The heterogeneity of CAFs and immune cell populations in the tumor microenvironment of pancreatic adenocarcinoma

Over the past decade, researchers have identified and characterized the diverse cell populations within the tumor microenvironment of pancreatic cancer. The interplay between these cells in the TME either promotes or inhibits the malignant behavior of pancreatic cancer cells. Cancer-associated fibroblasts, previously thought to be one main subset, can now be broadly subclassified into three main types: inflammatory, myofibroblastic, and antigen-presenting, with the former and the latter two exerting pro-tumoral and anti-tumoral functions, respectively. Myeloid cells include myeloid-derived suppressor cells and tumor-associated macrophages. Myeloid-derived suppressor cells can be further divided into polymorphonuclear and monocytic and exhibit pro-tumoral activities. Tumor-associated macrophages exhibit M1 (anti-tumoral) or M2 (pro-tumoral) phenotypes, which are present in a dynamic fashion between the two phenotypes. Other constituents of the immune make-up of the tumor microenvironment include T and B cells and less described subsets which include natural killer cells, γδ T cells, and group 2 innate lymphoid cells. This review provides an overview of the studies that lead to the discovery of those cellular populations and highlights the recent efforts to utilize them as therapeutic targets in pancreatic cancer.