Cells in Nonhuman Primates IL-2 Immunotoxin on NK and Regulatory T Differential Effects of Denileukin Diftitox

Denileukin diftitox (DD), a fusion protein comprising IL-2 and diphtheria toxin, was initially expected to enhance antitumor immunity by selectively eliminating regulatory T cells (Tregs) displaying the high-affinity IL-2R (a-b-g trimers). Although DD was shown to deplete some Tregs in primates, its effects on NK cells (CD16 + CD8 + NKG2A + CD3 2), which constitutively express the intermediate-affinity IL-2R (b-g dimers) and play a critical role in antitumor immunity, are still unknown. To address this question, cynomolgus monkeys were injected i.v. with two doses of DD (8 or 18 mg/kg). This treatment resulted in a rapid, but short-term, reduction in detectable peripheral blood resting Tregs (CD4 + CD45RA + Foxp3 +) and a transient increase in the number of activated Tregs (CD4 + CD45RA 2 Foxp3 high), followed by their partial depletion (50–60%). In contrast, all NK cells were deleted immediately and durably after DD administration. This difference was not due to a higher binding or internalization of DD by NK cells compared with Tregs. Coadministration of DD with IL-15, which binds to IL-2Rb-g, abrogated DD-induced NK cell deletion in vitro and in vivo, whereas it did not affect Treg elimination. Taken together, these results show that DD exerts a potent cytotoxic effect on NK cells, a phenomenon that might impair its antitumoral properties. However, coadministration of IL-15 with DD could alleviate this problem by selectively protecting potentially oncolytic NK cells, while allowing the depletion of immunosuppressive Tregs in cancer patients. D enileukin diftitox (DD) is a fusion protein composed of IL-2 and diphtheria toxin (DT), which was designed to kill cells expressing IL-2R. Because regulatory T cells (Tregs) constitutively express all three subunits of the high-affinity IL-2R (a, b, and g), DD was expected to preferentially bind and deplete this T cell subset. Based upon this principle, DD has been administered to cancer patients to eliminate presumably immu-nosuppressive Tregs and to enhance antitumor immunity (1–5). DD treatment has been tested in patients exhibiting IL-2Ra (CD25) + cutaneous T cell lymphoma and other cancers, including renal cell carcinoma (6), melanoma (7), B cell non-Hodgkin lymphoma, and lung cancer (8–13). Limited antitumoral effects were observed in these patients, an outcome that has been attributed to incomplete and short-lasting Treg depletion (4, 14–16). However, an alternative hypothesis, that DD could also delete some effector immune cells and, thereby, impairs its antitumor efficacy, has not been thoroughly investigated. DD is most toxic …