Difficulty of achieving long-term graft survival of MHC-disparate composite graft using CTLA4IG.

Because of recent advances in immunosuppressive drugs and microsurgical techniques, human hand transplantation has been attempted. However, the composite tissue has the strongest antigenicities in the body organs, and then requires heavy immunosuppressant therapy (1, 2). As a strategy for overcoming this aspect, blockade of the co-stimulatory pathway such as B7-CD28 or CD40-CD154 may reduce the need for chronic use of immunosuppressive agents, as shown experimentally (3, 4). We are interested in a recent study by Iwasaki et al. (5). These investigators showed experimentally the immunosuppressive potential of cytotoxic T-lymphocyte antigen-4 immunoglobulin (CTLA4Ig) against the allograft rejection of rat composite tissue. Survival times of composite allograft in a high responder combination of ACI (major histocompatibility complex haplotype; RT1) to LEW (RT1) were significantly prolonged in the rats that received a single dose of CTLA4Ig on day 2 after transplantation (mean SD, 20.5 1.0 days) compared with that of a control with human immunoglobulin G (mean SD, 8.4 0.9 days) (5). We also confirmed the effect of CTLA4Ig on composite tissue allograft in the similar high responder combination of DA (RT1) to LEW using adenovirus-mediated CTLA4Ig gene transfer. However, the grafts were rejected at days 28 and 36 after limb transplantation (unpublished data). The further study of composite tissue allograft transplantation to induce permanent acceptance without chronic use of immunosuppressants should be the goal (6). Here, we tested the combination effect of tacrolimus plus CTLA4Ig on this composite tissue allograft. Nonviral gene transfer to the graft was attempted because of a great advantage for compromised patients with chronic immunosuppression. In the separated issue, we have developed a novel method for enhancing protein expression in the rat muscle by using a nonviral, particle-mediated gene gun (7). Using this procedure, we attempted to induce the CTLA4Ig gene into the muscle and skin of the donor limb. Male adult DA and LEW rats were used as donor and recipient, respectively. Tacrolimus (0.64 mg/kg) was injected intramuscularly for 14 days postoperatively. The composite allograft received combination therapy of tacrolimus plus the CTLA4Ig gene and showed a significant prolongation of graft survival (32.5 2.1 days) compared with that of controls (5.3 1.0 days, P 0.001) or with tacrolimus treatment alone (26.2 1.1 days, P 0.001). A synergistic effect of tacrolimus with CTLA4Ig was observed, but we could not achieve a longsurviving composite tissue allograft. Finally, we would like to ask Iwasaki et al. (5) what the serum CTLA4Ig level in their model was. Our result showed that local gene therapy of CTLA4Ig using a gene gun could permit the prolonged survival of composite allograft, whereas concentrations of CTLA4Ig in these rats was low. We considered that the localized expression of the CTLA4Ig gene in the donor limb might provide a benefit for controlling the local immune response. However, we also felt insufficiency of costimulatory blockade for tolerance induction of major histocompatibility complex-disparate composite graft.