Macrophage Phenotype Modulation by CXCL4 in Atherosclerosis

Abstract

During atherogenesis, blood monocytes transmigrate into the subendothelial space and differentiate toward macrophages and foam cells. The major driver of monocyte-macrophage differentiation is macrophage colony-stimulating factor (M-CSF). M-CSF-induced macrophages are important promoters of atherogenesis as demonstrated in M-CSF and M-CSF receptor knock out mice. However, M-CSF is not the only relevant promoter of macrophage differentiation. The platelet chemokine CXCL4 also prevents monocyte apoptosis and promotes macrophage differentiation in vitro. It is secreted from activated platelets and has effects on various cell types relevant in atherogenesis. Knocking out the Pf4 gene coding for CXCL4 in Apoe(-/-) mice leads to reduced atherogenesis. Thus, it seems likely that CXC4-induced macrophages may have specific pro-atherogenic capacities. We have studied CXC4-induced differentiation of human macrophages using gene chips, systems biology, and functional in vitro and ex vivo experiments. Our data indicate that CXCL4-induced macrophages are distinct from both their M-CSF-induced counterparts and other known macrophage polarizations like M1 macrophages (induced by lipopolysaccharide and interferon-gamma) or M2 macrophages (induced by interleukin-4). CXCL4-induced macrophages have distinct phenotypic and functional characteristics, e.g., the complete loss of the hemoglobin-haptoglobin (Hb-Hp) scavenger receptor CD163 which is necessary for effective hemoglobin clearance after plaque hemorrhage. Lack of CD163 is accompanied by the inability to upregulate the atheroprotective enzyme heme oxygenase-1 in response to Hb-Hp complexes. This review covers the current knowledge about CXCL4-induced macrophages. Based on their unique properties, we have suggested to call these macrophages "M4." CXCL4 may represent an important orchestrator of macrophage heterogeneity within atherosclerotic lesions. Further dissecting its effects on macrophage differentiation may help to identify novel therapeutic targets in atherogenesis.

Keywords: CXCL4; M4; atherosclerosis; differentiation; macrophage; polarization.

Figure 1

Schematic overview of macrophage heterogeneity within atherosclerotic plaques. Monocytes entering the vascular wall may develop differentially depending on the predominant micromilieu. Different growth factors, chemokines, or lipoproteins may induce specific types of macrophage polarization. Dotted arrows indicate monocyte–macrophage differentiation, while solid arrows signify macrophage polarization (for details see text). Arrows are labeled with the key drivers of differentiation or polarization. Colors and distances indicate similarities and differences between the different macrophage types. M-DC, dendritic cells; M-ox, foam cells induced by oxidized low density lipoprotein (oxLDL); M-HA, hemorrhage-associated macrophages. CXCL4, platelet factor 4; GM-CSG, granulocyte–macrophage colony-stimulating factor; Hb/Hp hemoglobin–haptoglobin complexes; IFN-γ, interferon-gamma; IL, interleukin; LPS, lipopolysaccharide.

Figure 2

Specific functional and phenotypic features of CXCL4-induced M4 macrophages (blue/left) and M0 macrophages (green/right). M4 macrophages lose the ability to take up hemoglobin–haptoglobin (Hb–Hp) complexes via the receptor CD163 and subsequently upregulate the atheroprotective enzyme heme oxzgenase-1 as seen in M-CSF-induced macrophages. Furthermore, M4 macrophages express lover levels of scavenger receptors CD36/SR-A leading to fewer uptake of oxidized or acetylated low density lipoprotein (oxLDL/acLDL) as compared to M0 macrophages. The lower part of the figure indicates a selection of relevant cytokines, chemokines, chemokine receptors, surface receptors, molecule involved in T cell activation, and matrix metalloproteinases overexpressed in M4 or M0 macrophages (mRNA).