Expression of M2-associated molecules in circulating monocyte subsets in fertile non-pregnant women and pregnant women with uncomplicated pregnancy

In humans circulating monocytes include classical (CD14++CD16- ), intermediate (CD14++CD16+) and non-classical/alternative (CD14+CD16++) monocytes, which in turn can be activated via the classical or alternative pathway. Pregnancy is accompanied by significant changes in the monocyte compartment, which is manifested by an increase in the number of circulating monocytes, including the proportion of intermediate monocytes, and a change in their function. However, the functional properties of monocyte subsets during gestation remain largely unexplored. We hypothesized that circulating monocytes may be activated in an alternative pattern and acquire features of M2 polarization (anti-inflammatory / immunosuppressive properties). The aim of the investigation was to study M2-associated markers that characterize the anti-inflammatory and immunosuppressive potential of myeloid cells in subpopulations of circulating monocytes in fertile nonpregnant women and women with uncomplicated pregnancy in the 2nd trimester. It was shown that in fertile non-pregnant women intermediate and non-classical monocytes are characterized by a higher expression of M2-associated markers (CD206, Arginase 1, MerTK) compared to classical monocytes. In the 2nd trimester of pregnancy, the expression of these molecules on monocytes increases significantly, which is manifested by 1) an increase in the proportion of CD206+ cells in subpopulations of classical and intermediate monocytes, 2) an increase in the mean fluorescence intensity of Arginase 1 in all monocyte subsets, 3) an increase in the proportion of MerTK+ cells in subpopulations of classical and intermediate monocytes and mean fluorescence intensity across all monocyte subsets. The highest content of CD206+ and MerTK+ cells in pregnant women is detected in the subpopulation of intermediate monocytes, and the highest values of the mean fluorescence intensity of Arginase 1 and MerTK – in the subpopulations of intermediate and non-classical monocytes. The data obtained demonstrate that monocytes of pregnant women in the 2nd trimester of pregnancy are characterized by signs of M2 polarization. This is confirmed not only by an increase in the expression of the M2-associated mannose receptor CD206, but also by an increase in the expression of Arginase 1 and MerTK, which mediate the immunosuppressive activity of myeloid cells and, in particular, macrophages of the M2 phenotype. Further studies of M2-associated markers in monocyte subpopulations during gestation will allow a more detailed characterization of the regulatory role of circulating myeloid cells during pregnancy. 