Prolongation of Cardiac Allograft Survival by Selective Injection of Donor Liver Leukocytes in Non-Immunosuppressed Rats

Liver grafts are spontaneously accepted in several animal combinations and are able to induce acceptance of another organ originating from the same donor, which would be rejected when transplanted alone. However, the exact mechanism of this unique tolerance induction capability remains unclear. The aim of our study was to investigate the ability of nonparenchymal liver cells to induce tolerance when they were separated from their parenchymal environment. In the murine combination we used (BN --> LEW), heart transplants were constantly tolerated after combined liver plus heart grafting, but rejected when transplanted alone. Nonparenchymal liver cells were isolated from BN rat livers by enzymatic digestion and injected, at different times, to LEW rats, which were recipients of BN heart transplants. The average number of mononuclear cells obtained after isolation was 20 x 10(6)/5 g of rat liver. Immediate trypan-blue exclusion test showed more than 95% of viable cells. Phenotypic studies showed a predominant (47%) lymphocyte population, 7% were monocytes and 46% were cellular debris. Among the lymphocyte population, the majority of cells were bearing the NKR-P1 receptor and about 30% CD3 receptors. Inoculation of nonparenchymal liver cells 7 and 30 days prior to heart transplantation significantly prolonged graft survival compared to controls (14.6 and 12.7 vs. 8.1 days; p = 0.0008 and 0.0059, respectively), whereas simultaneous injection (day 0) had no effect. Injection of donor splenocytes or nonparenchymal liver cells from a third party, at any time, had no effect on rejection. These results provide some more evidence about the specific role of liver lymphocytes in allogenic unresponsiveness. They also suggest that the hepatic parenchymal environment is necessary for the optimal development of this phenomenon.