Mucosal Immunity

Question 1

Mucosal surfaces are continually exposed to external infectious agents, and consequently, immunologic defense against pathogens is paramount at these surfaces. Specific immunologic defense at mucosal surfaces is mediated by a specialized arm of the immune system that is
termed the mucosal immune system.

What does the mucosal immune system include?
Describe the muscosal or secretory branch of the immune system.

Answer 1

The mucosal immune system includes lymphoid tissues
of the gastrointestinal tract, respiratory tract, salivary glands, lacrimal glands, mammary
glands, and genito-urinary tract.

The mucosal, or secretory, branch of the immune system is
quite extensive, as the mucosal surfaces of the human body represent an area 100 times greater that of the skin. The importance of this system is underscored by the fact that 70 to 80% of all immunoglobulin producing cells in the body are physically located within the tissues of the mucosal immune system.

Question 2

Mucosal tissues are exposed to a large number of both potentially harmful and benign antigens from the environment, food, and microorganism. Give an example.

Answer 2

the intestine is host to
hundreds/thousands of different bacteria. The mucosal immune system must therefore
continually control responsiveness and unresponsiveness.

Question 3

Mucosal immunity is triggered by the coordinated interaction of multiple cell types within
the mucosal tissues. Describe what this process involves (Where is initiation of response? What does this lead to?)

Answer 3

The process involves the initiation of the response at an inductive site, leading to an immune response at multiple effector sites.

Question 4

Components of the mucosal immune system (MALT) include:

Answer 4

Gastrointestinal tract – gut associated lymphoid tissue (GALT)

Respiratory tract – bronchial associated lymphoid tissue (BALT)

Nasal associated lymphoid tissue (NALT)

Genitourinary tract

Lacrimal glands

Salivary glands

Mammary glands

Question 5

Lymphocytes reside in defined compartment of MALT (GALT is best defined example). Describe, mechanistically, the induction process in steps.

Answer 5

Antigens entering the digestive tract are taken up by specialized mucosal cells called M cells.

M cells internalize the antigen and transport it across the epithelium where antigen can be
taken up by APCs such as dendritic cells (DC). “M” cells are formed in mucosal
epithelium in response to signals from lymphocytes.

Antigen can be taken up by DC that have dendrites extending through the epithelial tight
junction into the lumen (drawing on right).

Antigens are then presented to lymphocytes (in the intestine, these are located in Peyer’s
patches).

Question 6

How is antigen captured from lumen?

Answer 6

Antigen is captured from the lumen by dendritic cells that extend across the epithelial layer.

Question 7

Describe the role of M cells.

Answer 7

M cells take up antigen by endocytosis and phagocytosis.

Antigen is transported across the M cells in vesicles and released at the basal surface.

Antigen is bound by DC, which activate T cells.

Question 8

Describe how lymphocytes (both B and T cells) leave the mucosal site and travel to the mesenteric lymph nodes, then into the lymph.

Answer 8

Lymphocytes (both B and T cells) leave the mucosal site and travel to the mesenteric lymph
nodes, then into the lymph.

Via the thoracic duct, the lymphocytes exit the lymph and enter the circulation.

Circulating lymphocytes “home” to positions within the mucosal lamina propria throughout
the body, including sites distant from the original antigenic encounter. Homing of
lymphocytes to mucosal sites involves specific interactions of both adhesion molecules and
chemokines.

B Lymphocytes within the peripheral tissues proliferate and differentiate into IgA secreting plasma cells at effector sites.

Question 9

What does administration of antigen at one mucosal site result in?

Discuss any examples of regional preference.

Answer 9

The administration of antigen at one mucosal site results in specific antibody production at
distant mucosal sites. Some regional preference seems to occur, however. For example,
induction via NALT leads to a more robust response in the respiratory sites than in
gastrointestinal sites.

Question 10

What do B cells in mucosa tend to produce?

Answer 10

B cells in the mucosa are selectively induced to produce dimeric IgA rather than other
isotypes. The selective switch of B cells to IgA is believed to be mediated by specific
cytokines produced by T cells in the inductive sites.

Question 11

What type of T cells are important in the mucosal immune response?

Answer 11

Conventional T cells, particularly CTLs, are also an important component of the mucosal
immune response. The induction and homing requirements for these cells are not as well
described as those for mucosal B cells.

Question 12

What does induction of a response via a mucosal site generally elicit?

Answer 12

Induction of a response via a mucosal site generally elicits a systemic immune response as
well, such that serum antibodies can be detected. This indicates that a mucosal encounter with
antigen generates subsets of T and B cells that home to mucosal sites and also to spleen and
regional nodes.

Question 13

What type of cell is predominantly found in the gut epithelium?

Answer 13

A distinct population of lymphocytes, mostly CD8+ T cells are found in the gut epithelium. The function of these cells is still not clear but they may readily kill infected epithelial
cells.

Question 14

Describe what happens when virus infects mucosal epithelium cell.

