T helper cell-dependent, microbial superantigen-induced murine B cell activation: polyclonal and antigen-specific antibody responses.

Microbial superantigens (SA), bound to human B cell surface MHC class II molecules, have been shown to promote direct, "cognate" interaction with SA-reactive autologous Th cells, resulting in polyclonal Ig production. To investigate the potential for microbial SA to support Th cell-dependent, Ag-specific antibody responses, we have extended our studies to the murine system. BALB/c Th cell lines (TCL), specific for either the Mycoplasma arthritis-derived SA or the Staphylococcus aureus-derived toxic shock syndrome toxin-1) were generated. These TCL cells are SA-specific, functionally noncross-reactive, and utilize distinct TCR V beta gene families. Coculture of SA-reactive TCL cells and syngeneic B cells bearing the relevant SA results in B cell proliferation and polyclonal IgM and IgG production. In contrast, Ag-specific (SRBC-specific) antibody-forming cells are only generated in cultures that also contain SRBC. Thus, microbial SA-mediated Th-B cell interactions induce both polyclonal B cell activation and provide selective help for the proliferation and/or differentiation of B cells that have encountered specific Ag. In additional studies, we determined that the in vivo administration of toxic shock syndrome toxin-1 to young, athymic (nude) BALB/c mice results in SA binding to splenic B cells, rendering these B cells effective stimulators of and targets for SA-reactive helper TCL cells. Taken together, these results demonstrate that microbial SA mediate productive Th-B cell interactions analogous to those that occur during allospecific Th-B cell interactions in vitro and GVHD in vivo. These findings are consistent with the hypothesis that microbial SA represent environmental factors that may trigger autoimmune disease in the genetically susceptible host.