Keratinocytes and cytokines.

The skin has long been recognized as a major producer of cytokines, but the keratinocyte as principal epidermal cell has received less attention as potential source and target of cytokines. Nevertheless, keratinocytes produce a plethora of cytokines including interleukin (IL)-1, -6, -7, -8, -10, -12, -15, -18, and -20, and tumor necrosis factor alpha (TNF). The production by keratinocytes of pro-inflammatory (IL)-1, -6, -8, and TNF was recognized early and is well studied. Keratinocyte-derived IL-7 and -15 are considered to be significant in T-cell trafficking, possibly even in the pathogenesis of cutaneous T-cell lymphoma. Immunomodulatory IL-10 and -12 originating from keratinocytes are considered to be responsible for systemic effects, and IL-18 perhaps has a similar action. Keratinocytes were fairly recently recognized as being source or target of other IL-10 family members like IL-20 and IL-24 and the role of these cytokines in specific diseases is under investigation. In addition, a variety of cytokine receptors are present on keratinocytes like those for IL-4, -13, and -17 and to lesser degree IL-2. The ability to study the expression of cytokines by keratinocytes in vivo and in vitro using primary cells, immortalized cells or even organotypic culture systems offers many possibilities to further investigate the role of cytokine production in keratinocyte biology and disease.