Answer 14

Infected cell displays viral peptide to CD8 IEL via MHC Class I

Activated IEL (intraepithelial lymphocyte) kills infected epithelial cell by perforin/granzyme and Fas-dependent pathways

Question 15

How are endocytosed bacteria recognized?

Answer 15

Endocytosed bacteria are recognized by TLRs in intracellular vesicles.

Bacteria or their products directly entering the cytosol are recognized by NOD1 and NOD2.

TLRs, NOD1 and NOD2 acivate NFkB, inducing the epithelial cell to epxress a number of inflammatory cytokines, chemokines, and other mediators. These in turn activate neutrophils, macrophages and DCs.

Question 16

Describe the mucosal response to infection. What type of immune mechanism can handle these infections?

How are local inflammatory cells activated?

Answer 16

Innate immune mechanisms eliminate most intestinal infections rapidly.

Local inflammatory cells are activated via pattern-recognition receptors, e.g., TLRs.

Question 17

Give an example of how innate immune mechanisms eliminate most intestinal infections rapidly using example of Shigellae.

Answer 17

Shigellae penetrate gut epithelium through M cells

Shigellae invade basal surface of epithelial cells and spread to other eipthelial cells

Shigellae LPS binds and oligomerizes NOD1, activating NFkB pathway

Activated epithelium secretes CXCL8, recruiting neutrophils

Question 18

Describe the most common primary immune deficiency.

Answer 18

Selective IgA deficiency is the most common primary immune deficiency, with an estimated
incidence of 1 per every 500 to 1000 persons. The precise characteristics of the deficiency
are variable, as some patients have complete IgA deficiency but others have decreased but
detectable levels of IgA.

Question 19

In patients that present with low or no serum IgA, how will their cell meidate immunity and serum antibody responses be affected?

Answer 19

Patients present with low or no levels of serum IgA, but have normal cell mediated immunity
and serum antibody responses. Not all patients exhibit increased susceptibility to infection.

Question 20

What might lead you to suspect a patient has IgA deficiency?

Answer 20

Reasons to suspect selective IgA deficiency include

1) a family history of IgA deficiency of
agammaglobulinemia,

2) a high incidence of oral infections,

3) frequent respiratory
infections, and

4) chronic diarrhea.

Question 21

What diseases are often associated with IgA deficiency?

Should patients be treated with immunoglobulin therapy, why or why not?

Answer 21

Autoimmune diseases, including SLE, juvenile rheumatoid arthritis, and thyroiditis, are often
associated with selective IgA deficiency.

Immunoglobulin therapy is generally not indicated, as the patient’s normal antibody response can produce anti-IgA antibody in response to IgA treatment.

People with a complete absence of IgA may develop allergies or even anaphylactic shock if given gammaglobulin.

Question 22

What is the predominant immunoglobluin in mucosal secretions?

Serum Ig levels? What form?
Secreted at muscosal sites? Form/how much?

Answer 22

The predominant immunoglobulin in mucosal secretions is IgA.

Serum Ig – 12% IgA class, primarily monomeric
Secreted Ig at mucosal sites – 96% IgA, primarily dimeric
IgA in mucous secretions is called secretory IgA, or sIgA.

Question 23

What does production of secretory IgA (sIgA) require?

Answer 23

The production of secretory IgA (sIgA) requires both plasma cells in the lamina propria and
epithelial cells of the mucosa.

Question 24

What produces dimergic IgA?

Answer 24

Dimeric IgA (2 monomeric IgA units covalently joined a J chain) is produced by plasma cells
within the mucosal lamina propria.

Question 25

Describe dimeric IgA receptors. Where are they located? What happens once IgA binds?

Answer 25

Dimeric IgA binds to the polymeric immunoglobulin receptor (pIGR) on the basal surface of mucosal epithelial cells.

The IgA-pIGR complex is endocytosed and transported through the epithelial cell to the
lumenal surface for release.

During this transport, the pIGR is cleaved and a small fragment is lost.

The remaining large component, secretory component, is covalently bound to the dimeric
IgA.

Question 26

How is IgA secreted at the mucosal surface?

What does secretory IgA production require?

Answer 26

IgA is secreted at the mucosal surface as dimeric IgA covalently bound to secretory
component.

Secretory IgA production requires two different cell types.

Question 27

What are the types of immunoglobulins that are capable of binding and being transported by pIgR?

Answer 27

Only polymeric immunoglobulins (dimeric IgA or pentameric IgM) are capable of binding and being transported by pIgR.

Question 28

What will mice that are genetically deficient in pIgR exhibit?

What will a pIgR deficiency lead to?

Answer 28

Mice that are genetically deficient for pIgR exhibit the expected decreases in IgA transport.

PIgR deficiency also leads to an increased mucosal leakiness.

Question 29

Describe the functions of IgA at mucosal surfaces.

Answer 29

Secreted IgA on the gut surface can bind and neutralize pathogens and toxins.

IgA is able to bind and neutralize antigens internalized in endosomes.

