Lymphocytic infiltrates in rheumatoid arthritis (RA) synovium often resemble lymphoid follicles and contain clonally related Ig transcripts, suggesting in situ antigen-dependent B cell selection. Recent reports have shown expression of recombination-activating genes (RAGs) and concurrent secondary rearrangement of Ig genes in normal peripheral lymphoid organs (receptor revision). We sought to determine if RAG-mediated receptor revision of Ig kappa light chains occurs in B cells within the RA synovium. Because we previously reported enhanced N-region addition at V(L)-J(L) joins in clonally expanded light-chain transcripts from RA synovium, we also sought expression of terminal deoxynucleotidyl transferase (TdT), which is normally expressed only in B cell precursors or immature B cells.
Reverse transcription-polymerase chain reaction (PCR) was used to detect RAG and TdT transcripts from unselected and B cell-enriched synovial and peripheral blood mononuclear cells obtained from 12 RA patients. Activity of RAG protein was sought using ligation-mediated PCR to detect recombination intermediates, and immunohistochemistry was performed to identify RAG+ cells within synovia.
We found evidence of RAG-mediated secondary Ig kappa light chain rearrangements in about one-third of RA synovia. TdT expression was found in several samples, but did not correlate with RAG expression.
RAG-mediated secondary Ig rearrangements of kappa light chains may contribute to the local production of antibodies to autoantigens (e.g., rheumatoid factor) or exogenous antigens, or it may represent a failed attempt at immune tolerance. TdT expression suggests the presence of immature B cells in RA synovia. These findings have important implications for the local generation of antibodies in RA and other chronic inflammatory diseases.