Highlights d Of several cytokines tested, only synergism of TNF-a and IFN-g induces PANoptosis d TNF-a and IFN-g-mediated PANoptosis perpetuates cytokine storm d TNF-a and IFN-g shock mirrors cytokine storm syndromes, including COVID-19 d Neutralizing TNF-a and IFN-g protects against SARS-CoV-2, HLH, and sepsis in mice

The COVID-19 pandemic has caused significant morbidity and mortality. Currently, there is a critical shortage of proven treatment options and an urgent need to understand the pathogenesis of multi-organ failure and lung damage. Cytokine storm is associated with severe inflammation and organ damage during COVID-19. However, a detailed molecular pathway defining this cytokine storm is lacking, and gaining mechanistic understanding of how SARS-CoV-2 elicits a hyperactive inflammatory response is critical to develop effective therapeutics. Of the multiple inflammatory cytokines produced by innate immune cells during SARS-CoV-2 infection, we found that the combined production of TNF-α and IFN-γ specifically induced inflammatory cell death, PANoptosis, characterized by gasdermin-mediated pyroptosis, caspase-8-mediated apoptosis, and MLKL-mediated necroptosis. Deletion of pyroptosis, apoptosis, or necroptosis mediators individually was not sufficient to protect against cell death. However, cells deficient in both RIPK3 and caspase-8 or RIPK3 and FADD were resistant to this cell death. Mechanistically, the STAT1/IRF1 axis activated by TNF-α and IFN-γ co-treatment induced iNOS for the production of nitric oxide. Pharmacological and genetic deletion of this pathway inhibited pyroptosis, apoptosis, and necroptosis in macrophages. Moreover, inhibition of PANoptosis protected mice from TNF-α and IFN-γ-induced lethal cytokine shock that mirrors the pathological symptoms of COVID-19. In vivo neutralization of both TNF-α and IFN-γ in multiple disease models associated with cytokine storm showed that this treatment provided substantial protection against not only SARS-CoV-2 infection, but also sepsis, hemophagocytic lymphohistiocytosis, and cytokine shock models, demonstrating the broad physiological relevance of this mechanism. Collectively, our findings reveal that blocking the COVID-19 cytokine-mediated inflammatory cell death signaling pathway identified in this study may benefit patients with COVID-19 or other cytokine storm-driven syndromes by limiting inflammation and tissue damage. The findings also provide a molecular and mechanistic description for the term cytokine storm. Additionally, these results open new avenues for the treatment of other infectious and autoinflammatory diseases and cancers where TNF-α and IFN-γ synergism play key pathological roles.

Heme metabolism is central to malaria parasite biology. The parasite acquires heme from host hemoglobin in the intraerythrocytic stages and stores it as hemozoin to prevent free heme toxicity. The parasite can also synthesize heme de novo, and all the enzymes in the pathway are characterized. To study the role of the dual heme sources in malaria parasite growth and development, we knocked out the first enzyme, δ-aminolevulinate synthase (ALAS), and the last enzyme, ferrochelatase (FC), in the heme-biosynthetic pathway of Plasmodium berghei (Pb). The wild-type and knockout (KO) parasites had similar intraerythrocytic growth patterns in mice. We carried out in vitro radiolabeling of heme in Pb-infected mouse reticulocytes and Plasmodium falciparum-infected human RBCs using [4-14C] aminolevulinic acid (ALA). We found that the parasites incorporated both host hemoglobin-heme and parasite-synthesized heme into hemozoin and mitochondrial cytochromes. The similar fates of the two heme sources suggest that they may serve as backup mechanisms to provide heme in the intraerythrocytic stages. Nevertheless, the de novo pathway is absolutely essential for parasite development in the mosquito and liver stages. PbKO parasites formed drastically reduced oocysts and did not form sporozoites in the salivary glands. Oocyst production in PbALASKO parasites recovered when mosquitoes received an ALA supplement. PbALASKO sporozoites could infect mice only when the mice received an ALA supplement. Our results indicate the potential for new therapeutic interventions targeting the heme-biosynthetic pathway in the parasite during the mosquito and liver stages.

Cell death provides host defense and maintains homeostasis. Za-containing molecules are essential for these processes. Z-DNA binding protein 1 (ZBP1) activates inflammatory cell death, PANoptosis, whereas adenosine deaminase acting on RNA 1 (ADAR1) serves as an RNA editor to maintain homeostasis. Here, we identify and characterize ADAR1's interaction with ZBP1, defining its role in cell death regulation and tumorigenesis. Combining interferons (IFNs) and nuclear export inhibitors (NEIs) activates ZBP1-dependent PANoptosis. ADAR1 suppresses this PANoptosis by interacting with the Za2 domain of ZBP1 to limit ZBP1 and RIPK3 interactions. Adar1 fl/fl LysM cre mice are resistant to development of colorectal cancer and melanoma, but deletion of the ZBP1 Za2 domain restores tumorigenesis in these mice. In addition, treating wild-type mice with IFN-g and the NEI KPT-330 regresses melanoma in a ZBP1-dependent manner. Our findings suggest that ADAR1 suppresses ZBP1-mediated PANoptosis, promoting tumorigenesis. Defining the functions of ADAR1 and ZBP1 in cell death is fundamental to informing therapeutic strategies for cancer and other diseases.

Resistance to cell death is a hallmark of cancer. Immunotherapy, particularly immune checkpoint blockade therapy, drives immunemediated cell death and has greatly improved treatment outcomes for some patients with cancer, but it often fails clinically. Its success relies on the cytokines and cytotoxic functions of effector immune cells to bypass the resistance to cell death and eliminate cancer cells. However, the specific cytokines capable of inducing cell death in tumors and the mechanisms that connect cytokines to cell death across cancer cell types remain unknown. In this study, we analyzed expression of several cytokines that are modulated in tumors and found correlations between cytokine expression and mortality. Of several cytokines tested for their ability to kill cancer cells, only TNF-a and IFN-g together were able to induce cell death in 13 distinct human cancer cell lines derived from colon and lung cancer, melanoma, and leukemia. Further evaluation of the specific programmed cell death pathways activated by TNF-a and IFN-g in these cancer lines identified PANoptosis, a form of inflammatory cell death that was previously shown to be activated by contemporaneous engagement of components from pyroptosis, apoptosis, and/or necroptosis. Specifically, TNF-a and IFN-g triggered activation of gasdermin D, gasdermin E, caspase-8, caspase-3, caspase-7, and MLKL. Furthermore, the intratumoral administration of TNF-a and IFN-g suppressed the growth of transplanted xenograft tumors in an NSG mouse model. Overall, this study shows that PANoptosis, induced by synergism of TNF-a and IFN-g, is an important mechanism to kill cancer cells and suppress tumor growth that could be therapeutically targeted.