Abstract 5154: The cytosolic branched chain aminotransferase is an important metabolic checkpoint of T cell function in the lymphoma microenvironment
Authors: Wetzel Tanner J. , Figueroa Lucas , Wheeler Leighton , Martin Alexander , Adam Christie , Boyer Michael , Ananieva Elitsa 
Published: April 2023

Abstract
Rapidly growing lymphoma cells deplete the tumor microenvironment (TME) of nutrients needed to maintain anti-tumor T cell immunity. This metabolic disadvantage leads to diminished T cell function and reduced clearance of tumor cells. The branched chain amino acids (BCAAs) are required for growth of tumor and immune cells. In the TME, BCAAs are preferentially taken up by tumor cells for protein synthesis or energy production. The first step in BCAA degradation is reversible transamination catalyzed by the cytosolic branched chain aminotransferase, BCATc. Recent investigations suggest that BCATc is immunosuppressive in the TME. We found BCATc is induced upon T cell activation, during which time, BCATc negatively regulates T cell metabolism. We hypothesized that a loss of BCATc from T cells restores effector T cell function and limits the immunosuppressive Treg cells in the TME. Tumor studies were conducted using mice with BCATc deleted from CD4+ and CD8+ T cells (T-BCATcKO mice) and corresponding controls expressing the floxed transgene of BCATc in T cells (T-BCATcfl/fl mice). Mice were subcutaneously injected with 2.5 × 105 murine EL-4 lymphoma cells [tumor-injected mice, n=8-9/group] or phosphate-buffered saline [vehicle-injected mice, n=4-6/group]. Tumor growth, body weight, and food intake were monitored for 15 days followed by collection of tumors and organs. Splenocytes were purified and stained for expression of Foxp3 or used to isolate CD4+ and CD8+ T cells that were activated with anti-CD3 and anti-CD28, and, after initial expansion with cytokine IL-2, reactivated to hypotolerant or fully activated CD4+ Th1 cells, or expanded to effector CD8+T cells for 96h, in the presence or the absence of the leucine antagonist, N-acetyl-leucine amide (NALA). Supernates were tested for secretion of IFNγ (CD4+T cells) or perforin and granzyme B (CD8+T cells). T-BCATcKO mice demonstrated significant delay in lymphoma appearance and a 50% reduction in tumor volumes when compared to control mice. The removal of BCATc from T cells did not cause systemic overactivation of the immune system, with no changes in size or appearance of the spleen, liver, or intestines. Splenic Foxp3 expression was reduced by 80% in lymphoma challenged T-BCATcKO mice when compared to T-BCATcfl/fl controls. Hypotolerant and fully activated T-BCATcKO CD4+ T cells released significantly more IFNγ when compared to T-BCATcfl/fl T cells. The addition of NALA reduced the IFNγ levels to those found in control cells. CD8+ T cells from T-BCATcKO mice released significantly more granzyme B and perforin compared to T-BCATcfl/fl T cells. These findings demonstrate that BCATc-deficient T cells provide better systemic response to lymphoma likely due to improved Th1 and effector CD8+ T cell with diminished Treg function. Thus, BCATc may act as an important metabolic checkpoint of the lymphoma TME
Citation Format: Tanner J. Wetzel, Lucas Figueroa, Leighton Wheeler, Alexander Martin, Christie Adam, Michael Boyer, Elitsa Ananieva. The cytosolic branched chain aminotransferase is an important metabolic checkpoint of T cell function in the lymphoma microenvironment. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5154.