Innate and Adaptive Immunity

• This topic covers innate and adaptive immunity mechanisms at body surfaces
• Invading microorganisms are first encountered and repelled/destroyed at mammal surfaces
• Recall physical barriers, physiological processes that eliminate invaders, and the roles of immunoglobulin A

The innate mechanisms

• The skin is an obvious surface but represents a smaller body area exposed to the exterior
• Mucous membranes within the intestine and respiratory tracts are at least 200x larger
• The immune system keeps the body's interior free of microbes, but surfaces cannot be sterile

The body's microflora

• Nearly every surface is populated by a large, stable population of microorganisms called microflora/microbiota in a symbiotic relationship

• Microbes thrive and animals benefit from these commensal microbes

• The intestine has the largest microflora make up, which consists of > 1000 bacterial species in humans, with each individual housing ~160 species and ~one hundred trillion (10^14) bacterial cells

• The immune system must adapt to the presence of this microflora, retaining its ability to fight pathogens

• Two threats animals face:

• The importance of keeping normal microflora in place

• The microflora and their molecules might trigger innate immune responses like inflammation

• These are only triggered after bacteria penetrate the epithelial barrier

• Microbial invasion prevention helps prevent the development of inflammation within the intestinal epithelium

Controlling the microflora

• Tight junctions between intestinal epithelial cells exclude commensals

• Tight junctions between intestinal epithelial cells, coating of attached mucins forming a glycocalyx, and multiple antimicrobial peptide production

• Paneth cells are specialized intestinal epithelial cells

• These cells expresses toll-like receptors (TLRs) and secrete multiple antimicrobial peptides

• Enteric defensins, known as cryptdins, accumulate within intestinal crypts at high local concentrations

• Enterocytes are protected from invasion because commensals are prevented from entering the crypt space

• The gastrointestinal mucus layer is critical to commensal and pathogen exclusion

• The mucus layer consists of a gel made of mucins, glycoproteins, and lipids

• It prevents bacteria from contacting the epithelium, acting as a lubricant, blocking chemical insults, and capturing/expelling pathogens

Benefits of Microflora

• Intestinal microflora acts competitively against potential invaders

• The physical defenses of the system are supplemented by occupying and exploiting the intestinal microenvironment

• Commensals block subsequent colonization by pathogenic bacteria

• The immune system fails to develop in some germ-free animals

• Pigs or mice that are microbiologically sterile have fewer and smaller Peyer's patches, smaller mesenteric lymph nodes, and fewer cells in the lamina propria when compared to animals with a complete microflora

• The development of Peyer's patches in pigs, and the appendix in rabbits depends upon stimulation provided by the intestinal microflora

• There are reduced numbers of intestinal lymphocytes and systemic defects also apparent in the absence of microflora

• Enterocytes express fewer TLRs and MHC class II molecules

• They are less cytotoxic

• There are fewer CD4+ T cells in the spleen

• They have fewer and smaller germinal centers, resulting from reduced B cell numbers

• Immunoglobulin levels are only about 2% of normal

• Intestinal dendritic cells can extend their processes into the intestinal lumen and capture commensal bacteria

• The dendritic cells can carry these bacteria into the mucosa and the mesenteric lymph nodes and present them to B cells over several days

• This induces a local IgA response to block mucosal penetration by these commensals

• Some commensal bacteria are taken up by specialized antigen capturing M cells, penetrate the Peyer's patches, and become resident within the tissues

• Macrophages kill most invading bacteria, but some are also presented to B cells

• The interaction between the intestinal microflora and the intestinal-associated lymphoid tissues regulates the B cell repertoire

• B cells produce IgA, which may modify the composition of the intestinal microflora

• An ongoing IgA response prevents commensals from breaching the mucosal barrier

• Mesenteric lymph nodes are an additional barrier

• Molecules produced by commensal bacteria can influence the functions of all the major T cell subsets

• The bacterial molecule polysaccharide A (PSA) is key

• PSA suppresses proinflammatory cytokine responses in the intestine and inhibits lymphocyte infiltration

• Treg populations are reduced in germ-free mice

• Microflora also affect Th17 cell development in the intestine

• Germ-free mice are deficient in IL-17

• Segmented filamentous bacteria drives T cell development and stimulates Th17 production

• SFBs also promote germinal center development, IgA production, and recruitment of intraepithelial lymphocytes

• The microflora may regulate the Th17-Treg switch, since Th17 cells may originate from T reg precursors

• Changes in intestinal microflora composition have been implicated in allergy development

• The hygiene hypothesis suggests that a lack of exposure to certain commensal bacteria early in life affects the development of the immune system, thus increasing the chances of developing allergic diseases

• Piglets with a more diverse intestinal flora expressed more genes related to T cell function

• Intestinal microflora alterations also influence the development of autoimmune diseases such as rheumatoid arthritis, some mouse models of autoimmune arthritis, ankylosing spondylitis, insulin-dependent diabetes mellitus, and experimental autoimmune encephalitis

• The surface of the intestine is covered by a layer of enterocytes that form intercellular tight junctions, forming an effective barrier to both microbes and macromolecules

