Human monocyte-derived macrophages spontaneously differentiated in vitro show distinct phenotypes

Tissue macrophages are resident phagocytes that acquire specific phenotypes according to the microenvironment. Morphological and functional heterogeneity has been evidenced in different homeostatic and pathological conditions. Indeed, the nature of macrophage subsets may have either harmful or beneficial functions in disease progression/resolution. Therefore the possibility to pharmacologically manipulate heterogeneity represents a relevant challenge. Since human tissue macrophages are not easily obtained, various in vitro models are currently used that do not adequately reflect the heterogeneity and plasticity of tissue macrophages. We had previously reported that two dominant and distinct macrophage morphotypes co-exist in the same culture of human monocytes spontaneously differentiated for 7 days in autologous serum. The present study was aimed to the phenotypic characterization of these morphotypes, that is, round- and spindle-shaped. We observed that, besides substantial differences in cytoskeleton architecture, round monocyte-derived macrophages (MDMs) showed higher lipid content, increased macropinocytosis/efferocytosis capacity, and overexpression of CD163, interleukin (IL)-10, and transforming growth factor (TGF) β2. Conversely, spindle MDMs exhibited enhanced respiratory burst and higher expression of the chemokine (C-C motif) ligands 18 and 24 (CCL18 and CCL24). Overall, round MDMs show functional traits reminiscent of the non-inflammatory and reparative M2 phenotype, whereas spindle MDMs exhibit a pro-inflammatory profile and express genes driving lymphocyte activation and eosinophil recruitment. MDMs obtained in the culture condition herein described represent a valuable model to disentangle and manipulate the functional heterogeneity of tissue macrophages that has been disclosed in scenarios spanning from inflammatory and wounding responses to atherosclerotic lesions.