The study of peripheral blood microchimerism in kidney transplantation

Beneficial effects of pretransplantation microchimerism on rejection-free survival in HLA-haploidentical family donor renal transplantation

BACKGROUND

Fetal-maternal microchimerism (MC) can develop during pregnancy and may persist for decades. Pretransplantation fetal-maternal MC may be present between mother and child and between siblings; however, its effect on renal transplantation is not known. We investigated the effects of pretransplantation MC on allograft outcomes in human leukocyte antigen (HLA)-haploidentical family donor transplantation.

METHODS

A total of 106 cases transplanted from 1996 to 2004 were retrospectively studied, with median follow-up of 96 months. The study and control groups included 63 and 43 cases of HLA-haploidentical and HLA-identical donor transplantations, respectively. MC against mismatched donor HLA-DRB1 allele was detected in the recipient's peripheral blood using nested polymerase chain reaction-single-strand conformation polymorphism method. The allograft outcomes of HLA-haploidentical MC (+) and (-) subgroups were compared with those of HLA-identical group.

RESULTS

Pretransplantation MC in the HLA-haploidentical recipients was detected in 22.2% (14 of 63). Compared with HLA-identical group, MC (-) subgroup showed significantly inferior allograft outcomes: higher acute rejection rate (11.6% vs. 42.9%; P=0.001), higher 5-year serum creatinine level (1.1 vs. 1.4 mg/dL; P=0.009), and lower 10-year rejection-free survival rate (83.7% vs. 54.5%; log-rank P=0.001). In contrast, MC (+) subgroup showed no significant differences from HLA-identical group in acute rejection rate (14.3%), 5-year serum creatinine level (1.1 mg/dL), and 10-year rejection-free survival rate (85.7%). Multivariate analysis revealed that pretransplantation MC is associated with a significantly lower risk of acute rejection (odds ratio=0.10; P=0.021).

CONCLUSION

Pretransplantation MC present in the recipient may have beneficial effects on rejection-free allograft survival in renal transplantation.

Microchimerism evaluation in recipients of living-related or unrelated deceased allograft renal transplants

The presence of microchimerism in the peripheral blood of solid organ graft recipients has been associated with long-term solid organ acceptance, immunologic tolerance, and less aggressive immunosuppressive therapy. Molecular biology assays are among the most sensitive methods to detect microchimerism, primarily to evaluate Y chromosome sequences in females as indirect evidence of circulating male nucleated donor cells. We screened for the presence of the SRY sequence region in peripheral blood of 13 female recipients of male kidney grafts: 5 living-related and 8 deceased grafts. Only patients who received grafts from related living donors exhibited microchimerism. Five of 13 patients studied exhibited better graft outcomes, including the 4 who were positive for the SRY sequences.

DNA released from rejecting organs is an indicator of the degree of graft cellular damage

Warm and cold ischemia as well as rejection of the transplanted organ or tissue cause destructive changes in the graft parenchyma. Fragments of disintegrated cellular organelles are phagocytized and digested by recipient scavenger cells in lymph nodes, spleen, and liver. Some fragments engulfed by dendritic cells are processed including donor DNA present in the ingested cellular debris. The question arises as to whether the DNA from the disintegrated cells may be used as a measure of graft damage. In this study we provide evidence that both syngeneic and allogeneic organ transplantation followed by "seeding" of donor DNA from graft cells is internalized in recipient macrophages and dendritic cells in lymphoid organs. The kinetics of accumulation of donor DNA in recipient tissues reflected the degree of ischemic and immune graft damage. Immunosuppression with cyclosporine or tacrolimus did not significantly attenuate the DNA release. Measurements of the concentration of donor DNA gives insight into the kinetics of allograft rejection. Real-time polymerase chain reaction for donor DNA in recipient serum and blood leukocytes that have engulfed donor cell debris may be useful for clinical diagnostic application.

Transplantation of organs is transplantations of donor DNA: fate of DNA disseminated in recipient

Microchimerism after allogeneic organ transplantation has been widely documented using DNA identification techniques. However, the question as to whether the detected donor DNA is present in the surviving donor passenger cells, recipient macrophages phagocytizing rejected donor cells, or dendritic cells (DC) internalizing donor apoptotic bodies or cell fragments has not been answered. We provide evidence that allogeneic organ transplantation is followed not only by cellular microchimerism caused by release of graft passenger cells but also dissemination of donor DNA from the ischemic rejecting graft cells and its internalization in recipient DC. The high levels of donor DNA at the time of heart rejection were inversely proportional to the concentration of donor passenger cells detected with use of flow cytometry. Depending on the type of graft, the kinetics of DNA distribution in recipient tissues were different. Immunosuppressive drugs attenuated the rejection reaction and release of DNA from grafts. Allogeneic but not syngeneic donor DNA fragments were found in recipient splenic DC-enriched population. Interestingly, that donor DNA fragments could be detected in recipient tissue at high levels on day 30. This challenges the notion that fragments of DNA are immediately cleaved by cell plasmatic enzymes. The biologic significance of our findings is not clear. We speculate that donor DNA fragments in recipient DC may play a, so far unknown, role in the immunization/tolerance process to allogeneic antigens.