Calpains Released from Necrotic Tumor Cells Enhance Antigen Cross-Presentation to Activate CD8+ T Cells In Vitro.

The initiation of CD8+ T cell responses against dead cell-associated Ags is tightly regulated, facilitating adaptive immunity against pathogens and tumors while preventing autoimmunity. It is now well established that dying cells actively regulate the generation of CD8+ T cell responses via the release or exposure of damage-associated molecular patterns. However, it is unclear whether nonproteasomal proteases (activated in stressed and dying cells) can influence the availability of Ags for cross-presentation. Using a mouse model of immunogenic necrosis, we investigated the role of tumor-derived proteases in the priming of CD8+ T cells. We demonstrate that proteases released from necrotic tumor cells can degrade whole-protein Ag, generating proteolytic intermediates that are efficiently cross-presented by dendritic cells and enhance CD8+ T cell cross-priming. We identify a dominant role for calpain proteases, which are activated during necrotic cell death induced by severe heat shock. Mechanistically, proteolytic intermediates generated by tumor-derived proteases associate with necrotic tumor cell debris, which acts as a vehicle for Ag transfer that facilitates highly efficient cross-presentation in dendritic cells. Our results suggest that proteolytic systems activated in Ag donor cells during cell death may influence the availability of antigenic substrates for cross-presentation, thereby regulating the antigenicity of cell death.