Divergent Roles for TLR7 and TLR9 in Modulating the NFκB Pathway in B Cells

Abstract BCR signal strength plays a key role in the fate decision of B cells with strong BCR signals promoting plasma cell differentiation and weaker BCR signals driving germinal center formation. Signals through toll-like receptors (TLRs) also influence B cell differentiation. It is still unclear how the BCR and TLR signaling cascades interface to modulate B cell responses and differentiation. To address this question, we used a BCR sdTg mouse model, AM14, in which all mature B cells have a naïve follicular phenotype. The AM14 BCR recognizes IgG2a-bound immune complexes (ICs) and B cell activation depends on TLR7 and/or TLR9. ICs formed using the auto-antibody (autoAb) PL2-3 can activate AM14 B cells through either BCR/TLR9 or BCR/TLR7, allowing us to use TLR knockout lines to dissect signaling pathways unique to each TLR. Using this system, we reported earlier that BCR/TLR7-activated AM14 B cells differentiate into short-lived plasmablasts while plasma cell development does not occur in BCR/TLR9-activated AM14 B cells. Intriguingly, we now show that BCR/TLR9 activation of AM14 B cells leads to sustained activation of the NFκB pathway. BCR/TLR7-activated AM14 B cells are unable to sustain the initial NFκB response and fail to express the non-classical IκB member, IκBζ. IκBζ is known to be important in regulating cytokine responses in T cells and macrophages. However, its role in B cells is not well defined. We find that several IκBζ regulated cytokines are expressed after BCR/TLR9 dependent signaling but not after BCR/TLR7 dependent signaling. Our results point to divergent roles for TLR7 and TLR9 in modulating the NFκB signaling cascades, leading to differential expression of NFκB target genes which may influence B cell fate decisions.