Pre-transplant administration of anti-thymocyte globulin increases its efficacy in inhibiting anti-donor memory T cell responses

Katayoun Ayasoufi¹, Ran Fan¹, Hong Yu¹, Anna Valujskikh¹, John M. Williams²

¹Department of Immunology, The Lerner Research Institute, The Cleveland Clinic Foundation, Cleveland, OH, USA ; ²Genzyme Corporation

Antithymocyte globulin (ATG) is a lymphoablative agent used to treat or prevent acute transplant rejection. The effects of ATG on anti-donor immune responses, especially on those by donor-reactive memory T cells, are poorly understood. The goal of this study was to evaluate anti-donor T cell responses in mouse heart allograft recipients treated with a mouse analog of ATG (mATG).

C57BL/6 (B6, H-2^b) or BALB/c (H-2^d) mice received fully MHC-mismatched heart allografts and were treated with 25-50 mg/kg mATG or control rabbit IgG (rIgG) either on d. 0 and 4 (peri-TP) or on d. -7 and -4 (pre-TP) relative to transplantation. Regardless of donor/recipient strains or mATG dose, pre-TP mATG resulted in better allograft outcome than peri-TP treatment (Table 1). Both pre-TP and peri-TP mATG regimens induced comparable T cell depletion, with the enrichment of the residual T lymphocytes for cells with CD44^hiCD62L^lo effector/memory phenotype. The percentages and total numbers of CD44^hi CD4 and CD8 T cells were similar after pre- or peri-TP mATG. However, peri-TP treated recipients contained significantly higher numbers of IFNg-secreting donor-reactive T cells than pre-TP treated groups (spots/million CD44^hi cells in B6 recipients: 72000, peri-TP; 12500, pre-TP; 18800, rIgG).

Next, we performed mATG treatment in recipients injected with tracer subsets of congenic donor-reactive memory or naïve T cells. Notably, heart transplantation induced more IFNg-producing effector T cells from pre-existing memory than from the naïve T cell pool (4800 vs 1800 spots/million for memory and naïve CD8 T cells at d. 10 post transplant in rIgG treated recipients). While both mATG regimens depleted naïve CD4 and CD8 T cells, pre-TP mATG was four times more efficient in depleting circulating donor-reactive memory T cells than peri-TP mATG.

Our results indicate that the administration of ATG prior to alloantigen exposure is more efficient in targeting pre-existing alloreactive memory T cells and inhibiting anti-donor T cell responses. Studies of the mechanisms underlying this increased efficacy may guide the future use of ATG in sensitized transplant patients.

Table 1