Transient low-dose methotrexate induces tolerance to murine anti-thymocyte globulin and together they promote long-term allograft survival.

Rabbit anti-thymocyte globulin (Thymoglobulin) effectively treats transplant rejection but induces anti-rabbit Ab responses, which limits routine readministration. Aiming to tolerize anti-rabbit responses, we coadministered a brief methotrexate regimen with a murine version of Thymoglobulin (mATG) for effects on anti-mATG Abs and cardiac allotransplantation in mice. Although both single and three courses of methotrexate could significantly inhibit anti-drug Ab titers to repeated mATG treatment, surprisingly, the single course given at the first mATG administration was most effective (>99% reduction). The transient methotrexate treatment also significantly improved pharmacokinetics and pharmacodynamics of repeated mATG administration. In the cardiac allograft model, the combination of transient mATG and methotrexate given only at the time of transplant dramatically improved allograft survival (>100 d) over either agent alone (<30 d). Anti-drug Ab titers were reduced and mATG exposure was increased which resulted in prolonged rather than enhanced mATG-mediated effects when combined with methotrexate. Moreover, methotrexate administration significantly reduced alloantibodies, suggesting that methotrexate not only decreases anti-drug Ab responses but also reduces Ab responses to multiple tissue-derived alloantigens simultaneously. These data suggest that mATG and methotrexate together can provide long-term allograft survival potentially through the induction of immune tolerance.