A Signaling Polypeptide Derived from an Innate Immune Adaptor Molecule Can Be Harnessed as a New Class of Vaccine Adjuvant

Modulation of intracellular signaling using cell-permeable polypeptides is a promising technology for future clinical applications. To develop a novel approach to activate innate immune signaling by synthetic polypeptides, we characterized several different polypeptides derived from the caspase recruitment domain (CARD) of IFN-β promoter stimulator 1, each of which localizes to a different subcellular compartment. Of particular interest was, N′-CARD, which consisted of the nuclear localization signal of histone H2B and the IFN-β promoter stimulator 1CARD and which localized to the nucleus. This polypeptide led to a strong production of type I IFNs and molecular and genetic analyses showed that nuclear DNA helicase II is critically involved in this response. N′-CARD polypeptide fused to a protein transduction domain (N′-CARD-PTD) readily transmigrated from the outside to the inside of the cell and triggered innate immune signaling. Administration of N′-CARD-PTD polypeptide elicited production of type I IFNs, maturation of bone marrow-derived dendritic cells, and promotion of vaccine immunogenicity by enhancing Ag-specific Th1-type immune responses, thereby protecting mice from lethal influenza infection and from outgrowth of transplanted tumors in vivo. Thus, our results indicate that the N′-CARD-PTD polypeptide belongs to a new class of vaccine adjuvant that directly triggers intracellular signal transduction by a distinct mechanism from those engaged by conventional vaccine adjuvants, such as TLR ligands.