M-CSF regulates macrophage proliferation, survival, and anti-inflammatory responses in largemouth bass (Micropterus salmoides).

Macrophage colony-stimulating factor (M-CSF) is a key hematopoietic cytokine that governs the proliferation, survival, and functional specialization of monocytes and macrophages in vertebrates. Although its roles are well characterized in mammals, knowledge of M-CSF-mediated immune regulation in teleost fish remains limited. In this study, we characterized the M-CSF homolog from largemouth bass (Micropterus salmoides) and investigated its structural features, tissue distribution and immunomodulatory functions. Sequence alignment revealed that largemouth bass M-CSF possesses a signature CSF domain and shares conserved cysteine and histidine residues essential for dimerization and receptor binding in mammals, indicating evolutionary conservation of its functional motifs. Quantitative expression analysis showed broad tissue distribution, with predominant expression in the head kidney and spleen. Recombinant M-CSF (rM-CSF) significantly enhanced monocyte/macrophages (MO/MФ) proliferation and upregulated the anti-apoptotic gene Bcl2, suggesting its role in promoting MO/MФ survival. Moreover, rM-CSF increased arginase activity and induced the expression of anti-inflammatory genes (i.e., IL-10 and TGF-β), indicative of M2-like polarization. Together, these findings demonstrate that largemouth bass M-CSF promotes macrophage proliferation, survival, and modulates functional phenotypes, thereby providing new insight into the evolutionarily conserved mechanisms of innate immune regulation in teleost fish.