Efficacy and Limitations of Natural Killer Cell Depletion in Cyclophosphamide-Induced Tolerance

Purpose We previously developed a cyclophosphamide (CP)-induced tolerance protocol, consisting of an intravenous injection of 1 × 10⁸ donor spleen cells (SC) given on day 0 and an intraperitoneal injection of 200 mg/kg CP given on day 2. In the present study, we modified this protocol with natural killer cell (NK) depletion in recipient mice, and evaluated the efficacy of tolerance induction. Methods We used B10.D2 (H-2^d; IE⁺) and B10 (H-2^b; IE⁻) mice as both donors and recipients. The recipient mice were treated with donor SC, CP, and donor bone marrow cells (BMCs) with or without NK depletion. Results A higher level of mixed chimerism was achieved in the NK-depleted recipients. Survival of both the skin and heart donor grafts was significantly prolonged in the NK-depleted recipients. Donor reactive Vβ11⁺ T cells were found at the same level as in untreated control mice. Pretreatment with recipient NK cell depletion was effective in inducing higher levels of donor mixed chimerism; however, permanent engraftment of donor bone marrow was not achieved. Conclusion Survival of donor grafts was remarkably prolonged in the NK cell-depleted group, but transplantation tolerance could not be induced. Our results suggest that NK cell depletion in CP-induced tolerance conditioning has some effect on the induction of donor-specific tolerance.