Engineered exosomes emerging from muscle cells break immune tolerance to HER2 in transgenic mice and induce antigen-specific CTLs upon challenge by human dendritic cells.

ABSTRACT

We recently described a novel biotechnological platform for the production of unrestricted cytotoxic T lymphocyte (CTL) vaccines. It relies on in vivo engineering of exosomes, i.e., nanovesicles constitutively released by all cells, with full-length antigens of choice upon fusion with an exosome-anchoring protein referred to as Nefmut. They are produced upon intramuscular injection of a DNA vector and, when uploaded with a viral tumor antigen, were found to elicit an immune response inhibiting the tumor growth in a model of transplantable tumors. However, for a possible application in cancer immunotherapy, a number of key issues remained unmet. Among these, we investigated (i) whether the immunogenic stimulus induced by the engineered exosomes can break immune tolerance, and (ii) their effectiveness when applied in human system. As a model of immune tolerance, we considered mice transgenic for the expression of activated rat HER2/neu which spontaneously develop adenocarcinomas in all mammary glands. When these mice were injected with a DNA vector expressing the product of fusion between Nefmut and the extracellular domain of HER2/neu, antigen-specific CD8+ T lymphocytes became readily detectable. This immune response associated with a HER2-directed CTL activity and a significant delay in tumor development. On the other hand, through cross-priming experiments, we demonstrated the effectiveness of the engineered exosomes emerging from transfected human primary muscle cells in inducing antigen-specific CTLs. We propose our CTL vaccine platform as part of new immunotherapy strategies against tumors expressing self-antigens, i.e., products highly expressed in oncologic lesions but tolerated by the immune system. KEY MESSAGES We established a novel, exosome-based method to produce unrestricted CTL vaccines. This strategy is effective in breaking the tolerance towards tumor self-antigens. Our method is also useful to elicit antigen-specific CTL immunity in humans. These findings open the way towards the use of this antitumor strategy in clinic.