Abstract 4513: The role of HPK1 in the regulation of T cell function and anti-tumor immune activity

Antigen recognition and T-cell receptor (TCR) activation are fundamental processes that drive anti-tumor T cell responses. HPK1 has been identified as a negative regulator of TCR activation, as well as BCR activation, and is a potential anticancer target for immuno-oncology. We show that genetic ablation of HPK1 in human T cells resulted in increased cytokine production upon cell stimulation. Jurkat cells with genetic HPK1 knockout were unable to phosphorylate SLP76, a direct downstream target of HPK1, upon cell stimulation which was associated with increased IL-2 production. Genetic deletion of HPK1 in human RAMOS B cells reduced phosphorylation of BLNK upon IgM stimulation and led to increased TNF alpha and TNF beta production. Treatment of HPK1 knockout primary human T cells with pembrolizumab enhanced IFN gamma secretion compared to the knockout cells alone. Based on these genetic data, HPK1 may be an attractive target for immuno-oncology. We describe herein the in vitro and in vivo profile of several small molecule HPK1 inhibitors and report a surprising disparity between in vitro and in vivo findings. In vitro, the HPK1 inhibitors tested phenocopied the genetic data by potently inhibiting pSLP76 and enhancing IL-2 production in Jurkat cells following stimulation. Similarly, HPK1 inhibitors enhanced IL-2 production in human PBMCs, and increased IFN gamma production in combination with atezolizumab in a primary T cell co-culture assay. In vivo, the HPK1 small molecule inhibitors inhibited tumor growth in the MC38 model, which was further enhanced in combination with anti-PD-L1 in 3 different syngeneic models (MC38, CT26 and MBT-2). Despite these results, tumor growth inhibition was not observed in the GL261 glioma syngeneic model whose growth was recently shown to be inhibited when implanted into HPK1 kinase-dead mice. Further, although several in vitro assays demonstrated increased functional cytokine production with HPK1 inhibitor treatment, the compounds did not increase in vivo cytokine production in tumors. Moreover, combining HPK1 inhibitors with a PD-L1 antibody in vivo abolished the anti-PD-L1-induced production of IFN gamma in the CT26 model. In summary, while our genetic data support the role of HPK1 as a negative regulator of T and B cells, the in vitro activity of HPK1 kinase activity inhibitors was not correlated with functional effects in in vivo syngeneic tumor models. These results underline the complexity of interpreting HPK1 biology and also highlight challenges for the development of clinically active compounds targeting this pathway. Citation Format: Yaoyu Chen, Jonathan Rios-Doria, Michelle Pusey, Kerri Lasky, Min Ye, Pramod Thekkat, Karen Gallagher, Kristine Stump, Patricia Conlen, Christine Gardiner, Hui Wang, Alexander Sokolsky, Mark Rupar, Luping Lin, Elham Behshad, Maryanne Covington, Holly Koblish, Oleg Vechorkin, Wenqing Yao, Sunkyu Kim, Yingnan Chen. The role of HPK1 in the regulation of T cell function and anti-tumor immune activity [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 4513.