IgA can export toxins and pathogens from the lamina propria while being secreted.

Question 30

Describe the barrier functions of IgA at mucosal surfaces.

Be specific.

Answer 30

Secretory IgA can bind to bacteria and viruses and prevent their adherence and invasion into mucosal tissues.

Secretory IgA can neutralize many viruses in this way, including polio, herpesvirus, coxsackie virus, and rotaviruses. Secretory IgA can also neutralize bacterial
toxins at mucosal surfaces

Question 31

Describe how IgA is involved in intraepithelial viral neutralization.

Answer 31

IgA that is internalized by mucosal epithelial cells (via the pIgR) may contribute to
intracellular viral inactivation.

Question 32

Describe the role of IgA in excretory immunity.

Answer 32

Viral particles that complex with dimeric IgA in the lamina propria may be endocytosed and
transported out by the pIgR pathway.

Question 33

How is IgA involved in passive immunity?

Answer 33

sIgA in breast milk provides passive immunity to the infant.

Question 34

Mucosal surfaces are portals of entry for many pathogens (e.g. cholera, HIV, influenza).

The development of immunization strategies that would produce a robust mucosal immune response is a high priority.

When compared to systemic immunization by intramuscular, intraperitoneal, or intradermal
routes, immunization with mucosally administered antigens has both advantages and disadvantages.

Describe the advantages and disadvantages.

Answer 34

Advantages:

• ease of administration (oral)
• generates both mucosal and systemic immunity

Disadvantages:

• difficulty in eliciting robust response
• response may not be long-lasting

Question 35

What is an example of an effective oral immunization? Effective nasal spray?

Answer 35

An example of an effective oral immunization is the polio vaccine. Effective nasal spray
vaccines for influenza have recently been developed.

Question 36

Describe some new strategies for oral immunization.

What do these strategies attempt to do?

Answer 36

New strategies for oral immunization include the use of cholera toxin chimeric molecules as well as recombinant avirulent bacteria (e.g. avirulent salmonella expressing S. pneumoniae
proteins).

Can also target M cells using bacteria and viruses that preferentially bind M cells
or antigen encased in biodegradable particles such as latex.

These strategies attempt to boost the uptake of foreign antigen at mucosal induction sites.

Question 37

The mucosa is exposed to many environmental antigens such as food that are not
infectious. To operate in an effective manner, the mucosal immune system must
distinguish between pathogenic antigens, which require a response, and non-dangerous
antigens, such as those in food and in the commensal bacteria that make the gut their
home. The response to most antigens is tolerance, and the type of antigen is critical to
eliciting the appropriate response.

Describe the induction of tolerance via mucosal sites.

What is the key feature that appears to distinguish between the induction of a response and the induction of tolerance?

Answer 37

The key feature that appears to distinguish between the
induction of a response and the induction of tolerance is inflammation.

Antigen encounters
that occur alongside inflammation generally illicit an immune response.

Antigen encounters in the absence of inflammation generally induces tolerance.

Question 38

What usually induces tolerance? What induces immune response?

Food antigens?
Microbes (bacteria and viruses)?
Peptides?

Answer 38

Food antigens generally induce tolerance.

Microbes (bacteria and viruses) that cause inflammation generally evoke a
mucosal immune response.

Peptides generally induce tolerance, unless attached to a mucosal adjuvant, such
as cholera toxin.

The induction of tolerance might be exploited therapeutically in autoimmune diseases, or to
limit transplant rejection.

Question 39

Describe the interaction between gut bacteria and the intestine.

Answer 39

greater than 1000 commensal bacterial species coinhabit the gut; 10X more bacterial cells than total human cells

Intestinal bacteria responsible for development of immune system; germfree animals have
almost no secondary lymphoid tissues including mucosal tissues

The mechanism by which the mucosal administration of some antigens induces tolerance,
rather than immunity, is incompletely understood. Recent studies suggest that mucosal tolerance is mediated by mucosal dendritic cells.

Question 40

How is DC maturation affected in the presence of commensal bacteria, production of PGE, TGF-B and TSLP?

Answer 40

DC maturation is inhibited

so immature DC (mesenteric lymph node) give weak co-stimulatory signals and induce CD4 T cells to differentiate into (regulatory TH2) or Treg cells.

Question 41

Describe DC response when invasive microorganisms penetrate epithelium to activate DC.

Answer 41

(mesenteric lymph node) activated DC express strong co-stimulatory ligands and induce CD4 T cells to differentiate into effector Th1 and Th2 cells, and Th17

Question 42

Describe the result from treatment with antibiotics.

Answer 42

Treatment with antibiotics eliminates a large proportion of the commensal organisms and
allows pathogens to proliferate and cause disease, e.g., Clostridium difficile.

Colon is colonized by large numbers of commensal bacteria

Antibiotics kill many of these commensal bacteria

Clostridium difficile gains a foothold and produces toxins that cause mucosal injury

Neutrophils and RBC leak into gut between injured epithelial cells