Horseshoe Bat 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, differentiation, and activation of myeloid immune cells. In horseshoe bats (Rhinolophus species), GM-CSF is likely produced by activated T lymphocytes, macrophages, endothelial cells, and fibroblasts in response to infection, inflammatory signals, or immune activation. GM-CSF binds to the GM-CSF receptor (CSF2R) 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, differentiation, and activation of granulocytes, macrophages, and dendritic cells. In horseshoe bats, GM-CSF likely contributes to regulation of innate immune responses and development of antigen-presenting cells, supporting antiviral defense and immune homeostasis. Horseshoe bats are well known as reservoir hosts for several coronaviruses, including relatives of SARS-related viruses, and studying cytokines such as GM-CSF in these species helps researchers understand myeloid cell regulation and inflammatory control in bat immune systems, providing insight into how bats can harbor viral pathogens while limiting excessive inflammatory responses that are often associated with disease in other mammals.