Mode of action of Mucosal Immunity against viral infection

Mucosal Immunity comprises the “surface exposed” parts of the body (gut, mouth, genital tract, lung, eye, etc.)

Mucosal-Associated Lymphoid Tissue (MALT)

•       Each mucosal surface has immune tissue, more or less organized

–      GALT (gut): Peyer’s Patches, appendix

–      NALT (nasopharynx): Adenoids, Tonsils

–      BALT (bronchial): iBALT

–      Genital tract?  Not as clear, no lymphoid follicles.

- The mucosal immune system forms the largest part of the body’s immune tissues

      ㆍ3/4 of all lymphocytes producing the majority of immunoglobulin

-The mucosal surfaces are the entry point of many pathogens and

are the first line of defense against foreign antigens, including microbial and dietary antigens.

-Mucosal pathogens are the highest killers of young animals because of,                        neonates are born with relatively undeveloped mucosal immune systems.

-So, the two important periods of maximum exposure to novel antigens occur in the young animal, immediately after birth and at weaning.

-In both cases the antigenic composition of the intestinal contents can shift suddenly, as a result of a novel diet and of colonization by novel strains and species of bacteria.

The Challenges Faced by the Mucosal Immune System:

The MALT has two challenges:

1-    The most frequent portal of entry for harmful substances.

The MALT must recognize the harmful substances and surmount an effective response.

2-The mucosal membranes, especially of the digestive tract, must allow nutrient absorption. So the MALT must ignore a lot of harmless substances.

A very complex decision to make: to differentiate between harmful antigens and harmless antigens. The cells are specialized to face these two challenges simultaneously.

Unique features of Mucosal I.S.

Close association of mucosal epithelial layers and lymphoid structures

–      Diffuse or organized

–      Example: Microfold (M) cells in the gut

Effectors

–      IgA

–      Specialized T’s, activated even w/o infection

Strong immunosuppressive environment

–      Suppressive responses to commensals

Specialization of Cells

B cells mediate the mucosal immune response by secreting IgA at least 20X more than IgG+ and by secreting IgM

Critical Features of Secretory IgA:

1. resistant against common intestinal proteases

- the mucosal immune system makes special secretory IgA resistant to this proteolytic environment which would normally cleave protein IgA

2. does not interact with complement or cells in a way to cause inflammation. 

- keeps harmful Ags in the lumen

So how does secretory IgA protect?

1.    inhibits the adherence of different pathogens to mucosal surfaces

- dimeric IgA creates a negatively-charged hydrophilic shell around pathogens and thereby prevents their adherence to the mucosal surfaces: pathogens retained in the lumen

2. neutralizes viruses

- blocks viral ability to interact with cellular receptors used for viral endocytosis

3. neutralizes a variety of enzymes and toxins

ex. Cholera toxins

4. immune exclusion: pathogens remains in the lumen
1. IgA inhibits antigen absorption
2. minimizes the inflammatory responses because the antigen is never presented
3. minimizes the chance of pathogen replication or pathogenic cause of infections

Factors Controlling IgA Isotype Switching

An APC picks up the antigen, processes it, and presents it in the context of class II MHC to T cells.

 This occurs in the presence of co-stimulatory signals (B7:CD28).

The APC presents antigen to the T cell, and the T cells in turn gets activated.

 Now the T cell can provide help to B cells in two forms:

(1) contact dependent interaction (e.g. CD40:CD40L) and

(2) production of cytokines.

 TGF-β is the key cytokine implicated as the switch factor for IgA. 

TGF- β is key in this primary event. 

These two events drive B cells from IgM+ to IgA+.

Other cytokines important (after the B cell has switched to IgA) to help in the terminal differentiation of B cells into plasma cells include IL-2, IL-4, IL-5, IL-6, IL-10.

These are called TH2 type cytokines.  (More later)

Finally, the plasma cell starts secreting a large amount of IgA. 

In addition to IgA production, you need a J chain

The J chain:

-          a small polypeptide

-          binds to the tail of dimers,

-          production found in plasma cells at mucosal surfaces… where you need secretory IgA

o   not in the bone marrow: there you secrete monomeric IgA

-          stabilizes the dimers: the homomeric form of IgA

-          interacts with the secretory component (the polymeric IgA receptor)

o   secretory component allows IgA to travel from the mucosal surface into the gut

The Secretory Component/Polymeric Ig Receptor

IgA gets from the lamina propria to the lumen of any mucosal surface by attachment of the secretory component to J chain portion of dimeric IgA

The Secretory Component:

-          a receptor in the basolateral membrane of a mucosal epithelial cell

-          binds IgA:J chain,

-          the complex is internalized, endocytosed, transported to the other side of the epithelial cells, and cleaved off into the lumen:

-          the intra-cytoplasmic and transmembrane portions of S.C./receptor stays within the cell.

