Decidual and endometrial macrophages

Generic macrophage (Wikimedia Commons)

Macrophages are ancient cells. There are multiple lineages in mammals, some of which originate in the yolk sac rather than being the progeny of bone marrow-derived monocytes (reviewed here).

Macrophages constitute some 25% of the leukocyte population in the human uterus. Because there is cyclical recruitment of macrophages to the uterus, they are thought to be recruited from Ly6c⁺ inflammatory monocytes. Interestingly, however, when monocytes are differentiated in vitro, the cells resemble but are not identical to decidual macrophages (here).

One of the factors acting on decidual macrophages is human leukocyte antigen G (HLA-G) derived from trophoblast.  An HLA-G homodimer can bind to the LILRB1-Fc fusion protein on the surface of macrophages. However, HLA-G is a novel member of MHC Class I found only in higher primates, so this mechanism is not relevant to all mammals.

Ruminants

Macrophage differerentiation from Oliveira et al. (here) CC licence

Macrophages are abundant in the endometrial stroma of the bovine uterus especially in the regions between placentomes. There is evidence that these belong to the alternatively activated macrophage category (M2 pathway) and are endowed with immunosoppressive properties (here).