Mucosal immune system - Block D lecture one

Question 1

what is mucosal immunity

Answer 1

Mucosal immunity is the immune system present in the mucosa, including gut, lungs and urogenital tract

Question 2

what is the disease with the highest deaths per year ?

Answer 2

diarrhoea

Question 3

what does the mucosal immune system make up percentage of the body ?

Answer 3

Mucosal immune system makes up 70% of the body, more important than thymus and spleen.

Question 4

why do we need to know the mechanisms of mucosal immunity ?

Answer 4

Most pathogens enter via mucosal route

Is it better to immunise via mucosal route?

You need to understand mechanisms so that you can devise strategies against inflammatory bowel disease, asthma and COPD

Could you use oral tolerance to stop responding to food we eat but could this be used to prevent autoimmune diseases?

Question 5

what are the most important cells im mucosal immunty ?

Answer 5

Dendritic cells

Epithelial cells

T cells

B cells

Question 6

describe the GI tract ?

Answer 6

GI tract has large surface area of 400m2 - about the floor area of a 65 x 65 ft room. This is 200 times the area of your skin covered by single layer of epithelial cells to allow efficient nutrient absorption.

Question 7

microbiome ?

Answer 7

Inside the gut there are 100 million million (10^14) commensal organisms of 400 species, the microbiome. This is 2,500 kg food antigens (proteins) in a lifetime. Mucosal immune system has to be able to discriminate between antigens with no pathogenic potential (dietary proteins and commensal organisms) and antigens associated with potentially harmful microbes.

Question 8

mechanical immune response?

Answer 8

Mechanical immune responses to prevent infection include the single cell layer epithelial barrier which contains tight junctions to prevent the microbes entering between them.

Peristalsis moves the infection out of the gut and diarrhoea is an increase in fluid secretion in the gut is good to get rid of worms which and large this is called the weep and sweep action. This is non specific

Question 9

humoral defences ?

Answer 9

Humoral defences are secreted and nonspecific this

Gastric acid - pH 1

Lysozyme - break down bacteria

Peroxidase - break down bacteria

Mucin - part of mucus which impeded interaction of pathogen and mucous surface

Anti-microbial peptides - prevent growth

Defensins - prevent growth

Trefoil proteins – prevent growth

Question 10

mucosal assocaiyed lymphoid tissue (MALT) ?

Answer 10

Mucosal associated lymphoid tissue (MALT) is the mucosal immune system including the lung , gut and urogenital tract.

Question 11

gastrointestinal associated lymphoid tissue (GALT) ?

Answer 11

gastrointestinal associated lymphoid tissue (GALT) is just the gastrointestinal tract. It has features including Mesenteric lymph nodes, Peyer’s patches, Lamina propria and Cryptopatches (in mice) which are collections of immune cells to help to prevent infection and controlling cancer.70 % of ALL lymphocytes are found in the gut

Question 12

villi ?

Answer 12

In the small intestine, there are long finger like projections called villi, which are there to allow an increase in absorptive surface area

Question 13

crypt of lieberkuhn ?

Answer 13

Crypt of Lieberkühn is where the epithelial cells proliferate and pan F cells such as trefoil proteins and defenisns

Question 14

proliferation of epithelial cells ?

Answer 14

The epithelial cells proliferate then move up the epithelial escalator until it reaches the top where it undergoes apoptosis before reabsorption. This process takes 2 to 3 days, continually renewing the epithelial cells, this is beneficial because if a pathogen has entered it doesn’t have time to replicate before the cell is removed by apoptosis. If infection has occurred, then the rate of epithelial renewal increases, and the depths of crypts increase

Question 15

IEL ?

Answer 15

IEL are intracellular epithelial lymphocytes which control their patch for infection or transformed epithelial cells, then apoptosis occurs.

Question 16

lamina propria ?

Answer 16

In the lamina propria this underlies the crypts in germinal centres (aggregation of T, B and dendritic cells)

Question 17

dendritic cells ?

Answer 17

The dendritic cells circulate in the lymphatic system and can receive signals from IEL, if the dendritic cell identifies a pathogen or antigen then they recruit T cells and alert B cells to produce antibodies

Question 18

peyers patch ?

Answer 18

The peyers patch is an area of interaction between B, T, macrophages and dendritic cells. This are contains a special M cell which is a microfold cell. This M cell is lacking the brush border present in the other cells, the role of the cell is to present antigen from the gut lumen to the cells underneath in the Peyer’s patch. The cells in the peyers patch, lymphatic system then leave and enter the mesenteric lymph node, and this is a passive circulation of cells.

Question 19

FAE ?

