Dendritic cells in antitumor immune responses. II. Dendritic cells grown from bone marrow precursors, but not mature DC from tumor-bearing mice, are effective antigen carriers in the therapy of established tumors.

Antitumor CTL responses were studied in a model tumor hearing a mutant human p53 gene. We found ineffective induction of antitumor CTL in mice bearing these tumors associated with measurable defects in the function of dendritic cells (DC) from these animals. In this study we investigate the mechanism of this defect in mature DC and find that functional DC can be generated by growth from the bone marrow of tumor-hearing animals. Tumor cell supernatants did not affect the function of mature DC obtained from the spleen of tumor-bearing animals, but significantly suppressed the ability to generate functional DC from the bone marrow of control mice in vitro. This suggests that tumor cells may release factors which block early stages of DC maturation from precursors. DC generated from the bone marrow of tumor-bearing mice showed normal potential to stimulate allogeneic T cells, to stimulate anti-mutant p53 peptide-specific cytotoxic T cells, and to induce anti-p53 CTL responses in vivo in control mice. Repeated immunization with peptide-pulsed DC generated from the bone marrow of control mice (every 4-5 days) blocked progression of established tumors. Immunization of mice with peptide-pulsed DC obtained from the spleen of tumor-bearing mice (4 weeks after tumor injection) did not affect the tumor growth, whereas immunization with peptide-pulsed DC generated from bone marrow of tumor-bearing mice resulted in significantly prolonged survival and delayed tumor growth. Tumor progression was associated with change of the balance Th1/Th2 cells in favor of the Th2-like cytokine profile, while effective immunization was associated with a shift to the Th1 phenotype. Thus, frequent immunization of mice with mutant p53 peptide-pulsed DC generated from stem cells of tumor-bearing hosts can induce effective antitumor CTL responses associated with production of Th1 cells and lead to significant antitumor effects.