Background. Continued follow-up of a series of donor bone marrow cell (DBMC)-infused first cadaver renal transplant recipients is described (n=58), now at 36-month actuarial time point postoperatively. Serial polymerase chain reaction-flow cytometry (PCR-Flow) and cellular immune assays of iliac crest bone marrow aspirates and peripheral blood have begun to be compared with concomitantly transplanted recipients of living-related donor (LRD) kidneys and donor marrow infusions given the same immunosuppressive regimen (n=16). There have also been comparisons (36 months) with 188 controls transplanted concomitantly, i.e., recipients of first cadaver kidney transplants, who did not receive bone marrow. Methods. Each group was given equivalent immunosuppressive regimens of OKT3 anti-T cell induction and maintenance tacrolimus, mycophenolate mofetil, and methylprednisolone. Actuarial patients and graft survival have been 96% and 93%, respectively, in the controls and 91% and 91%, respectively, in the DBMC-infused recipients. Trough levels of tacrolimus were significantly lower in the DBMC-infused group. Results. In PCR-Flow measurements, in peripheral blood up to 6 months postoperatively, there were higher levels of chimerism, i.e., in the total number of donor cells, as well as the donor CD3+ and CD34+ subsets in the LRD recipients administered DBMC infusions, compared with cadaver DBMC recipients, supporting the notion of a positive effect of histocompatibility on chimerism levels. In PCR-Flow measurements of recipient iliac crest bone marrow aspirates as in previous studies on peripheral blood, early acute rejection episodes (<1 month) were found to be associated with a later (6-14 months) decrease in donor cell lineage chimerism. However, a trend toward recovery of chimeric levels occurred by 21-28 months in a second iliac crest marrow aspirate 1 year after the first aspirate in the DBMC-infused recipients who experienced in such early rejection episodes. This was in contrast to the controls in whom there were sustained low levels of iliac crest bone marrow chimerism at both the earlier and later intervals (i.e., no chimeric recovery), with 17/183 surviving controls progressing into chronic rejection. This has not yet been seen in the DBMC-infused group (0/54). In in vitro observations on cellular immune reactivity at 1 year postoperatively, decreased peripheral blood lymphocyte proliferative reactions were seen in response to phytohemagglutinin and Staph-A mitogens, as well as to cytomegalovirus and Epstein-Barr viral protein antigens in the DBMC-infused group versus the controls. Chronic immunosuppression did not seem to effect a vigorous in vitro inhibitory (regulatory) activity of bone marrow taken from these transplant recipients 2 years postoperatively in mixed lymphocyte culture and cell-mediated lympholysis reactions, using allogeneic responding cells from 'normal' laboratory volunteers. Autologous peripheral blood lymphoproliferative responses to phytohemagglutinin and Staph-A mitogens, as well as to cytomegalovirus and Epstein-Barr virus protein antigens, were also regulated by either organ donor (non-immunosuppressed) bone marrow cells or by transplant recipients (immunosuppressed) bone marrow cells. What appeared to be disparate between the DBMC-infused and control groups (both immunosuppressed) was the trend for the (autologous) bone marrow suppressive effect on antiviral lymphoproliferative responses, to be stronger in the DBMC-infused group, who also had significantly (> one order of magnitude) higher levels of chimerism (P=0.01). Conclusions. It is concluded that the establishment of a chimeric state in DBMC-infused recipients, albeit of relatively low magnitude (~1% at 2 years in recipient iliac crest bone marrow), has had a definite regulatory effect on immune responses. These results, therefore, add weight to the 'causal' horn of the dilemma as to whether donor cell chimerism is a cause or an effect of organ transplant acceptance.
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