The lymphocytic infiltrates sometimes organized in ectopic germinal centres in rheumatoid arthritis (RA) synovium contain locally activated B cells expressing hypermutated immunoglobulin transcripts and recombination-activating genes (Rag), suggesting an ongoing antigen driven process directly in synovium.

To test this hypothesis, mutational frequencies of immunoglobulin mRNAs, signs of isotype switching and Rag gene expression in individual RA synovial B cells were analyzed.

Single-cell RT-PCR was used to analyze individual synovial CD19⁺CD38⁺ and CD19⁺IgM⁺ B cells (as the reference population) from two RA patients.

We found significantly reduced frequencies of peripheral blood CD19⁺CD38⁺ B cells from RA patients as compared with controls, suggesting their possible migration into the site of inflammation. Three subsets of CD19⁺CD38⁺ B cells in RA synovium were detected, expressing (1) only IgM transcripts (IgM⁺, 13.5%), (2) only IgG transcripts (IgG⁺, 48.7%), and (3) both IgM and IgG mRNAs (IgM⁺IgG⁺, 37.8%). The differences in mutational frequencies between them were significant (Table 1). Over 40% of analyzed cells coexpressed Rag mRNAs. Similar subsets and expression patterns were found in CD19⁺IgM⁺ B cells; however, IgG⁺ cells displayed significantly decreased mutational frequencies (3.6%; P < 0.0001). This might reflect the presence of synovial B cell populations that differ in their maturational status or biological function.