*2.5. Macrophages in Preeclampsia*

The increase in non-classical monocyte subset may a ffect the composition of tissue macrophages in the endometrium and be responsible for the poor placentation in PE [104]. Appropriate balance between pro- and anti-inflammatory macrophages in the placenta is essential for healthy pregnancy and its optimal outcome. It has been suggested that transition to the M2 profile, which normally occurs in the second trimester, is blocked in PE; more specifically, it is canceled at the early stages of the disease [105]. As a consequence, M1 responses remain unsuppressed, and cytokines exhibit a pro-inflammatory profile with elevated levels of IFN-γ, TNFalpha, IL6 and reduced levels of IL-4 and IL-10 [105,106].

Behavior of resident placental macrophages in PE has not received the proper attention of the researchers as yet. This can be possibly explained by the complexity of the biomaterial collection and the time-consuming procedure of isolation and phenotyping of the cells from this material in contrast to the easily obtained blood samples. However, this subject requires a comprehensive study. In the context of any pregnancy complication, placental macrophages should be considered as two populations: Hofbauer cells of the fetal placenta and decidual macrophages of the maternal placental part.

Recently published works comprise somewhat controversial numerical estimates for both Hofbauer cells and decidual macrophages in preeclamptic placentas. Yang and colleagues observed significantly lower numbers of CD14 + Hofbauer cells in PE placenta as compared with the healthy control but no corresponding significant di fference in CD68+ cell numbers was found [94]. Tang et al. observed significantly declined numbers of CD68+ Hofbauer cells in PE group in comparison with the gestation age-matched preterm birth control group [95]. By contrast, Evsen and colleagues, on the contrary, observed increased Hofbauer cell numbers in PE complicated by HELLP syndrome group compared to the control group; the authors also used CD68 as a macrophage marker [96]. Saeed et al. report a two-fold increase in the Hofbauer cell number in preeclamptic placentas in comparison to normotensive pregnancy [98]. As for decidual macrophages, their comparative abundance in the preeclamptic placenta is also a controversial subject. Schonkeren and colleagues reported an increase in number of CD14+ cells in the decidua basalis for preterm preeclamptic pregnancies compared with preterm control pregnancies [50]. Milosevic-Stevanovic et al. observed higher numbers of CD68 + decidual cells in PE compared to healthy control placentas [100]. In one study, a significant increase in number of CD68+ cells, in both fetal and decidual parts of placenta, was observed in preeclamptic group as compared to controls [97]. At the same time, several research groups report that decidual macrophage numbers in PE placentas are reduced [94,101,102]. Apparent inconsistencies between the studies may be explained by the use of di fferent technical approaches (cell markers, antibodies, signal detection protocols, etc.) and the di fference in the formation of studied groups.

The issue of macrophage polarization in preeclamptic placenta looks less ambiguous. Yang and colleagues showed that the level of CD163 + Hofbauer cells is significantly downregulated in PE compared with healthy pregnancies [94]. In work of Tang's and colleagues they also observed a decrease in CD163 + cell numbers compared with the preterm control [95]. Przybyl et al. reported reduced CD74+ cell numbers in preeclamptic placentas; according to the proposed model, this may lead to a pro-inflammatory signature [99]. Ma et al. observed an increase in the percentage of CD11b-iNOS co-labeled cells and a concomitant decrease in the percentage of CD11b-Arg1 co-labeled cells in preeclamptic placentas as compared with normal ones [93]; the combinations of markers reflect M1 and M2 phenotypes, respectively. The ratio of CD163/CD14 decidual cells was also found to be declined in placental samples collected from women with PE [50].

The shift in the balance of M2/M1 macrophages towards M1 is explained by high levels of pro-inflammatory cytokines and low levels of anti-inflammatory cytokines within the preeclamptic placenta [107,108]. In addition to the altered cytokine production, there is also a cellular axis in the process of polarization. A number of studies sugges<sup>t</sup> an essential role of placental mesenchymal stem cells in macrophage polarization and their ability to a ffect their activation [109,110]. Wang and colleagues revealed the role of hyaluronan in maintaining normal pregnancy. Their findings indicate

that high levels of hyaluronan induce M2 polarization and regulate production of cytokines (e.g., IL-10) by decidual macrophages [111].

The summarizing scheme is presented in Figure 1. Despite the large body of available data, several questions are still remaining unanswered. When do the observed changes in macrophage polarization really emerge—a<sup>t</sup> the early stages of pregnancy or after the PE manifestation? At what gestational age could they be valid as markers? Can we use macrophages and monocytes for therapeutic purposes?

**Figure 1.** Summarizing scheme. Monocyte–macrophage system in decidua, fetal and maternal placental parts and blood flow of mother and fetus. Classical (CD14++CD16−), non-classical (CD16++CD14+) and intermediate (CD14+CD16+) monocyte populations as well as proinflammatory (M1) and anti-inflammatory (M2) macrophages are shown. Modified from [26] (distributed under CC-BY).
