CELLULAR IMMUNE-RESPONSES OF HUMAN CADAVER DONOR BONE-MARROW CELLS AND THEIR SUSCEPTIBILITY TO COMMONLY USED IMMUNOSUPPRESSIVE DRUGS IN TRANSPLANTATION

Background. The cascade of immunological effects brought about by dono r bone marrow cell (DBMC) infusions in human organ transplantation, es pecially in the context of continuous pharmacologic immunosuppression, is not fully understood. Yet, in inbred rodents and even primates, ad ministration of specific bone marrow cells has caused a state of acqui red immunologic tolerance. Methods. In vitro mixed lymphocyte culture (MLC) and cell-mediated lympholysis (CML) culture systems were used to compare the responding and regulatory properties of DBMC and individu al bone marrow cell subsets versus spleen cells in the presence or abs ence of pharmacologic immunosuppression. Results. In the absence of im munosuppressive drugs, the DBMC proliferated in MLC and in response to phytohemagglutinin, but to a lower magnitude than donor spleen cells. In CML assays, DBMC failed to function as cytotoxic cells. Removal of both CD3(+) and CD34(+) cells together (not just singly) had to occur for complete abrogation of the proliferative response of DBMC evoked in the presence of allogeneic stimulating cells. Testing several exper imental variables using flow cytometric analysis led to the conclusion that when purified DBMC CD34(+) cells were placed in coculture with i rradiated allogeneic peripheral blood mononuclear cells, such CD34(+) cells give rise both to CD3(-) TCR alpha beta(+) as well as to dimly s taining CD3(+) TCR alpha beta(+) cells. Low pharmacologic concentratio ns of tacrolimus/cyclosporine (CsA) and mycophenolic acid (MPA) singly or in combination had no effect on the spontaneous proliferation of D BMC and had significantly less inhibitory activity on MLC responses of DBMC and its purified CD3(+) or CD34(+) subpopulations, compared with the responses of spleen cells. Moreover, the previously described reg ulatory effects of DBMC on the MLC responses of peripheral blood or sp lenic responding cells were not inhibited by these immunosuppressive d rugs. Conclusions. Taken together, these results support the notion th at in vitro DBMC subpopulations, which proliferate as responding cells in co-culture with x-irradiated allogeneic cells and which cause regu latory effects when added as a third component to MLC reactions, seem to be culture-generated lymphoid cell lineage(s) progeny of CD34(+) ce lls. This possibly includes unique CD3(+) ''primitive'' (dimly stainin g) T cells, which are not as inhibited in their function by tacrolimus /CsA and MPA, as are postthymic (splenic) T cells.