Molecular IgV(H) analysis demonstrates highly somatic mutated B cells in synovialitis of osteoarthritis: A degenerative disease is associated with a specific, not locally generated immune response

In osteoarthritis (OA), the synovial tissue exhibits a nonfollicular inflam matory infiltration with a characteristic arrangement of lymphocytes and pl asma cells. These arrangements are either small perivascular aggregates wit h plasma cells surrounding the lymphocytes or small groups of plasma cells, located in the vicinity of small blood vessels. These patterns suggest tha t B lymphocytes directly differentiate into plasma cells. To understand the B-cell response in OA, we analyzed the V-H genes from B cells of synovial tissue of nine OA patients (average age, 71.5 +/- 10.5 years; six female an d three male). V-H gene repertoires were determined from RNA prepared from tissue cryosections and from DNA of single isolated B lymphocytes and plasm a cells. The inflammatory infiltrate was analyzed immunohistochemically by detecting CD20, Ki-M4 (follicular dendritic cells), CD4, IgG, IgM, IgA, Ki- 67, and by simultaneous demonstration of the plasma-cell-specific antigen C D138 (syndecan-1) and factor VIII. The molecular data demonstrate B cells w ith a high number of somatic mutations (average, 16.5 to 19.8), and high ra tios of replacement to silent mutations in the small lymphocytic/plasmacell ular aggregates of OA. In the tissue cryosections, the values of the Sigma R/Sigma S at the complementarity determining regions were 5.3 and 2.0 in th e framework regions. For both the isolated B lymphocytes and plasma cells, the value of this ratio in the complementarity determining regions was 3.5. In the framework regions, the values of this ratio were 2.0 for the isolat ed B cells and 1.8 for the plasma cells. B lymphocytes and plasma cells exh ibited a distribution not described thus far. Two patterns of B-cell distri bution could be observed: (a) Centrally located CD20(+) B and CD4(+) and CD 8(+) T lymphocytes were surrounded directly by IgG (predominantly) or IgA a nd IgM plasma cells. No proliferating Ki-67-positive cells and no follicula r dendritic cells (germinal centers) could be detected in the aggregates; ( b) Plasma cells (predominantly IgG) were located directly near endothelial cells of small blood vessels. The finding of highly mutated V-H genes in B lymphocytes and the characteristic arrangement of B lymphocytes and plasma cells suggests that B cells, which participate in OA synovialitis, have und ergone germinal center reaction at different sites. This may explain the lo w inflammatory infiltration without germinal centers in OA, which is a feat ure of this primarily degenerative joint disease.