Alopecia of IFN- γ Knockout Mouse as a Model for Disturbance of the Hair Cycle: A Unique Arrest of the Hair Cycle at the Anagen Phase Accompanied by Mitosis

Interferon-γ ^-/- (IFN-γ ^-/-) and IFN-γ ^+/+ C57BL/6 mice (3 weeks of age) completed the production of morphogenesis-derived hair. Around 6 weeks of age, however, most of the IFN-γ ^-/- but none of the IFN-γ ^+/+ mice began to lose hairs in the dorsal and occipital areas in the absence of inflammatory reactions, and the alopecia was sustained for at least several 10-week periods of observation. A single subcutaneous injection of IFN-γ to IFN-γ ^-/- mice at 3, but not 4, 5, or 8 weeks of age could protect all the mice from alopecia, revealing that the lack of IFN-γ around 3 weeks of age is directly responsible for the alopecia. Histologic features showed that the hair follicles of the IFN-γ ^+/+ mice passed through the anagen (4-5 weeks of age) and catagen/telogen (≈6 weeks of age) phases, whereas those of IFN-γ ^-/- mice (5 weeks of age or older) stayed in the anagen phase. TUNEL and bromodeoxyuridine experiments suggested that an arrest with unlimited DNA synthesis of the hair cycle in the anagen phase by the lack of IFN-γ-dependent apoptosis in the midfollicle region and diffuse shedding of previously formed hair induced alopecia in IFN-γ ^-/- mice.