Cross-Presentation Protein 90 of Dendritic Cells in Antigen Essential Role of Endogenous Heat Shock

Extracellular HSP90 associated with Ag peptides have been demonstrated to efficiently cross-prime T cells, following internal-ization by dendritic cells (DCs). In addition, the nature of cell-associated Ags required for cross-priming is implicated as peptides and proteins chaperoned by heat shock protein (HSP). However, the role of endogenous HSP in DCs during cross-presentation remains elusive. In this paper, we show that endogenous HSP90 is essential for cross-presentation of both soluble and cell-associated Ags in DCs. Cross-presentation of soluble OVA and OVA-loaded transporter associated with Ag processing-1– deficient cells by bone marrow-derived DCs and DC-like cell line DC2.4 was profoundly blocked by HSP90 inhibitors, whereas presentation of endogenously expressed OVA was only partially suppressed. Assays using small interfering RNA and heat shock factor-1–deficient DCs (with defective expression of HSP90a) revealed the pivotal role of HSP90a in cross-presentation. The results suggest that in addition to HSP90 in Ag donor cells, endogenous HSP90 in DCs plays an essential role during Ag cross-presentation and, moreover, points to a link between heat shock factor-1–dependent induction of HSP90a within DC and cytotoxic T cell immunity. T he heat shock proteins (HSPs) are known as molecular chaperones and many members of this family are induced by heat shock, which is called " heat shock response " (1). HSP is also known as a stress protein whose expression is increased by conditions that cause protein denaturation or unfolding in the cells (2). Thus, HSP is engaged in: 1) promoting protein folding/refolding and preventing aggregation of misfolded proteins ; 2) targeting misfolded proteins for degradation by the pro-teasome; and 3) facilitating protein transport. In addition to the fundamental roles in the life cycle of cells, HSP was implicated as a player in adaptive immunity, especially cross-priming (3). HSP90–peptide complexes reconstituted in vitro were shown to be efficient in promoting cross-presentation (4). Moreover , HSP90 in Ag donor cells is essential for cross-priming of certain Ags, because cells treated with HSP90-specific inhibitor lose their immunogenicity (3, 5). The rationale behind this observation is based on the concept that antigenic peptides associated with HSP90 in Ag donor cells are released by the HSP90 inhibitor, becoming free forms that might become inefficient in terms of cross-priming. This idea is also supported by the genetic evidence that heat shock factor-1 (HSF-1)–deficient Ag donor cells (H-2 d) where HSP90 is mainly downregulated is less capable of cross-priming (6). Zheng and Li (6) have proposed that …