Modification in the function of dendritic cells (DC), such as that achieved by microbial stimuli or T cell help, plays a critical role in determining the quality and size of adaptive responses to Ag. NKT cells bearing an invariant TCR (iNKT cells) restricted by nonpolymorphic CD1d molecules may constitute a readily available source of help for DC. We therefore examined T cell responses to i.v. injection of soluble Ag in the presence or the absence of iNKT cell stimulation with the CD1d-binding glycolipid α-galactosylceramide (α-GalCer). Considerably enhanced CD4⁺ and CD8⁺ T cell responses were observed when α-GalCer was administered at the same time as or close to OVA injection. This enhancement was dependent on the involvement of iNKT cells and CD1d molecules and required CD40 signaling. Studies in IFN-γR^−/− mice indicated that IFN-γ was not required for the adjuvant effect of α-GalCer. Consistent with this result, enhanced T cell responses were observed using OCH, an analog of α-GalCer with a truncated sphingosine chain and a reduced capacity to induce IFN-γ. Splenic DC from α-GalCer-treated animals expressed high levels of costimulatory molecules, suggesting maturation in response to iNKT cell activation. Furthermore, studies with cultured DC indicated that potentiation of T cell responses required presentation of specific peptide and α-GalCer by the same DC, implying conditioning of DC by iNKT cells. The iNKT-enhanced T cell responses resisted challenge with OVA-expressing tumors, whereas responses induced in the absence of iNKT stimulation did not. Thus, iNKT cells exert a significant influence on the efficacy of immune responses to soluble Ag by modulating DC function.

The fate of dendritic cells (DC) after they have initiated a T cell immune response is still undefined. We have monitored the migration of DC labeled with a fluorescent tracer and injected s.c. into naive mice or into mice with an ongoing immune response. DC not loaded with Ag were detected in the draining lymph node in excess of 7 days after injection with maximum numbers detectable ∼40 h after transfer. In contrast, DC that had been loaded with an MHC class I-binding peptide disappeared from the lymph node with kinetics that parallel the known kinetics of activation of CD8⁺ T cells to effector function. In the presence of high numbers of specific CTL precursors, as in TCR transgenic mice, DC numbers were significantly decreased by 72 h after injection. The rate of DC disappearance was extremely rapid and efficient in recently immunized mice and was slower in “memory” mice in which memory CD8⁺ cells needed to reacquire effector function before mediating DC elimination. We also show that CTL-mediated clearance of Ag-loaded DC has a notable effect on immune responses in vivo. Ag-specific CD8⁺ T cells failed to divide in response to Ag presented on a DC if the DC were targets of a pre-existing CTL response. The induction of antitumor immunity by tumor Ag-loaded DC was also impaired. Therefore, CTL-mediated clearance of Ag-loaded DC may serve as a negative feedback mechanism to limit the activity of DC within the lymph node.

Plasmacytoid dendritic cells (PDCs) are a unique leukocyte population capable of secreting high levels of type I interferon (IFN) in response to viruses and bacterial stimuli. In vitro experiments have shown that upon maturation, human and murine PDCs develop into potent immunostimulatory cells; however, their ability to prime an immune response in vivo remains to be addressed. We report that CpG-matured murine PDCs are capable of eliciting in naive mice antigen-specific CTLs against endogenous antigens as well as exogenous peptides, but not against an exogenous antigen. Type I IFN is not required for priming, as injection of CpG-matured PDCs into type I IFN receptor–deficient mice elicits functional CTL responses. Mature PDCs prime CTLs that secrete IFN-γ and protect mice from a tumor challenge. In contrast, immature PDCs are unable to prime antigen-specific CTLs. However, mice injected with immature PDCs are fully responsive to secondary antigenic challenges, suggesting that PDCs have not induced long-lasting tolerance via anergic or regulatory T cells. Our results underline the heterogeneity and plasticity of different antigen-presenting cells, and reveal an important role of mature PDCs in priming CD8 responses to endogenous antigens, in addition to their previously reported ability to modulate antiviral responses via type I IFN.