In vivo depletion of CD4+FOXP3+ Treg cells by the PC61 anti-CD25 monoclonal antibody is mediated by FcγRIII+ phagocytes

Depletion of CD4(+)CD25(+)FoxP3+ Treg using PC61 mAb (anti-murine CD25 rat IgG1) is widely used to characterize Treg function in vivo. However, the mechanism of Treg depletion remains largely unknown. Herein, we report the PC61 mAb's mechanism of action. In peripheral blood, a single injection of PC61 mAb eliminated similar to 70% of CD4(+)FoxP3(+) cells with the remaining Treg expressing low or no CD25. Functional blockade of Fc gamma receptors with 2.4G2 mAb significantly inhibited PC61 mAb activity. Furthermore, Fc gamma receptor (Fc gamma R)III-/- mice were resistant to Treg depletion. Fc gamma RIII is expressed on immune cells including NK cells and macrophages that are the major effector cells for Ab-dependent-cellular-cytotoxicity and Ab-dependent-cellular-phagocytosis, respectively. Depletion of NK cells had no effect, whereas depletion of phagocytes, including macrophages, by clodronate liposome significantly inhibited Treg depletion. Furthermore, in vitro, PC61 mAb can mediate Ab-dependent-cellular-phagocytosis of CD25(+) cells by WT or Fc gamma RIIB-/-, but not Fc gamma RIII-/-, macrophages. Altogether these data demonstrate the critical role of Fc gamma RIII+ phagocytes in mediating Treg depletion by PC61 mAb. This finding may be useful in guiding the development of human Treg targeting therapy.