Molecular characterization of dendritic cell-derived exosomes: Selective accumulation of the heat shock protein hsc73

Exosomes are membrane vesicles secreted by hematopoietic cells upon fusion of late multivesicular endosomes with the plasma membrane. Dendritic cell ( DC)-derived exosomes induce potent antitumor immune responses in mice, resu lting in the regression of established tumors (Zitvogel, L., A. Regnault, A . Lozier, J. Wolfers, C. Flament, D. Tenza, P. Ricciardi-Castagnoli, G. Rap oso, and S. Amigorena. 1998. Nat. Med. 4:594-600). To unravel the molecular basis of exosome-induced immune stimulation, we now analyze the regulation of their production during DC maturation and characterize extensively thei r protein composition by peptide mass mapping. Exosomes contain several cyt o cytosolic proteins (including annexin II, heat shock cognate protein hsc7 3, and heteromeric G protein Gi2 alpha), as well as different integral or p eripherally associated membrane: proteins (major histocompatiblity complex dass II, Mac-1 integrin, CD9, milk fat globule-EGF- factor VIII [MFG-E8]). MFG-E8, the major exosomal component, binds integrins expressed by DCs and macrophages, suggesting that it may be involved in exosome targeting to the se professional antigen-presenting cells. Another exosome component is hsc7 3, a cytosolic heat shock protein (hsp) also present in DC endocytic compar tments. hsc73 was shown to induce antitumor immune responses in vivo, and t herefore could be involved in the exosome's potent antitumor effects. Final ly, exosome production is downregulated upon DC maturation, indicating that in vivo, exosomes are produced by immature DCs in peripheral tissues. Thus , DC-derived exosomes accumulate a defined subset of cellular proteins refl ecting their endosomal biogenesis and accounting for their biological funct ion.