Cynomolgus Monkey GM-CSF (Granulocyte–Macrophage Colony-Stimulating Factor, CSF2) is a hematopoietic cytokine belonging to the colony-stimulating factor family, which also includes G-CSF (CSF3) and M-CSF (CSF1) that regulate the development, proliferation, and activation of myeloid immune cells. In cynomolgus monkeys (Macaca fascicularis), GM-CSF is produced by activated T lymphocytes, macrophages, endothelial cells, fibroblasts, and epithelial cells in response to infection, inflammatory cytokines, or immune stimulation. Cynomolgus monkey GM-CSF binds to the GM-CSF receptor (CSF2R) expressed on hematopoietic progenitor cells and mature myeloid cells, activating intracellular signaling pathways such as JAK/STAT, MAPK/ERK, and PI3K–AKT, which promote the proliferation and differentiation of granulocytes, macrophages, and dendritic cells and enhance their immune effector and antigen-presenting functions. Because cynomolgus monkeys share strong genetic and immunological similarities with humans, they are widely used as preclinical animal models for immunology, infectious disease, and vaccine development. GM-CSF–mediated immune responses have been studied in nonhuman primate vaccine models, including SARS-CoV and other viral vaccine studies, where GM-CSF is used as an immune adjuvant to enhance dendritic cell activation and antigen presentation, providing insights into immune mechanisms and vaccine efficacy relevant to human disease.