The C8/144B monoclonal antibody recognizes cytokeratin 15 and defines the location of human hair follicle stem cells

Stem cells are vital for the homeostasis of self-renewing tissues such as t he hair follicle, Epithelial stem cells have been implicated in tumorigenes is and wound healing, and their manipulation may have wide ranging applicat ions including gene therapy and tissue transplantation. Rodent hair follicl e stem cells have been localized to an area of the follicle called the bulg e, however, the identification and characterization of human hair follicle stem cells has been hampered by a lack of cellular markers for this area. W e have determined that the C8/144B monoclonal antibody, originally generate d against a short intracytoplasmic peptide of CD8, preferentially immunosta ins hair follicle bulge keratinocytes without staining the remaining hair f ollicle. Using expression cloning, we identified cytokeratin 15 as the kera tinocyte protein recognized by the C8/144B monoclonal antibody. By delineat ing the bulge using this antibody, we demonstrated that bulge cells possess a stem cell phenotype characterized by their slowly-cycling nature, prefer ential proliferation at the onset of new hair follicle growth, high level o f beta 1 integrin expression, and expression of cytokeratin 19.