Superiority of rapamycin over tacrolimus in preserving nonhuman primate treg half-life and phenotype after adoptive transfer

Many critical issues remain concerning how best to deploy adoptive regulatory T cell (Treg) immunotherapy to the clinic. These include a determination of their pharmacokinetic characteristics, their optimal dose, their phenotypic stability and the best therapies with which to pair Tregs. By performing a CFSE-labeled autologous Treg pulse experiment, we determined that the accessible peripheral blood Treg pool in rhesus macaques is quite large (75-±-11-×-10⁶-Tregs/kg). Pharmacokinetic analysis revealed that Tregs have two phases of elimination: an α phase, with a T_1/2 in the peripheral blood of 32.4-±-11.3-h and a β phase with a T_1/2 of 120.4-±-19.7-h. In addition to their short initial half-life, Tregs underwent rapid phenotypic shifts after infusion, with significant loss of both CD25 and FoxP3 by day +6. While tacrolimus stabilized CD25 expression, it did not improve T_1/2, nor mitigate the loss of FoxP3. In contrast, rapamycin significantly stabilized both CD25 and FoxP3, and supported an increased half-life, with an α phase of 67.7-±-6.9-h and a β phase of 252.1-±-54.9-h. These results suggest that rapamycin may be a necessary addition to Treg immunotherapy, and that tacrolimus may be deleterious to Treg integrity posttransfer. Adoptively transferred Tregs exhibit two-phase elimination kinetics after transfer and demonstrate a loss of phenotypic integrity, both of which are unchanged with concomitant tacrolimus therapy but improved with rapamycin. See editorial by Tang on page 2679.