**2.4 Keratin 17**

Keratin 17 (K17) is a type I intermediate filament mainly expressed in the basal cells of epithelial in hair follicles. As a complex cytoskeletal protein, K17 regulates a numereous biological processes in epithelial layer, including cell proliferation and growth, skin inflammation and hair follicle cycling. Abnormal expression of K17 is found in various diseases ranging from psoriasis to malignancies such as breast, cervical, oral squamous and gastric carcinomas [63, 64].

Many studies have showed the aberrant expression of K17 in psoriatic epidermis [35, 65, 66]. This aberrant expression is mainly due to interferon-γ (IFN-γ) produced by activated T cell [67]. Various inflammatory mediators like IFN-γ, IL-6, IL-17 A, IL-22 derived from T cells induced transcription of keratin K17 through STAT3- and ERK1/2-dependent mechanisms [64, 68, 69]. Thus, all these cytokines are involved in the induction of K17/T cell/cytokine autoimmune loop and play an important role in the progression of psoriasis [68].

*Immunomodulatory Effect of Methotrexate Abruptly Controls Keratinocyte Activation in Psoriasis DOI: http://dx.doi.org/10.5772/intechopen.102811*
