Attenuation of B cell receptor-toll like receptor responses by Fc gamma receptor IIB
Moody, Krishna Laroche
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The pathogenesis of lupus and other autoimmune diseases driven by antibody-antigen complexes involves interactions between genetic and environmental factors. The genetic factors can be separated into factors that dysregulate adaptive immunity, innate immunity or cell death. One genetic risk factor that can affect both innate and adaptive immunity is the inhibitory Fcγ receptor, FcγRIIB. Reduced or loss of function mutations in FcγRIIB lead to an increased risk of autoimmunity. Using the murine IgG2a specific B cell receptor (BCR) transgenic (Tg) mouse, AM14, our lab discovered that delivery of nucleic acid ligands via the BCR activates B cells by dual engagement of the BCR and endosomal toll like receptors (TLR) 7 and/or 9. Mechanistic studies interrogating the role of downstream signaling effectors and intracellular trafficking in the attenuation of BCR-TLR responses by FcγRIIB were limited by our inability to deliver immune complexes (IC) to non-Tg B cells or form brightly fluorescent IC. To deliver IC to non-Tg B cells, I developed a BCR adapter (BCRAM) that delivers IC to IgM-positive B cells. To track the uptake and trafficking of IC, I developed a panel of antibodies specific for streptavidin (SA). Complexes formed with biotinylated molecules and fluorescent streptavidin could be delivered to AM14 B cells or macrophages and tracked via flow cytometry and/or confocal microscopy. BCRAM and fluorescent IC were used to understand how FcγRIIB attenuated BCR-TLR responses. I found that both DNA IC and RNA IC responses were enhanced by FcγRIIB ablation. Interestingly, a naturally-occurring somatic mutation in the Fc domain of the nucleic acid-binding antibody PL2-3 prevented regulation by FcγRIIB and reduced binding to activating FcγR. Paradoxically, I found that SHIP-1, a negative regulator activated downstream of FcγRIIB engagement, promoted BCR-TLR9 responses independent of FcγRIIB. I hypothesized that FcγRIIB attenuates BCR-TLR9 responses by interfering with sensing by the endosomal TLRs. Using a pH sensing IC, I found that engagement of FcγRIIB leads to residence of the IC in a higher pH compartment. These findings demonstrate that FcγRIIB regulates the activation of autoreactive B cells by modulating the trafficking of nucleic acid containing IC to TLR7 and TLR9 associated intracellular compartments in B cells.
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