CpG-A stimulates Hsp72 secretion from plasmacytoid dendritic cells, facilitating cross-presentation

• Immunization with Hsp90-peptide complex in combination with CpG-Awas more effective for CTL induction than was immunization with Hsp90-peptide complex alone.
• PlasmacytoidDCs secreted heat shock protein 72 (Hsp72) in response to CpG administration in a TLR9-dependent manner.
• Extracellular Hsp72 bound to an Hsp90-peptide complex and enhanced binding of Hsp90-peptide complex to pDC, resulting in efficient cross-presentation and peptide-specific CTL induction.

Plasmacytoid dendritic cells (pDCs) are the main producers of IFN-α in response to unmethylated DNA molecules, including cytosine guanine dinucleotide (CpG)-DNA in vivo. pDCs specifically express toll-like receptor (TLR) 9 and are therefore able to recognize the unmethylated DNAs. It has recently been shown that not only conventional DCs (cDCs) but also pDCs efficiently cross-present exogenous antigens after TLR9 activation. However, the precise molecular mechanism has remained unclear. Here, we show that pDCs secreted heat shock protein 72 (Hsp72) in response to CpG-A administration in a TLR9-dependent manner. Extracellular Hsp72 bound to an Hsp90-peptide complex and enhanced binding of Hsp90-peptide complex to pDC, resulting in efficient cross-presentation. Our experiments therefore suggest a mechanism for orchestration of immune responses by stimulation of pDCs with CpG-A.