Immature dendritic cells (imDCs) can have a tolerizing effect under normal conditions or after transplantation. However, because of the significant heterogeneity of this cell population, it is extremely difficult to study the mechanisms that mediate the tolerance induced or to harness the application of imDCs for clinical use. In the present study, we describe the generation of a highly defined population of imDCs from hematopoietic progenitors and the direct visualization of the fate of TCR-transgenic alloreactive CD4(+) and CD8(+) T cells after encountering cognate or noncognate imDCs. Whereas CD4(+) T cells were deleted via an MHC-independent mechanism through the NO system, CD8(+) T-cell deletion was found to occur through a unique MHC-dependent, perforin-based killing mechanism involving activation of TLR7 and signaling through Triggering Receptor-1 Expressed on Myeloid cells (TREM-1). This novel subpopulation of perforin-expressing imDCs was also detected in various lymphoid tissues in normal animals and its frequency was markedly enhanced after GM-CSF administration.