Immune Dysregulation by the Rheumatoid Arthritis Shared Epitope

Abstract

Rheumatoid arthritis (RA) is closely associated with HLA-DRB1 alleles that code a five-amino acid sequence motif in positions 70–74 of the HLA-DRβ–chain, called the shared epitope (SE). The mechanistic basis of SE–RA association is unknown. We recently found that the SE functions as an allele-specific signal-transducing ligand that activates an NO-mediated pathway in other cells. To better understand the role of the SE in the immune system, we examined its effect on T cell polarization in mice. In CD11c+CD8+ dendritic cells (DCs), the SE inhibited the enzymatic activity of indoleamine 2,3 dioxygenase, a key enzyme in immune tolerance and T cell regulation, whereas in CD11c+CD8− DCs, the ligand activated robust production of IL-6. When SE-activated DCs were cocultured with CD4+ T cells, the differentiation of Foxp3+ T regulatory cells was suppressed, whereas Th17 cells were expanded. The polarizing effects could be seen with SE+ synthetic peptides, but even more so when the SE was in its natural tridimensional conformation as part of HLA-DR tetrameric proteins. In vivo administration of the SE ligand resulted in a greater abundance of Th17 cells in the draining lymph nodes and increased IL-17 production by splenocytes. Thus, we conclude that the SE acts as a potent immune-stimulatory ligand that can polarize T cell differentiation toward Th17 cells, a T cell subset that was recently implicated in the pathogenesis of autoimmune diseases, including RA.