Dendritic cells play a key role in establishing the class of immune response against invading pathogens. Upon engagement with double-stranded RNA, a major bioactive constituent of many virus types, immature dendritic cells develop into type 1 immunostimulatory dendritic cells that promote Th1 responses. Immature dendritic cells reside in the epithelia and are in close contact with keratinocytes. We studied to what extent dendritic cells can also adopt a type 1 immunostimulatory dendritic cell phenotype indirectly, as a result of the interaction with keratinocytes responding to double-stranded RNA. In contrast to supernatants from keratinocytes activated by the combination of tumor necrosis factor alpha and interleukin-1beta, supernatants from keratinocytes activated by synthetic double-stranded RNA, polyriboinosinic polyribocytidylic acid, comprised tumor necrosis factor alpha and type I interferons, which induced maturation of human monocyte-derived immature dendritic cells. In addition, dendritic cells matured in the presence of these supernatants strongly biased the development of Th1 cells from naive Th cells. This bias was dependent on keratinocyte-derived interferon-alpha/beta and interleukin-18, as neutralization of both interferon-alpha/beta and interleukin-18 in the keratinocyte culture supernatant reduced the development of interferon-gamma-producing Th cells. These findings suggest that keratinocytes can contribute to the development of selective Th1/Th2 responses through the induction of maturation and functional polarization of dendritic cells, indicating a novel role for keratinocytes as initiators and regulators of cutaneous T-cell-mediated inflammation. In addition, these results support the concept that, in addition to direct interaction with pathogens, dendritic cells may also be activated and primed by pathogen indirectly, via the effect of resident tissue cells responding to pathogen.