Immature dendritic cells (DCs) use chemokines and intercellular adhesion molecule (ICAM)-1, but not DC-specific ICAM-3-grabbing nonintegrin, to stimulate CD4+ T cells in the absence of exogenous antigen.

Dendritic cells (DCs) possess a number of unique features that distinguish them from other APCs. One such feature is their ability to trigger Ag-independent responses in T cells. Previous studies have focused on mature DCs, but the prevalence of this phenomenon in the resting-state immature DCs has never been considered. In this study, we show that, in the absence of Ag, human immature DCs trigger multiple responses in autologous primary CD4+ T cells, namely, increased motility, small Ca2+ transients, and up-regulation of CD69. These responses are particularly marked in CD4+ memory T cells. By using several experimental approaches, we found that DC-specific ICAM-3-grabbing nonintegrin plays no role in the induction of T cell responses, whereas ICAM-1/LFA-1 interactions are required. In addition, DC-produced chemokines contribute to the Ag-independent T cell stimulatory ability of DCs, because pertussis toxin-treated T cells exhibit diminished responses to immature DCs. More particularly, CCL17 and CCL22, which are constitutively produced by immature DCs, mediate both T cell polarization and attraction. Thus, immature DCs owe part of their outstanding Ag-independent T cell stimulatory ability to chemokines and ICAM-1, but not DC-specific ICAM-3-grabbing nonintegrin.