T cell subsets including effector (T), regulatory (T), and memory (T) cells are characterized by distinct metabolic profiles that influence their differentiation and function. Previous research suggests that engagement of long-chain fatty acid oxidation (LC-FAO) supports Foxp3 T cell and T cell survival. However, evidence for this is mostly based on inhibition of Cpt1a, the rate-limiting enzyme for LC-FAO, with the drug etomoxir. Using genetic models to target Cpt1a specifically in T cells, we dissected the role of LC-FAO in primary, memory, and regulatory T cell responses. Here we show that the ACC2/Cpt1a axis is largely dispensable for T, T, or T cell formation, and that the effects of etomoxir on T cell differentiation and function are independent of Cpt1a expression. Together our data argue that metabolic pathways other than LC-FAO fuel T or T differentiation and suggest alternative mechanisms for the effects of etomoxir that involve mitochondrial respiration.

Highlights d Developed 13 C-infusion method for studying T cell metabolism in vivo d T cell glucose use and bioenergetics differ between cell culture and mouse models d Glucose metabolism in T cells changes dynamically over an immune response d Glucose-dependent serine biosynthesis supports T cell proliferation in vivo

Triple-negative breast cancer (TNBC) is an aggressive disease for which treatment options are limited and associated with severe toxicities. Immunotherapeutic approaches like immune checkpoint inhibitors (ICIs) are a potential strategy, but clinical trials have demonstrated limited success in this patient cohort. Clinical studies using ICIs have revealed that patients with preexisting anticancer immunity are the most responsive. Given that oncolytic viruses (OVs) induce antitumor immunity, we investigated their use as an ICI-sensitizing approach. Using a therapeutic model that mimics the course of treatment for women with newly diagnosed TNBC, we demonstrate that early OV treatment coupled with surgical resection provides long-term benefits. OV therapy sensitizes otherwise refractory TNBC to immune checkpoint blockade, preventing relapse in most of the treated animals. We suggest that OV therapy in combination with immune checkpoint blockade warrants testing as a neoadjuvant treatment option in the window of opportunity between TNBC diagnosis and surgical resection.