Intrinsic B cell TLR-BCR linked coengagement induces mature and protective antibody responses in the absence of T cells

Abstract Somatic hypermutation (SHM) and class-switch DNA recombination (CSR) together with plasma cell and memory B cell differentiation are central to the maturation of the antibody response. Prompted by our finding that CSR and plasma cell differentiation can be induced in vitro by a TLR ligand together with BCR crosslinking, we hypothesized that TLR and BCR coengagement can induce mature antibody responses in the absence of T cells. To test this hypothesis, we injected Tcrβ−/−Tcrδ−/− mice, which are devoid of T cells, with NP-LPS or Salmonella Typhimurium flagellin, which coengages B cell TLR4 or TLR5 with BCR. These mice generated NP or flagellin-specific IgG class-switched antibodies and plasma cells. The IgG response to NP evoked formation of germinal center microclusters, a high frequency of point-mutations in IgH V186.2 (region which encodes NP-binding BCRs), including the affinity-enhancing Trp33Leu mutation that underpins the production of high-affinity anti-NP antibodies, and generation of memory B cells. The T cell-independence of the class-switched antibody response was further emphasized by the anti-NP IgG response in immune-deficient NSG mice engrafted with purified mouse B cells. It was mimicked by the specific TLR5-dependent IgG response to S. Typhimurium flagellin in Tcrβ− /−Tcrδ−/− mice. The LPS-immunized and flagellin-immunized Tcrβ−/−Tcrδ−/− mice generated LPS-specific and flagellin-specific IgG antibodies that neutralized E. coli and S. Typhimurium in vitro and protected them from lethal infection in vivo. Thus, TLR-BCR coengagement can induce a class-switched, hypermutated and protective antibody response in the absence of T cells.