Abstract
The tumor promoting role of M2 macrophages has been described in in vivo models and the presence of macrophages in certain tumor types has been linked to a poor clinical outcome. In light of burgeoning activities to clinically develop new therapies targeting tumor-associated macrophages (TAMs), reliable in vitro models faithfully mimicking the tumor promoting functions of TAMs are required. Generation and activation of human monocyte-derived macrophages (MDM) in vitro, described as M1 or M2 macrophages attributed with tumoricidal or tumor-promoting functions, respectively, has been widely reported using mainly serum containing culture methods. In this study, we compared the properties of macrophages originating from monocytes cultured either in media containing serum together with M-CSF for M2 and GM-CSF for M1 macrophages or in serum-free media supplemented with M-CSF or GM-CSF and cytokines such as IL-4, IL-10 to induce activated M2 or LPS together with IFN-γ to generate activated M1 phenotype. We observed differences in cell morphology as well as increased surface receptor expression levels in serum-containing culture whereas similar or higher cytokine production levels were detected under serum-free culture conditions. More importantly, MDM differentiated under serum-free conditions displayed enhanced tumoricidal activity for M1 and tumor promoting property for M2 macrophages in contrast to MDM differentiated in the presence of serum. Moreover, evaluation of MDM phagocytic activity in serum free condition resulted in greater phagocytic properties of M2 compared to M1. Our data therefore confirm the tumor promoting properties of M2 macrophages in vitro and encourage the targeting of TAMs for cancer therapy.
Highlights
Macrophages derive from two different pools of circulating inflammatory or resident monocytes recruited to tissues in response to inflammation or infection as well as homeostasis to replace apoptotic resident macrophages [1,2]
Enriched CD14+, CD16+or2 round-shaped monocytes were further cultured in the presence of GM-CSF or M-CSF for 6 days, in either RPMI 10% FBS or XVivo 10 media, respectively to allow differentiation of monocytes into M1 or M2 macrophages
Stimulation with GM-CSF in XVivo 10 revealed a different morphology compared to RPMI 10% FBS which was visible for M-CSF differentiated macrophages in serum-free and serum-containing media
Summary
Macrophages derive from two different pools of circulating inflammatory or resident monocytes recruited to tissues in response to inflammation or infection as well as homeostasis to replace apoptotic resident macrophages [1,2]. The role of macrophages in anti- and pro-inflammatory immune processes results from their great heterogeneity and plasticity. Their phenotype is highly dependent on the specific microenvironment cues. M1 originate from monocyte stimulated with GM-CSF or MCSF in the presence of IFN-c and/or bacterial products such as LPS [6]. They are characterized by a pro-inflammatory phenotype, display microbicidal activity which results in tumor suppression, whereas M2 macrophages can promote tissue repair, matrix remodelling and angiogenesis supporting tumorigenesis. M2a, M2b and M2c are induced by M-CSF and IL-4 or IL-13, immune complexes together with LPS or IL-1b, and M-CSF and IL-10, respectively [4]
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