Complexed BCR-TLR9 stimulation induces an activation associated cell death in B cells which is rescued by BLyS (60.12)

Abstract

Abstract Combined B cell receptor (BCR) and Toll-like receptor 9 (TLR9) signaling has been associated with the T cell Independent (TI) activation of autoreactive B cells in the presence of self-DNA. However, TLR9 knockout models of humoral autoimmunity have increased disease severity, suggesting the receptor has a more complex role. We have recently found that while both WT and AM14 RF producing B cells stimulated with complexed BCR-TLR9 agonists indeed proliferate, they die rapidly within 48-72hrs of initial stimulus, implying that combined BCR:TLR9 signaling has a role in limiting the T cell independent activation of B cells. Importantly, this activation associated cell death is unique to complexed BCR-TLR stimulation, as BCR agonist (anti-IgM) alone, TLR9 agonist (CpG ODN) alone, or both BCR + TLR9 agonists uncomplexed induces cells to continue to survive and proliferate robustly within this time frame. Further, this apoptosis involves the intrinsic mitochondrial cell death pathway, as it involves cleavage of caspase 9, occurs independently of caspase 8, and is rescued by the presence of a BCLxL transgene. Finally, we have found that addition of the survival cytokine BLyS rescues B cells proliferating in response to complexed BCR-TLR9 from cell death through BLyS binding to BR3, providing insight into how increased BLyS titers contribute to the onset of autoreactive B cell activation.