Lecture 26: Mucosal Immunity

• The presentation discusses Mucosal Immunity and Barrier Immunity, highlighting their roles in protecting vital organs, regulating the immune system, and preventing disease

Mucosal Immune System

• The mucosal immune system is a large apparatus that plays a crucial role in health
• It protects physiologically vital organs
• It helps regulate the tone of the entire immune system and prevent disease.
• Mucosal surfaces are vulnerable to infection and possess innate and adaptive immunity mechanisms.
• The adaptive immune system of mucosa-associated lymphoid tissues differs from the peripheral lymphoid system.
• Features of the adaptive Immune System in Mucosa
• Immediate juxtaposition of mucosal epithelium and lymphoid tissue.
• Presence of diffuse lymphoid tissue and organized lymphoid organs.
• Specialized antigen uptake mechanisms and dendritic cells.
• Predominance of activated/memory lymphocytes, even without infection.
• Production of dimeric secretory IgA as the predominant antibody.
• Downregulation of immune responses to innocuous antigens like food antigens and commensal microbes.

Mucosal Immunity Concepts

• Involves an inductive component: M cell uptake of Ag, CD103+ DC activation of T cells, and B cell differentiation into IgA-secreting PC.
• These PCs traffic through lymphatics to the blood and back to the lamina propria via alpha-4 beta-7 integrin and CCR9.
• The normal state is tolerogenic.
• Involves an effector component: activation of memory B and T cells, IgA, IgM or IgG-secreting PC, IEL, and CD103- CD11b+ DC uptake and activation of inflammatory T cells in the MLN.
• IgA, IgM, and IgG can cross the gut epithelia to prevent infection and eliminate pathogens; microbes can evade Ig neutralization.
• ILCs play a role in barrier immunity; they respond to cytokines, generating T helper immune responses.

Learning Objectives

• Identify unique features of mucosal immunity.
• Recognize major cellular components and functions within the inductor site, including M cells, B cells, T cells, Treg cells, macrophages, and CD103+ DC.
• Understand the mechanism that promotes the predominance of Th2 and Treg differentiation and IgA production in the mucosa.
• Define the mechanism by which CD103+ DCs direct tolerogenic responses against commensal microbes.
• Discuss commensal microbes' role in maintaining the tolerogenic effect in mucosal immunity.
• Discuss the role of intestinal macrophages in maintaining the tolerogenic effect.
• Identify major cellular components and functions within the effector site, including B and T cells, PC, IEL, mast cells, eosinophils, and CD103- CD11b+ DC.
• Define the mechanism by which CD103- CD11b+ DCs direct immunogenic responses against invading commensals and pathogenic microbes.
• Identify the mechanisms of Ig release into the gut lumen.
• Differentiate between ILC1, ILC2, and ILC3 responses, including involved cytokines.

Barrier Organs

• Barrier organs include the skin and intestinal, respiratory, and urogenital tracts.
• They provide a boundary between the microbiota, pathogens, and our tissues.
• Microbiota: collection of commensal microbes.
• Microbiome: collective activities and genetic potential of microbes in their environment.
• Barrier organs provide opportunities for our microbiota to communicate with our immune system.
• Human body surfaces are populated by microorganisms, monitored by barrier immune systems, and lined by epithelial cells interacting with immune cells
• The surfaces are populated by microorganisms and monitored by barrier immune systems
• Each surface is lined by one or more layers of epithelial cells to interacts with a variety of immune cells at and below these cell layers
• The interaction between Microbiota vs Innnate/Adaptive cells regulates the balance between homeostasis, tolerence, inflammation, health and disease

Role of Barrier Immunity

• Healthy equilibrium between microbes and immune systems at barrier surfaces is known as homeostasis.
• Homeostasis is maintained by mechanisms that inhibit inflammation and promote tolerance
• Homeostasis requires immune cell activity, known as physiological inflammation.
• All barrier surfaces are lined by one or more layers of epithelial cells.

