Dendritic cells (DC) purified from murine spleen or generated in vitro from bone marrow precursors were compared for their respective abilities to stimulate T cell responses and provide tumor protection in vivo. In vitro incubation with synthetic tumor peptide conferred on both DC populations the ability to induce proliferation of tumor-peptide-specific T cells in vitro. Spleen DC were reproducibly about twofold more effective than bone-marrow-derived DC in this assay. Both DC populations could also induce cytotoxic activity in vivo. In vitro cytoxicity assays showed that, while cytotoxic activity induced by immunization with spleen DC was clearly peptide-specific, a high non-specific cytotoxic activity was consistently observed after immunization with bone-marrow-derived DC, whether peptide-pulsed or not. Regardless of such high non-specific activity in vitro, only tumor-peptide-pulsed DC could provide protection against subsequent inoculation of tumor cells. DC not pulsed with tumor peptide were ineffective. We conclude that DC isolated from spleen or generated in vitro from bone marrow precursors are suitable reagents for use in tumor vaccination studies.

The contribution of CD4+ T cells to dendritic cell (DC) activation and to the induction of CD8+ T cell responses in vivo was investigated using a model of antitumor immune responses. Immunization with peptide-loaded MHC class II-deficient (MHC class II−/−) DC induced the activation of Ag-specific CD8+ T cells and their accumulation in the lymph nodes and spleens of immunized mice. The accumulation induced by MHC class II−/− DC immunization was lower than the accumulation observed after immunization with MHC class II+/+ DC. Similarly, immunization with peptide-loaded, MHC class II−/− DC induced some degree of protection against tumor challenge, but this protection was lower than the protection achieved after immunization with MHC class II+/+ DC. Incubation with a membrane-associated form of CD40 ligand resulted in the up-regulation of costimulatory molecules on MHC class II−/− DC and fully rescued their ability to induce antitumor immunity. We conclude that CD4+ T cells play a critical role in the generation of antitumor immune responses through their capacity to induce the activation of DC via CD40/CD40 ligand interaction, and thus maximize CD8+ T cell responses.

SummaryWe describe a novel activation antigen recognized by the monoclonal antibody CMRF-37. which is absent or only weakly expressed on resting cells but is rapidly induced on T cells, B cells and monocytes following stimuiation. Its kinetics of expression (12-24 h after the addition of the inductive signal) indicate that it is an early activation antigen. The cellular distribution and expression kinetics of the CMRF-37 antigen differ from other known activation antigens and as such should be a useful marker of human leucocyte activation.