IN-VIVO DETECTION OF DENDRITIC CELL ANTIGEN PRESENTATION TO CD4(-CELLS() T)

Although lymphoid dendritic cells (DC) are thought to play an essentia l role in T cell activation, the initial physical interaction between antigen-bearing DC and antigen-specific T cells has never been directl y observed in vivo under conditions where the specificity of the respo nding T cells for the relevant antigen could be unambiguously assessed . We used confocal microscopy to track the in vivo location of fluores cent dye-labeled DC and naive TCR transgenic CD4(+) T cells specific f or an OVA peptide-I-A(d) complex after adoptive transfer into syngenei c recipients. DC that were not exposed to the OVA peptide, homed to th e paracortical regions of the lymph nodes but did not interact with th e OVA peptide-specific T cells. In contrast, the OVA peptide-specific T cells formed large clusters around paracortical DC that were pulsed in vitro with the OVA peptide before injection. Interactions were also observed between paracortical DC of the recipient and OVA peptide-spe cific T cells after administration of intact OVA. Injection of OVA pep tide-pulsed DC caused the specific T cells to produce IL-2 in vivo, pr oliferate, and differentiate into effector cells capable of causing a delayed-type hypersensitivity reaction. Surprisingly, by 48 h after in jection, OVA peptide-pulsed, but not unpulsed DC disappeared from the lymph nodes of mice that contained the transferred TCR transgenic popu lation. These results demonstrate that antigen-bearing DC directly int eract with naive antigen-specific T cells within the T cell-rich regio ns of lymph nodes. This interaction results in T cell activation and d isappearance of the DC.