Background Donor organ rejection continues to be a significant problem for patients receiving transplants.We therefore tested whether transferring a donor's major histocompatibility complex (MHC) gene to the recipient would mitigate the rejection of transplanted hearts in mice.Methods H-2Kkgene from donor mice was amplified using nested polymerase chain reaction (PCR) and ligated into a mammalian expression vector,which was then transfected into thymus ground mass cells collected from the recipients.Clones stably expressing the transgene were then injected into the recipients' thymus visualized using ultrasound.Control mice were administered cells previously transfected with empty vector.Following heart transplantation,cardiac activity was monitored electrocardiographically.Recipient thymus cells were tested for MHC antigenicity using flow cytometry and spleen cells were subjected to mixed lymphocyte culture tests.Finally,the transplanted hearts were sectioned,stained and examined under light microscopy.Results Southern analysis following nested PCR revealed clear expression of H-2Kk gene.Following transplantation,electrocardiosignals were detectable highly significantly longer in recipients administered thymal cells expressing donor H-2Kk than in those receiving control cells.Flow cytometric analysis using an anti-H-2Kk antibody confirmed its expression in H-2Kk treated recipients but not in control mice.Mixed lymphocyte cultures containing H-2Kk treated cells showed significantly less proliferation than those containing control cells.Hearts from control mice showed substantially greater lymphocyte infiltration than those from H-2Kk treated mice and large areas of necrosis.Conclusion Rejection of transplanted hearts can be mitigated substantially by introducing the donor's MHC into the recipient.