Barrier Immune Systems

• Systemic immune system defends against pathogens penetrating the skin (lymph node and spleen) - Reactive.
• Mucosal immune system defends against pathogens breaching mucosal surfaces but is tolerant to food antigens and commensal microbes - Proactive.
• Mucosal Immune System
• Protects against harmful pathogens
• Cultivates harmonious interactions with microbes
• Commensal microbes provide vital nutrients and help establish a healthy immune system
• Secretory IgA is the predominant antibody

Systemic Immunity

• Pathogen encounter leads to macrophage production of cytokines, resulting in inflammation.
• Neutrophils, NK cells, infiltrate tissue, and DC capture antigens and migrate to lymph nodes to activate T cells while B cells are activated by antigens in the follicles.
• Effector T cells and antibodies travel to the infected tissue to eliminate the pathogen.
• Inflammation is suppressed and wound repair engages to repair damaged tissues.

Mucosal Immunity

• Pathogen encounter is continuous, leading to immune activation that creates memory B and T cell responses; rapid response with minor tissue damage
• Inflammation is terminated by IL-10-producing T regulatory (Treg) cells by suppressing promoters for inflammatory cytokines.
• Peyer's patches and mesenteric lymph nodes develop independently of the spleen and lymph nodes = unique populations and activities imprinted early in life

Unique Features of Mucosal Immunity

• Mucosal epithelia are directly linked with lymphoid tissue.
• Discrete compartments of lymphoid tissue are present (Peyer's patches, lymphoid follicles, tonsils, etc.).
• Specialized antigen uptake occurs via microfold cells (M cells).
• Activated/memory T cells predominate, even in the absence of infection.
• Specialized natural effector cells are present.
• There is active suppression of immune responses to antigen predominates.
• Gut-associated lymphoid tissues (GALT) must suppress responses to commensal bacteria and foreign food substances, but respond to pathogenic bacteria.
• M cells are phagocytic and direct gut materials to follicles and lymph nodes.
• Macrophages, T cells, and dendritic cells promote IgA production by B cells.
• IgA limits mucosal colonization by pathogens.
• Bronchial associated lymphoid tissue in the lungs has similar architecture and function to the GALT.

Barrier Microanatomy

• There are differences and similarities in the microanatomies of barrier tissues.
• Major cell types are skin (keratinocytes, Langerhans cells, CD8+ T cells), gastrointestinal tract (Goblet cells, M cells, IEL, DC, Entero-endocrine cell, lamina propria cells), and respiratory tract (Club cell, DC cell, interstitial macrophage, goblet cell, IgA+ B Cell, Alveolar macrophage)
• Each of these tissues is separated by on or more epithelial layers
• The skin Epidermis consist of multiple layers of epithial cells called Keratinocytes that ineract with Langerhans cells and multiple lympcytes
• Intestine and Respiratory tracts contain single layer of epithelial cells or mucosa
• Epithelial cells interact directly and indirectly with hematopetic cells, macrophages, DCs, and lymophocytes
• Some cells are in the epithelial layer, but most are below

Selected Barrier Epithelial Cells:

• Barrier tissues have physical barriers between epithelial cells and microbiota
• Skin - layers of epithelial keratinocytes.
• GI & urogenital tracts - specialized epithelial cells (goblet cells) generate mucus.
• Gut & respiratory tract – single layer of diverse epithelial cells:
  • Enterocytes – absorptive cells.
  • Paneth cells – secrete antimicrobial peptides.
  • Tuft cells secrete cytokines.
  • Alveolar type 1 and type 2 (AT1 and AT2) cells – generate surfactant.
  • Microfold (M) cells – carry antigen via transcytosis form lumen to subepithelial space.
• Keratinocytes are important for protection and waterproofing.
• Antimicrobial peptides (AMP), like defensins, are produced by epithelial cells and leukocytes.
• Cathelicidin is produced by neutrophils and epithelial cells, protecting against infections.
• M cells are concentrated in Peyer's patches.

