10 - Preventing Infxn at Mucosal Surfaces

What is systemic immunity?

infxn in tissue/skin

• initiate an immune response (previously discussed)

• tissues have blood supply

What is mucosal immunity?

infxn at mucosal surfaces

• have to secrete across epi layer

• mucosa exposed to more microbes, have some there all the time

What important task does the immune system do in mucosal immunity?

has to distinguish b/w what’s supposed to be there/not

• microbiota vs pathogen

What do the lacrimal & salivary glands secrete?

secrete specific:

• Abs

• peptides

• antimicrobial mols

• enzymes

Which places shouldn’t ever have microbes in the body?

bladder, lungs, kidney, etc

• further in body = want less microbes

What key task does the mucosal surface do?

produce mucus

What does mucus contain?

proteoglycans, enzymes, glycolipids, peptides

What is the collective goal of mucosal components?

collective goal is to

• protect from damage → reduced inflammation

• limit infxn

What would occur in an incident of poorly containing inflammation?

asthma, allergies, IBS, Crohn’s

How are autoimmune disorders related to mucosal tissues?

autoimmune disorders create overly inflammatory enviro in mucosal tissues

• cause lots of damage & issues

• more susceptible to infxns

How are autoimmune disease pts more susceptible to infxns?

inflammation damages protective layer

• microbes get through epi layer easily to tissues underlying mucosal surface

What do mucosal epi cells interact w/?

various foreign particles

• proteins, carbs, lipids, nucleic acids, microbes

• can interact w/ each of individual molecules

Where do the nucleic acids that mucosal epi cells interact w/ come from?

from diet

Where do the microbes that mucosal epi cells interact w/ come from?

from microbiota or commensal

What is the goal of mucosal epi cells?

• elim pathogens before breaching cell layer

• restrict commensals

• X inhibit nutrition (GI tract)

• have to decide if non/pathogenic

How do mucosal epi cells decide if non/pathogenic?

assess how much commensals present (can have too much)

• restrict at mucosal surface

• need to balance # of microbes

What occurs if there are too many commensals at mucosal surface?

take nutrition away, leave malnourished

How to commensals get restricted at mucosal surfaces?

• coughing → remove mucus, trapped inside

• mucociliary escalator (resp tract) → push mucus up & away from lungs

• peristalsis (GI tract) → muscle contractions pushing mucus out

What are mucins?

make mucus sticky

What are mucins made of?

• polypeptide chain

• globular domains

• polymer formation

What does the polypeptide chain of mucins have?

Ser/Thr glycosylation, branched structure

What do the globular domains of mucins have?

Cysteine residues to form disulfide bonds

Why do mucins form polymers?

do polymer formation & crosslinking

• make a net trapping microbe

What are 2 forms of mucins?

secreted & membrane bound mucins

Where are commensal organisms located?

live in mucosal tissues of the GI tract

How many bacteria/mL are in the stomach?

10^3 → lower # since higher acidity keeps low

How many bacteria/mL are in the small intestines?

10^5-8 → depends on where, when & who is sampled

How many bacteria/mL are in the large intestines?

10^12 → more than 10^13 of human cells

What are benefits of commensal microbes?

• synthesis of essential metabolites

• breakdown of indigestible foods

• inactivate toxic substances

• prevent access of pathogens to human gut

• interact w/ epi to trigger development of secondary lymphoid tissue

How do commensal microbes make essential metabolites?

metabolize things & produce metabolites that we use

How do commensal microbes breakdown indigestible plant fibers?

breakdown indigestible plant fibers into digestible food/units

How do commensal microbes inactivate toxic substances?

inactivate or breakdown toxic substances consumed, neutralizing substances in food or from pathogens

How do commensal microbes prevent access of pathogens to human gut?

limit pathogenic microbe replication, present but low #s

• take up space & nutrients needed to expand, replicate, & invade into tissues

How do commensal microbes interact w/ epithelium?

interact to help MALT/secondary lymphoid tissue development

• GALT, BALT, etc

How were the benefits of commensal microbes determined?

germ free mice studies

• treat mom in pregnancy w/ antibiotics

• pups delivered in germ-free environment

• no microbiota, no normal MALT

• need exposure to commensal microbes to develop MALT

How do infxns of intestines occur?

commensal species can cause infxn

Which commensal species can cause infxn?

