Abstract 3465: T cells on a crossroad between cytosolic and mitochondrial BCAA transamination in the lymphoma microenvironment

Abstract Neoplastic cells of the lymphoma microenvironment demonstrate substantial plasticity allowing their escape from immune surveillance and subsequent rapid growth. A distinct feature of this plasticity is the release of immunosuppressive signals that lead to T cell exhaustion. Metabolic enzymes, including the cytosolic (BCATc) and mitochondrial (BCATm) branched chain aminotransferases may contribute to the immunosuppressive repertoire of lymphoma cells as these enzymes break down and thus limit the availability of the branched chain amino acids (BCAAs) for helper and cytotoxic T cells. Because of the significant impact BCATc and BCATm have on the biosynthetic and energy demands of T cells, we hypothesized that T cell differentiation and effector function depend on a switch between cytosolic and mitochondrial BCAA transamination. Mice with a single or a combined deletion of BCATc and BCATm in T cells were subcutaneously injected with 2.5x105 murine EL4-OVA lymphoma cells [n=8-9/group] for 15 days and tumor development and growth were monitored. In addition, splenic and lymphatic T cells were isolated, activated with anti-CD3/CD28, and differentiated into CD4+ T cell subsets or CD8+ T cells, followed by RT-PCR, Flow Cytometry and ELISA to measure lineage-specific transcription factors and effector cytokine secretion. Results showed that T cells deficient in BCATm afforded mice with a local (as seen during an in vitro cell killing assay) but not with a systemic resistance to EL4-OVA lymphoma. In contrast, T cells with a single (BCATc), or a combined (BCATc-BCATm) deletion, led to ∼ 50 % reduction in tumor growth over time. This difference in response was further observed at the level of T cell subset differentiation, where a loss of BCATc caused an increased secretion of IFNγ (53 %) and upregulation of Tbet expression by 31 % in Th1 cells. In contrast, BCATm had no impact on Th1, however, in Th2 cells, a loss of BCATm expression resulted in a 50 % decrease in the secretion of IL13. A loss of both isoenzymes from Treg cells, however, favored a switch to Th1-like phenotype where Treg cells demonstrated 2.3- and 1.7- fold increases in the expression of Tbet in the absence of BCATc or BCATm, respectively. Lastly, deletion of BCATc positively affected CD8+ T cell activation, while deletion of BCATm stimulated CD8+ T cell expansion. The observed effects were directly bound to the transaminase activity of the BCAT enzymes as seen by the reverse effects of the leucine structural antagonist, n-acetyl-leucine amide [NALA], on T cell differentiation and function. These findings suggest that T cells switch between cytosolic and mitochondrial transamination of BCAAs during their activation and lineage commitment, a behavior that may likely impact the interactions of T cells with cancer cells in the lymphoma microenvironment. Citation Format: Tanner Wetzel, Christie Adam, Sheila Erfan, Lucas Figueroa, Lindsey Croll, Anthony Diehl, Taha Khan, Alexander Martin, Maxwell Swain, Michael Boyer, Elitsa Ananieva. T cells on a crossroad between cytosolic and mitochondrial BCAA transamination in the lymphoma microenvironment [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 3465.