Apoptotic tumour cells express neo-antigens that are sensed by the immune system and drive effective antitumour responses. This ‘immunogenic cell death’ (ICD) can be elicited in vivo following bortezomib (26S proteasome inhibitor) treatment in some cancers. Here, we test whether bortezomib-induced ER stress and apoptosis in melanoma promotes ICD. We demonstrate that sub-clinical doses of bortezomib induced apoptosis in vitro. Classic hallmarks of ICD, including cell-surface expression of calreticulin, HSP70 and HSP90 and the secretion of HMGB1 and ATP by apoptotic melanoma cells, were detected following bortezomib treatment in human and murine melanoma cells, suggesting  ICD initiation.  Biosensor-mediated real-time imaging confirmed bortezomib-induced ER stress in melanoma cells. We next asked whether bortezomib-treated melanoma cells could elicit anti-tumour immunity in a functional immune system in vivo. Mice were primed subcutaneously with melanoma cells that are undergoing ICD. Mice were then challenged 10 days later subcutaneously on the opposing flank with matched, live melanoma cells, and tumour growth was monitored. We observed a delay in tumour initiation and growth in mice that were primed by ICD compared to controls. These data present evidence that bortezomib induces ER stress, apoptosis and subsequently ICD of melanoma cells and promotes effective anti-tumour immunity. We therefore propose that utilising ICD to recruit an immune response could further improve current immune therapies.

Bortezomib induces immunogenic cell death in melanoma and enhances immune responses in vivo