Background/Purpose: The products of rheumatoid arthritis (RA)-associated MHC class II risk alleles HLA-DRB1*04:01 and *04:04 (DR4) differ only by two amino acids in the peptide binding groove. Upon analyses of >21,000 seropositive RA cases and controls (from EIRA, NARAC and WTCCC cohorts), we could show that combined HLA-DRB1*04:01/04:04 genotype is more frequent for seropositive RA than either of the homozygous DR4 allomorphs compared to controls. Here, we aimed to study the association of increased disease risk and antigen-specific T cell responses in RA.

Methods: We used DR4-restricted T cell epitopes from citrullinated cartilage intermediate layer protein (cit-CILP_297-311) and influenza matrix protein (MP_97-116) to investigate antigen-specific responses in RA patients carrying HLA-DRB1*04:01+ (n=7), HLA-DRB1*04:04+ (n=8) or the combination of the two (n=5). We used HLA-class II tetramer staining, crystallography and peptide-HLA binding studies, TCR sequencing and artificial TCR re-expression to investigate different aspects of peptide-HLA-TCR interactions.

Results: Cit-CILP_297-311 is nearly identically presented in the two DR4 molecules as assessed by crystallographic studies, despite the amino acid differences influencing the peptide binding pockets, P1 and P4. Same was seen for MP_97-116. This similarity implicates the existence of TCRs that cannot distinguish between the peptide-DR4 allomorph complexes. Such T cells were indeed present amongst in-vitro propagated cells as visualized by HLA tetramer technology. MP97 and Cit-CILP specific double-positive T cells from heterozygous *04:01/*04:04 patients (n=5) expressed higher levels of activation marker CD137 in response to antigen-stimulation than T cells binding to one HLA allomorph. We could validate such elevated responses using artificial TCR re-expression. We could also find cit-CILP specific ‘public’ double positive TCRs shared between heterozygous and *04:01+ RA patients.

Conclusion: Altogether, our data demonstrate how products of two different HLA-DR alleles can work in synergy and implicate that this co-existence may be detrimental in the context of an autoimmune disease. The observed synergy between two allomorphs and presence of shared TCRs between genotypes supports the finding of increased risk in heterozygous patients. It also gives a rationale for therapeutic targeting of specific T cell signatures in RA.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/shared-epitope-allomorphs-directly-influence-antigen-specific-t-cell-responses-in-ra/