Abstract
Background/Objectives: Pancreatic ductal adenocarcinoma (PDAC) is one of the most lethal malignancies, with limited therapeutic options despite rapid progress in the immunotherapy era. The balance among CD4(+) helper T cells (Th), CD8(+) cytotoxic T cells (Tc), and regulatory T cells (Tregs) is a central determinant of tumor immune dynamics and clinical outcomes. The profound immune suppression in PDAC, driven largely by regulatory T cells (Tregs), remains a major barrier to successful immunotherapy response. Tregs enforce tolerance, shape fibroblasts' immunosuppressive effect, and reprogram the tumor metabolic niche. This study describes the effect of the relative abundance of effector T cell subtypes and Tregs on survival outcomes in metastatic pancreatic cancer patients and reviews how Tregs and other effector T cell subtypes regulate PDAC immunobiology and influence clinical outcomes. Methods: This retrospective study provides immunohistochemical profiling of 62 metastatic PDAC patients, revealing differential prognostic associations among intratumoral and peritumoral subsets of Th, Tc, and Tregs. For each immunostaining, the immune cell infiltrates were evaluated by counting the number of positive cells under the objective of X20 magnification per 0.125 mm(2). Results: While high intratumoral Th (>16.8) and Tc (>19.6) abundances correlated with improved overall survival and progression-free survival, Treg infiltration (both IT and PT) showed no significant prognostic effect. Conclusions: The effector Th and Tc are the dominant prognostic T cell subsets in PDAC, whereas Treg abundance alone is an incomplete surrogate of immunosuppression. These findings describe the immunobiological landscape of PDAC.