Assistant Professor UMass Chan Med. Sch. Worcester, Massachusetts, United States
Disclosure(s):
Kerstin Nundel, PhD: No financial relationships to disclose
Introduction/Rationale: 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. Using the BCR sdTg mouse model AM14 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.
Methods: To better understand the signaling differences in BCR/TLR7 and BCR/TLR9 activated B cells, we activated purified AM14 B cells with RNA and DNA containing immune complexes and measured the activation of NFkB by Western blot analysis. We measured IkBz and BATF expression in these B cells at different timepoints by RT-PCR and Western blot. Furthermore, B cells from IkBz-deficient and BATF-deficient mice were activated with immune complexes and plasmablast differentiation was measure after 72 hours by flow cytometry.
Results: Intriguingly, we now show that BCR/TLR9 activation induces a more pronounced NFkB signal and the expression of the non-classical IkB member IkBz. BCR/TLR7-activated AM14 B cells are unable to sustain the initial NFkB response and fail to express IkBz. IkBz 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 IkBz regulated cytokines are expressed after BCR/TLR9 dependent signaling but not after BCR/TLR7 dependent signaling. Furthermore, we show that expression of IkBz inhibits plasma cell formation by regulating the expression of the transcription factor BATF.
Conclusion: Our results point to divergent roles for TLR7 and TLR9 in modulating the NFkB signaling cascades, leading to differential expression of NFkB target genes which influence B cell fate decisions.
