PDCD1 and neoplasm: Depletion of gut bacteria can lead to immunogenic changes in the pancreatic ductal adenocarcinoma (PDAC) microenvironment, including increased differentiation of myeloid-derived suppressor cells (MDSCs) and M1-like tumor-associated macrophages (TAMs), as well as the activation of CD4+ Th1 and CD8+ T cells, which can enhance the effectiveness of checkpoint immunotherapy by increasing PD-1 expression (67).