M2 macrophages promote wound-induced hair neogenesis

De novo hair regeneration occurs in scars of normal adult mice. This interesting phenomenon is termed as wound-induced hair neogenesis (WIHN). We hypothesized that M2 macrophages are crucially involved in WIHN.

To clarify the contribution of M2 macrophages to WIHN.

We established a mouse model of WIHN. A full thickness skin excision was implemented on the back of C57BL/6 (B6) mice. Newly developing hair follicles were detected by a whole-mount assay. WIHN took place 2 weeks after wounding.

At first, flow cytometry revealed increased infiltration of CD11b⁺/CD206⁺ M2 macrophages at the 2nd and 3rd week after wounding. Immunohistochemistry also showed the existence of CD206⁺ M2 macrophages in the vicinity of regenerated hair follicles. Secondly, the productions of growth factors were confirmed by culturing M2 macrophages isolated from the skin in a comparison with CD11b⁺ spleen cells. Array for 84 genes revealed increased expressions of various growth factors including Igf1 and Fgf2. Thirdly, we verified the effect of the growth factors on WIHN. WIHN was increased by 2 folds in mice treated with Fgf2 (p=0.05) or by 1.5 folds with Igf1 (p=0.05). Finally, we used B6.Tg(ITGAM-DTR) mice in which macrophages are ablated by diphtheria toxin. We depleted macrophages at one to 2 weeks after wounding when M2 macrophages were dominant. WIHN was attenuated to one third (P=0.05) by the ablation of macrophages.

Our study suggests that M2 macrophages could promote WIHN through producing a panel of growth factors.