• Molecules > ~2 kDa are excluded

• Aggressive invasive bacteria may damage this barrier and trigger local inflammatory and immune responses

• Organisms and macromolecules can penetrate the intact intestinal wall via two alternative routes towards the intestinal lymphoid tissues:

• By penetrating specialized epithelial cells (M cells) found in the epithelium directly over aggregates of lymphoid tissue or Peyer's patches

• Via dendritic cells that reside in the submucosa but extend their cytoplasmic processes between the enterocytes into the intestinal lumen

• Tight junctions remain intact in the latter route, but antigen samples can enter within the dendritic cell cytosol

• This route supports noninvasive bacteria and macromolecules being sampled and presented to nearby T cells

Mucosal lymphoid tissues

• Two groups exist:
• Inductive sites where antigens are processed and immune responses initiated
• Effector sites, where antibodies and cell-mediated responses are generated
• IgA-producing B cells are produced in inductive sites (e.g. Peyer's patches) when stimulated by antigen
• They then leave the intestine and circulate in the bloodstream before settling on other surfaces like the lung, mammary gland, and other regions of gastrointestinal tract effector sites
• This transfer in the mammary gland ensures milk contains IgA antibodies against intestinal pathogens

Adaptive Protective Mechanisms

• Both antibody- and cell-mediated immune processes protect body surfaces

• The antibodies produced on mucosal surfaces include IgA, IgM, IgE, and IgG

• Some, most notably IgA and possibly IgM, act by immune exclusion

• The others, especially IgE and IgG, destroy antigen within the surface tissues by immune elimination

• IgA predominates in surface secretions; it is found in enormous amounts in saliva, intestinal fluid, nasal and tracheal secretions, tears, milk, colostrum, urine, and the secretions of the urogenital tract.

• IgA has three locations it can operate:

• In tissue fluid

• In enterocytes

• In the intestinal lumen

• The bound antigen in tissues or enterocytes is carried to the intestinal lumen

• Some IgA, instead of being secreted directly into the intestine, may be carried to the liver, where it is passed through the hepatocytes into the bile duct

• This route is very important in some species, such as the rat

• Only ~5% of IgA reaches the intestine by this route in humans

• IgA does not activate complement, so it functions by immune exclusion

• Immune elimination destroys antigen that penetrates the mucosal barrier (second line of defense)

• IgE mediates this

• IgE-producing cells are mainly found on body surfaces instead of lymph nodes or spleen

• IgE binds to mast cell Fc receptors in the mucosa of the intestine, respiratory tract, and skin

• If invading organisms evade IgA and gain access to the tissues, IgE-mediated responses are triggered

• The IgE response in the intestinal wall

• Antigen reaches IgE-sensitized mast cells to cause their degranulation

• This causes vasoactive factor release

• This increased vascular permeability and exudation of serum IgG antibodies

• IgA and IgE work in concert

• IgA normally is the first line of defense, and IgE serves as a backup system

• If IgA production is defective, the IgE response may be triggered to excess

• In ruminants (especially cattle), IgG1, not IgA, is the major secretory immunoglobulin in colostrum and milk, due to the selective transfer from the bloodstream into the mammary gland

• However, IgA remains the predominant immunoglobulin on other body surfaces in ruminants, although IgG1 is also present

• IgG2 is also transferred into the intestine and saliva in ruminants

• IgG may be of greater protective significance in the respiratory tract than in the intestine because it is less likely to be degraded by proteases

• Stimulating a mucosal IgA response makes sense when animals are vaccinated against organisms that infect body surfaces in the intestinal or respiratory tracts

• The vaccine antigen can simply be ingested or inhaled

• Unfortunately, such vaccines aren't always effective

• Inactivated antigens commonly fail to trigger an IgA response because they are immediately washed or sneezed off when applied to mucous membranes

• High levels of vaccine antigens incorporated in feed is a notable exception

• Using live vaccines where the vaccine organism can invade mucous membranes is the only way to trigger a significant IgA response

• Intestinal responses also have a high threshold due to the abundant intestinal microflora

• They lack memory and tend to fade rapidly

• The body tightly regulates antigen input across epithelial cells

• Regulatory effects on IgA production constantly adapt the IgA response to the intestinal microflora

• Respiratory tract vaccines against bovine or feline rhinotracheitis are good examples,

• Systemic vaccination against these surface infections can provide adequate immunity (e.g. in human influenza and polio vaccines), since some IgG may be transferred from serum to the mucosal surface

• Many available vaccines stimulate high levels of IgG antibodies in blood

• Once an invading organism causes tissue damage and triggers inflammation, IgG floods the invasion site, making this effective

• Oral tolerance remains a challenge for mucosal vaccines

• Administering some antigens to the respiratory or intestinal tracts may promote mucosal and T cell systemic unresponsiveness

• Inducing secondary immune responses on surfaces is sometimes difficult, and multiple vaccine doses may not increase the local immune response intensity or duration

• Intrinsic defects do not cause this, but rather high IgA levels blocking antigen absorption and preventing it from reaching antigen presenting cells