The persistence of regulatory cells developing after rat spontaneous liver acceptance

We have previously reported that the spontaneous acceptance of Lewis (LEW, RT1 l) to Dark Agouti (DA, RT1 a) rat orthotopic liver transplant (OLT) is eliminated by donor γ-irradiation. The acceptance of the irradiated LEW liver is also reestablished in a naïve rat after the adoptive transfer of T regulatory (T-reg) cells from a LEW to a DA liver–tolerant long-term (>60 days) survivor (LTS) into a naïve DA rat. However, little is known about the growth conditions required to maintain T-reg function. In this study, we examined the need for continued donor-specific alloantigen stimulation for the maintenance and function of T-reg cells. Splenocytes (SCs; 1.5 × 10 8 cells) from a LEW liver allograft–tolerant LTS DA recipient were adoptively transferred fresh or after in vitro stimulation into another naive DA rat on day 1, 4, or 7 before an irradiated (1000R) LEW liver transplant. For in vitro alloantigen stimulation, SCs from LEW to DA LTS were co-cultured with mitomycin-C (MMC)–treated naïve LEW (donor alloantigen–specific) or Brown Norway (BN) (RT1 n; third party) SCs for 72 hours. Graft rejection, as defined by death of the recipient, was confirmed histologically. All LEW liver grafts were accepted spontaneously by DA recipients for more than 60 days (n = 32), while all irradiated LEW livers were acutely rejected (n = 9; mean survival time [MST] = 12.8 ± 4.0 days). When LTS DA SCs were adoptively transferred into a naive DA rat 1 day before OLT, all irradiated LEW grafts were accepted greater than 60 days (n = 9). However, when fresh LTS DA SCs were transferred to a new naïve DA rat on 4 or 7 days before OLT, all irradiated LEW liver grafts were acutely rejected (MST = 10.2 ± 0.5 days [n = 4] and MST = 13.5 ± 5.0 days [n = 4], respectively). When LTS DA SCs were stimulated in vitro before adoptive transfer, irradiated LEW liver grafts after 4 days (n = 5) were then accepted. In vitro culture of LTS DA SCs with MMC–treated BN SCs (third-party) for 72 hours before adoptive transfer resulted in 3 of 5 irradiated LEW livers at day 4 being accepted (n = 5). The maintenance of T-reg function requires continuous LEW donor–specific alloantigen stimulation.