Loss of tolerance of anti-dsDNA B cells in mice overexpressing CD19

Mice transgenic for the R4A-Cμ heavy chain of an anti-dsDNA antibody, maintain tolerance by anergy and deletion. In C57BL/6 mice overexpressing CD19, a molecule, which lowers the threshold for B cell activation, elevated levels of serum autoantibodies have been observed. In the present study, we wished to determine whether CD19 overexpression could alter the induction of tolerance in R4A-Cμ mice and lead to the secretion of transgenic anti-dsDNA antibodies. We, therefore, bred R4A-Cμ transgenic mice-to-mice transgenic for human CD19 (hCD19) and generated R4A-Cμ mice heterozygous and homozygous for hCD19. We, now report the spontaneous secretion of transgenic IgM anti-dsDNA antibody in the sera of R4A-Cμ mice overexpressing CD19, indicative of a loss of B cell tolerance. We observe that transgenic B cells secreting anti-dsDNA antibody in these mice are T independent and display a marginal zone like phenotype althought they do not reside in the MZ. In addition, they appear to be derived from the conventional B2 subset rather than the B1 subset. Interestingly, a subset of the anti-dsDNA B cells in these mice still display the phenotype and functional characteristics of anergic B cells. These B cells cannot be activated to secrete antibody following BCR crosslinking, however, they are hyper-responsive to activation by innate signaling mechanisms. This suggests that CD19 overexpression may promote anergic B cells to escape tolerance by converging with BCR independent pathways, thereby rendering these B cells hyper-responsive to innate signaling.