Intrathymic inoculation of donor antigen: an ineffective strategy for prolonging xenograft survival

The aim of this study was to determine whether intrathymic inoculation of t he recipient with donor antigen and short-term depletion of peripheral lymp hocytes would lead to donor-specific unresponsiveness to rat pancreatic isl et xenografts. The results were compared directly with an allograft model t o determine whether there were substantial differences in the mechanisms of graft prolongation between allografts and xenografts using an identical co nditioning regimen. Streptozotocin-induced diabetic C57B6 mice were injecte d with up to 0.3 ml of rat anti-mouse lymphocyte serum (ALS) 1 day before i ntrathymic injection of donor splenocytes. DA rat islet xenografts or Balb/ c mouse islet allografts were transplanted 3 days later. ALS depleted CD3() and CD4(+) peripheral blood T lymphocytes to less than 5% of values in co ntrol mice by 24 h. Median islet xenograft survival (MGS) was 9 days in unt reated mice, 28 days in mice receiving 0.2 ml of ALS and 32 days in mice re ceiving 0.3 mi of ALS alone. Intrathymic injection of 5 x 10(6) DA splenocy tes plus 0.2 mi of ALS did not improve islet xenograft survival beyond 28 d ays. Increasing the intrathymic inoculum to 10(7) DA splenocytes with or wi thout a higher dose of AES (0.3 mi) did not increase MGS beyond 26 days, al though 2 out of 18 animals survived beyond 100 days. These long-term surviv ing mice rejected a second DA rat islet graft in less than 22 days, indicat ing that tolerance was not achieved. To confirm the efficacy of this treatm ent regimen in allotransplantation, diabetic C57B6 mice received 10(7) Balb /c splenocytes intrathymically and 0.3 mi of ALS. A Balb/c islet allograft was performed 3 days later. Allograft survival was similar to that of rat i slet xenografts with 40% (4 out of 10) of grafts surviving beyond 100 days. In contrast to the xenograft recipients, a second Balb/c islet allograft s urvived indefinitely, indicating that tolerance was achieved. Histology of the long-surviving allografts showed intact islets with a sparse cellular i nfiltrate, whereas the long-surviving xenografts (> 100 days) showed a larg e cellular infiltrate and significant islet destruction. To investigate fur ther the role of the thymus, adult thymectomized C57B6 mice were treated wi th 0.3 mi of ALS and received a DA rat islet xenograft. The median graft su rvival was 52 days and no graft survived beyond 80 days, suggesting that pe ripheral xenoreactive T cells remained after ALS treatment and greater T-ce ll depletion was necessary to obtain permanent engraftment. These results s how that peripheral xenoreactive T cells which remain after profound T-cell depletion are capable of rejecting an islet xenograft despite intrathymic inoculation of donor antigen. The T-cell-mediated xenograft response appear s to be stronger and more difficult to suppress than the allograft response using this strategy.