Antibodies and B cells recognising citrullinated proteins display a broad cross-reactivity towards other post-translational modifications.

OBJECTIVE: Autoantibodies against antigens carrying distinct post-translational modifications (PTMs), such as citrulline, homocitrulline or acetyllysine, are hallmarks of rheumatoid arthritis (RA). The relation between these anti-modified protein antibody (AMPA)-classes is poorly understood as is the ability of different PTM-antigens to activate B-cell receptors (BCRs) directed against citrullinated proteins (CP). Insights into the nature of PTMs able to activate such B cells are pivotal to understand the 'evolution' of the autoimmune response conceivable underlying the disease. Here, we investigated the cross-reactivity of monoclonal AMPA and the ability of different types of PTM-antigens to activate CP-reactive BCRs. METHODS: BCR sequences from B cells isolated using citrullinated or acetylated antigens were used to produce monoclonal antibodies (mAb) followed by a detailed analysis of their cross-reactivity towards PTM-antigens. Ramos B-cell transfectants expressing CP-reactive IgG BCRs were generated and their activation on stimulation with PTM-antigens investigated. RESULTS: Most mAbs were highly cross-reactive towards multiple PTMs, while no reactivity was observed to the unmodified controls. B cells carrying CP-reactive BCRs showed activation on stimulation with various types of PTM-antigens. CONCLUSIONS: Our study illustrates that AMPA exhibit a high cross-reactivity towards at least two PTMs indicating that their recognition pattern is not confined to one type of modification. Furthermore, our data show that CP-reactive B cells are not only activated by citrullinated, but also by carbamylated and/or acetylated antigens. These data are vital for the understanding of the breach of B-cell tolerance against PTM-antigens and the possible contribution of these antigens to RA-pathogenesis.

De novo gene mutations in normal human memory B cells.

In the past years, the genomes of thousands of tumors have been elucidated. To date however, our knowledge on somatic gene alterations in normal cells is very limited. In this study, we demonstrate that tetanus-specific human memory B lymphocytes carry a substantial number of somatic mutations in the coding regions of the genome. Interestingly, we observed a statistically significant correlation between the number of exome mutations and those present in the immunoglobulin heavy variable regions. Our findings indicate that the majority of these genomic mutations arise in an antigen-dependent fashion, most likely during clonal expansion in germinal centers. The knowledge that normal B cells accumulate genomic alterations outside the immunoglobulin loci during development is relevant for our understanding of the process of lymphomagenesis.