Transdifferentiated Monocytes: a Novel Source of Lymphatic Endothelial-like Cells.

Although monocytes have previously been demonstrated to contribute to lymphatic vessel formation in vivo, monocyte transdifferentiation into lymphatic endothelial cells and the specific conditions required remain unclear. In this study, monocyte cultures isolated from human peripheral blood were stimulated to transdifferentiate into lymphatic endothelial cells under specific in vitro induction conditions. These results demonstrate primary isolates of CD14 (+) monocytes express low levels of lymphatic endothelial cell specific markers or pan-endothelial markers under routine culture conditions. Using fibronectin (FN) coated flasks and EGM-2 supplemented culture medium, monocytes were induced to express lymphatic endothelial markers Prox-1, VEGFR-3, LYVE-1, Podoplanin, and pan-endothelial markers vWF, CD144, and VEGFR-2. Furthermore, using the FN/EGM-2/lipopolysaccharide (LPS) culture conditions, monocytes displayed dramatically increased expressions of Prox-1, VEGFR-3, Podoplanin, LYVE-1 and vWF, while the expression of CD144 and VEGFR-2 sharply decreased. In addition, VEGF-C secretion by monocytes exposed to fibronectin coated plates with EGM-2 medium with FN/EGM-2/LPS in vitro was significantly increased over levels seen in routine culture conditions. These findings demonstrate that monocytes can be induced to undergo transdifferentiation becoming more lymphatic endothelial-like cells and increase their VEGF-C production in an FN/EGM-2/LPS environment.

Monocytes can be induced to express lymphatic phenotypes.

Although it has been recently shown that monocytes can transdifferentiate into blood vascular endothelial cells which are involved in angiogenesis, little attention has been paid to their potential to transdifferentiate into lymphatic endothelial cells. Therefore, we examined this question in our study. We first stimulated monocytes with either fibronectin (FN), VEGF-C, TNF-alpha, LPS, or IL-3 for 24h. Then we examined the expression of several markers of lymphatic endothelium and found that the monocytes expressed specific lymphatic endothelial markers, LYVE-1, Podoplanin, and Prox-1, but not common endothelial markers vWF or eNOS. Next, monocytes were incubated in endothelial growth medium with FN and VEGF-C for 6d. These monocytes were also found to express LYVE-1, Podoplanin and Prox-1, but not vWF or eNOS. Our results indicate that monocytes in vitro can be easily induced to present lymphatic phenotypes in an inflammatory environment.