Molecular characterization of dendritic cell (DC)-derived exosomes reveals selective accumulation of the heat shock protein hsc73. Exosomes are membrane vesicles secreted by hematopoietic cells, and DC-derived exosomes induce potent antitumor immune responses in mice, leading to tumor regression. To understand the molecular basis of exosome-induced immune stimulation, the regulation of exosome production during DC maturation and their protein composition were analyzed. Exosomes contain several cytosolic proteins, including hsc73, and membrane proteins such as MHC class II, Mac-1 integrin, and CD9. The heat shock protein hsc73 is a cytosolic protein that induces antitumor immune responses in vivo and accumulates in exosomes. Exosome production is downregulated upon DC maturation, indicating that exosomes are produced by immature DCs in peripheral tissues. Exosomes originate from endocytic compartments, such as multivesicular bodies (MVBs), and are released upon fusion with the plasma membrane. Exosomes from DCs contain MHC class II and MHC class I molecules, and those pulsed with tumor-derived antigenic peptides induce potent immune responses, leading to tumor regression in mice. The mechanisms of this antitumor effect are still unknown. The study identified eight major exosomal proteins, including MHC class II molecules, cytosolic proteins potentially involved in exosome formation, and membrane proteins necessary for exosome targeting. The protein hsc73 is a potent inducer of antitumor immune responses and accumulates in exosomes. The study also showed that hsc73 is selectively accumulated in endocytic compartments, suggesting its role in exosome biogenesis and function. The presence of hsc73 in exosomes is interesting in relation to exosome biology, as it may contribute to their antitumor effects. The study also identified other exosomal proteins, such as MFG-E8, which binds to integrins expressed by DCs and macrophages, suggesting a role in exosome targeting. The study highlights the importance of exosome composition in their biological functions and their potential as therapeutic agents in cancer treatment.Molecular characterization of dendritic cell (DC)-derived exosomes reveals selective accumulation of the heat shock protein hsc73. Exosomes are membrane vesicles secreted by hematopoietic cells, and DC-derived exosomes induce potent antitumor immune responses in mice, leading to tumor regression. To understand the molecular basis of exosome-induced immune stimulation, the regulation of exosome production during DC maturation and their protein composition were analyzed. Exosomes contain several cytosolic proteins, including hsc73, and membrane proteins such as MHC class II, Mac-1 integrin, and CD9. The heat shock protein hsc73 is a cytosolic protein that induces antitumor immune responses in vivo and accumulates in exosomes. Exosome production is downregulated upon DC maturation, indicating that exosomes are produced by immature DCs in peripheral tissues. Exosomes originate from endocytic compartments, such as multivesicular bodies (MVBs), and are released upon fusion with the plasma membrane. Exosomes from DCs contain MHC class II and MHC class I molecules, and those pulsed with tumor-derived antigenic peptides induce potent immune responses, leading to tumor regression in mice. The mechanisms of this antitumor effect are still unknown. The study identified eight major exosomal proteins, including MHC class II molecules, cytosolic proteins potentially involved in exosome formation, and membrane proteins necessary for exosome targeting. The protein hsc73 is a potent inducer of antitumor immune responses and accumulates in exosomes. The study also showed that hsc73 is selectively accumulated in endocytic compartments, suggesting its role in exosome biogenesis and function. The presence of hsc73 in exosomes is interesting in relation to exosome biology, as it may contribute to their antitumor effects. The study also identified other exosomal proteins, such as MFG-E8, which binds to integrins expressed by DCs and macrophages, suggesting a role in exosome targeting. The study highlights the importance of exosome composition in their biological functions and their potential as therapeutic agents in cancer treatment.