PMI 126 M3

CH 12 THE BARRIER IMMUNE SYSTEM

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mucosal surfaces

require very close contact with the external environment

- urogenital tract: reproductive tract --> survival

- GI tract: intestine --> nutrition

- respiratory tract: lung --> oxygen

types of mucosal tissue

- simple columnar epithelium

- pseudostratified columnar epithelium

- nonkeratinized stratified squamous epithelium

simple columnar epithelium corresponds to

intestine/stomach

pseudostratified columnar epithelium

trachea/bronchi

nonkeratinized stratified squamous epithelium

oral cavity/rectum

Pathogens have evolved to exploit our vulnerabilities

many annual diseases from mucosal infections: ex respiratory infections, diarrheal diseases, etc

MALT

mucosa associated lymphoid tissue

what are types of MALTS

gut-associated (GALT)

nasal-associated (NALT)

bronchus-associated (BALT)

GI tract anatomy

- lymphatic drainage that drain to local lymph nodes covering each segment of the intestine

intestinal epithelial surface is greater or smaller than skin surface

way greater (2200 vs 20 sq feet)

ILF

- isolated lymphoid follicle

- lymphoid structure that places a "lymph node-like" structure in direct contact with the epithelial layer without lymphatic drainage

M cells

- found in ILF

- can sample the outside world

- capture particulates (viruses, bacteria, and even latex beads) and transfer them inside the body

- constantly sampling intestine so immune system knows what's normal

is mucous found on the ILF

no - directly in contact with gut lumen contents

Cryptopatches

- present at birth

- small collections of Dendritic Cells and Lymphoid Tissue Inducer cells

ILF

- grow from cryptopatches after microbiota colonization of gut and will contain mostly B cells

- develops after birth

Peyer's patches

- present at birth, increased size after birth

- only found in distal ileum and are larger and have more T cells and B cells (yellow=germinal centers which only exist where there is CD4 T cell help for antibody responses)

inductive sites

- organized lymphoid tissue

- where naïve T cells and B cells get activated

Effector site

- "rest of the intestine," not organized lymphoid tissue

- no naïve lymphocytes, but activated and memory lymphocytes patrol

- include IELs, Lamina Propria, AMPs, ILC

IELs

Intraepithelial lymphocytes sit between the tight junctions of epithelial cells

Lamina Propria

the tissue under the epithelium - contains a variety of innate cells and lymphocytes

AMPs

antimicrobial peptides secreted by epithelium (mostly by Paneth cells)

ILC

innate lymphoid cells

difference between small and large intestine effector sites

large intestine has 2 layers of mucus

Gut microbiota (non-pathogens) develop over time

- These bacteria provide many health benefits (as well as allowing maturation of the gut lymphoid tissue)

- One critical function is that they provide COLONIZATION RESISTANCE

colonization resistance

taking up "space" that pathogens might use

what are the benefits of colonization resistance

ex) commensal bacterial colonization:

prevents pathogens like clostridium dificile from producing toxins that cause mucosal injury

what does clostridium dificile do

- produces toxins that cause mucosal injury

- neutrophils and red blood cells leak into gut between injured epithelial cells

- connective tissue degradation leads to colitis and psuedomembrane formation

what has been useful in restoring colonization resistance

fetal microbiota transplant (fmt)

what is considered a "sealed barrier"

innate immunity

how is innate immunity a "sealed barrier"

- on the apical end, membranes of adjacent epithelial cells form an intercellular barrier

- adhesion complexes

- claudin and occludin associate with zonulin to regulate the permeability of tight junctions

mucous barrier of innate immunity

- thickness of the mucus layer correlates with the amount of bacteria - much more in the large intestine

- Inner layer is usually devoid of microbes so less thick mucus

what cell types make mucus, how?

- goblet cells

what exactly is mucus

- glycoproteins that undergo crosslinking

- traps microbes seeking to invade the epithelium and expels them by peristalsis

secreted antimicrobial proteins

- innate immunity secretes many types of antimicrobial products

- Paneth cells and regular epithelial cells (enterocytes) provide simple antimicrobial defense within the lumen

2 functions of TLR

- toll-like receptors

1. protect against pathogen: recognize harmless bacteria because TLRs cannot distinguish between pathogen and non-pathogen bacteria -> TLR secrete AMP

2. keep microbiota away from epithelial layer

4 types of intraepithelial lymphocytes

intraepithelial lymphocytes: positioned within intestinal epithelium

1. unconventional gd T cells

2. unconventional ab T cel

3. conventional ab T cells

4. ILC1 (no TCR)

1. unconventional gd T cells

2. unconventional ab T cel

Both recognize nonclassical MHC molecules and butyrophilins

conventional ab T cells

Recognize regular MHC

ILC1

recognize cytokines

what is a shared property between the 4 types of intraepithelial cells

- can be cytotoxic and kill infected or damaged epithelial cells

- Because of the size of the intestine, IELs are one of the largest populations of T cells in the body

how do "normal" (conventional) IELs kill infected epithelial cells

- infected cells displays viral peptide to CD8 IEL through MHC class 1

- the activated IEL kills the infected cells by perforin/grazyme and Fas-dependent pathways

