Membrane‐bound HSP70‐engineered myeloma cell‐derived exosomes stimulate more efficient CD8 + CTL‐ and NK‐mediated antitumour immunity than exosomes released from heat‐shocked tumour cells expressing cytoplasmic HSP70

Exosomes (EXO) derived from tumour cells have been used to stimulate antitumour immune responses, but only resulting in prophylatic immunity. Tumour‐derived heat shock protein 70 (HSP70) molecules are molecular chaperones with a broad repertoire of tumour antigen peptides capable of stimulating dendritic cell (DC) maturation and T‐cell immune responses. To enhance EXO‐based antitumour immunity, we generated an engineered myeloma cell line J558 HSP expressing endogenous P1A tumour antigen and transgenic form of membrane‐bound HSP70 and heat‐shocked J558 HS expressing cytoplasmic HSP70, and purified EXO HSP and EXO HS from J558 HSP and J558 HS tumour cell culture supernatants by ultracentrifugation. We found that EXO HSP were able to more efficiently stimulate maturation of DCs with up‐regulation of Ia b , CD40, CD80 and inflammatory cytokines than EXO HS after overnight incubation of immature bone‐marrow‐derived DCs (5 × 10 6 cells) with EXO (100 μg), respectively. We also i.v. immunized BALB/c mice with EXO (30 μg/mouse) and assessed P1A‐specific T‐cell responses after immunization. We demonstrate that EXO HSP are able to stimulate type 1 CD4 + helper T (Th1) cell responses, and more efficient P1A‐specific CD8 + cytotoxic T lymphocyte (CTL) responses and antitumour immunity than EXO HS . In addition, we further elucidate that EXO HSP ‐stimulated antitumour immunity is mediated by both P1A‐specific CD8 + CTL and non‐P1A‐specific natural killer (NK) responses. Therefore, membrane‐bound HSP70‐expressing tumour cell‐released EXO may represent a more effective EXO‐based vaccine in induction of antitumour immunity.