Human α1-Antitrypsin and Temporary T Cell Depletion Synergize to Expand Regulatory T Cells and to Extend Allogeneic Skin Graft Survival.

Abstract

Introduction: Treatment with α1-antityrpsin (AAT), an anti-inflammatory tolerogenic circulating protein that lacks direct effect on T cells, prolongs rat-to-mouse pancreatic islet xenograft survival in combination with temporary T cell depletion, yet treatment with each drug separately results in graft failure. Aim: To elucidate the mechanism behind the synergy and examine its effect on skin allograft transplants. Methods: Wild type (WT) and human AAT transgenic mice (hAAT+/+) (both C57BL/6; H-2b background) were used as graft recipients. Islet xenotransplantation. Temporary T cell depletion was performed with either anti-CD4 or anti-CD8-depleting antibodies. Circulating T-cell subtypes were monitored. Rat islets were grafted into non-depleted untreated mice, as well as to depleted mice that were either untreated or treated with hAAT (240 mg/kg every 3 days). Graft survival was determined. Skin allotransplantation. Mice received a single dose of anti-CD4/CD8 depleting antibodies and then grafted with skin from BALB/c mice (H-2d). Graft survival was monitored. Results: T cell repopulation profile. hAAT group displayed significantly higher levels of CD4+ cells and lower levels of CD8+ after depletion (6.47±1.56 and 2.12±0.27-fold compared to WT mice, respectively), as well as a greater proportion of foxp3+ cells after depletion (6.18±0.86-fold compared to WT mice). Islet xenograft survival. Without depletion, untreated mice rejected islet xenografts on day 13.2 (mean, n=5); CD4-depleted mice rejected on day 26.4 (mean, n=5), and CD8-depeleted mice on day 14 (mean, n=5). None displayed graft survival after day 35. However, in the hAAT group, graft function was evident after day 35 in 75% CD8-depleted and 25% CD4-depleted mice. Skin allograft survival. Non-depleted, WT mice rejected skin allografts on day 11 (mean, n=5) and hAAT+/+ mice rejected on day 9.5 (mean, n=4). WT, T cell depleted mice rejected skin grafts on day 13.8 (mean, n=5). None displayed graft survival after day 20. However, 57% of the T cell depleted hAAT+/+ mice exhibited skin allograft survival after day 35. Conclusion: The combination between AAT and temporal T cell depletion appears to facilitate regulatory T cells expansion in favor of both islet-xenograft and skin-allograft transplantation outcomes. Considering the safety of both regimens, their use in treatment-resistant transplants should be considered.