Salmonella, Shigella, Helicobacter, Escherichia

• diff parts of GI tract

• can become infxn if certain conditions met

How do virulence factors allow for invasion?

help commensal species/microbes escape lumen

• breach epi layer & invade tissues

Which virulence factors allow for invasion?

• adhesins, toxins, and invasins that facilitate microbial attachment and damage to host tissues

• can become primary pathogen, cause D

• X make toxin = commensal

What are mucosal lymphoid tissues?

in mouth & intestines

Which mucosal lymphoid tissues are in the mouth?

Palatine tonsils, adenoids, lingual tonsils

Which mucosal lymphoid tissues are in the intestines?

Peyer’s patches, isolated lymphoid follicle, appendix

Which mucosal lymphoid tissues are removed in tonsillectomies?

palatine tonsils and adenoids

What are isolated lymphoid follicles?

mucosal lymphoid tissues in intestines

• more numerous, scattered throughout digestive tract

• smaller & less organized lymphoid tissues

Where do isolated lymphoid follicles drain into?

scattered around digestive tract, drain & go out into mesenteric LN

What occurs in isolated lymphoid follicles during infxns?

w/ infxn → Ags go to mesenteric/draining LN

• try to do response at lymphoid tissues

What are the steps for systemic immunity?

1. breach mechanical barriers

2. cross epi layer, immune cells interact w/ microbe immediately

3. mphages release cytokines, inflame

4. infxn terminated, leave damaged tissue to repair

What are the steps for mucosal immunity?

1. breach mechanical barriers

2. cross epi layer, immune cells interact w/ microbe immediately

What is unique about systemic immunity?

• skin infxn → recruit immune factors from bloodstream

• want tissue resident macrophages → start inflammatory response, make BV permeable to let things go to infxn site

• all steps fo activating immune response

  • started by macrophages

  • calls in other effectors/innate immunity cells

  • get adaptive immunity, help clear infxn

• slower overall, faster on next time in same place

• want/do inflammation → get immune cells needed from bloodstream

• wait for effector T cells to show up

What is unique about mucosal immunity?

not just macrophages, Abs at site from coming into contact more w/ microbes

Where are immune cells in mucosal immunity?

have immune cells in tissue

• Peyer’s patches, basically LN underneath

• less organized follicles underneath in lamina propria

• can have immune response @ mucosal surface under epi cell layer

Does mucosal immunity focus on inflammation?

No, goal is to have a response but limited non-inflammatory response

Why doesn’t mucosal immunity do inflammation?

Because it aims to prevent excessive tissue damage and maintain homeostasis while still defending against pathogens.

• ex → GI = stomach hurts, lungs = hard to breathe

• don’t need it to recruit things, MALTs in mucosal surfaces

Where are memory cells in mucosa?

deposited periodically throughout tissue

What immune cells are in the mucosa?

macrophages, plasma memory cells, Tregs (anti-inflammatory compounds), gen effector T & B cells, tissue resident T cells

What is the lamina propria?

The lamina propria is a layer of connective tissue found beneath the epithelium of mucosal surfaces, containing immune cells, blood vessels, and supports the epithelium's structural integrity.

What are tissue resident macrophages?

Tissue resident macrophages are immune cells that reside in tissues and play a crucial role in maintaining homeostasis, responding to infections, and facilitating tissue repair. They are a specialized subset of macrophages distinct from those that circulate in the blood.

• mucosal immunity only

How do epithelial cells recognize microbes?

w/ innate immune strategies such as pattern recognition receptors (PRRs) that detect pathogen-associated molecular patterns (PAMPs). These receptors trigger signaling pathways that initiate antimicrobial responses and inflammation.

Which PRRs do epithelial cells use to recognize microbes?

TLRs on lumen/apical side, tissue/basolateral sides

• recog b/w what’s in lumen & transversed epi cell

How do epi cells recog microbes using TLRs?

1. microbe comes in & interacts w/ TLR

2. activates NF-kB (TF) & increased gene expression

3. make diff types of mols

How do epi cells recog microbes using NOD receptors?

1. microbe comes & interacts

   • 1b) big microbe actually comes in, goes to NOD

   • 1c) stays outside & releases products, ex: Ecoli

2. products or microbe itself interact w/ NOD-like receptors

3. activate NF-kB (TF) & increased gene expression

4. make diff types of mols

Which molecules are made after epithelial cells recognize microbes?

chemokines & cytokines, antimicrobial proteins

• interact w/ microbes or other cells in area to call to location

What is the purpose of the basolateral sided TLR?

