Tumor-derived autophagosome vaccine: induction of cross-protective immune responses against short-lived proteins through a p62-dependent mechanism.

PURPOSE Tumor-specific antigens of 3-methylcholanthrene (MCA)-induced sarcomas were defined by the narrow immune responses they elicited, which uniquely rejected the homologous tumor, with no cross-reactions between independently derived syngeneic MCA-induced tumors. This study examines whether an autophagosome-enriched vaccine derived from bortezomib-treated sarcomas can elicit an immune response that cross-reacts with other unique sarcomas. EXPERIMENTAL DESIGN Mice were vaccinated with either MCA-induced sarcomas or autophagosomes derived from those tumors and later challenged with either homologous or nonhomologous sarcomas. In addition, 293 cells expressing a model antigen were used to understand the necessity of short-lived proteins (SLiP) in this novel vaccine. These findings were then tested in the sarcoma model. Autophagosomes were characterized by Western blotting and fluorescent microscopy, and their ability to generate immune responses was assessed in vitro by carboxyfluorescein succinimidyl ester dilution of antigen-specific T cells and in vivo by monitoring tumor growth. RESULTS In contrast to a whole-cell tumor vaccine, autophagosomes isolated from MCA-induced sarcomas treated with a proteasome inhibitor prime T cells that cross-react with different sarcomas and protect a significant proportion of vaccinated hosts from a nonhomologous tumor challenge. Ubiquitinated SLiPs, which are stabilized by proteasome blockade and delivered to autophagosomes in a p62/sequestosome-dependent fashion, are a critical component of the autophagosome vaccine, as their depletion limits vaccine efficacy. CONCLUSION This work suggests that common short-lived tumor-specific antigens, not physiologically available for cross-presentation, can be sequestered in autophagosomes by p62 and used as a vaccine to elicit cross-protection against independently derived sarcomas.