Innate Immunity Is Responsible for Early Allorejection of Donor Bone Marrow Cells.

Abstract 3523

Poster Board III-460

Adaptive immunity, especially T cells, has long been believed to be the dominant immune response in allogeneic transplantation. However, innate immunity has been recently shown to pose a significant barrier to the induction of tolerance to solid organ transplants. The role of the innate immune system in bone marrow cell alloreactivity has not been addressed. The innate immune system provides the first line of defense in the removal of pathogens because of the delay in generation of adaptive immune responses. Our present findings show that innate immunity is a significant first line barrier in bone marrow cell (BMC) rejection. To determine the effect of innate immune cell populations on rejection of donor BMCs, T cell deficient mice (TCR β/δ^−/−) were used as BMC recipients in in vivo cytotoxicity assays (Figure A). TCRβ/δ^−/− mice have normal innate immune cell populations but do not initiate adaptive cell-mediated cytotoxic or humoral responses. 20 × 10⁶ CFSE labeled donor (high CFSE fluorescence intensity) and recipient control splenocytes (low CFSE fluorescence intensity) were injected into TCRβ/δ^−/− and wildtype control B6 recipient mice. Donor cell survival was compared over time. Donor BALB/c splenocytes were eliminated in wildtype B6, with rejection complete by day 3. The kinetics of elimination of donor cells in TCRβ/δ^−/− mice was similar to that for wildtype B6 controls, with donor cells eliminated by day 3. These results indicate that early rejection of the splenocytes in wildtype mice was T cell-independent. The acute rejection of BMC in wildtype B6 recipients occurred within 3 days, which is prior to the time required for T cell activation. Thus the effectors mediating BMC rejection would be the innate immune cells: macrophages, neutrophils, or NK cells. To rule out potential involvement of natural Abs in the cytotoxicity we observed in the TCRβ/δ^−/− mice, Rag^−/− mice were used as recipients (Figure B). Rag^−/− mice do not produce mature T cells or B cells. 20 × 10⁶ CFSE labeled donor (high CFSE intensity) and recipient control BM cells (low CFSE intensity) were injected into Rag^−/−mice. Rag^−/− and wildtype B6 control mice exhibited similar kinetics of donor cell cytolysis. The rapid elimination of allogeneic cells from immunocompetent mice is comparable with that observed in T cell- (TCRβ/δ^−/−) or T and B cell- (Rag^−/−) deficient mice indicates that allogeneic cells are subject to T cell-independent rejection at the early time period after cell infusion (≤ 3 days). As the kinetics of cytotoxicity were similar in experiments using either splenocytes or BMCs as target cells in our later experiments, our data suggest that the innate immune system is responsible for early allorejection of donor BMC at the early inductive period for adaptive immunity. These findings may have significant impact on the development of immune-based nonmyeloablative conditioning strategies and show for the first time that a dominant factor in BMC rejection is contributed by innate immune responses.

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