The HLA-DRB1*14:02 allele predisposes indiginous North American populations to rheu- matoid arthritis based on a unique molecular mechanism

HLA-DR*14:02 is an HLA molecule, prevalent in indigenous North Americans (INA), that contains the rheumatoid arthritis (RA) associated shared epitope (SE) sequence. This sequence forms an electropositive pocket that was shown to preferentially accommodate citrulline residues in contrast to RA-resistant HLA-molecules, providing an explanation for the link between the HLA-system, RA and citrullination. Recent studies suggested an additional role for specific HLA-DRB1-residues at position β11 and β13 that are not carried by HLA-DRB1*14:02. We show, using a large cohort of INA RA patients, a significant association between HLADRB1*14:02 and seropositive RA (OR=2.46). In contrast to other HLA-SE-alleles, HLA-DRB1*14:02 does not prefer citrulline in its electropositive pocket 4 or any of its other pockets as revealed by HLA-peptide-binding studies. Solved structures of HLA-DRB1*14:02 in complex with citrulline/arginine containing vimentin-epitopes revealed that citrulline and arginine residues are presented in a different orientation in pocket 4, allowing for the citrulline to interact with T cell receptors. The differences between the SE alleles are attributable to the β11/β13 polymorphisms influencing the packing of P4 residues These findings deepen our understanding of the association between the HLAsystem and RA by showing that different HLA-SE molecules present citrullinated ligands in structurally different ways, that nonetheless allow the unique presentation of citrullinated ligands to T cells. HLA-DRB1*14:02 predisposes to RA based on a unique molecular mechanism 159 Ch ap te r 9 INTRODUCTION Rheumatoid arthritis is a chronic systemic inflammatory disease characterized by extensive inflammation of synovial joints[1]. The presence of Anti-citrullinated protein antibodies (ACPA) is an important hallmark of RA[2]. These autoantibodies specifically target proteins that have undergone a post-translational modification converting positively charged arginine residues in uncharged citrulline residues[3, 4]. The HLA class II locus is the most important risk factor for ACPA+ RA and contains genes encoding for the alpha and beta chains of HLA-DR and HLA-DQ molecules[5-7]. It was previously hypothesized that the contribution of the HLA class II region is explained by the HLA-DR locus when it was shown that RA risk associates with HLA-DRB1 alleles that encode the amino-acids (aa) sequences QKRAA, QRRAA and RRRAA in position 70-74. This sequence is known as the shared epitope (SE) motif [8, 9]. SE-residues β70, β71 and β74 shape peptide binding pocket 4 of HLA-DR molecules and it was proposed that the SE are directly involved in the presentation of a shared (type of) antigen to CD4+ T cells[8, 9]. To date, such an antigen has not been identified. However, accumulating data have shown that pocket 4 of different SE-alleles display an increased affinity for citrulline over arginine residues, thereby permitting enhanced presentation of citrullinated peptide-ligands[10-16]. Most of this work is centered around HLA-DRB1*04:01, the strongest risk allele in Caucasians. Recently the genetic association of individual amino acids in the HLA-DRB1 locus with ACPA+ RA was studied in a large cohort of Caucasians. This study identified that next to SE-residues β70 and β71, also β11 and β13, that are in tight LD, are associated with RA risk[17]: β13His (e.g. present in SE-allele HLA-DRB1*04:01) and β13Phe (e.g. present in SE-allele HLA-DRB1*01:01) are most strongly associated with risk, while β13Ser is not associated with predisposition[17]. Interestingly, β13 is, like the SE-motif, involved in shaping the peptide binding properties of pocket 4. To understand and uncouple the relative contribution of the SE-motif and β13 to presentation of citrullinated peptides, we wished to study HLA-DRB1*14:02. HLADRB1*14:02 is an SE-allele that is rare in Caucasians, but prevalent in indigenous North Americans (INA). Interestingly, this SE-allele carries the non-predisposing β13Ser residue. We aimed to assess if HLA-DRB1*14:02 associates with seropositive RA as well as the relative capacity of the HLA-DRB1*14:02 to accommodate arginine and citrulline residues in the various peptide binding pockets, particularly in pocket 4.