Lymphoblastoid cells express HLA‐B27 homodimers both intracellularly and at the cell surface following endosomal recycling

Abstract

The MHC class I allele HLA-B27 is very strongly associated with development of autoimmune spondyloarthritis, although the disease mechanism remains unknown. Class I molecules classically associate in the endoplasmic reticulum (ER) with g 2-microglobulin ( g 2 m) and antigenic peptides for cell surface expression and presentation to T cells. We have previously shown that HLA-B27 is capable of forming g 2 m-free disulfide-bonded homodimers in vitro. Here we show that HLA-B27 forms disulfide-bonded homodimers in vivo by two distinct pathways. HLA-B27 homodimers form in the ER but appear unable to egress to the cell surface in human cells. Cell surface HLA-B27 homodimers are abundantly expressed in a variety of lymphoid cell lines. Experiments with inhibitors indicate that HLA-B27 homodimers can arise from cell-surface heterodimers via an endosome-dependent recycling pathway. HLA-B27 homodimer expression on the cell surface of 721.220 is dependent on the unpaired cysteine 67 and is inhibited by restoration of tapasin function or by incubation with peptides that bind strongly to HLA-B27 heterodimers. Cell surface expressed HLA-B27 homodimers are likely to be immunologically reactive ligands for NK family immunoreceptors and, hence, could play a pathogenic role in spondyloarthritis.

Similar cell surface expression of β2‐microglobulin–free heavy chains by HLA–B27 subtypes differentially associated with ankylosing spondylitis

“…This approach was based on using antibodies that recognized the B27 heterodimer or ␤ 2 m-free HLA class I heavy chains. In our study, we assumed that free B27 heavy chains arise at the cell surface from dissociation of the heterodimer, either directly or upon endosomal recycling (3). Whereas the possibility that some forms of ␤ 2 m-free heavy chains may escape the ER and reach the cell surface cannot be formally ruled out, this possibility has not been demonstrated and is unlikely because of impairment by quality control mechanisms operating in the assembly of class I MHC molecules (4).…”

“…However, since HC10 also reacts with unfolded heavy chains in monomeric form, the possibility that subtypes fail to form homodimers to an equal extent at the cell surface cannot be ruled out. Bird et al (3) showed that the S67 mutant failed to formed disulfide-linked homodimers, but still retained the capacity to form multimers. In our study, S67 showed the same behavior in the presence of exogenous peptide as Cys 67 -containing subtypes.…”

“…Thus, slow folding of HLA-B*2705 has inspired a hypothesis based on the induction of ER stress responses, which might lead to inflammation independently of antigen presentation (10,11). In addition, detection of B27 heavy chain homodimers at the cell surface (3,12,13) and their recognition by some innate immunity receptors (13,14) have suggested a possible pathogenetic involvement of these noncanonical forms of B27. Moreover, it has recently been suggested that B27 dissociation at the cell surface might lead to a low-level chronic release of ␤ 2 m, which if trapped in the synovium and deposited in the joints, might lead to chronic inflammation (15).…”

Critical Role of Endoplasmic Reticulum Aminopeptidase 1 in Determining the Length and Sequence of Peptides Bound and Presented by HLA–B27

“…The ERAP1 shRNA were then cloned into a lentiviral plasmid for stable transduction of 2 HLA-B27-expressing cell lines (C1R.B27 and HeLa.B27). C1R.B27 cells were previously generated by transfection of HLA-B27 into C1R cells, an HLA-A/B-negative mutant B lymphoblastoid cell line (15). HeLa.B27 cells were generated by transduction of HLA-B27 into HeLa cells, a cell line derived from cervical cancer cells.…”

HLA–B27 up‐regulation causes accumulation of misfolded heavy chains and correlates with the magnitude of the unfolded protein response in transgenic rats: Implications for the pathogenesis of spondylarthritis‐like disease

“…Lack of activation of the UPR in macrophages from B7-transgenic rats is consistent with evidence that HLA-B7 does not misfold and the hypothesis that the UPR may be involved in the pathogenesis of SpA-like disease in rats. Other unusual properties of HLA-B27 include the tendency to form cell-surface homodimers via a mechanism distinct from ER misfolding (7,12,13); these homodimers have been hypothesized to be recognized in vivo by leukocyte receptors and to exert immunoregulatory effects (14). HLA-B7 also forms cell-surface homodimers in human cells as well as rat cells, although these homodimers are less abundant with HLA-B7 than with HLA-B27 (7,9).…”