The tumor microenvironment (TME) has multiple mechanisms of immune-suppression, one being recruitment of arginase (ARG) expressing myeloid cells. ARG is an enzyme that catalyzes hydrolysis of L-arginine into urea and L-ornithine. L-arginine is a semi-essential amino acid required for optimal function of T cells and natural killer (NK) cells. Arginine depletion in the TME inhibits T cell and NK cell function. Therefore, inhibition of ARG can reverse immune-suppression and optimize anti-tumor immunity. To determine the dependence to arginine, we cultured human T- and NK cells in arginine concentrations ranging from 0 to 150 µM. Both human CD8+ T cells and NK cells showed decreased proliferation with decreased L-arginine concentration, and a complete proliferation arrest in the absence of L-arginine. L-arginine threshold for optimal proliferation was determined to be ~30 µM for CD8+ T cells and ~ 9 µM for NK cells. Furthermore, both CD8+ T cells and NK cells showed decreases in cytotoxic molecules (e.g. granzyme A, granzyme B, perforin) when cultured under reduced arginine conditions. In addition, NK cells showed a decreased ability to kill target cells in low arginine conditions. We next explored whether arginase inhibition could reverse the immune-suppressive environment in vivo. Administration of arginase inhibitor to tumor bearing mice resulted in a dose dependent increase in both plasma (up to 4 fold) and tumor arginine (up to 12 fold) in all models tested including MC38-ova, CT26, and Lewis Lung. In addition, ARG inhibitor showed signs of in vivo immune cell activation including ~doubling of the number of CD8+ T cells, NK cells, and CD103+ dendritic cells in the tumor microenvironment. Combining with anti PD-L1 further increased the number of CD8+ T cells to ~4-fold of control levels. Arginase inhibitor also increased the activation state of CD8 T cell as determined by percent of granzyme, IFNg, and Ki67 positivity. Moreover, ARG inhibitor resulted in an increase of IFNg and TNFa producing CD8+ T cells in tumor draining lymph nodes. Treatment of tumor bearing mice with ARG inhibitor as monotherapy resulted in modest, but consistent, tumor growth inhibition (TGI) of ~30% in multiple syngeneic models (LL, MC38-ova, CT26). Combining ARG inhibitor with anti-PDL1 significantly improved efficacy reaching ~87% TGI in the MCA38-ova model, with 7/10 mice showing tumor regression. In summary, our pre-clinical data demonstrates that an ARG inhibitor in combination with a checkpoint inhibitor can increase plasma and tumor arginine levels and reverse tumor immune-suppression leading to strong immune activation and anti-tumor responses, suggesting arginase inhibitors could provide an opportunity to increase activity of checkpoint inhibitors clinically.

Citation Format: Alwin Schuller, Aatman Doshi, Susan Cantin, Michael Secinaro, Yanjun Wang, Laura Prickett, Sharon Tentarelli, Eric Gangl, Horma Ghadially, Kristina Ilieva, Theresa Proia, Scott Mlynarski, Ray Finlay, Wenlin Shao. Inhibition of arginase in combination with anti-PDL1 leads to increased infiltration and activation of CD8+ T cells, NK cells, and CD103+ dendritic cells in mouse syngeneic tumor models [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 4523.