The role of regulatory cells in miniature swine rendered tolerant to cardiac allografts by donor kidney cotransplantation

To determine the mechanism by which cotransplantation of a kidney allograft induces tolerance to a donor heart in miniature swine, we examined the role of CD25⁺ cells in heart/kidney recipients. Tolerance was induced to class-I MHC mismatched hearts by cotransplanting a donor-specific kidney with a 12-day course of cyclosporine. Peripheral blood leukocytes (PBL) were isolated from tolerant heart/kidney recipients and used in cell-mediated lympholysis (CML) coculture assays as either unmodified PBL, PBL enriched for CD25⁺ cells or PBL depleted of CD25⁺ cells to assess their ability to suppress CML responses of naïve recipient-matched leukocytes against mismatched target cells. Primed PBL from tolerant heart/kidney recipients completely suppressed lysis by naïve cells. Complete suppression of the response of naïve recipient-matched leukocytes against donor-matched target cells was lost following the depletion of CD25 ⁺ cells from tolerant heart/kidney animal PBL, but it was reestablished by incubation of naïve cells with small populations of CD25⁺ cells from tolerant heart/kidney animals. These data suggest that peripheral blood from tolerant heart/kidney recipients contains regulatory cells that, upon priming, can suppress the response of naïve-matched PBL in coculture CML assays, and that suppression appears to be dependent on cells expressing CD25.