BackgroundImmunotherapy represents the future of clinical cancer treatment. The type of cancer cell death determines the antitumor immune response and thereby contributes to the efficacy of anticancer therapy and long-term survival of patients. Induction of immunogenic apoptosis or necroptosis in cancer cells does activate antitumor immunity, but resistance to these cell death modalities is common. Therefore, it is of great importance to find other ways to kill tumor cells. Recently, ferroptosis has been identified as a novel, iron-dependent form of regulated cell death but whether ferroptotic cancer cells are immunogenic is unknown.MethodsFerroptotic cell death in murine fibrosarcoma MCA205 or glioma GL261 cells was induced by RAS-selective lethal 3 and ferroptosis was analyzed by flow cytometry, atomic force and confocal microscopy. ATP and high-mobility group box 1 (HMGB1) release were detected by luminescence and ELISA assays, respectively. Immunogenicity in vitro was analyzed by coculturing of ferroptotic cancer cells with bone-marrow derived dendritic cells (BMDCs) and rate of phagocytosis and activation/maturation of BMDCs (CD11c+CD86+, CD11c+CD40+, CD11c+MHCII+, IL-6, RNAseq analysis). The tumor prophylactic vaccination model in immune-competent and immune compromised (Rag-2−/−) mice was used to analyze ferroptosis immunogenicity.ResultsFerroptosis can be induced in cancer cells by inhibition of glutathione peroxidase 4, as evidenced by confocal and atomic force microscopy and inhibitors’ analysis. We demonstrate for the first time that ferroptosis is immunogenic in vitro and in vivo. Early, but not late, ferroptotic cells promote the phenotypic maturation of BMDCs and elicit a vaccination-like effect in immune-competent mice but not in Rag-2−/− mice, suggesting that the mechanism of immunogenicity is very tightly regulated by the adaptive immune system and is time dependent. Also, ATP and HMGB1, the best-characterized damage-associated molecular patterns involved in immunogenic cell death, have proven to be passively released along the timeline of ferroptosis and act as immunogenic signal associated with the immunogenicity of early ferroptotic cancer cells.ConclusionsThese results pave the way for the development of new therapeutic strategies for cancers based on induction of ferroptosis, and thus broadens the current concept of immunogenic cell death and opens the door for the development of new strategies in cancer immunotherapy.

BackgroundAnti-cancer therapy is more successful when it can also induce an immunogenic form of cancer cell death (ICD). Therefore, when developing new treatment strategies, it is extremely important to choose methods that induce ICD and thereby activate anti-tumor immune response leading to the most effective destruction of tumor cells. The aim of this work was to analyze whether the clinically widely used photosensitizers, photosens (PS) and photodithazine (PD), can induce ICD when used in photodynamic therapy (PDT).MethodsCell death in murine glioma GL261 or fibrosarcoma MCA205 cells was induced by PS- or PD-PDT and cell death was analyzed by MTT or flow cytometry. Intracellular distribution of PS and PD was studied by using the laser scanning microscope. Calreticulin exposure and HMGB1 and ATP release were detected by flow cytometry, ELISA and luminescence assay, respectively. Immunogenicity in vitro was analyzed by co-culturing of dying cancer cells with bone-marrow derived dendritic cells (BMDCs) and rate of phagocytosis and maturation (CD11c+CD86+, CD11c+CD40+) of BMDCs and production of IL-6 in the supernatant were measured. In vivo immunogenicity was analyzed in mouse tumor prophylactic vaccination model.ResultsWe determined the optimal concentrations of the photosensitizers and found that at a light dose of 20 J/cm2 (λex 615–635 nm) both PS and PD efficiently induced cell death in glioma GL261 and fibrosarcoma MCA205 cells. We demonstrate that PS localized predominantly in the lysosomes and that the cell death induced by PS-PDT was inhibited by zVAD-fmk (apoptosis inhibitor) and by ferrostatin-1 and DFO (ferroptosis inhibitors), but not by the necroptosis inhibitor necrostatin-1 s. By contrast, PD accumulated in the endoplasmic reticulum and Golgi apparatus, and the cell death induced by PD-PDT was inhibited only by z-VAD-fmk. Dying cancer cells induced by PS-PDT or PD-PDT emit calreticulin, HMGB1 and ATP and they were efficiently engulfed by BMDCs, which then matured, became activated and produced IL-6. Using dying cancer cells induced by PS-PDT or PD-PDT, we demonstrate the efficient vaccination potential of ICD in vivo.ConclusionsAltogether, these results identify PS and PD as novel ICD inducers that could be effectively combined with PDT in cancer therapy.

Ferroptosis is a recently discovered form of regulated cell death that is morphologically, genetically, and biochemically distinct from apoptosis and necroptosis, and its potential use in anticancer therapy is emerging. The strong immunogenicity of (early) ferroptotic cancer cells broadens the current concept of immunogenic cell death and opens up new possibilities for cancer treatment. In particular, induction of immunogenic ferroptosis could be beneficial for patients with cancers resistant to apoptosis and necroptosis. However, ferroptotic cancer cells may be a rich source of oxidized lipids, which contribute to decreased phagocytosis and antigen cross-presentation by dendritic cells and thus may favor tumor evasion. This could explain the non-immunogenicity of late ferroptotic cells. Besides the presence of lactate in the tumor microenvironment, acidification and hypoxia are essential factors promoting ferroptosis resistance and affecting its immunogenicity. Here, we critically discuss the crucial mediators controlling the immunogenicity of ferroptosis that modulate the induction of antitumor immunity. We emphasize that it will be necessary to also identify the tolerogenic (ie, immunosuppressive) nature of ferroptosis, which can lead to tumor evasion.