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Donor bone marrow potentiates the effect of tacrolimus on nonvascularized heart allograft survival: association with microchimerism and growth of donor dendritic cell progenitors from recipient bone marrow.

BACKGROUND
The influence of donor hematopoietic cell microchimerism on organ allograft survival has been studied largely in vascularized transplant models. Here, we examine the impact of donor bone marrow (BM) cells administered intravenously together with transient systemic tacrolimus therapy on microchimerism, the survival of nonvascularized cardiac allografts, and growth of donor antigen-presenting cells [dendritic cells (DCs)] from recipient BM.


METHODS
Adult male C3H (H2k) mice received heterotopic heart transplants from B10 (H2b) donors in the dorsal ear pinna. They were given no further treatment, or either a short course of tacrolimus (FK506; 2 mg/kg i.p. from day 0 to day 13), unmodified donor BM cells (50x10(6) i.v. on day 0) or both treatments. Grafts were examined daily for contractile activity. Anti-donor cytotoxic T lymphocyte responses were determined in recipients' spleens. Microchimerism (IAb+ cells) was demonstrated by immunocytochemical staining of spleens, and of cells expanded from recipient BM using cytokines and culture conditions that promote the growth of DCs.


RESULTS
Tacrolimus alone significantly prolonged median heart graft survival time from 10 to 22 days (P<0.001). BM alone failed to prolong graft survival. By contrast, tacrolimus + donor BM resulted in a mean survival time of 42 days (P<0.01 compared with tacrolimus treatment alone). This marked increase in heart allograft survival was associated with reduced anti-donor cytotoxic T lymphocyte responses attributable to a nonspecific effect of tacrolimus. In addition, however, a link was observed between the beneficial effect of donor BM and comparatively large numbers of donor major histocompatibility complex class II (IAb+)-positive cells in recipients' spleens, and in cultures of granulocyte-macrophage colony-stimulating factor + interleukin-4-stimulated DCs from recipients' BM. No donor-derived cells were propagated from heart graft recipients given either tacrolimus or donor BM alone.


CONCLUSIONS
This nonvascularized organ transplant model demonstrates the positive effect on allograft survival of donor BM given at the time of transplant to transiently immunosuppressed recipients. The findings also reveal links between hematopoietic cell chimerism, the presence of donor DC progenitors in recipient BM, and organ allograft survival.