Secretory IgM and IgA

• Secretory IgM and IgA protect mucosal surfaces from microbial invasion
  • Antibodies are retained in the mucus and form bonds with the mucin molecules
  • IgM is secreted at mucosal surfaces and important for mucosal immunity
  • immunoglobulins coat mircobial surfaces and impede microbial invasion and proliferation (Neutralization)
  • IgA is anti-inflammatory and IgG is important in fighting infection

IgA in Barrier Immunity

• All barrier tissues include IgA antibody-secreting cells (microenvironment enriched with TGF-beta)
• IgA
• Anti-inflammatory
• Cross epithelial cell barriers
• Recognize broad microbes or be very specific
• Inhibits microbes from penetrating epithelial barriers
• Promotes immune tolerance

Myeloid Cells in Barrier Immunity

• Antigen-presenting cells (APCs)
• Closely affiliated with barrier epithelial cells
• Include dendritic cells (DCs) and macrophages, expressing pattern recognition receptors (PRRs).
• Some DCs and macrophages are permanent residents in barrier tissues, whereas others are migratory -Langerhans cells (LCs) - specialized macrophages that reside in the skin -Alveolar macrophages (dust cells) - monitor lower airways for pathogens DCs vary by expression of CD103, CD11b, and CD8-alpha
  • Can induce tolerance, contribute to type 1 and 2 responses, or homing
  • Plasmacytoid dendritic cells (pDCs) promote tolerance
  • Produce antiviral type I interferons, produce cytokines that enhance IgA formation, stimulate T cells to produce anti-inflammatory cytokine IL-10 Macrophages CX3CR1 receptor are resident and extends protrusions sampling antigens by regulatory T cells

Innate Lymphoid Cells in Barrier Immunity

• Innate lymphoid cells (ILCs) play an important role in barrier immunity
• They are abundant in healthy barrier tissues
• Do not express antigen-specific receptors BUT respond to cytokines
• ILC1 - Intracellular pathogens generate a type 1 response (IL-12, IL-18) and contribute to inflammatory responses to INTRACELLULAR pathogens
• ILC2 - Helminth worms generate a type 2 response (IL-25, IL-33, TSLP) and release type 2 cytokines (IL-4, IL-5, IL-13); TSLP: thymic stromal lymphopoietin
• ILC3-Fungi and extracellular pathogens induce a type 3 response (IL-1, IL-23); roles in tolerance and inflammation
• Cytokines from IILC's often mirror T Memory differentiation in th1//2/17
• ILC's express no antigen receptors BUT respond to cytokines to create differernt subsets

Overview of Barrier Tissue Immune Cell Interactions

• Immune cells in barrier tissues interact to produce type 1, type 2, and type 3 responses.
• Epithelial cells and APCs use PRRs to sense pathogens.
• Cytokine production guides the immune response.
• Activated APCs migrate to lymphoid tissue, activate/polarize T cells, and coordinate the effector response.

Adaptive Immune Cells in Barrier Immunity

• A mixture of conventional lymphocytes and regulatory lymphocytes -Regulatory T cells are found in the mucosa and skin and help stop immune responses to antigens and microbes that are not a threat
  • TH17 cells important in maintaining barrier tissue homeostasis -TFH cells help induce and maintain IgA production in barrier tissues -Other CD4+ TH cells like TH1, TH2, TH9 are recruited during response to pathogens -CD8+ T cells (intraepithelial lymphocytes - IELs) intercalated between epithelial cells of barrier tissues a mixture of memory CD8+ T cells, other conventional T cells, and invariant T cells and IgA-secreting plasma cells -Enrichement of TGFbeta will give rise to most cells -Antigen cells must present antigens to IEL's for effector functions -CD4+T cells will respond to cooridinate

Lymphoid Tissue Associated with Barrier Organs

• Infections enter through mucosal surfaces; response depends on the specific surface
• Local lymph nodes serve barrier tissues
• Migratory APCs travel via lymphatics to draining lymph nodes, establishing system responses
• Unique lymphoid microenvironments offer direct access Mucosal-associated lymphoid tissue (MALT) is most organized and includes BALT, NALT, GALT, and SALT; GALT is well characterized

