T-independent activation of B cells by vesicular stomatitis virus: no evidence for the need of a second signal.

Vesicular stomatitis virus (VSV) induces a T helper cell-independent IgM antibody response, whereas the IgG response is strictly T helper cell dependent. Since VSV induces B cells in complete absence of T helper cells, the question arises as to whether this induction occurs in the absence of a second signal or whether it depends upon an alternative or replacing signal 2. We therefore asked whether VSV has polyclonal B cell stimulator activity and/or whether B cell induction by VSV needs costimulation via complement or tumor necrosis factor (TNF) receptor or by natural killer (NK) cell activity. In vitro B cell proliferation assays and analysis of the in vivo antibody response in CD40-deficient mice excluded that VSV has properties of a polyclonal B cell stimulator. C3 depletion by cobra venom factor and application of anti-complement receptor antibodies showed that the T-independent IgM response was largely C3-independent except under very limiting antigen doses. Immunization of TNF receptor-deficient mice showed a normal anti-VSV IgM response, and in a cytotoxicity assay on YAC target cells there was no evidence of NK cell activation by VSV. Thus, VSV seems to induce B cells without polyclonal activation and/or C3, TNF, or NK cells functioning as a replacing second signal.