Glucocorticoid-induced tumor necrosis factor (TNF) receptor-related protein ligand (GITRL) is a recently described co-stimulatory molecule expressed by antigen-presenting cells (APCs). Activated keratinocytes are known to engage intraepithelial T cells through co-stimulatory molecules. This study investigated the expression and function of GITRL in resting keratinocytes. We showed by immunofluorescence and flow cytometry that keratinocytes from Balb/C and C57Bl/6 mice, as well as PAM 212 murine cell line keratinocytes and human epidermal keratinocytes (HEK), express cell-surface GITRL. Stimulation of murine skin biopsies and HEK with GITR fusion protein (GITR: Fc FP) resulted in mRNA induction for chemoattractants: cutaneous T-cell-attracting chemokine (CTACK), thymus and activation-regulated chemokine (TARC), IL-8, monocyte chemoattractant protein-1 (MCP-1), and murine beta-defensin 3 (MBD3). Immunofluorescent studies on mouse biopsies treated with GITR: Fc FP confirmed corresponding TARC and MCP-1 protein production by keratinocytes. Chemokine induction was shown to be NF-kappaB-mediated. T-cell proliferation was enhanced by the addition of keratinocytes. This was reversed by pretreatment with an anti-GITRL antibody. We conclude that keratinocytes express GITRL, and that through this important co-stimulatory molecule, they have the potential to influence T-cell numbers in the skin through chemokine production and through a direct cell-cell effect on T-cell proliferation.Journal of Investigative Dermatology advance online publication, 18 June 2009; doi:10.1038/jid.2009.163.