Is ferroptosis immunogenic? The devil is in the details!

Correspondence to: Dr. Oliver Kepp (captain.olsen@gmail. fr), Gustave Roussy Cancer Center, Villejuif, France; Prof. Dr. Guido Kroemer (kroemer@orange.fr), Centre de Recherche des Cordeliers, Paris, France. Ferroptosis has emerged as an iron-dependent type of regulated cell death in which excessive membrane peroxidation results into non-apoptotic, non-necroptotic demise. Ferroptosis is implicated in pathologies such as ischemia reperfusion injury, neurodegenerative disorders and cancer. It is molecularly distinct from other forms of regulated cell death including apoptosis, necroptosis and pyroptosis. Mechanistically, ferroptosis is characterized by the excessive oxidation of polyunsaturated fatty acid (PUFA) and the accumulation of cytotoxic lipid peroxides in response to increasing concentrations of free intracellular iron. The pharmacological inhibition of the selenoenzyme glutathione (GSH) peroxidase 4 (GPX4) or the amino acid antiporter system xc−-mediated uptake of cystine further upstream in the same pathway can precipitate ferroptosis. Moreover, the concomitant inhibition of the oxidoreductase ferroptosis suppressor protein 1 (FSP1) synergizes with GPX4 inhibition in the induction of ferroptosis in a number of cancer types. The increased expression of acylCoA synthetase long-chain family member 4 (ACSL4) in cancerous cells catalyzes the incorporation of oleate and palmitate into phospholipids prone to lethal oxidation, thus sensitizing cells to ferroptosis, making it an interesting target for antineoplastic therapy. In addition to the induction of cell death in cancer cells, ferroptosis can impact on systemic immunity by directly affecting innate and adaptive immune cells such as macrophages and cytotoxic T lymphocyte, respectively. Moreover, it has been suggested that ferroptosis can modulate the immunogenicity of cells through the release of danger associated molecular patterns (DAMPs) which in turn can be perceived by pattern recognition receptors (PRR) expressed on immune cells including dendritic cells (DC), such as advanced glycosylation end-product–specific receptor (AGER), Toll-like receptor 4 (TLR4), and cyclic guanosine monophosphate–adenosine monophosphate synthase (CGAS) all of which have been identified to integrate DAMP signaling emanating from ferroptotic cells. Thus, it has been reported that the proteoglycan decorin (DCN) is released from ferroptotic cells and then acts as an extracellular DAMP on the PRR AGER to trigger the production of immunostimulatory cytokines by macrophages. Moreover, early ferroptosis reportedly stimulates the release of immunogenic cell death (ICD)-associated danger associated molecular patterns (DAMPs) such as ATP and high mobility group box 1 (HMGB1), altogether facilitating T cell mediated adaptive immune responses. ICD can also occur in the context of necroptosis, especially when the lethal phase of the process is preceded by activation of the NF-kB pathway. In stark contrast, Wiernicki et al. recently described ferroptosis as a tolerogenic cell death modality that, despite the emission of ICD-associated DAMPs, decreases the phagocytic potential and maturation of DC and dampens antigen cross-presentation, thus impeding dendritic cell-mediated anti-tumor immunity. The research on ferroptosis often focuses on the inhibition of the GSH pathway converging on GPX4 and its control by radical-trapping antioxidants. An ever-increasing number of pro-ferroptotic agents are being employed for scientific exploration, each of which burdened with a panoply of side effects that might blur the assessment of biological downstream effects other than the induction of cell death. Moreover, the order and magnitude of cellular stress and death routines probably affects the spatial and temporal appearance of ICDrelevant DAMPs and might also impact the manifestation of immunosuppressive signals such as oxidized phospholipids and cyclooxygenase 2 (COX2), which generates immunoinhibitory prostaglandin E2 (PGE2). At this point, research on the immunological consequences of ferroptosis used a heterogeneous panoply of distinct cell types, ferroptosis inducers and inhibitors (Table 1). Thus, further investigation should aim at standardizing the experimental assessment of ferroptosis and its downstream immunological consequences including the potential molecular brakes that modulate the engagement of immune effectors. Of note, it is possible that the question whether ferroptosis elicits immunostimulatory or immunosuppressive effects is intrinsically erroneous. Indeed, it is a non sequitur to state that apoptosis would be immunologically neutral or suppressive, while necroptosis would be immunogenic. It has become clear that the immunological consequences of apoptosis and necroptosis are not only determined by the cell death modality but also – to a large extent – by the premortem stress responses. Thus, apoptosis is only immunogenic if it is preceded and accompanied by the induction of autophagy (for optimal release of the DAMP ATP), a peculiar form of endoplasmic reticulum stress (for exposure of the DAMP calreticulin at the cell surface), as well as a Type-1 interferon response. Similarly, necroptosis is only immunogenic if it is preceded by the premortem activation of the pro-inflammatory NF-kB pathway. Hence, it may be important to understand which stress pathways have been activated before cells ignite ONCOIMMUNOLOGY 2022, VOL. 11, NO. 1, e2127273 (3 pages) https://doi.org/10.1080/2162402X.2022. 2127273