Targeting regulatory T cells in tumor immunotherapy

Activating Fcγ receptors (FcγRs) stimulate immune cell effector mechanisms, such as antibody-dependent cell-mediated cytotoxicity (ADCC) and phagocytosis (ADCP), which combine to facilitate antibody-mediated tumor cell clearance.1 The importance of FcγR-mediated immune effector cell function has been demonstrated in preclinical efficacy studies for antibodies targeting a range of tumor cell-expressed receptors, including trastuzumab (HER2) and rituximab (CD20). But until recently, it has remained unclear what contribution FcγR biology was making in the effectiveness of antibody therapeutics that target immune cell surface receptors, such as those in the T-cell checkpoint or co-stimulatory class. Here in this issue, Bulliard et al. describe the ability of antibodies reactive with mouse OX40 (CD134), a key co-stimulatory tumor necrosis factor receptor expressed on activated CD4+ T cells (clone OX86, rat IgG1), to deplete intratumoral regulatory T cells (Tregs) in an activating FcγR-dependent manner2 Figure 1). These findings corroborate other studies that support a critical role of Tregs in suppressing effective T helper 1 responses in tumors and set the scene for the design of new antibody therapeutics that may achieve even greater clinical outcomes in the treatment of cancers with an existing immune reaction. Tumors often contain neoplastic and non-neoplastic stromal cells embedded in an active extracellular matrix microenvironment. A spectrum of soluble chemokines and cytokines regulate the entry of different types of infiltrating immune cells, which have various effects on tumor progression. These cells can be located in the tumor center, in the invasive margin or in the adjacent tertiary lymphoid structures. Notably, immune infiltrates are highly heterogeneous, and not only between tumor types, but also from patient to patient. The demonstration that a T helper 1 immune polarity is prognostic of good clinical outcome and the recent remarkable successes of immune checkpoint blockade inhibitors, such as the US Food and Drug Administration-approved anti-CTLA-4 monoclonal antibody (mAb) (ipilimumab) and the mAbs targeting anti-PD-1 or PD-1 ligand (PD-L1), have recently revolutionized the cancer therapy field.3 A consistent hallmark of improved immune response and tumor shrinkage for many of these successful immune therapies is a reduction in the ratio of intratumoral Tregs to effector T cells (Teffs, generally CD8+). It has been recognized for sometime, from both mouse and human studies, that Tregs suppress many effective arms of the immune response4, 5 and their presence in high proportions in tumors is generally a poor prognostic feature.6, 7 Remarkably, despite the revolutionary success of antibody therapeutics against T-cell checkpoints, and the well-known importance of Fc receptor (FcR) in antitumor activity, it is only now that it has become apparent that antibodies thought to mediate their actions primarily by blocking checkpoints or agonizing co-stimulatory molecules, may in fact be simply causing ADCC and ADCP of Tregs in the tumor microenvironment (Figure 1). The current thinking has been that agonistic antibodies targeting OX40, GITR (TNFRSF18; CD357) and other co-stimulatory molecules on T cells directly improve the effector function of T cells while counteracting the immunosuppressive effects of Tregs.8 However, in a well-controlled series of experiments using FcR knockout mice and FcR competent and incompetent versions of the OX86 clone, Bulliard et al.2 show that FcR-activating receptors and intratumoral Treg depletion are necessary for the tumoricidal activity mediated by anti-OX40 mAb. The mechanism by which ipilimumab mediates its antitumor activity in melanoma patients has also remained controversial. Substantial data, however, now support a mechanism where anti-CTLA-4 antibodies engage activating FcγRs expressed by tumor-associated leukocytes; thus facilitating the selective elimination of intratumoral T-cell populations, particularly Tregs.9 This evidence in mice has implications for the clinic, as ipilimumab is an IgG1 that binds to most human Fc receptors, and may therefore mediate Treg reduction at the tumor site by inducing ADCC and ADCP. A similar mechanism of action has recently been described for antibodies targeting GITR.10 Collectively, these studies provide a conceptual framework for using antibodies to target and eliminate Tregs in tumors. Clearly, the outcome (tumor immunity and/or autoimmune events) of FcR activation and ADDC/ADCP will depend upon the relative expression of the target of the antibody (for example, OX40, GITR, CTLA-4) on Tregs, Teffs and other key leukocytes in the tumor and the periphery. CTLA-4 is widely expressed on Tregs outside the tumor microenvironment and thus CTLA-4+ Treg in the periphery may act as a sink for the antibody and serve to cause generalized Treg depletion in tissues leading to immune-related adverse events. Other targets, like Tim 3, may be more attractive given their selectively higher expression on intratumoral Tregs.11 An intratumoral balance of Treg depletion and Teff depletion must also be considered, and thus the ability of the antibody to reduce the Treg:Teff ratio selectively in the tumor is critical. Thus, a focus on strategies to target anti-CTLA-4 activity specifically within the tumor microenvironment only (for example, by using local delivery)12 is warranted. One must recognize that the ADCC/ADCP mechanisms will only be applicable in tumors where a substantial leukocyte infiltrate is found including myeloid cells, neutrophils and natural killer cells with activating FcRs that are in proximity to the tumor cells. Many cancers do not have an obvious immune reaction at diagnosis and it is not yet clear whether Tregs have any part in their control. In a tumor containing Tregs, but lacking leukocytes with activating FcR, or if the antibody does not have potent FcR function, the therapeutic can still be rendered useful by linking it to an appropriate toxin molecule. Other parameters like Treg versus Teff division and senescence may then become important factors in its antitumor efficacy. Overall this study highlights the general importance of Treg depletion in the mechanism of action of anticancer antibodies directed at immune molecules on T cells and suggests that a single manipulation that specifically and significantly depletes intratumoral Tregs may be a very effective monotherapy for many tumors and an agent that can be combined well with other new cancer therapeutics. MWLT is supported by a National Health and Medical Research Council of Australia (NH&MRC) CDF1 Fellowship and Project Grants, and grant from the Prostate Cancer Foundation of Australia. MJS and SFN are supported by a NH&MRC Australia Fellowship and Program Grant. Mechanism of action of anti-OX40 antibody. The depletion of OX40+ Tregs in tumors by ADCC and ADCP mediated by intratumor natural killer (NK) cells, macrophages and neutrophils, swings the balance toward CD8+ T-cell effector function, resulting in tumor regression.