CD56+ human dendritic cells (DCs) have recently been shown to differentiate from monocytes in response to GM-CSF and type 1 interferon in vitro. We show here that CD56+ cells freshly isolated from human peripheral blood contain a substantial subset of CD14+CD86+HLA-DR+ cells, which have the appearance of intermediate-sized lymphocytes but spontaneously differentiate into enlarged DC-like cells with substantially increased HLA-DR and CD86 expression or into fully mature CD83+ DCs in response to appropriate cytokines. Stimulation of CD56+ cells containing both DCs and abundant gammadelta T cells with zoledronate and interleukin-2 (IL-2) resulted in the rapid expansion of gammadelta T cells as well as in IFN-gamma, TNF-alpha, and IL-1beta but not in IL-4, IL-10, or IL-17 production. IFN-gamma, TNF-alpha, and IL-1beta production were almost completely abolished by depleting CD14+ cells from the CD56+ subset before stimulation. Likewise, depletion of CD14+ cells dramatically impaired gammadelta T-cell expansion. IFN-gamma production could also be blocked by neutralizing the effects of endogenous IL-1beta and TNF-alpha. Conversely, addition of recombinant IL-1beta, TNF-alpha, or both further enhanced IFN-gamma production and strongly up-regulated IL-6 production. Our data indicate that CD56+ DCs from human blood are capable of stimulating CD56+ gammadelta T cells, which may be harnessed for immunotherapy.