TLR/BCR co-engagement induces mature and protective T-cell independent antibody responses through MyD88 and PI(3)K signalings

Abstract

Abstract B cells are central to immune responses to infections and vaccines by producing high-affinity and class-switched protective antibodies, often in a T cell-dependent manner. Prompted by our findings that dual TLR/BCR engagement induces efficient CSR in vitro, to a high level comparable to that induced by CD40 engagement, here we have analyzed induction of T cell-independent CSR/SHM and antibody responses by TLR-BCR co-engagement. Immunization of wildtype C57 mice with NP hapten conjugated to TLR4-engaging LPS (NP-LPS), but not NP-Ficoll admixed with LPS, elicits high-affinity and class-switched IgGs (IgG1, IgG2a, IgG2b, IgG3), at levels comparable to those elicited by a T cell-dependent antigen NP-CGG. NP-LPS also induces sustained and high levels of IgG2a, IgG2b and IgG3, but not IgG1, in Tcrβ−/−Tcrδ−/− mice, which are devoid of T cells and, as expected, fail to mount any response to NP-CGG. Immunization of both wildtype and Tcrβ−/−Tcrδ−/− mice with highly purified Salmonella typhimurium (S. typhimurium) flagellin, which has dual BCR and TLR5 engaging activities, elicits flagellin-specific antibodies that can opsonize S. typhimurium. T cell-independent immune responses, which are impaired in mice lacking Myd88 or Pik3r1 in B cells, are associated with high mutation loads in VH genes, generation of switched B cells, and B cell differentiation into long-lived plasma cells and memory B cells, which can be re-activated for anamnestic antibody responses. Thus, SHM/CSR and B cell differentiation into plasma cells and memory B cells can occur independently of T cells, leading to protective mature antibody responses.