Background. In order to evaluate whether immuno-regulatory mechanisms are brought about by human donor bone marrow cell infusions accompanying organ transplantation, we established in vitro culture systems analogous to the transplant model. Methods. Cell-mediated lympholysis (CML) and mixed lymphocyte culture (MLC) responses of peripheral blood lymphocytes or spleen cells stimulated with irradiated cadaver donor spleen cells in the presence of specific donor vertebral-body bone marrow cell (DBMC) modulators were tested. Results. When compared with spleen cells as modulator controls, DBMC inhibited both the proliferative and cytotoxic responses in a dose-dependent manner. Use of unirradiated and T cell-depleted DBMC was required for detection of the inhibitory activity. Furthermore, DBMC had to be added within the first 2 days after the initiation of the cultures for the down- regulation of CML (MLC) to occur. The down-regulation of MLC responses could not be shown to be antigen (donor) specific. Physical separation of DBMC from the responder-stimulator cells using the trans-well system abrogated modulation of the CML (and MLC) reactions, suggesting the requirement of cell-cell contact for modulatory effect. The inhibitory activity by DBMC could not be overcome by addition of up to 200 U/ml of exogenous recombinant interleukin 2 to the cultures. However, it could be abrogated by restimulation with donor spleen cells, indicating that donor reactive cells were not deleted by DBMC in short-term cultures. Conclusions. These results showed a regulatory role for DBMC in cellular immune responses against donor cell alloantigens, supporting the rationale for DBMC for facilitating clinical allograft acceptance.
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