Mucosal Origin of Autoimmunity Flashcards
what is the structure of the surface of the colon?
Single cell layer thick barrier between 10^13 bacteria and body - fragile
- Thin barrier is maintained – bilayer of mucous prevents entry of bacteria
- Enormous gut surface full of bacteria – prevents bacteria entering, but enables uptake of metabolites from bacteria to supply immune system
how does the gut microbiome engage with the immune system?
- Epithelial layer of gut
- Large surface – 400 square meters which can engage/interact with bacteria
- M cells – specialised in small particle uptake (not bacteria) to interact with immune system – interaction occurs in nodule follicles sitting beneath surface which function like mini lymph nodes – first place where immune system interacts with bacteria of gut
- Mesenchymal draining lymph nodes with T and B cell zones – control immune tolerance to bacteria
why are interactions with the microbiome crucial?
- Don’t want an immune response to these bacteria as it is a symbiotic relationship with microbiota
- Interaction with healthy microbiome is crucial – metabolites produced by microbiome directly regulate lymphocyte gene transcription
- complex crosstalk
how can autoimmunity be triggered in the gut?
Unbalanced interaction with microbiota can lead to inflammation in the gut and triggering of autoimmunity
- e.g. colitis and Crohn’s
- Fine balance of microbiome and immune system
where can interactions with the gut microbiome go wrong?
- Balance between Tregs and Th17 T cells can go wrong – induces gut inflammation – T cell regulation/homeostasis is crucial
- Disequilibrium in response to bacteria - lack of regulation
- Defects in the thin mucosal barrier
All of these can induce autoreactive T and B cell responses – autoantibody production
what are the roles of the mucosa in initiation of autoimmunity (AI)?
- it is the site of initial contact and as portal of entry - some bacteria that enter may be more pro-inflammatory
- it regulates susceptibility to AI - can form regional ectopic tertiary lymphoid structures
- there may be cross-reactivity between foreign and self antigens - molecular mimicry
- alteration of self proteins e.g. citrullination in RA - Modified proteins from neutrophils – AI response to these proteins
what is a common theme in arthritis?
- damage or dysfunction may occur at the entry surfaces for bacteria, which can lead to increased risk of arthritis
how is inflammatory bowel disease (IBD) linked to arthritis?
40% IBD patients tend to develop enteropathic arthritis - chronic inflammation of joints, similar to RA, but different autoantibodies
- loss of barrier function
- Gut permeability and inflammation can trigger destructive arthritis
how are periodontitis and arthritis linked?
Close association between gum health and RA – patients with RA have high risk of periodontitis and vice versa
- gum inflammation and tooth loss and RA are highly associated
how is psoriasis linked with arthritis?
Psoriatic arthritis – psoriasis = inflammatory response in skin – 30% of these patients can also develop psoriatic arthritis long term – different autoantibodies, so not same as RA
- chronic, destructive joint disease
what tends to be a patient’s journey towards RA?
1st hit: environmental or genetic risk factor
- e.g. HLA-DRB1, or smoking
2nd hit: development of anti-CCPs due to increased citrullination - can still be healthy, so aren’t yet diagnosed
- could treat these people but difficult - need to consider scale of risk and side effects
3rd hit: may develop swelling at finger joints over the next few years
4th hit: meet full diagnostic criteria for RA: swollen joints and anti-CCP
- disease progression can take time
what are ACPA?
anti-citrullinated protein antibodies
- Anti-CCP recognise neoantigen of arginine residue being modified by PAD which can remove amino group to produce modified residue citrulline
- PAD requires calcium
- Citrulline is not made de novo, it is generated by this secondary modification by PAD
how is ACPA implicated in RA?
- highly specific, found in 50-70% of RA patients
- detected years before onset of symptoms
- correlated with disease severity
are all patients seropositive for ACPA?
no, there are seronegative patients which lack anti-CCP
More anti-CCP = more likely to develop worse RA
what has been implicated to produce citrullinated autoantigens?
Samples from RA joints – found NET structures (neutrophils spill out chromatin to capture bacteria and prevent them spreading)
- Lots of neutrophils and NETs in joints of RA patients
- In the patient NETs, there are high CCP levels
- neutrophils generate the CCP autoantigens
how do neutrophils produce citrullinated peptides?
