**3.2 Maintenance of activation**

TNF-α induced by IL-1 can maintain KCs in an activated state [75]. In psoriasis, a wide variety of cells produce TNF-α, primarily macrophages and monocytes but also epithelial cells including KCs [89]. TNF-α activates immune responses by inducing production of additional signaling molecules, cytokines, growth factors, their receptors, and adhesion proteins (e.g., amphiregulin, TGF-α, IL-1 α, IL-1 receptor antagonist, epidermal growth factor receptor (EGFR), and intercellular adhesion molecule (ICAM-1) [90]. In response to the activation of the EGFR, KCs proliferate, degrade components of the extracellular matrix, and become migratory [91].
