Functional outcome of B cell activation by chromatin immune complex engagement of the B cell receptor and TLR9

We have previously shown that rheumatoid factors produced by Fas-deficient autoimmune-prone mice typically bind autologous IgG2a with remarkably low affinity. Nevertheless, B cells representative of this rheumatoid factor population proliferate vigorously in response to IgG2a/chromatin immune complexes through a mechanism dependent on the sequential engagement of the BCR and TLR9. To more precisely address the role of both receptors in this response, we analyzed the signaling pathways activated in AM14 B cells stimulated with these complexes. We found that the BCR not only serves to direct the chromatin complex to an internal compartment where it can engage TLR9 but also transmits a suboptimal signal that in combination with the signals emanating from TLR9 leads to NF-kappaB activation and proliferation. Importantly, engagement of both receptors leads to the up-regulation of a group of gene products, not induced by the BCR or TLR9 alone, that include IL-2. These data indicate that autoreactive B cells, stimulated by a combination of BCR and TLR9 ligands, acquire functional properties that may contribute to the activation of additional cells involved in the autoimmune disease process.