Involvement of CD56brightCD11c+ Cells in IL-18–Mediated Expansion of Human γδ T Cells

γδ T cells are considered to be innate lymphocytes that play an important role in host defense against tumors and infections. We recently reported that IL-18 markedly amplified γδ T cell responses to zoledronate (ZOL)/IL-2. In an extension of this finding, we analyzed the mechanism underlying the IL-18-mediated expansion of γδ T cells. After incubation of PBMCs with ZOL/IL-2/IL-18, the majority of the cells expressed γδ TCR, and the rest mostly exhibited CD56(bright)CD11c(+) under the conditions used in this study. CD56(bright)CD11c(+) cells were derived from a culture of CD56(int)CD11c(+) cells and CD14(+) cells in the presence of IL-2 and IL-18 without the addition of ZOL. They expressed IL-18Rs, HLA-DR, CD25, CD80, CD83, CD86, and CD11a/CD18. In addition, they produced IFN-γ, TNF-α, but not IL-12, when treated with IL-2/IL-18, and they exerted cytotoxicity against K562 cells, thus exhibiting characteristics of both NK cells and dendritic cells. Incubation of purified γδ T cells with CD56(bright)CD11c(+) cells in the presence of ZOL/IL-2/IL-18 resulted in the formation of massive cell clusters and led to the marked expansion of γδ T cells. However, both conventional CD56(-/int)CD11c(high) dendritic cells induced by GM-CSF/IL-4 and CD56(+)CD11c(-) NK cells failed to support the expansion of γδ T cells. These results strongly suggest that CD56(bright)CD11c(+) cells play a key role in the IL-18-mediated proliferation of γδ T cells.