To recognize pathogens that breach the epithelial barrier and initiate immune responses.

What other cells are in the intestinal epithelium?

Paneth cells, goblet cells, intestinal macrophages, M cells

What are Paneth cells?

produce AMPs & antimicrobial enzymes

• cells in intestinal epithelium

What are Goblet cells?

produce mucus, throughout gut wall

• cells in intestinal epithelium

What are intestinal macrophages?

less inflammatory forms of macrophages

• phagocytic but not inflammatory

• turned off transcription of inflammatory genes = X inflammation

• cells in intestinal epithelium

What are M cells?

sample what’s in lumen/apical side, on top of Peyer’s patch

• cells in intestinal epithelium

Why do M cells sample the lumen?

done to see if need to decrease microbiota

• capture bact & deliver them & Ags to DCs & lymphocytes in Peyer’s patch

• pathogenic microbes hijack method

How do M cells sample the lumen?

trancytosis of microbes & individual Ags

• breakdown & deliver broken down contents to lamina propria’s Peyer’s patches

• can deliver whole microbe

Where is a weak spot in the epithelium & why?

easily co-opted by microbes w/ delivery occurring

• less mucus to help transport whole microbes = less immune cells & general open area

What occurs after M cells bring in microbes (intact/whole)?

microbes (broken down/whole) get across Peyer’s patch

• DCs present Ag to T cells & B cells & activate

• result in localized immune response

What is an example of M cell mechanisms being taken advantage of?

Shigella & polio take advantage, replicate inside cells via intracellular infxn

• trancytose & infects cells

• spread from that cell to neighboring cell w/ lateral vesicle transport

• come in through M cell, infecting w/o actually leaving cell

Where can DCs get Ags from?

DCs in lamina propria

• squeeze b/w tight junctions & reach out w/ dendrites to take up Ags directly from lumen

• or get from M cells/directly from lumen

What is an important role of DCs w/in Ag recognition?

interact w/ microbe, have to determine if pathogenic or not

• make IL-10 w/ nonpathogenic, block T cell activation

• want anti-inflammatory response

What is IL-10?

DC secreted, anti-inflammatory

What do effector B & T cells do in mucosal surfaces?

protect mucosal surfaces, in Peyer’s patches

• after activation → move into lymph/blood stream, exit along mucosal surfaces at multiple locations

Where do effector T & B cells get delivered to?

• can also be delivered to microvilli after establishing there’s an infxn

• move to lymph/bloodstream & exit multiple places such as mucosal tissues throughout the body.

• come in & go somewhere else, any mucosal surface & deposit

Why are effector cells deposited at other locations?

GI tract goes directionally, continually have them coming out of lumen into tissues downstream

• have more local infxns down GI tract

• need to respond to potential infections, ensuring rapid immune responses where needed.

What are 2 waves of B cells activation?

pentameric IgM & dimeric IgA

What is the pentameric IgM wave of B cell activation?

1st made by low affinity IgM plasma cells

• agglutinate → make large immune complexes that get trapped in mucus

• microbes X itneract w/ epi cells or breach mechanical layer

Why are pentameric IgM & dimeric IgA involved in B cell activation in mucosal immunity?

only ones that can trancytose

How are pentameric IgA & dimeric IgM trancytosed into lumen?

bind to poly Ig receptor (pIgR), use J chain to transport

What is the dimeric IgM wave of B cell activation?

2nd wave, emerge after geminal center rxn

What determines which Igs are in secretions?

depends on where in mucosa, can add in other Ab types

Where is IgA secreted?

tears, saliva, milk, intestinal fluids

• help us not infected by things we consume/breathe in

Where is IgG secreted?

• secretions of nose/upper resp tract (help stop early before going to lungs)

• lower resp tract (X want in lungs)

• fe/male urogenital tract

• generally has most functions & restriction of mucus

Where is IgE secreted?

small concentration of saliva, gut & resp tract

• X really want/problematic → allergies

How is IgG transported to the mucosa?

IgG is actively transported through the endothelial cells via the neonatal Fc receptor (FcRn), which protects it from degradation.

How does FcRn transport IgG to the mucosa?

across endo & epi cells

• on the apical side/lumen of mucosal surface

• take to lamina propria & onto gut lumen w/ recycling of it