Psoriasis vulgaris is an autoimmune dermatosis characterized by type 1 T cell infiltration. Prolactin may be involved in the pathogenesis of psoriasis. CXC ligand 9 (CXCL9), CXCL10, and CXCL11 recruit type 1 T cells, and their production by keratinocytes is enhanced in psoriatic lesions. CXCL9, CXCL10, and CXCL11 production by keratinocytes depends on nuclear factor-kappaB (NF-kappaB) and signal transducer and activator of transcription (STAT)1 and that of CXCL11 depends on interferon (IFN)-regulatory factor (IRF)-1. We examined in vitro effects of prolactin on CXCL9, CXCL10, and CXCL11 production in human keratinocytes. Although prolactin alone was ineffective, it enhanced IFN-gamma-induced secretion and mRNA expression of CXCL9, CXCL10, and CXCL11 in parallel to the activation of STAT1, NF-kappaB, and IRF-1. Inhibitors of Janus kinase (JAK), p38 MAPK, and MAPK/ERK kinase (MEK) suppressed prolactin- plus IFN-gamma-induced CXCL9, CXCL10, and CXCL11 production and NF-kappaB, STAT1, and IRF-1 activities. Prolactin induced phosphorylation of JAK2 and ERK, whereas IFN-gamma induced phosphorylation of JAK1, JAK2, and p38 MAPK. Prolactin modestly or IFN-gamma greatly induced tyrosine phosphorylation of STAT1, and both were suppressed by JAK inhibitor. Prolactin modestly or IFN-gamma greatly induced serine phosphorylation of STAT1, which was suppressed by MEK or p38 MAPK inhibitor, respectively. Prolactin induced phosphorylation of inhibitory kappaBalpha and NF-kappaB p65, which was suppressed by MEK inhibitor. These results suggest that prolactin may enhance IFN-gamma-induced CXCL9, CXCL10, and CXCL11 production in keratinocytes via activation of STAT1, NF-kappaB, and IRF-1 through JAK2 and MEK/ERK pathways. Prolactin may promote type 1 T cell infiltration into psoriatic lesions via these chemokines.