Synovial B Cells in Rheumatoid Arthritis: Clonal Expansion, Diversification, and Persistence
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KeywordsRheumatoid Arthritis Rheumatoid Arthritis Patient Plasma Cell Synovial Fluid Synovitis
The extent of B cell clonal diversity in the blood, synovial fluid (SF) and synovial tissue (ST) of rheumatoid arthritis (RA) patients was studied. We measured diversity using an lg VH gene fingerprinting assay that identifies B cell clones based on VH CDR3 length. By comparing fingerprints from genomic (g)DNA vs cDNA, clones that were expanded in number could be distinguished from those that were activated and contained increased amounts of lg VH gene mRNA (but were not necessarily numerically-expanded). These assays were performed on total B cells and on naïve, germinal center, and memory B cells and plasma cells. The data were confirmed by VH gene sequence analyses.
Comparisons of the gDNA and cDNA assays indicated that certain clones were expanded at the DNA but not at the RNA level, suggesting increased numbers of resting memory cell-like B cells. By contract, some clones were less expanded at the DNA level and much expanded at the RNA level, indicating activated B cells.
Studies of paired blood and SF or ST samples identified clones that were joint-specific, blood-specific and blood-joint common. When B cell clones in the SF from the same joint of the same patient were studied over time, most of the B cell clones changed. However, a few remained constant. The persisting clones exhibited VH gene diversification with time. We also compared the B cell clones present in the ST of two hip joints surgically removed from the same RA patient at the same time. These expanded clones also exhibited intraclonal sequence diversity.
Finally, ST B cells were sorted into memory and plasma cell subsets and analyzed. Certain B cell clones were unique to the memory compartment, while others were also expressed as plasma cells. Some of these B cell clones may play a role in the synovitis of RA.