Pancreatic cancer, one of the deadliest human malignancies, is associated with oncogenic Kras and is most commonly preceded by precursor lesions known as pancreatic intraepithelial neoplasias (PanIN). PanIN formation is accompanied by the establishment of an immunotolerant microenvironment. However, the immune contribution to the initiation of pancreatic cancer is currently poorly understood. Here, we genetically eliminate CD4⁺ T cells in the iKras* mouse model of pancreatic cancer, in the context of pancreatitis, to determine the functional role of CD4⁺ T cells during mutant Kras-driven pancreatic carcinogenesis. We show that oncogenic Kras-expressing epithelial cells drive the establishment of an immunosuppressive microenvironment through the recruitment and activity of CD4⁺ T cells. Furthermore, we show that CD4⁺ T cells functionally repress the activity of CD8⁺ T cells. Elimination of CD4⁺ T cells uncovers the antineoplastic function of CD8⁺ T cells and blocks the onset of pancreatic carcinogenesis. Thus, our studies uncover essential and opposing roles of immune cells during PanIN formation and provide a rationale to explore immunomodulatory approaches in pancreatic cancer. Cancer Immunol Res; 2(5); 423–35. ©2014 AACR.