In recent years, the decades-long promise of tumor immunotherapy has finally begun to come to fruition. Checkpoint blockade, for example, represents a critically important intervention for potentiating the antitumor immune response. In these therapies, blockade of T cell intrinsic negative regulators (such as CTLA-4 and PD-1 signaling) releases the brake on effector T cells in the tumor, resulting in substantial, durable antitumor immunity, and clinical responses.
While negative regulators on the effector T cells can be relieved through these interventions, effector T cells still face a variety of cell extrinsic modes of immune suppression, notably through suppression via regulatory T (Treg) cells. Treg cells play critical roles in preventing autoimmune responses to self tissues as well as limiting immunopathology during exuberant immune responses. However, Treg cells represent a major barrier to antitumor immunity. Many tumors recruit, activate, and expand large numbers of Treg cells, which can be specific for any number of normal, self antigens expressed by the tumor. While depletion of total Treg cells can result in autoimmune pathologies, inhibition of Treg cell stability or function has been shown to allow for local inhibition of Treg cell suppression in the tumor, while sparing normal tissues from an autoimmune response.
Thus, finding phenotypic, signaling, or functional parameters that distinguish intratumoral Treg and conventional T (Tconv) cells could shed light on mechanisms by which Treg cells could be targeted to allow for a greater antitumor response. Recent studies have found that Tconv and Treg cells have distinct metabolic requirements. Not unlike cancer cells, conventional T cells undergo aerobic glycolysis (the 'Warburg effect') when undergoing robust expansion. However, regulatory T cells utilize alternative sources of fuel. Our initial findings in the laboratory suggest that not only do intratumoral Treg cells utilize distinct fuel from their conventional brethren, but engage different metabolic pathways from Treg cells in normal tissues and lymphoid organs. This suggests that metabolic pathways, or their downstream targets, could be targeted in order to inhibit intratumoral Treg cells specifically, releasing a crucial cell extrinsic brake on the antitumor immune response. The goal is to provide alternative modalities of therapy that could be utilized alone or in combination with other immunotherapeutic strategies, to allow for robust and durable immune responses for the eradication of cancer.