Introduction Cardiovascular disease, especially atherosclerosis and related pathologies, is one of the leading causes of death globally. It is well established that macrophages play crucial role in the formation and progression of atherosclerotic plaques. Macrophages are very plastic and can alter their phenotypes and functions according to their environment. Distinct macrophage phenotypes within atherosclerotic plaques have been described with M1 and M2 macrophages being most studied. Different macrophage phenotypes can have different functional characteristics and their contributions towards lesion development are not well understood. Hence, a better understanding of the functional characteristics of macrophage phenotypes could potentially lead to novel treatment methods. The aim of our study is to characterise five different macrophage phenotypes, namely M1, M2a, M2c, Mox and M4 for their ability to metabolise lipids and the propensity to form foam cells.
Methods Macrophages were differentiated from blood monocytes with 100 ng/ml M-CSF in RPMI media containing FBS for 7 days before they were polarised to M1, M2a, M2c, Mox and M4 by treating with 20 ng/ml hrIFNg + 100 ng/ml LPS, 20 ng/ml hrIL-4, 20 ng/ml hrIL-10, 25 μg/ml oxPAPC and 1 μM hrCXCL4 respectively for 24 h. Unpolarised macrophages were used as control. Lipid uptake was tested by FACS using 488-labelled AcLDL. Foam cells formation was induced with 25 μg/ml AcLDL and stained with Oil red O. Enumeration of foam cells were performed by microscopy and lipid accumulation was quantified by isopropanol wash assay.
Results We observed that AcLDL uptake by M1 and M4 macrophages were lower when compared to M2a, M2c and Mox phenotypes. And the amount of AcLDL uptake correlated with the surface expression of CD36 on the various phenotypes, in line with the notion that CD36 is the receptor involved in the binding and uptake of lipoproteins. Interestingly, the ability to form foam cells is lowest in M1 and Mox macrophages, followed by M4 and then M2a and M2c phenotypes formed the highest percentage of foam cells.
Conclusion Distinct polarised macrophage phenotypes exhibit significant differences in AcLDL uptake, which do not entirely correlate with their ability to form foam cells. This may be attributed to differences in lipid accumulation and cholesterol efflux that is currently under investigation.
- macrophage phenotype
- foam cell formation
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