Antibodies targeting checkpoint inhibitors or co-stimulatory receptors on T cells have shown significant antitumor efficacy in preclinical and clinical studies. In mouse tumor models, engagement of activating Fcc receptor (FccR)-expressing immune cells was recently shown to be required for the tumoricidal activity of antibodies recognizing the tumor necrosis factor superfamily receptor (TNFR) GITR (CD357) and CTLA-4 (CD152). In particular, activating FccRs facilitated the selective elimination of intratumoral T-cell populations. However, it remains unclear whether FccRs contribute to the antitumor efficacy of other immunomodulatory antibodies. Here, we explored the mechanism of antitumor activity mediated by an agonistic antibody (clone OX86) to the co-stimulatory TNFR OX40 (CD134). OX40 was highly expressed by intratumoral T cells, particularly those of the FoxP3 þ regulatory T-cell (Treg) lineage. OX86 administration resulted in the depletion of intratumoral regulatory T cells in an activating FccR-dependent manner, which correlated with tumor regression. Together with previous data from our group and others, these findings support a mechanism whereby antibodies targeting antigens highly expressed by intratumoral T cells can mediate their elimination by FccR-expressing immune cells, and facilitate subsequent antitumor immunity. Agonistic antibodies targeting co-stimulatory tumor necrosis factor superfamily receptors (TNFRs) expressed by T cells, such as GITR (CD357) or OX40 (CD134), have been shown to enhance the proliferation and activation of isolated T cells. [1][2][3][4] Moreover, in preclinical tumor efficacy studies, these agonistic antibodies have shown potent tumoricidal activity. [5][6][7] The current model stipulates that agonistic antibodies targeting GITR, OX40 and other costimulatory receptors on T cells directly enhance the cytotoxic effector function of T cells, while counteracting the immunosuppressive effects of immune-regulatory populations, such as regulatory T cells (Tregs). [8][9][10][11] We recently reported that an agonistic antibody targeting GITR (DTA-1) requires the engagement of activating FcgRs but not the inhibitory FcgR (FcgRIIB) to mediate antitumor activity in mice. 12 This finding challenges the notion that FcgRIIB-mediated crosslinking is generally required for the in vivo activities of agonistic antibodies targeting TNFRs. [13][14][15] Importantly, DTA-1 administration triggered the rapid and selective elimination of intratumoral T cells, particularly those of the Treg lineage, as defined by FoxP3 expression. The extent of Treg depletion in the tumor microenvironment directly correlated with the antitumor efficacy. A similar mechanism was observed for antibodies targeting the non-TNFR immunoregulatory molecule CTLA-4. 12,16 A common feature of GITR and CTLA-4 is their high expression by intratumoral populations of T cells, with Tregs expressing the highest levels of each receptor. 12,17,18 Here, we investigated whether antibody-mediated intratumoral T-cell depletion might con...