Induction of tumor-specific acquired immunity against already established tumors by selective stimulation of innate DEC-205+ dendritic cells

Two major distinct subsets of dendritic cells (DCs) are arranged to regulate our immune responses in vivo; 33D1⁺ and DEC-205⁺ DCs. Using anti-33D1-specific monoclonal antibody, 33D1⁺ DCs were successfully depleted from C57BL/6 mice. When 33D1⁺ DC-depleted mice were stimulated with LPS, serum IL-12, but not IL-10 secretion that may be mediated by the remaining DEC-205⁺ DCs was markedly enhanced, which may induce Th1 dominancy upon TLR signaling. The 33D1⁺ DC-depleted mice, implanted with syngeneic Hepa1-6 hepatoma or B16-F10 melanoma cells into the dermis, showed apparent inhibition of already established tumor growth in vivo when they were subcutaneously (sc) injected once or twice with LPS after tumor implantation. Moreover, the development of lung metastasis of B16-F10 melanoma cells injected intravenously was also suppressed when 33D1⁺ DC-deleted mice were stimulated twice with LPS in a similar manner, in which the actual cell number of NK1.1⁺CD3⁻ NK cells in lung tissues was markedly increased. Furthermore, intraperitoneal (ip) administration of a very small amount of melphalan (l-phenylalanine mustard; l-PAM) (0.25 mg/kg) in LPS-stimulated 33D1⁺ DC-deleted mice helped to induce H-2K^b-restricted epitope-specific CD8⁺ cytotoxic T lymphocytes (CTLs) among tumor-infiltrating lymphocytes against already established syngeneic E.G7-OVA lymphoma. These findings indicate the importance and effectiveness of selective targeting of a specific subset of DCs, such as DEC-205⁺ DCs alone or with a very small amount of anticancer drugs to activate both CD8⁺ CTLs and NK effectors without externally added tumor antigen stimulation in vivo and provide a new direction for tumor immunotherapy.