pIEL

- peripheral IEL

- express CD8a/b

- considered memory cell because this cell was generated in a lymph node in response to previous infection and then took up residence in the epithelial layer (it came from outside the intestine)

how are "Innate-like" IELs different than "normal" IELs

- they kill stressed cells

- use NKG2D

how are cells stressed, what do they express

- Cell can be stressed by infection or by exposure to gluten components

- express non-classical MHC molecules MIC-A and MIC-B and IL-15 which NKG2D from "Innate-like" IELs recognize

nIEL

- Natural IEL expressing CD8a/a instead of the usual CD8a/b

- These cells populate the gut early in life after exposure to microbiota and thus do not come from normal naïve thymic-derived T cells

Butyrophilin

- activate gd IELs (unconventional)

- sensed by the TCR (gd) of these unusual IELs to activate cytotoxicity

- allows epithelial cells to be killed if they have come in contact with microbes or are undergoing transformation (cancer)

what is mucosal tolerance

- prevents immune response to one thing that is tolerated

- generation of peripheral regulatory CD4 T cells which can actively suppress future responses to the same antigen

mechanistic pathways of mucosal tolerance

• Deletion of T cells

• Anergy (T cells survive but can't respond)

• Immune regulation

describe the mucosal tolerance mouse experiment

mice only suppress immune response to correct antigen

breakdown of this immune tolerance examples

Celiac disease, IBD

how do DCs sample the outside world in the intestinal lumen

bulk and receptor-mediated transport across M cells

FcRn-dependent transport

uptake of sample via goblet cells, which DC then senses

what is another function of FcRn besides the one in the gut

transport of IgG from mother to baby

How DCs sample the outside world (lumen)

- paracellular transport across tight junctions

- apoptosis-dependent transfer

- antigen capture by transepithelial dendrites (TEDs)

multiple factors maintain DC tolerance mechanisms

- short chain fatty acids, vitamin A, bacteria all condition cDCs to maintain mucosal tolerance thru various mechanisms

what might defects in the processes of DC tolerance lead to

- Defects in any one of these processes might cause a loss of immune tolerance and allow inappropriate responses to a non- threatening stimuli - Inflammatory Bowel Disease

what disease do intestinal Tregs protect us from

IBD

what does mice treg experiment show

- Transfer of T cells to mice without Tregs causes rapid inflammatory responses like IBD

- This means that our T cells would kill us unless they are constantly restrained by Tregs

how are antibody isotopes distributed

- IgA: on external mucosal surface (lung/intestine etc)

- IgM: blood

- IgG: blood, delivered to a fetus across placenta in some mammals

- IgE is mostly at epithelial surfaces on mast cells (skin, gut and respitory tract)

what part of the body mostly lacks antibodies

The brain is largely devoid of Ig

Transport mechanism for IgA

- dimeric IgA is transpretd to into gut lumen thrugh epithelial cells at the base of crypts

- binds to mucus overlaying the gut epithelium

- ensures that IgA covers mucosal surfaces, helps keep bacteria from the epithelial layer

function of IgA

- Primary function is neutralization of viruses, bacteria, toxins

- modifies the microbiota

- IgA is unable to fix and activate complement thus avoiding inflammation at mucosal surfaces

lung mucosal tissue similarities and diff to intestine

- no colonization resistance because lungs are usually sterile below the pharynx

- goblet cells secrete a layer of mucus and the bronchial glands also produce antimicrobial peptides (unlike immune cells where many are in the lamina propria underneath the epithelium)

- no organized lymphoid tissues in the lungs, but lymphatics drain the lamina propria into local lymph nodes where T cells and B cell can respond (like the skin)

- Deeper in the alveoli, there is no mucus layer to protect the epithelium because gas exchange has to occur across the epithelial layer

immune response to flu induces ---

CD8 T cells and antibody defense

- virus infects epithelial cell, stimulating release of Type 1 and 3 interferons and pro-inflammatory cytokines

- intraepithelial cd1 cells engulf the infected epithelial cell and lamina propria 2 cd2 cells take the virus and traffic it to draining lymph node, inducing CTL and B cell responses

mechanisms of immune response

- antibody production

- Th1/Th2/Th17 development

- cytotoxic t cells

FAILURES OF HOST DEFENSE MECHANISMS

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what does bursa chicken failure show

- "father of b cells"

- removal of Bursa has absolutely no effect on chicken growth/development

- Bursectomized chickens make no antibody response therefore bursa is immune organ

Primary immune deficiency leads to disease susceptibility

- Primary immune deficiencies are caused by inheriting genetic defects

- Patients suffer from repeated infections - often from opportunistic pathogens

is defficiency with adapative or innate immunity more serious

- innate immune system, not compatible with life and may appear early on in life

- microogranism count skyrockets

X-linked Severe Combined Immune Deficiency (SCID)

