Screening of novel immunogenic cell death inducers within the NCI Mechanistic Diversity Set

Immunogenic cell death (ICD) inducers can be defined as agents that exert cytotoxic effects while stimulating an immune response against dead cell-associated antigens. When initiated by anthracyclines, ICD is accompanied by stereotyped molecular changes, including the pre-apoptotic exposure of calreticulin (CRT) on the cell surface, the lysosomal secretion of ATP during the blebbing phase of apoptosis, and the release of high mobility group box 1 (HMGB1) from dead cells. By means of genetically engineered human osteosarcoma U2OS cells, we screened the 879 anticancer compounds of the National Cancer Institute (NCI) Mechanistic Diversity Set for their ability to promote all these hallmarks of ICD in vitro. In line with previous findings from our group, several cardiac glycosides exhibit a robust propensity to elicit the major manifestations of ICD in cultured neoplastic cells. This screen pointed to septacidin, an antibiotic produced by Streptomyces fibriatus, as a novel putative inducer of ICD. In low-throughput validation experiments, septacidin promoted CRT exposure, ATP secretion and HGMB1 release from both U2OS cells and murine fibrosarcoma MCA205 cells. Moreover, septacidin-killed MCA205 cells protected immunocompetent mice against a re-challenge with living cancer cells of the same type. Finally, the antineoplastic effects of septacidin on established murine tumors were entirely dependent on T lymphocytes. Altogether, these results underscore the suitability of the high-throughput screening system described here for the identification of novel ICD inducers.