Transfer of Endogenous Retroviral Superantigen from Donor to Recipient B Cells Following Priming to Induce Peripheral T Cell Tolerance

Endogenous retroviral superantigens such as vSAG-7 are highly stimulatory for T cells through interaction with the T cell receptor on the basis of Vβ usage. Priming of adult MMTV 7-negative mice with vSAG-7-expressing cells has been shown to result in peripheral Vβ6/CD4+ T cell activation followed by tolerance to further interaction with the superantigen. The goal of the current study was to examine the cells presenting vSAG-7 during this initial burst of in vivo T cell activation. Priming of MMTV 7-negative BALB/c (H-2d) mice with DBA/2 (H-2d, MMTV 7+) spleen cells resulted in a 5- to 12-fold increase in the number of B cells in the lymph nodes. These B cells expressed increased levels of I-A and I-E class II MHC determinants. Use of MMTV 7-negative CB.17 (H-2d, Ighb) mice as recipients of DBA/2 (Igha) cells indicated that the increased number of B cells was of host, rather than donor, origin. The number of donor-derived (IgM/B220+) B cells observed during the course of vSAG-7-reactive Vβ6/CD4+ T cell activation was very low. Proliferation of unprimed T cells from MMTV 7-negative mice was induced during coculture with B cells from the lymph nodes of vSAG-7-primed recipients and was blocked by anti-class II MHC antibodies, as well as by an anti-vSAG-7 antibody. Highly purified host B cells from vSAG-7-primed recipients specifically stimulated the blastogenesis of Vβ6/CD4+ T cells in vitro. Collectively, the results indicate that following priming to induce peripheral tolerance, vSAG-7 is transferred from donor cells to class II MHC determinants on recipient B cells and is presented to the T cell repertoire by the autologous B cells.