Expression of the CD8 alpha alpha homodimer has been used to differentiate lymphoid (CD8alpha(+)) from myeloid (CD8alpha(-)) dendritic cells (DCs). We have reported that CD8alpha(+) and CD8alpha(-) DCs have differential abilities to stimulate proliferation in allogeneic T cells. However, no specific function has been attributed to DC-derived CD8alpha. The current study examines the hypothesis that CD8 alpha alpha expression on DCs regulates DC-induced T cell activation. CD8alpha(-) transduced bone marrow-derived DCs were more potent stimulators of T cell proliferation, and produced significantly greater quantities of IL-12 in co-culture with T cells. LCK, a kinase whose expression is reported to be T cell-restricted and known to bind to the cytoplasmic tail of CD8 alpha beta in T cells, was detected readily in primary CD8alpha(+) splenic DCs and at greater levels than CD8alpha(-) DCs from the same tissues. LCK also co-precipitated with CD8alpha on immunblots strongly suggesting its role in CD8alpha(+) DC-induced T cell activation. Collectively, these data show that CD8alpha expressed on DC may not only be a lineage/maturation marker but also contribute to DC function.