To release NETs, PAD is activated in neutrophils, leading to citrullination of arginine side chains in proteins such as collagen II, fibrinogen
- Autoantibodies form immune complexes with CCPs on NETs
– these proinflammatory complexes will lead to phagocytosis by macrophages to act as APCs for the adaptive response, leading to further adaptive mobilisation
how does autoimmunity travel from the mucosa to the joints?
Inflammatory trigger in gut leads to the production of CCP antigens
- This leads to ACPA response
- Inflammatory response may occur a while before the development of RA symptoms
- ACPA antibodies can be in healthy people
- then second inflammatory hit in joint causes neutrophils to infiltrate the joint
- existing ACPA form immune complexes with CCP in NETs and induce uptake of neoantigens
what cytokines are produced in RA joints?
Sorted cell populations from synovial fluid for cytokine mRNA:
- Neutrophils produce pro-inflammatory cytokines in RA e.g. IL-1B (doesn’t just produce NETs, also drives inflammation in joint)
- In B cells, there is massive production of RANK ligand = cytokine which can cause bone damage
what B cell population expresses RANKL?
New B cell population in joint:
- expresses FcRL4 (receptor for IgA)
- These B cells produce RANKL which can damage joints – induces formation of osteoclasts which degrade bone
- Rituximab is a monoclonal antibody that depletes B cells – can be used for RA patients to prevent B cell-induced bone damage by reducing RANKL production in the joint
how do FcRL4+ B cells contribute to RA inflammation?
FcRL4 is an IgA receptor, and these pathogenic B cells were enriched with IgA, acting as though they were at the mucosa
- Mucosal phenotype of B cells present at the joint, contributing to inflammation
are these FcRL4+ B cells specific for a particular antigen?
Cloned recombinant antibodies from FcRL4+ B cells with same the antigen-binding site from RA patients into vectors and ran over ELISA for CCPs
- Showed that FcRL4+ B cells were enriched for recognition of CCP autoantigens
- Not high-affinity antibodies, but there is reactivity to CCPs from this B cell subset – association of B cell mucosal immune response within the joint
what are the FcRL4+ B cells enriched for on their surface? what does this enable?
FcRL4 isotype of these B cells from RA synovial fluid were enriched for IgA1, even though they’re not at the mucosa
- IgA immune complexes can be internalised by the FcRL4 B cells – act as APC
- These complexes are then presented to T cells
- Both specificity and ability to capture IgA isotype suggests FcRL4 B cell role in autoimmunity
Do FcRL4+ B cells link joint and bowel inflammation?
FcRL4 B cells found in the inflamed gut and in synovial fluid
- Inflammatory bowel disease patients have increased risk of RA – maybe due to FcRL4+ B cells
can FcRL4+ B cells be targeted?
- this receptor is specifically expressed on these B cells
- high-specificity target, so less side effects
- delete these cells and their mucosal inflammatory role in RA
how was the role of FcRL4+ B cells in gut inflammation studied?
Sorted FcRL4+ and - cells
- Cloned recombinant IgA antibodies
- tested which of these antibodies recognised bacteria with flow cytometry
- labelled the antibodies from stool samples
- sorted bacteria which were labelled with antibody using FACS
- Sequenced sorted bacteria
do antibodies from RA joint B cells recognise gut bacteria?
Some antibody clones from B cells recognise subgroups of bacteria
But:
- Particles of bacteria may travel into joint?
- Or B cells from gut travel into joint?
how do B cells from the RA joint recognise bacteria?
Tiny particles 20nm are shed by bacteria which can travel through body barriers
- Is there enrichment of these particles in joints of RA patients and do they trigger local inflammation?
is there a link between mucosal and joint inflammation?
Strong link between mucosal inflammation and arthritis via FcRL4+ B cells
- IgA+ B cells overrepresented in FcRL4+ population, found in inflamed mucosa and synovium
- can capture IgA immune complexes in mucosa and synovium, enabling ACPA repsonse
- Caused by travelling cells from site to site which may or may not be caused by bacteria
- The B cells produce cytokines such as RANKL which can cause damage
- FcRL4+ B cell subset may travel from mucosal inflammation or develop in joint – in both cases showing strong link to bacterial infection
