infected cells displaying Ag are killed by Ag-specific CTLs. Adaptive responses to pathogens: cont’d If high local concentration of IL-12 in the site of assault, the differentiating Th effectors are biased towards Th1 & Th17. In contrast to gut, Th1 and Th17 responses are well tolerated by the skin due to its inherent toughness. IL-17 stimulates production of other inflammatory cytokines & promotes CTL-mediated cytotoxicity against infected cells. Interaction of Th0 cells in the epidermis with LCs sometimes results in differentiation of Th22 cells & they produce IL-22. In normal skin, IL-22 promotes keratinocyte proliferation & injury repair & blocks keratinocyte terminal differentiation. Adaptive responses to pathogens: cont’d Upon activation of epidermal T cells, IFN-g & bacterial products diffuse into dermis & stimulate dermal macrophages & DCs, and increase their migration to epidermis and phagocytic activities. LCs bearing Ag may enter dermis & access lymphatics leading to the local lymph node. Then naïve T cells in the node are activated and generate Th effector cells and CTLs that express homing receptor cutaneous lymphocyte antigen (CTA) which direct them back to inflammatory site in the dermis. Humoral responses can be generated in the skin if needed.  Next Lecture Chapter 13: Immunity to infection.