A serum-free medium for differentiation of monocytes to macrophages

Abstract

Macrophages play an important role in defense against pathogens and in tissue homeostasis. They are classified into two main groups depending on how they are activated: 1) M1 (activated by IFN-γ and LPS) and 2) M2, which include M2a (activated by IL-4 or IL-13), M2b (immune complexes with IL-1β or LPS) and M2c (IL-10) subsets. It is difficult to control macrophage activation and polarization in cultures containing serum, in which variable amounts of M1- and M2-specific cytokines and other factors may be present. Here we describe a serum-free culture system that selectively supports the differentiation of monocytes into either M1 or M2a macrophages. Monocytes were isolated using EasySep immunomagnetic cell separation and cultured for 4 or 6 days in serum-free ImmunoCult medium with 50 ng/mL M-CSF. The cells were then stimulated by adding 10 ng/mL LPS plus 50 ng/mL IFN-γ for M1 or 10 ng/mL IL-4 for M2a macrophage polarization and cultured for 2 more days. The yields of M1 and M2a macrophage were 51±18% and 58±14% (mean±SD), respectively. M1 macrophages expressed high levels of CD80 and CCR7 (96±3% and 62±15%, n=57), but very low levels of CD206 and CD209. In contrast, M2a macrophages expressed CD206 and CD209 (90±10% and 92±13%, n=59), but were negative or low for CD80 and CCR7. M1 macrophages produced TNF-α and IL-12 (mean±SEM: 2821±396 and 656±86, pg/mL, n=24). M2a macrophages produced small amounts of IL-10 (29±6 pg/mL, n=21). Both M1 and M2a macrophages were functional as demonstrated in a phagocytosis assay using fluorescently-labeled E.coli. By selecting appropriate stimuli this culture method can be easily adapted to generate other macrophage subsets as well and should prove useful for the study of macrophage biology.