-          The rest gets released into the lumen with the IgA.

This new complex in the lumen consists of: two IgA molecules, the J chain, and a piece of the receptor.  The receptor piece protects IgA from proteolytic cleavages, so IgA can mount an effective immune response in the face of a proteolytic environment.

Cytokine Production in Mucosal Surfaces: 

TH1 cytokines are mostly IFNγ and  IL2: proinflammatory

TH2 cytokines are IL-4,5,6,10,13: not as inflammatory

at Inductive sites (where pathogen enters):

          - a balance of TH1 and TH2 cytokines: both pro- and anti-inflammatory

- mostly CD4+

at Effector sites

                  

Mucosal T cells have specialized subsets of cells: lamina propria lymphocytes & interepithelial cells

Lamina Propria Lymphocyte: T cells (LPLs)

-          LPLs scattered diffusely throughout the small intestine in the lamina propria

-          The lamina propria is the single largest T cell site

o   most are CD4+ cells

-          shift in balance of cytokines to TH2 cells: shift toward anti-inflammatory and produce terminal differentiation of sIgA+ B Cells to IgA secreting plasma cells

The Intraepithelial Lymphocytes (IELs): 

The second specialized subset of mucosal T cells

-          between columnar epithelial cells in the small & large intestine

-          mostly CD8+ cells

-          balance of TH1 and TH2 cytokines

-          approximately 10% are γ/δ cells

-          IEL T cell receptor shows limited diversity:

o   IELs likely to recognize common pathogens

-          IELs express a novel integrin HML-1 so IELs can migrate to its correct location between epithelial cells

o   Note: many mucosal T cells express integrins or addressins to get to a specific location

Functional Properties of IELs (speculative)  

-          first immune cell line of defense in the intestine

-          lymphocytes against many common pathogens by:

(1)  killer, cytotoxic activity (CD8+)

(2)  secretion of large amounts of cytokines incl. IFNγ and TNFα

-          monitor the epithelial cell layer

(1) if a cell infected, IELs kill the cell, then secrete growth factors to promote epithelial cell renewal -- the barrier stays intact

(2) play a regulatory role in tolerance of dietary antigens

•       M Cells: specialized epithelial cells - Specialized Ag-uptake mechanisms          

-          look like membranes in the gut

-          over the lymphoid follicle domes along small and large intestine

-          very thin

Features important for M cell functions:

1. Short irregular microvilli: M cells sample antigen, not for GI absorption through microvilli
2. Abundant endocytic vesicles: bring in antigen from the lumen to the lamina propria
3. Pockets in the basolateral surface: for APCs (DC,B cells, macrophages) which take antigen from the M cells: antigen sampling
4. Low lysosomal content: M cells are involved in transport, not Ag presentation or processing
5. Binding sites for secretory IgA, but not for secretory component
1. IgA:Ag binds M cells and moves from the lumen into the lamina propria
2. M cells do not transport IgA in the opposite direction

Peyer’s Patches    

-          the “afferent limb” of the immune response

-          organized mucosal lymphoid follicles lacking afferent lymphatics (so only bring antigen from lumen)

-          found in the small intestine

-          similar follicles in other mucosal sites found in the appendix, in the rest of the GI tract, and in the respiratory tract

1.    antigens get brought in through the M cells: endocytosis into the vesicles, transportation to the APCs

2.    APC processes and presents Ag to T cell

3.    if a T cell recognizes the Ag, it gets activated

4.    form a lymphoid follicle (under these M cells) = the Peyer’s Patch

5.    T cells secrete factors like TGFβ

a.    So B cells switch to surface IgA (switched from IgM)

6.    Surface IgA+ B cells and activated T cells migrate to the mesenteric lymph nodes, through the thoracic duct, and into the peripheral blood

7. Go to effector sites in the lamina propria and intraepithelium

Sum so Far: The antigen is seen in a few Peyer’s Patches.

The B cells and T cells which recognize the antigen migrate into many different sites.

(So more places in the mucosa can mount a response to the specific antigen.) 

8. surface IgA+ B cell (specific for this antigen) starts last step in differentiation: becomes a plasma cell
9. plasma cell secretes J chain with IgA (which dimerizes)
10. dimeric IgA + J chain bind to the secretory component:
1. transcytosis into the lumen
2. complex is secreted
11. Note: lamina propria lymphocytes provide a TH2 cytokine environment: IL-4, 5, 6, 10
1. B cells need TH2 help to complete their differentiation
12. IELs protect G.I.T against common pathogens

Note: This is very compartmentalized process.

 The B cells recognize antigens in the Peyer’s patches go back to the gut.

 B cells have a variety of receptors for B cell localization . 

The differences between receptors clearly bear clinical importance