­­­­The differential effect of M-CSF vs. GM-CSF on macrophage polarization

Macrophages are innate immune sentinels, which play a central role both in immunity and homeostasis. Not all macrophages are equally competent in performing these roles, displaying specialized phenotypes that favor anti-microbial or anti-inflammatory homeostatic functions. Acquisition of such phenotypes is influenced by exposure to defined cytokines, notably macrophage colony stimulating factor (M-CSF) and granulocyte macrophage colony stimulating factor (GM-CSF). M-CSF is abundant in healthy human serum, it is needed for the proper development of most tissue-resident macrophages and is constitutively secreted by a variety of cell types. In contrast, GM-CSF is rarely detectable in healthy tissues and is upregulated during injury, inflammation and in certain tumors. In-addition, GM-CSF is expressed in the lung, where it is necessary for the proper development of alveolar macrophages. We sought to compare and characterize the phenotypic differences exhibited by human blood monocytes following in vitro exposure to these cytokines. We found differential effects on both the morphology and function of the resulting macrophages. The macropinocytic and phagocytic ability of cells polarized with either M-CSF or GM-CSF also differed markedly. M-CSF-treated cells macropinocytosed and phagocytosed much more effectively than did GM-CSF-treated cells. These morphological and functional features suggest differential cytoskeletal organization. To establish the molecular basis of these differences we preformed both proteomic and transcriptomic studies. The results, which will be described in detail, showed differential expression of both membrane proteins and small-GTPases that shed light on the observed morphological and functional differences.