Abstract
Macrophage plasticity is a process that allows macrophages to switch between two opposing phenotypes based on differential stimuli. Interferon γ (IFNγ)-primed macrophages stimulated with lipopolysaccharide (LPS) [M(IFNγ+LPS)] produce high levels of pro-inflammatory cytokines such as IL-12, TNFα, and IL-6 and low levels of the anti-inflammatory cytokine IL-10, while those stimulated with LPS in the presence of the immune complex (IC) [M(IFNγ+LPS+IC)] produce high levels of IL-10 and low levels of IL-12. In this study, we investigated the plasticity between M(IFNγ+LPS) and M(IFNγ+LPS+IC) in vitro and compared one of the active histone marks [histone H3 lysine 4 trimethylation (H3K4me3)] between M(IFNγ+LPS) and M(IFNγ+LPS+IC) using murine bone marrow-derived macrophages. We found that in an in vitro system, macrophages exhibited functional plasticity from M(LPS) to M(LPS+IC) upon repolarization after 2 days of washout period while IFNγ priming before LPS stimulation prevented this repolarization. Phosphorylation of p38, SAPK/JNK, and NF-κB p65 in M(LPS+IC) repolarized from M(LPS) was similar to that in M(LPS+IC) polarized from resting macrophages. To obtain the epigenetic profiles of M(IFNγ+LPS) and M(IFNγ+LPS+IC), the global enrichment of H3K4me3 was evaluated. M(IFNγ+LPS) and M(IFNγ+LPS+IC) displayed marked differences in genome-wide enrichment of H3K4me3. M(IFNγ+LPS+IC) showed increased global enrichment of H3K4me3, whereas M(IFNγ+LPS) showed decreased enrichment when compared to unstimulated macrophages. Furthermore, M(IFNγ+LPS+IC) exhibited high levels of H3K4me3 enrichment in all cis-regulatory elements. At the individual gene level, the results showed increased H3K4me3 enrichment in the promoters of known genes associated with M(IFNγ+LPS+IC), including Il10, Cxcl1, Csf3, and Il33, when compared with those of M(IFNγ+LPS). Finally, we investigated the impact of M(IFNγ+LPS+IC) on the systemic immune response by adoptive transfer of M(IFNγ+LPS+IC) in an LPS-induced endotoxemia model. The cytokine profile revealed that mice with adoptively transferred M(IFNγ+LPS+IC) had acutely reduced serum levels of the inflammatory cytokines IL-1β and IL-p12p70. This study highlights the importance of epigenetics in regulating macrophage activation and the functions of M(IFNγ+LPS+IC) that may influence macrophage plasticity and the potential therapeutic use of macrophage transfer in vivo.
Highlights
Macrophages are cells of the innate immune system that are found in most tissues
The results showed that M(LPS+immune complex (IC)) polarized from resting macrophages and M(LPS) with a washout period of 48 h had increased IL-10 and decreased IL-12p70 to a comparable extent (Figure 2B), whereas the shorter washout durations (2, 6, and 24 h) between the primary LPS stimulation and secondary LPS/IC stimulation failed to yield M(LPS+IC) cytokine profiles (Supplementary Figure 1A)
This result suggests that Interferon γ (IFNγ) priming during M(IFNγ+LPS) and a resting duration between the two rounds of stimulation are the key determinants of the plasticity of M(LPS) to be repolarized to M(LPS+IC) in vitro
Summary
Macrophages are cells of the innate immune system that are found in most tissues. They respond to infection as the first line of defense against pathogens [1]. Macrophages play an important role in the immune response and homeostasis with functional diversity, such as inflammation, phagocytosis, metabolism, tissue remodeling, and immunoregulation. The functional plasticity of macrophages contributes to the pathogenesis of various diseases, such as cancer, metabolic diseases, autoimmune diseases, and systemic infections [3]. LPS and immune complex (IC)-stimulated macrophages [M(IFNγ+LPS+IC)] produce large amounts of the anti-inflammatory cytokine IL-10 and low levels of the pro-inflammatory cytokine IL-12 but produce high levels of tumor necrosis factor (TNFα), IL6 and IL-1β [6, 7]. IL-10 is a key multifunction regulatory cytokine that regulates the immune response during infection and dampens immune hyperactivation [9, 10]
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