Benefits of Commensal Microbiota

• Synthesis of essential metabolites
• Breakdown of plant fibers into digestible food
• Inactivation of toxic substances
• Prevention of pathogen access
• Interaction with the epithelium to trigger the development of the secondary lymphoid tissue
• Inapropriate responses to food create damages Celiac = wheat damage

Barrier Health

• Initiate tolerogenic and inflammatory responses to microorganisms, where homeostasis leads to a tolerogenic state
• Cell-cell and cell-microbe interactions produce anti-inflammatory molecules like TGF-β and IL-10, enhancing TREG cell and IgA-secreting plasma cell production.
• IgA interacts with commensal microbes, preventing penetration and inflammation
• Epithelial cells secrete defensins; metabolites generated by microbiota contribute
• Short-chain fatty acids and inositol phosphate
• Epithelial barriers share basic anatomnic, connective, immune layers that respons against invaders
• Secondary tissues help adaptive responses

Common Themes in Barrier Immune Responses

• Immune systems respond to microbiota with tolerogenic responses to commensal bacteria and inflammatory responses to pathogens, inducing interactions between microbes and APCs that generate cytokines
• Inflammatory responses initiated by microorganisms involve type 1, type 2, or type 3 responses, where type 1 (intracellular pathogens and fungi) is associated with helper 1 and ILC1 cell activation
• Responses depends of the presence of APC's and APC induction of interations
• T1 and T17 produce inflamatory markers and recruit cells

Immunity in the Gastrointestinal (GI) System

• Immunity in the GI System must maintain balance between defense against intestinal pathogens and tolerance to commensals and food antigens
• This is achieved with a thick mucus layer, antibiotic peptides, and IgA production

Gastrointestinal Gut Tract

• The gastrointestinal gut tract (GI) is organized with gut-associated lymphoid tissues, known as a GALT, that is composed of a Mucosa layer, containing resident Immune cells, Peyers Patches, and mesenteric nodes
• Cryptopatches are precursors to ILFs – isolated lymphoid follicles.

Small vs Large Intestine (Immune System)

• The immune systems in the small and large intestines differ in commensal diversity and vulnerability to specific pathogens and diseases

Antigen Delivery

• M cell receptors, Goblet cells, and resident APC's (macrophages) sample antigens from Intestinal Lumen
• Cells sample and transfer them

Maintenance of Intestinal Homeostasis

• Interactions generate a tolerizing environment with TGF-ẞ, retinoic acid, thymic stromal lymphopoietin, and IL-10.
• Key tolerogenic cells include intestinal APCs, TREG cells, IgA-secreting plasma cells, B10 B cells, TFH cells, TH17 cells, ILCs, and IELs
• T cells and regulatory lympcytes
• Key interactions Treq + BAFF (T and B cell factors for the interction)
• Il-22 increases cell produciton and epithelial cells

Mucosal CD103+ Dendritic Cells

• DCs can uptake Antigens from M cells that constitutively secrete Thymic Stromal Lymphopoiten, but can be further induced by NLR signalling resulting in T2 and T Cell Differentiation that promote tollerance

Mucosal CD103- Dendritic Cells

• DC's extend dendrites into the Lumen using ATP receptors to determine cyokine output
• Increased Bacterial Levels -DC creates more output increasing the chance of creating Th17 and Th1 cell differentiation that promotes inflamation

Intestinal Dysbiosis

• Results from enviromental changes that effects the gut barrier
• Pathogens win/colonize with antibiotics

Immune Response in the Gut

• Gut responses involve induction to generate inflammatory immune response from antigen and effector phase mount immune response clear organism and recruit cells Alveolar
• Involves induction (proinflammation) and effective for (actual clear)

Respiratory Immunity

• Response is triggered by microbe interactions
• Lamina Propaia contains lymphocytes and inter-action with alveolar macrophages
• Macrophages engaged and release cytokines
• Worms stimulate cells to trigger type 2 response