Exosomal pMHC-I complex targets T cell-based vaccine to directly stimulate CTL responses leading to antitumor immunity in transgenic FVBneuN and HLA-A2/HER2 mice and eradicating trastuzumab-resistant tumor in athymic nude mice

One of the major obstacles in human epidermal growth factor receptor 2 (HER2)-specific trastuzumab antibody immunotherapy of HER2-positive breast cancer is the development of trastuzumab resistance, warranting the search for other therapeutic strategies. Using mouse models, we previously demonstrated that ovalbumin (OVA)-specific dendritic cell (DC)-released exosome (EXOOVA)-targeted CD4(+) T cell-based (OVA-T-EXO) vaccine stimulates efficient cytotoxic T lymphocyte (CTL) responses via exosomal peptide/major histocompatibility complex (pMHC)-I, exosomal CD80 and endogenous IL-2 signaling; and long-term CTL memory by means of via endogenous CD40L signaling. In this study, using two-photon microscopy, we provide the first visual evidence on targeting OVA-T-EXO to cognate CD8(+) T cells in vivo via exosomal pMHC-I complex. We prepared HER2/neu-specific Neu-T-EXO and HER2-T-EXO vaccines using adenoviral vector (AdVneu and AdV(HER2))-transfected DC (DCneu and DCHER2)-released EXOs (EXOneu and EXOHER2), and assessed their stimulatory effects on HER2/neu-specific CTL responses and antitumor immunity. We demonstrate that Neu-T-EXO vaccine is capable of stimulating efficient neu-specific CTL responses, leading to protective immunity against neu-expressing Tg1-1 breast cancer in all 6/6 transgenic (Tg) FVBneuN mice with neu-specific self-immune tolerance. We also demonstrate that HER2-T-EXO vaccine is capable of inducing HER2-specific CTL responses and protective immunity against transgene HLA-A2(+)HER2(+) BL6-10(A2/HER2) B16 melanoma in 2/8 double Tg HLA-A2/HER2 mice with HER2-specific self-immune tolerance. The remaining 6/8 mice had significantly prolonged survival. Furthermore, we demonstrate that HER2-T-EXO vaccine stimulates responses of CD8(+) T cells capable of not only inducing killing activity to HLA-A2(+)HER2(+) BL6-10A2/HER2 melanoma and trastuzumab-resistant BT474(A2) breast cancer cells in vitro but also eradicating 6-day palpable HER2(+) BT474(A2) breast cancer (3-4 mm in diameter) in athymic nude mice. Therefore, the novel T cell-based HER2-T-EXO vaccine may provide a new therapeutic alternative for women with HER2(+) breast cancer, especially for trastuzumab-resistant HER2(+) breast cancer patients.