Partial reduction of human FOXP3+ CD4 T cells in vivo after CD25-directed recombinant immunotoxin administration

The regulation of tolerance to self-proteins and the suppression of T-cell responses have in part been attributed to the activity of CD25 ⁺CD4 ⁺ T regulatory (T _reg) cells. Further, T _reg cells can inhibit the antitumor effectiveness of adoptive immunotherapy and active immunization approaches in preclinical models. In an effort to selectively eliminate T _reg cells from human peripheral blood mononuclear cell to potentially bolster antitumor responses, we have evaluated the T _reg-cell depleting capacity of the CD25-directed immunotoxin, RFT5- SMPT-dgA. In preclinical studies, incubation of human peripheral blood mononuclear cell with RFT5-SMPT-dgA mediated a partial reduction in the levels of CD25 ⁺, Foxp3- expressing CD4 ⁺ T cells in vitro. Administration of RFT5- SMPT-dgA to 6 patients with metastatic melanoma induced a transient but robust reduction in the number of CD25high CD4 ⁺ T cells in vivo (a 97.5% mean reduction at nadir; from 69.4±12.4 cells/mL to 1.7±0.3 cells/μL). The reduction in FOXP3 ⁺ CD4 ⁺ T-cell number was less comprehensive (a 71.3% mean reduction at nadir; from 66.6±16.5 cells/mL to 14.2±3.9 cells/μL). This resulted in the selective persistence of a stable number of CD25low/neg FOXP3 ⁺ CD4 ⁺ T cells in vivo. No objective antitumor responses were seen in any patient. Our results indicate that the CD25-directed, RFT5-SMPT-dgA immunotoxin can mediate a transient, partial reduction in Treg-cell frequency and number in vitro and in vivo and suggest that comprehensive eradication of human T _reg cells in vivo may require the ability to target and eliminate FOXP3 ⁺ CD4 ⁺ T cells expressing both high and low levels of CD25.