Background/Purpose: Among genetic risk factors of RA, HLA class II molecules confers the highest risk of ACPA positive RA. The shared epitope (SE) hypothesis that initially involved positions 70 to 74 on the HLA molecule has been recently challenged by the influence of other positions such as the valine 11 position. The presence of the SE is thought to influence presentation of citrullinated peptides (Cit-P) by antigen presenting cells to T cells. Study of T-cell response in patients with various HLADRB1 haplotype is challenging. Rodent models lack the expression of the equivalent of the DRB1 molecule and transgenic mice for the SE have led to controversial results. Certain macaques express DRB1 molecules in a similar fashion compared to humans. They are thus a great tool to analyze the effect of the HLA molecule on T cell response to Cit-P. In these macaques, we previously demonstrated that immunization with Cit-P and an intra-articular boost are able to induce a specific T-cell anti-Cit-P response independent of the SE epitope and a monoarthritis (Bitoun et al. Frontiers Immunol 2017).

Methods: We selected two macaque haplotypes to have the greatest difference in the risk induced by the HLADRB1. The H6 haplotype has a similar sequence to the human RA risk-conferring HLADRB1*01:01 and displays QRRAA in the 70 to 74 positions and a F in the 11 position. The H3 haplotype is the closest to RA protective haplotypes HLA-DRB1 04:02 and 13:01 and expresses DRRAS (70-74 positions) but a V in position 11. Two groups of six animals were immunized with four Cit-P: vimentin (59–71) and (66–78), fibrinogen α (79–91) and aggrecan (89–103). These peptides were selected based on their ability to induce an anti-citrullinated T-cell response in RA patients carrying the SE. T-cell response was assessed using Interferon γ ELISPOT. Timepoints were compared using Mann Whitney tests.

Results: After a prime and 7 boosts, we obtained a detectable anti-citrullinated T cell response in only 7 out of twelve animals. The analysis dividing animals according to the presence of the SE motif (H6 in macaques) show a significantly diminished T-cell response in H6 macaques (Figure A). This was contradictory with the common description of higher T cell response in patients carrying the SE. When we reanalyzed the data based on the presence or not of a V in position 11, we found that this valine in position 11 was strongly associated with the presence of a T-cell response against citrullinated peptides, whatever the 70-74 sequence was (Figure B).

Conclusion: As recently described in humans for association to RA and anti-CCP positivity, T-cell response to Cit-P in macaques is more influenced by the presence of a valine at position 11 in the HLA-DRB1 than with the presence of the QRRAA 70-74 SE. The respective influence of the V in position 11 and the 70-74 QRRAA sequence should be studied regarding a better binding and presentation of Cit-P.