Dissecting the role of LDL uptake in tumor immunity and immunotherapy efficacy - Abstract Despite the success of immune checkpoint inhibitors (ICI), many patients fail to respond, often because tumorinfiltrating CD8+ T cells are functionally impaired or excluded from the tumor microenvironment. Interestingly, obesity is associated with improved ICI efficacy, suggesting that systemic factors linked to obesity may enhance CD8+ T cell function. Our preliminary data indicate that one such factor is elevated low-density lipoprotein (LDL), which boosts CD8+ T cell activation and cytotoxicity when available to these cells. However, tumors actively uptake LDL, limiting its availability in the tumor microenvironment and thereby impairing T cell responses and promoting immune evasion. The precise role of systemic LDL availability, the mechanisms by which CD8+ T cells take up LDL, and the contributions of specific LDL-derived lipids to T cell activation and effector function in the tumor microenvironment remain unknown. By understanding these factors, we could design strategies to enhance the efficacy of immune checkpoint–based immunotherapy. This proposal will investigate three LDL-related parameters in colorectal cancer, melanoma, and lung carcinoma. Because CD8+ T cell–mediated anti-tumor immunity, ICI efficacy, and systemic lipid metabolism can only be fully evaluated in the context of an intact organism, a subset of the proposed experiments will employ mice as a model system to capture physiological interactions between tumor growth, immune responses, and host lipid availability. First, we will define how systemic and tumor-local LDL availability influence tumor immune control, immune cell infiltration, and response to ICI, using isogenic tumors with or without LDL uptake, and genetic models with high or low systemic LDL. Second, we will test the necessity and sufficiency of LDL uptake in CD8+ T cells by knocking out or overexpressing the LDL receptor (Ldlr) and measuring T cell activation, tumor infiltration, and tumor-killing capacity in vitro and in vivo. Third, we have identified linoleic acid (LA), a polyunsaturated fatty acid sourced exclusively from the diet and enriched in LDL, as a candidate lipid that enhances CD8+ T cell function in vitro. We will use LA supplementation to dissect its mechanism in T cells and test whether it can promote anti-tumor immunity in vivo. Finally, we will measure LA levels in plasma from patients treated with ICI and correlate abundance with treatment outcomes, aiming to establish plasma LA levels as a biomarker of ICI response. Nutrient competition is a common mechanism of tumor immune evasion. This work will establish competition for LDL lipids between tumors and CD8+ T cells as a critical determinant of anti-tumor immunity. By enhancing dietary lipid influx into CD8+ T cells, we aim to shift this competition in favor of immune responses and improve ICI efficacy. Importantly, while obesity increases cancer incidence and worsens prognosis, it is paradoxically associated with higher ICI response. Thus, our studies may provide a mechanistic explanation for this clinical observation and lay the foundation for using circulating LA as a predictive biomarker in immunotherapy.
