The local production of pathogenic autoantibodies by plasma cells in synovium is one of the hallmarks of rheumatoid arthritis (RA). There may be a potential link between ectopic lymphoid neogenesis and the local autoimmunity in rheumatoid synovium. The unfolded protein response (UPR) has key roles in the development and maintenance of plasma cells secreting immunoglobulin. This study was designed to explore the potential links between the activation of the UPR of infiltrating plasma cells in inflamed peripheral joints and the histopathological variants of rheumatoid synovitis as well as the local production of pathogenic autoantibodies.
The variants of rheumatoid synovium were histopathologically classified into follicular and diffuse synovitis. Immunohistochemical and double-immunofluorescent stainings were performed to detect the expression of 78-kDa glucose-regulated protein (GRP78), a marker of activation of the UPR, in infiltrating plasma cells of synovium, and flow cytometry and immunoblotting analyses were performed to quantify GRP78 in plasma cells of synovial fluid in inflamed peripheral joints of RA. The detections were also taken in osteoarthritis (OA) as controls. The synovial fluid levels of anti-cyclic citrullinated peptide antibodies (anti-CCP) (IgG) were quantified with the enzyme-linked immunosorbent assay and corrected to those of total IgG in RA.
Expressions of GRP78 were more intensive in infiltrating plasma cells in RA synovium relative to those in OA synovium (P < 0.001) and in synovium with follicular synovitis relative to that with diffuse synovitis (P < 0.001). Analyses by flow cytometry and immunoblotting showed that there was a significant upregulation of GRP78 of plasma cells from synovial fluid of RA compared with that of OA (P < 0.05) and from synovial fluid of follicular synovitis relative to that of diffuse synovitis (P < 0.05). Moreover, a positive relationship between the expression of GRP78 of plasma cells from synovial fluid and the corrected synovial levels of anti-CCP (IgG) was seen in RA (P < 0.001).
There may be a link between enhanced activation of the UPR of plasma cells and ectopic lymphoid neogenesis as well as the local production of anti-CCP (IgG) in inflamed peripheral joints of RA.