Suppression of T cell function by phosphoethanolamine, a metabolite enriched in tumor interstitial fluid

Nutrient stress represents a significant barrier for antitumor immunity, and tumor interstitial fluid (TIF) often contains metabolites that hinder immune function. However, it is difficult to isolate the effects of tumor nutrient stress from other suppressive factors. Thus, we employed a chemically-defined cell culture medium based on the metabolomic profile of TIF: Tumor Interstitial Fluid Medium (TIFM). Culture of CD8+ T cells in TIFM limited cell expansion and impaired CD8+ T cell effector functions upon restimulation, suggesting tumor nutrient stress alone is sufficient to drive T cell dysfunction. We identified phosphoethanolamine (pEtn), a phospholipid intermediate, as a driver of T cell dysfunction. pEtn dampened TCR signaling by depleting T cells of diacylglycerol required for TCR signal transduction. Reduction of pEtn accumulation in tumors improved intratumoral T cell function and tumor control, suggesting pEtn accumulation plays a dominant role in TME immunosuppression.