Answer 19

FAE is the follicular associated epithelial which overlays the peyers patch

Question 20

goblet cell ?

Answer 20

produces mucus

Question 21

endocrine cell ?

Answer 21

produces hormone

Question 22

tuft cell ?

Answer 22

controls immune responses

Question 23

Pan F cells ?

Answer 23

produces Trefoil factors

Question 24

stem cells ?

Answer 24

at the bottom is where the stem cells are that produce the epithelial cells

Question 25

what are the mucosal surfaces protected by ?

Answer 25

MALT

Question 26

B cells ?

Answer 26

B cells produce antibodies, this can occur in the germinal centre of the lamina propria

Question 27

what makes up the IEL ?

Answer 27

alpha, beta and gamma T cells make up the intracellular epithelial lymphocytes (IEL)

Question 28

what immunoglobulins are present in the gut lumen ?

Answer 28

IgM, IgG and IgE and IgA are all present in the gut lumen

Secreted or leakage from the systemic gut.

Question 29

IgA ?

Answer 29

IgA is the major class of antibody actively secreted into the gut lumen and lungs; it contains a protective element that prevents it from being degraded from pathogens. A normal adult secretes ~ 3g IgA/day, about 70% of total Ab output. It prevents attachment of bacteria, toxins, viruses, absorption of foreign substances to epithelial cells. Most recent Ig to evolve, only found in mammals and birds, not reptiles or fish. Some bacteria produce proteases against the hinge region of IgA eliminating secondary effector function

Question 30

gamma delta T cells in the intestine

Answer 30

All T cells have a receptor made up of 2 chains part of CD3 molecule. Most are alpha and beta chains but gamma and delta T cells are found in the intestine.

gd cells make up about 50% of IEL in mice, about 20% in man, up to 70% in cows. Only 5% are present in other lymphoid tissues. But as 70% of all lymphocytes are in the gut they make up about 35% of T cells in mice and 15% in man

Role is that they are the first line of defence, also regulatory cells (mucosal homeostasis) and bridge between innate and adaptive responses

Question 31

gd cells and antigen processing ?

Answer 31

γδ T cells are peculiar - do not seem to require antigen processing and MHC presentation of peptide epitopes. γδ T cells may have important role in recognition of lipid antigens. This contrasts with alpha and beta T cells which are about proteins and peptides.They are of an invariant nature and may be triggered by alarm signals, such as heat shock proteins (HSP).

Question 32

what do gd cells interact with ?

Answer 32

The delta gamma T cells can interact with NK cells and other cells in adaptive and innate immunity. They act as a bridge, like NK cells as they have mechanisms of adaptive immunity and innate.

Question 33

alpha , beta T cells compared to gd T cells ?

Answer 33

ab T cells are tightly regulated within the thymus, gd T cells can develop extrathymically in liver and gut.gd T cells express RAG-1 so they can undergo recombination for diversity.

gd T cells are homodimeric for CD8 i.e. aa CD8

FceR1g chain acts as a component of the gd T cell CD3 complex , this is the receptor for mast cells (IgE)

gd T cells are oligoclonal i.e. not diverse, TCR gd repertoire is polyclonal in newborns
gd T cells may be important in mucosal defence early in life before ab T cell and IgA responses are developed.

Question 34

what has more variation gd or ab T cells ?

Answer 34

Gamma delta only has 840 variations while alpha beta has 3 million.

Question 35

function of gd IEL ?

Answer 35

Surveillance of intestinal epithelial layer against microbial invasion (20-100 epithelial cells/gamma delta T cell).

Have cytotoxic activity against microbial pathogens via lysis of infected epithelial cells. Furthermore, they are involved in providing B cell help through the production of cytokines and chemokines (e.g., IL-1, IL-2, IL-4, IL-5, IL-10, IFN-g, TNF-a, TGF-b, lymphotactin).

Also support of epithelial cell growth and the maintenance of epithelial barrier integrity. This occurs by production of growth factors (e.g. keratinocyte growth factor (KGF). Also involved in the removal of damaged or transformed epithelial cells.

Immunoregulatory by abrogating/promoting oral tolerance by production of cytokines (e.g. IL-4, IL-10, IFN- g, TGF-b ).

Question 36

description in depth of peyers patch ?

Answer 36

Described by de Peyer in 1667, it is an organised lymphoid aggregate. Comprised of specialised follicle-associated epithelium (FAE) overlying a sub epithelial dome (SED) , within the dome there are multiple B cell follicles that contain germinal centres (GC),

Within the germinal centres there are Interfollicular regions (IFR) contain T cells, high endothelial venules (HEV) and efferent lymphatics. All lymphoid migration occurs from the blood across HEV as there are no afferent lymphatics in the gut

Question 37

M microfold Cell ?

