NIV Congres

Introduction Repopulation of regulatory T cells (Treg) after rabbit antithymocyte globulin (rATG) induction therapy in kidney transplant patients occurs via either thymopoiesis or homeostatic proliferation. In addition, rATG converts effector T cells into induced Tregs in vitro. In this study, we investigated the origin and the compensatory mechanisms by which the immune system repopulates Tregs as well as their controlling function after T cell depletion therapy.

Methods Patients were treated with rATG (3x2mg/kg/day, n=9) or non-depleting anti-CD25 antibody (Basiliximab: day 0, 4; 20mg, n=10) induction therapy in combination with tacrolimus, MMF and steroids. Flow cytometric analysis was performed to analyze Tregs (CD4+CD25+CD127-FOXP3+) for the expression of Ki67 (marker for cell division), CD31 (marker for recent thymic emigrants), and Helios (marker for natural Tregs). Functional studies were performed with FACSsorted CD4+CD25+CD127- Treg in co-culture with effector T cells activated by irradiated donor or 3^rd party cells.

Results In the first six months after rATG induction therapy, the percentage of Tregs gradually increased (p=0.003 vs. baseline). At one month after rATG induction a higher proportion of Ki67 expressing Tregs was found than before transplantation (p<0.01) and compared to basiliximab treated patients (p<0.0001). The majority of proliferating cells expressed the memory marker CD45RO. No differences were observed in the percentage of thymic CD31+CD45RO-Treg between time points and treatment arms. The proportion of Helios+ Tregs was lower in the rATG group than in the basiliximab group (mean±sd, 62±19% vs. 81±14%, p=0.03) at one month. Particularly, fewer Ki67+CD45RO+ Treg expressed Helios (57±27% vs. 77±9%, p<0.05). At the functional level a clear difference was found between the rATG and basiliximab arm. After rATG therapy Tregs inhibited proliferation of donor antigen activated T cells while the proliferation of 3^rd party activated T cells was not inhibited (p<0.05). The inhibition of proliferation by Tregs in the basiliximab group was comparable for both donor and 3^rd party.

Conclusion rATG therapy allows the induction of functional donor specific regulatory T cells via the conversion of effector T cells which is followed by homeostatic proliferation of both induced and natural Tregs.