Pretransplant donor peripheral blood mononuclear cells infusion induces transplantation tolerance by generating regulatory T cells

Background. It was suggested that maintenance of tolerance to organ transplantation may depend on the formation of T regulatory cells. Methods. Lewis (LW) rats were made tolerant to a Brown Norway kidney by pretransplant donor peripheral blood mononuclear cells (PBMC) infusion. At greater than 90 days after transplantation, lymph node cells (LN) and graft-infiltrating leukocytes (GIL) alloreactivity was tested in mixed lymphocyte reaction (MLR), coculture, and transwell experiments. GIL phenotype was analyzed by FACS. mRNA expression of cytokines and other markers was analyzed. on CD4(+) T cells from LN. The tolerogenic potential of tolerant cells in vivo was evaluated by adoptive transfer. Results. Tolerant LN cells showed a reduced proliferation against donor stimulators but a normal anti-third-party alloreactivity. In coculture, these cells inhibited antidonor but not antithird-party reactivity of naive LN cells. Interleukin (IL)- 10 and FasL mRNA expression was up-regulated in tolerant CD4(+) T cells, but an anti-IL- 10 monoclonal antibody (mAb) only partially reversed their inhibitory effect. Immunoregulatory activity was concentrated in the CD4(+)CD25(+) T-cell subset. In a transwell system, tolerant T cells inhibited a naive MLR to a lesser extent than in a standard coculture. Regulatory cells transferred tolerance after infusion into naive LW recipients. CD4(+) T cells isolated from tolerized grafts were hyporesponsive to donor stimulators and suppressed a naive MLR against donor antigens. Conclusions. Donor-specific regulatory T cells play a role in tolerance induction by donor PBMC infusion. Regulatory activity is concentrated in the CD4(+)CD25(+) subset and requires cell-to-cell contact. Regulatory CD4(+) T cells accumulate in tolerized kidney grafts where they could exert a protective function against host immune response.