Lipid-free Apolipoprotein A-I Enhances Endothelium generation from Human Blood Monocytes

We have recently reported that human blood monocytes are capable of rapidly transforming into endothelial-like cells after adhering to preexisting endothelium. This indicates that blood monocytes are a potential source for the repair/maintenance of intact endothelial layers. In this study, we investigated the effect of lipid-free apolipoprotein (apo) A-I, the main apolipoprotein in HDL, on endothelium generation from endothelial-adherent blood monocytes. Peripheral blood mononuclear cells (PBMCs) obtained from healthy HLA-A2+ donors were co-cultured with HLA-A2− human umbilical vein endothelial cells (HUVECs), thus allowing the PBMC-derived cells to be tracked by their HLA-A2 expression. The co-cultures were exposed to lipid-free apoA-I at final concentrations of 0.5 and 2.0 mg/ml. The cell layers were analysed by immunofluorescence and dual-color flow cytometry at serial time points. After 24 hours of co-culture, the proportion of monocyte-derived endothelial-like cells (M-ELCs) in the controls (no apoA-I) was 17 ± 0.9% while the proportion of M-ELC in co-cultures containing 0.5 mg/ml and 2.0 mg/ml of apoA-I were increased to 33 ± 1.4 and 35.8 ± 1.7% (both P < 0.01 compared to control). At an earlier two hour time point, at which potential M-ELC proliferation is minimal, a similar two-fold increase in the proportion of M-ELC was also observed. Endothelial transformation of the adherent blood monocytes was characterised by the acquisition of CD105, CD144, and eNOS expression. This was not affected by exposure to apoA-I. In conclusion, a physiologically relevant concentration of apoA-I enhances the generation of endothelium from endothelial-adherent blood monocyte by increasing monocyte recruitment.