Recombinant Lactaptin Induces Immunogenic Cell Death and Creates an Antitumor Vaccination Effect in Vivo with Enhancement by an IDO Inhibitor

Natural compounds of various origins are intensively investigated for their antitumor activity. Potential benefits of antitumor therapy can be achieved when cytotoxic agents kill cancer cells and these dying cancer cells drive adoptive immunity to the tumor. This strategy was successfully demonstrated for chemotherapeutic drugs that induce immunogenic type of cell death (ICD) with release of DAMPs (danger associated molecular patterns) and exposure of eat me signals. In this study, we demonstrated that recombinant human milk peptide lactaptin (RL2) induces death of cancer cells with ICD hallmarks in vitro with the release of ATP and high-mobility group box 1 protein (HMGB1) and exposure of calreticulin and HSP70 on the external cell membrane. RL2-treated cancer cells were efficiently engulfed by phagocytic cells. Using the syngeneic mouse model, we demonstrated that RL2-treated MX-7 rhabdomyosarcoma cells confer long-term immune-mediated protection against challenge with live MX-7 cells. We also analyzed the combinatorial antitumor effect of vaccination with RL2-treated cells and the inhibition of indoleamine 2,3-dioxygenase (IDO) with ethyl pyruvate. Compared to solo anti-tumor immunization with RL2-treated cells, additional chemical inhibition of IDO demonstrated better long-term antitumor responses than vaccination alone.