Answer 37

This is the primary site of antigen handling in the gut, they are specialised epithelial cells. They have a poorly developed brush border, so aren’t there to absorb nutrients. No microvilli and Thin glycocalyx so easy for antigen to cross membrane. Absence of hydrolytic enzymes so don’t break down protein passing membrane. Express MHC class II on basolateral surface, the inside and can present antigen to B and T cells in the SED layer. Rich in pinocytotic vesicles, they can engulf and take up antigen present in the gut

Question 38

what happens to antigen in the gut ?

Answer 38

2% of dietary protein is found in portal venous of liver after a meal

Lumenal pre-processing

gastric acid,

gastric enzymes - digest

pancreatic proteases - digest

Antigen is absorbed but does not evoke an immune response

Question 39

what needs to occur for an immune response ?

Answer 39

For an immune response is the APC could be macrophage or dendritic cell which patrols around for antigens. When it encounters an antigen, it will process it and present it via it’s MHC II by peptide. It presents it to an effector CD4+ T cell. If the cell has encountered a danger signal it will be upregulated it’s co stimulatory molecules. These co stimulatory molecules then activate the T cell to produce cytokines.

On the antigen presenting cell, there must be peptide TCR and the costimulatory molecules CD28 CD80/86.

Question 40

antigen sampling in the gut dendritic cell route ?

Answer 40

Dendritic cells can push their cell extensions (arm) through the gaps of the epithelial cells and grab antigens, the antigen can then come into the area be processed and enter the lymphatic system, arrive at lymph nodes where the antigen is presented to T cells.

If there is a danger signal , then the gap junctions between the epithelial cells will open up , so that the dendritic cells can pass to process the antigens.

Question 41

epithelial cell route of antigen presenting ?

Answer 41

Antigens can be absorbed by the epithelial cells, it can then either enter the high endothelial venules and traffic into the blood, presented to T cells as epithelial cells contain MHC class II , or presented to macrophages. These are the 3 basic routes

Question 42

M cell route ?

Answer 42

This is the most important route; antigen is taken up by M cell and processed where it can be taken up by dendritic cells. It can also directly activate T cells, then once active these can help B cells in the germinal centre to produce IgA. Or they can exit via the high endothelial venules, to the mesenteric lymph nodes and enter the lymphatic system to activate immune cells.

Question 43

lymphocyte homing in the intestine ?

Answer 43

To enter the gut there is lymphocyte homing with tissue selective trafficking, they will try to enter where they came from. Initial exposure to antigen in mucosal inductive sites gut at

Peyer’s patches. Once in the Peyers patch it leaves via the efferent lymphatics and enter the mesenteric lymph node (MLN) then leaves here and enters the thoracic duct to the thymus , then enters the blood to be reintroduced in the gut tissue. This occurs by Mucosal homing with the expression of integrin (sticky molecule) a4b7 , this integrin will bind to an addresin at sites with the expression of MAdCAM , the muscosal effector site is the lamina propria.

Question 44

lymphocyte homing in the intestine ?

Answer 44

Everything leaves the gut via the high endothelial venules, into the mesenteric lymph node. The cells at the top of the villi are expressing a4b7 and will only bind to MAdCAM , which is only expressed on the high endothelial venules in the lamina propria of the gut in the body.

Question 45

vascular adhesion molecules ?

Answer 45

MAdCAM-1 is a mucosal addressin cell adhesion molecule – selectively expressed on postcapillary venules in the lamina propria and gut associated lymphoid tissue such as PP and MLN.

MAdCAM binds to cells expressing Intestinal homing receptor a4b7 , not naive cells they have to be activated.Heterodimeric integrin adhesion receptor family expressed in high levels on intestinal memory and effector cells. Binds MAdCAM-1

Question 46

why is this important ?

Answer 46

Vaccine development

How can you direct lymphocytes immunised in the gut systemically as they won’t leave the gut?

How can you direct systemically immunised cells to the gut?

Therapeutic implications

Block MAdCAM-a4b7 interactions and block lymphocytes inducing inflammation in the gut

Question 47

what can antigen be proccessed by ?

Answer 47

Antigen can be processed by:

M cells

Epithelial cells

Dendritic cells

Question 48

lymphocyes primed to antigen in the gut will return to the gut how ?

Answer 48

MAdCAM-1

Mucosal adhesion molecule – selectively expressed in gut associated lymphoid tissue such as PP and MLN.

Intestinal homing receptor a4b7

Integrin expressed by intestinal memory and effector cells. Binds MAdCAM-1