Dynamics of Macrophage Polarization Reveal Unique Phenotypes During Mouse Postpartum Uterine Remodeling

Macrophages play essential roles in uterine physiology during pregnancy and the postpartum (PP) period. In various organ systems, macrophages exhibit distinct M1 (pro-inflammatory) and M2 (anti-inflammatory) polarization states, the balance of which is key not only in scarless wound healing but also in pathological fibrosis. While macrophages are central players in PP uterine remodeling, their polarization dynamics during this period and associated spatial distribution remain largely unknown.

Uterine tissues were collected from C57BL/6J wild-type mice at various PP days (PPD 1, 3, 7, 14, 21, 28) and non-pregnant controls. Multicolor flow cytometry was used to analyze immune populations and macrophage polarization states. Macrophage polarization was determined using CD86 (M1 marker) and CD206 (M2 marker). Immunofluorescence (IF) and immunohistochemistry (IHC) were used to examine the colocalization of M1 and/or M2 markers alongside F4/80 to assess macrophage polarization status and tissue distribution.

On PPD1, uterine macrophage numbers were higher compared to all other time points, with significantly increased M2 macrophages (CD86--CD206+) (43% p<0.0001) compared to PPD3 (5.1%), PPD7 (2.4%), PPD14 (2.3%), and non-pregnant (1.9%). A rapid polarization shift to M1 macrophages (CD86+CD206-) occurred on PPD3 (50.6%), PPD7 (66.5%), and PPD14 (58.8%) compared to PPD1 (11.9% p<0.0001). Additionally, there was a trend of increased M2 macrophages on PPD21 (10%) and PPD28 (5%). We observed a unique macrophage subset of mixed phenotype (co-expressing CD86+CD206+) significantly higher on PPD 1 (37.2% p<0.05) compared to NP (7.8%), PPD7 (11.6%), and PPD14 (26.4%). IF co-staining confirmed these dynamics, showing the highest percentage of macrophages co-expressing CD206+F4/80+ on PPD1 (77.2%), significantly higher than all other time points. IHC revealed the specific temporal and spatial distribution of F4/80+ and CD206+ cells within the uterine endometrial stroma adjacent to the myometrial junction in the early PP period.

The predominance of M2-polarized macrophages on PPD1 followed by a rapid shift to M1 phenotype from PPD3 to PPD14 indicates an evolving inflammatory response during the early postpartum period. Repolarization to M2 profile on PPD21, along with increased abundance of a mixed population (M1/M2), uncovers the dynamic complexity of macrophage polarization during PP uterine remodeling, providing new insights into scarless uterine tissue repair.