Induction of class-switched and somatically hypermutated primary and anamnestic antibody responses by TLR-BCR co-engagement in Tcrβ −/− Tcrδ −/−and NSG/B-cell mice

Abstract

Abstract Ig gene CSR/SHM as well as plasma cell and memory B cell differentiation are central processes to the maturation of the antibody response and generally depend on T cell help. Prompted by our findings that CSR is efficiently induced by T cell-independent (TI) stimuli in vitro, e.g., dual engagement of TLR/BCR, here we have addressed the induction of TI antibody responses and underlying mechanisms in vivo using Tcrβ−/−Tcrδ−/− mice. These mice lack T cells and, as expected, failed to generate any class-switched antibodies to T-dependent antigens (e.g., NP-CGG and ovalbumin), even in the presence of adjuvants. They, however, displayed circulating IgG2b, IgG3 and IgA, at levels comparable to those in C57BL/6 mice, and to a lesser extent, IgG1 and IgG2a. Further, they generated high-affinity NP-specific IgG2b and IgG3 upon immunization with NP-LPS, but not NP-Ficoll mixed with LPS. The anti-NP-LPS response entailed a high-frequency of (replacement) mutations in the Ig variable region, a hallmark of antibody affinity maturation, CSR and generation of a high number of NP-binding IgG+B cells. Also generated at a high frequency were IgG+NP-specific plasma cells, which homed to the bone marrow, and memory B cells, which were instrumental in mounting an effective anamnestic response. These findings, together with the readily detectable NP-specific response to NP-LPS in immune-deficient NSG mice grafted with highly purified B cells, highlight a previously unsuspected but efficient TI and B cell-intrinsic mechanism, i.e., TLR/BCR co-engagement, in the generation of full-fledged antibody responses. The B cell-intrinsic nature of this mechanism was further emphasized by the failure of mice with B cell-specific Tlr4 deficiency to respond to NP-LPS.