Novel agents that can overcome the immunological barrier to CD8+ T cell infiltration and effector function have the potential to improve clinical outcomes for pancreatic ductal adenocarcinoma (PDAC) patients. We have found that targeting MNK kinases, which regulate mRNA translation, increases CD8+ T cell infiltration and expression of T cell chemo-attractants. However, MNK inhibitors induce tumor-associated macrophages (TAMs) to express the immunosuppressive protein Arginase-1, suggesting that MNK inhibitors polarize TAMs to cause CD8+ T cell exhaustion. The main objective in this application is to identify mechanisms by which the MNK kinase pathway regulates tumor infiltration by CD8+ T cells and modulates their effector function. The central hypothesis is that targeting the MNK kinase pathway can overcome the immunological barriers to CD8+ T cell infiltration. A second hypothesis is that MNK inhibitors demonstrate synergistic anti-tumor responses with therapies that overcome CD8+ T cell exhaustion. Two specific aims are proposed: 1) Determine the mechanism by which the MNK kinase pathway in cancer cells limits CD8+ T cell infiltration in PDAC tumors. 2) Determine the role of the MNK pathway in TAMs and its regulation of the effector function of infiltrating CD8+ T cells in PDAC tumors. Under the first aim, the mechanisms by which targeting the MNK pathway in PDAC cells increases T cell chemo-attractants will be evaluated, focusing on the role of the downstream MNK target hnRNPA1. The effects of targeting hnRNPA1 in cancer cells on CD8+ T cell infiltration will also be assessed in the syngeneic orthotopic mouse model. In additional studies, the effects of the combination therapy with a MNK inhibitor and an anti-PD-1 antibody on CD8+ T cell infiltration and activation will be evaluated in the KPC transgenic mouse model of PDAC. For the second aim, the contribution of MNK kinases and the MNK effector hnRNPA1 in macrophages to their polarization and modulation of the CD8+ T cell effector function will be evaluated. The effects of combining MNK and Arginase-1 inhibitors in ex vivo slice cultures of human PDAC tumors will also be assessed. In additional studies, the efficacy of the triple regimen of a MNK inhibitor, an anti-PD-1 antibody, and an Arginase-1 inhibitor will be evaluated in the KPC transgenic and the syngeneic orthotopic mouse models. There are several innovative elements in this proposal, including novel concepts on the role of the MNK kinase pathway in regulating CD8+ T cell infiltration and effector function, unique experimental approaches, and novel therapeutic approaches to enhance immunotherapy responses in PDAC patients. This proposed research is significant because it will have important clinical-translational implications and should result in the development of mechanism-based novel combination therapies for PDAC patients.