TREM2+ Macrophages Inhibit Hair Growth by Producing Oncostatin M

In the 2019 study by Wang et al., researchers discovered that a specific group of macrophages in the dermis, characterized by TREM2 expression, inhibit hair growth by producing Oncostatin M (OSM). This cytokine maintains hair follicle stem cell (HFSC) quiescence through the JAK-STAT signaling pathway, particularly affecting STAT5. Inhibiting OSM led to the activation of the hair growth phase (anagen). Additionally, removing CSF1R-expressing cells, which include these macrophages, caused an increase in hair follicle cell proliferation and induced the anagen phase locally. These findings indicate that the OSM-OSMRb-STAT5 signaling pathway in keratinocytes, mediated by TREM2+ macrophages or "trichophages," is a significant inhibitor of hair growth, playing a crucial role in keeping HFSCs inactive and thus restraining hair growth.