Agonism of 41BB (CD137), a costimulatory molecule in the TNFR family, has profound consequences on the metabolic state of CD8+ T cells, as revealed by Menk et al. Signaling through p38-MAPK and upregulation of PCG1α leads to increased mitochondrial mass, mitochondrial fusion, and increased available ADP, positioning the cell for a rapid metabolic response. 41BB agonism, even transiently, enhances the impact of anti-PD-1 therapy and adoptive T cell therapy, highlighting the importance of metabolic sufficiency of antitumor T cells.
Despite remarkable responses to cancer immunotherapy in a subset of patients, many patients remain resistant to these therapies. The tumor microenvironment can impose metabolic restrictions on T cell function, creating a resistance mechanism to immunotherapy. We have previously shown tumor-infiltrating T cells succumb to progressive loss of metabolic sufficiency, characterized by repression of mitochondrial activity that cannot be rescued by PD-1 blockade. 4-1BB, a costimulatory molecule highly expressed on exhausted T cells, has been shown to influence metabolic function. We hypothesized that 4-1BB signaling might provide metabolic support to tumor-infiltrating T cells. 4-1BB costimulation of CD8(+) T cells results in enhanced mitochondrial capacity (suggestive of fusion) and engages PGC1alpha-mediated pathways via activation of p38-MAPK. 4-1BB treatment of mice improves metabolic sufficiency in endogenous and adoptive therapeutic CD8(+) T cells. 4-1BB stimulation combined with PD-1 blockade results in robust antitumor immunity. Sequenced studies revealed the metabolic support afforded by 4-1BB agonism need not be continuous and that a short course of anti-4-1BB pretreatment was sufficient to provide a synergistic response. Our studies highlight metabolic reprogramming as the dominant effect of 4-1BB therapy and suggest that combinatorial strategies using 4-1BB agonism may help overcome the immunosuppressive metabolic landscape of the tumor microenvironment.