- Caused by mutations in IL-2R gamma chain

- This receptor is required for function of IL-2, 4, 7, 9, and

- Patients have no T cells or NK cells

- Usually affects boys because X- linked recessive disease

DNA pkcs mutation

- Enzyme required for TCR and BCR rearrangement

- Animal has no T cells or B cells

- severe Combined Immune Deficiency

- Highly susceptible to many infectious diseases

Passive transfer of immunity

- antibodes transfered from placenta to growing baby

- trasnient low level of IgG which is compenasated for by transfering this antibody through breast milk feeding

what might t and b cell immune deficiencies cause

the gene causing the defficiency can resykt in

- specific antibody isotype deficiency

- specific b cell deficiency

- combined immune deficieny (CID) --> both b and t cells defiency

2 b cell signals

signal 1: BCR

signal 2: cd4 ligand

what does the cd4 ligand tell the b cell

tells b cell it should form a germinal center (b cell activation and differentiation)

2 t cell signals

signal 1: cd8 on t cell receptor

signal 2: cd28

What does CD28 do?

helps bind b7 and tells t cell how b7 gets upregulated

what happens when DC enounters antigen that has TLR like LPS

- DC upregulates B7 before it gets to lymph node

- therefore impt that t cell has received both signal 1 and 2 so it can be activated

what happens if t cell only receives 1 signal

- no signal 2 means that there is low b7, so no infection.

- t cell turns itself off

hyper IgM syndrome

- Low or absent levels of IgG, IgA, IgE, but elevated levels of IgM

- Caused by a failure of class-switching• Most common cause is a mutation in CD40L gene (X-lined recessive) -- T cells can't help B cells

- Somatic hypermutation and class switching require germinal center development (CD4 T cell dependent CD40L-CD40) - -- Because CD40/CD40L plays a role in Th1 responses, these patients have CID (T cell and B cell disfunction)

what region does a somatic hypermutation of an antibody change

- Focuses on variable region

- results in Increased affinity

what region does a class switch of an antibody change

- Focuses on constant region

- results in Functional modification

what causes a lack of germinal centers in the lymph nodes of individuals with hyper IgM syndrome

CD40 deficienyc

Selective IgA deficiency

- Most common immune-deficiency in humans - 1/100-1/500 •- - Most are asymptomatic and undiagnosed

- Increased risk of mild respiratory of intestinal infections

- Underlying genetic defect is not known

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what is the name of the transport mechanism that allows IgA to coat mucosal surfaces

pIgR

what does pIgR also transport

IgM, presumably this can compensate for the lack of IgA

what type of infections do TH17 and ILC3 deficiencies results in? why?

- get more extracellular bacterial infections because 1L17 and ILC3 recruit phagocytes to kill extracellular bacteria

what are the mechanisms of cytotoxicity

activation, perforin, docking, priming, fusion

do cells with cytotoxic deficiencies get many viral infections?

no - their cd8 and nk cells can still secrete IFN-g and have anti-viral activity

what happens to cytotoxic deficient cells

- get hemophagocytic lymphohistocytic syndrome (HLH) - uncontrolled expansion and activation of CD8 T cells.

- This tells us that cytotoxicity plays a key role in immune-regulation

are complement defficiencies innate or adapative immunity defficiencies

innate immunity defficiencies

what do complement deficiencies cause

- Defects in complement lead to broad susceptibility to extracellular bacterial infection

- This is because antibody, complement and neutrophils work together to clear these infections

- defects in the MAC cause a specific deficiency in combating Neisseria infections

why do defects in the MAC cause a specific deficiency in combating Neisseria infections

Neisseria clearance being mediated by antibody-independent complement mediated cell lysis

phagocyte deficiency

- defects in innate immunity

- are often incompatible with life

- There can be defects in phagocyte production, adhesion/motility, activation, or intracellular killing

Canine cyclic neutropenia

- Phagocyte deficiencies

- Rare genetic disorder (Gray Collie Syndrome) mutation in the canine AP3B1 gene

-gray coat and lack of neutrophils every 10-15d

- affected pup die before 6 mo age

- susceptibility to bacterial infection

what fixes primary inmmune deficiencies and how

- stem cell transplantation

- Because most immune cells are bone marrow-derived, there is the possibility of "fixing" the problem with stem cell transplantation

- The donor and recipient would ideally share all MHC alleles but minimally have to share some MHCs

how do stem cell transplants work

- In the recipient, donor-derived lymphocytes can be selected on host MHC in the thymus (this is why they have to share MHC alleles) and develop a normal T cell pool

- Donor-derived dendritic cells and macrophages can also populate lymphoid tissues, meaning that most defects can be corrected with this donated bone marrow

why is it important that the donor bone marrow be depleted of mature T cells

- results in Graft versus Host disease

- If not, these graft (mature) T cells will react strongly to host MHC and cause a serious inflammatory response against host tissues (Graft v Host)

what is another important reason that the donor bone marrow be depleted of mature T cells

- (Host v Graft)

recipient T cells could recognize donor MHC and eliminate the grafted stem cells (Host v Graft)

what is the modern approach for fixing SCID (Severe Combined Immunodeficiency) that avoids the problems above

use the patients own stem cells and fix the immune deficiency prior to bone marrow reconstitution