Essential Role of Macrophages in Contact Hypersensitivity–Induced Hair Regeneration

Contact hypersensitivity (CHS) has long been used to treat alopecia areata, an autoimmune disease caused by autoreactive T cells ( Kaplan et al., 2012 Kaplan D.H. Igyártó B.Z. Gaspari A.A. Early immune events in the induction of allergic contact dermatitis. Nature Reviews Immunology. 2012; 12: 114-124 Crossref PubMed Scopus (423) Google Scholar , Vocanson et al., 2009 Vocanson M. Hennino A. Rozieres A. Poyet G. Nicolas J.F. Effector and regulatory mechanisms in allergic contact dermatitis. Allergy. 2009; 64: 1699-1714 Crossref PubMed Scopus (281) Google Scholar ). In alopecia areata, anagen hair follicles (HFs) enter telogen prematurely, followed by the prolonged arrest of HF stem cells (HFSCs) quiescence ( Pratt et al., 2017 Pratt C.H. King L.E. Messenger A.G. Christiano A.M. Sundberg J.P. Alopecia areata. Nature reviews Disease primers. 2017; 3: 1-17 Crossref Scopus (421) Google Scholar ). CHS can induce hair growth in patients with and mouse models of alopecia areata ( Pratt et al., 2017 Pratt C.H. King L.E. Messenger A.G. Christiano A.M. Sundberg J.P. Alopecia areata. Nature reviews Disease primers. 2017; 3: 1-17 Crossref Scopus (421) Google Scholar , Tang et al., 2003 Tang L. Lui H. Sundberg J.P. Bissonnette R. McLean D.I. Shapiro J. Restoration of hair growth with topical diphencyprone in mouse and rat models of alopecia areata. Journal of the American Academy of Dermatology. 2003; 49: 1013-1019 Abstract Full Text Full Text PDF PubMed Scopus (15) Google Scholar ). How CHS activates HFSCs remains unclear. The beneficial effect has been attributed to its immunomodulation, which alleviates local autoimmunity ( Pratt et al., 2017 Pratt C.H. King L.E. Messenger A.G. Christiano A.M. Sundberg J.P. Alopecia areata. Nature reviews Disease primers. 2017; 3: 1-17 Crossref Scopus (421) Google Scholar ). In addition to immunomodulation, whether CHS also directly stimulate HFSCs to accelerate anagen entry is unknown.