Bacterial products induce an alternate pathway in B cell receptor signaling

B cell receptor signaling may take place through a classical pathway that is dependent on signalosome mediators or by an alternate pathway that is signalosome-independent. The alternate pathway is known to be produced via cross-talk between the B cell receptor and T cell products such as CD40L and IL4. We sought to determine whether lipopolysaccharide (LPS) and other bacterial products can trigger this phosphoinositol-3 kinase (PI-3K), phospholipase C gamma (PLCgamma2), protein kinase C beta (PKCbeta), and Bruton's tyrosine kinase (BTK) independent BCR signaling pathway. Splenic B cells were treated with one or another bacterial product for 24 hours and were then exposed to the PI-3K inhibitor LY294002 (or other signalosome inhibitor) for 1 hour followed by stimulation with anti-Ig. BCR-triggered outcomes of extracellular signal regulated kinase (ERK) and NF-kappaB activation were completely blocked by LY294002 in naive B cells; however, pretreatment with LPS or other bacterial products (including the lipoprotein PAM3 CSK4, the lipoprotein MALP-2, and unmethylated CpG DNA) reprogrammed BCR-signaling so that it was no longer dependent on PI-3K. In addition, pre-treatment of B cells with bacterial products enhanced subsequent ERK phosphorylation and the rate at which IkappaB-alpha degradation occurred. Thus, bacterial products re-route the BCR signaling pathway. Additionally, the alternate BCR signaling pathway in combination with the classical signaling pathway, enhance the rate and strength of BCR-triggered signaling. Further, the alternate pathway was found to be dependent on a protein blocked by the inhibitor rottlerin. The LPS-induced alternate pathway represents a new paradigm for B cell activation and suggests the nature of BCR signaling depends on environmental cues for optimal responses.