Prevention of graft-versus-host disease and the induction of transplant tolerance by low-dose UV-B irradiation of BM cells combined with cyclosporine immunosuppression.

GVHD is prevented and stable chimerism is induced in the rat BMT model by 700 J/m2 but not 100-500 J/m2 UV-B irradiation of allogeneic BM cells. Paradoxically, CsA which prevents GVHD in clinical BMT causes an aggressive autoimmune disease termed syngeneic GVHD in irradiated syngeneic BMT recipients after its withdrawal. Recently, we have shown that while 500-700 J/m2 UV-B irradiation of syngeneic BM cells combined with a 30-day course of CsA recipient immunosuppression impairs hemopoiesis due to lack of hemopoietic factors, a low dose of 100-300 J/m2 UV-B is effective in preventing CsA-induced autoimmune disease without endangering BM engraftment. This study extends these findings to the P-to-F1 hybrid and fully allogeneic rat BMT models and examines the effectiveness of low-dose UV-B irradiation of BM cells combined with a short course of CsA treatment in the prevention of GVHD and induction of transplant tolerance. Lethally gamma-irradiated (10.5 Gy) LBNF1 recipients of naive or UV-B irradiated (100-700 J/m2) BMT were treated with CsA (12.5 mg/kg/day) for 30 consecutive days after BMT. All lethally irradiated LBNF1 that did not receive BMT died in < 16 days, while animals transplanted with UV-B (700 J/m2) BMT survived > 1 year without GVHD. In contrast, all recipients of naive BMT died of lethal GVHD in < 50 days. Similarly, all recipients of naive BMT that received a 30-day course of CsA therapy developed severe GVHD with 60% mortality after cessation of CsA therapy. CsA-treated recipients of BMT irradiated with 700 J/m2 died between 12 and 25 days from failure of hemopoiesis. In contrast, CsA-treated recipients of 100-200 J/m2 UV-B irradiated BMT showed full BM engraftment without GVHD after cessation of CsA and survived > 1 year. These results were reproducible in the fully allogeneic UV-B BMT model. To test for donor-specific tolerance, the animals challenged 100 days after BMT with cardiac allografts accepted permanently (> 100 days) Lewis but not BN (non-BMT parental donor) cardiac allografts. Our results confirm that 700 J/m2 UV-B irradiation of BM cells combined with CsA recipient immunosuppression impairs the recovery capacity of stem cells while the use of lower UV-B (100-200 J/m2) is effective in preventing CsA-induced autoimmune disease without endangering BM engraftment and leads to induction of transplant tolerance.