Induction of Germinal Centers by MMTV Encoded Superantigen on B Cells
It has not been established whether an endogenous superantigen (SAg) expressed on B cells can induce germinal centers (GCs). An interesting model is that of mammary tumor virus encoded viral SAgs, which induce vigorous T cell proliferation and are predominantly expressed on activated B cells. We have used this model to analyze the possibility that direct stimulation of Mtv7+ DBA/2 B cells by vSAg-responsive (Vβ6+) BALB/c T cells can give rise to GCs. Injection of BALB/c SCID mice iv with 2 × 106 DBA/2 B cells, together with LPS, followed by 2 × 106 BALB/c T cells induces numerous large splenic GCs within 3–5 days. The GCs are still large on day 7, but are very much reduced by day 10. B cell activation with LPS is needed for this effect. These GCs form in spite of the apparent absence of follicular dendritic cells (FDCs) as judged by staining for several FDC surface markers. Control mice receiving either BALB/c T or DBA/2 B cells + LPS alone or DBA/2 T + B cells + LPS fail to exhibit any GCs on days 3–7. Numerous small clusters of PNA+ cells, but few large GCs are observed when TNF-R(p55)-Ig is also injected, whereas LTβR-Ig treatment impeded the formation of aggregations of these cells even further, leaving scattered PNA+ single cells and very small clumps throughout the white pulp of the spleens. Anti-TNFα had no effect. These results suggest that endogenous vSAg mediated GC formation is independent of antigen trapping by FDCs.