The co-translocation of ERp57 and calreticulin determines the immunogenicity of cell death

The exposure of calreticulin (CRT) on the plasma membrane can precede anthracycline-induced apoptosis and is required for cell death to be perceived as immunogenic. Mass spectroscopy, immunofluorescence and immunoprecipitation experiments revealed that CRT co-translocates to the surface with another endoplasmic reticulum-sessile protein, the disulfide isomerase ERp57. The knockout and knockdown of CRT or ERp57 inhibited the anthracycline-induced translocation of ERp57 or CRT, respectively. CRT point mutants that fail to interact with ERp57 were unable to restore ERp57 translocation upon transfection into crt^-/- cells, underscoring that a direct interaction between CRT and ERp57 is strictly required for their co-translocation to the surface. ERp57^low tumor cells generated by retroviral introduction of an ERp57-specific shRNA exhibited a normal apoptotic response to anthracyclines in vitro, yet were resistant to anthracycline treatment in vivo. Moreover, ERp57^low cancer cells (which failed to expose CRT) treated with anthracyclines were unable to elicit an anti-tumor response in conditions in which control cells were highly immunogenic. The failure of ERp57^low cells to elicit immune responses and to respond to chemotherapy could be overcome by exogenous supply of recombinant CRT protein. These results indicate that tumors that possess an intrinsic defect in the CRT-translocating machinery become resistant to anthracycline chemotherapy due to their incapacity to elicit an anti-cancer immune response.