Elucidating synergistic and antagonistic effects of B cell receptor and toll-like receptors 3, 4, and 9 in B cell activation

Abstract B cells are key components of the humoral immune response. Two forms of B cell activation come from engagement of the B cell receptor (BCR) with foreign antigens and from pathogen-associated molecular patters (PAMPs) binding to toll-like receptors (TLRs). For TLRs, MyD88 and TRIF are the major adaptors that lead to NFkB and IRF3 activation respectively. Among various TLRs that are present in B cells, TLR9 is only able to use MyD88 and TLR3 only signals through TRIF, while TLR4 uses both pathways. Therefore, these TLRs provide excellent tools to investigate the roles of TRIF and MyD88 pathways in B cell function and the interplay between them. Here, using stimulation through these three different TLRs either individually or in combination with each other and with/or without anti-IgM, we observed that there is synergy and antagonism between these pathways in terms of activation induced metabolic boost, cytokine production, IgM secretion, expression of various cell surface markers, and antigen presentation. We identified the TRIF pathway as an inducer of Type-I interferon secretion by B cells, which eventually favors the expression of costimulatory molecules such as CD86. On the other hand, MyD88 stimulation antagonized the expression of costimulatory molecules and dramatically induced rapid proliferation and production of IgM as well as proinflammatory cytokines such as IL-6 and TNF. Using B cells obtained from MyD88 or TRIF KO mice, we showed that the interplay between pathways is of limited scale; however, for TLR4 signaling MyD88 has a largely permissive effect while TRIF determines the magnitude of response. These results further our understanding of the complex TLR signaling machinery in B lymphocytes.