The induction of skin xenograft tolerance in rat-to-mouse combination could be affected by DFR mediating cells and antibodies against rat bone marrow cells as well as NK cells in the cyclophosphamide-induced tolerance system.

We investigated whether the prolongation of skin xenograft survival was obtained by a tolerance-inducing method using cyclophosphamide (CY), by which long-lasting skin allograft tolerance could be induced. The long-lasting skin allograft survival could be obtained in the recipient C3H/HeN (C3H) mice which were given 100 micrograms of anti-CD4 mAb on day -3, 1 x 10(8) spleen cells (SC) plus 3 x 10(7) bone marrow cells (BMC) derived from C57BL/6 (B6) mice on day -2,200 mg/kg CY on day 0, and which were grafted with allogeneic B6 skin on day 14. When the C3H mice were treated with anti-CD4 mAb, 1 x 10(8) s.c. plus 5 x 10(7) BMC derived from F344 rat and CY, the F344 skin grafts survived slightly longer (about 15 days) than those in untreated recipients (about 8.4 days). Such a prolongation of skin xenograft survival was considered donor-specific because rejection of 3rd party skin grafts from BN rats occurred significantly earlier than that of F344 skin grafts. In the recipient C3H mice treated with anti-CD4 mAb, F344 s.c. plus BMC and CY, mixed chimerism in the periphery was detected for a few days after CY administration, although intrathymic chimerism was not detected throughout this study. In these recipient C3H mice, cytotoxic T lymphocytes (CTL) against F344 antigens were completely abrogated through the delayed footpad reaction (DFR) remained at a low but significant level. Moreover, though antibody (Ab) activity against F344 s.c. was completely abrogated, neither Ab activity against F344 BMC, which seemed to have a background common to natural Ab activity, nor NK activity were abrogated by this treatment. These results suggested that DFR mediating cells directly mediated skin xenograft rejection in the recipient mice treated with anti-CD4 mAb, F344 cells, and CY. Such cells may interfere with establishment of mixed chimerism and long-lasting skin xenograft tolerance, presumably in cooperation with CY-resistant Ab activity and NK cells.