Fatal cytokine release syndrome by an aberrant FLIP/STAT3 axis

Abstract

Inflammatory responses rapidly detect pathogen invasion and mount a regulated reaction. However, dysregulated anti-pathogen immune responses can provoke life-threatening inflammatory pathologies collectively known as cytokine release syndrome (CRS), exemplified by key clinical phenotypes unearthed during the SARS-CoV-2 pandemic. The underlying pathophysiology of CRS remains elusive. We found that FLIP, a protein that controls caspase-8 death pathways, was highly expressed in myeloid cells of COVID-19 lungs. FLIP controlled CRS by fueling a STAT3-dependent inflammatory program. Indeed, constitutive expression of a viral FLIP homolog in myeloid cells triggered a STAT3-linked, progressive, and fatal inflammatory syndrome in mice, characterized by elevated cytokine output, lymphopenia, lung injury, and multiple organ dysfunctions that mimicked human CRS. As STAT3-targeting approaches relieved inflammation, immune disorders, and organ failures in these mice, targeted intervention towards this pathway could suppress the lethal CRS inflammatory state.

A Complex Metabolic Network Confers Immunosuppressive Functions to Myeloid-Derived Suppressor Cells (MDSCs) within the Tumour Microenvironment

“…Interestingly, c-FLIP + CD14 + monocytes isolated from patients with pancreatic ductal adenocarcinoma (PDAC) expressed high levels of CD38 and programmed death-ligand 1 (PD-L1), and an increase in c-FLIP + PD-L1 + CD14 + cell number in combination with high levels of serum IL-6 has been identified as a negative independent prognostic factor for both overall survival and disease free survival (DFS) [65]. In agreement with these results, we demonstrated that in both SARS-CoV-2-infected hACE2 transgenic mice and autopsy samples from the lungs of patients with COVID-19, c-FLIP is overexpressed in myeloid cells, and c-FLIP-expressing human monocytes display immunosuppressive functions and release high amounts of pro-inflammatory cytokines [67]. Collectively, these data highlight FLIP as a key mediator for reprogramming monocytes into immune regulatory elements in both mice and humans.…”

Immune-Guided Therapy of COVID-19

“…Therefore, from an immune viewpoint, COVID-19 progresses from a STAT1/ISG/IFN type I/III antiviral immune response, which is sufficient to control disease in a vast majority of infected individuals, to an inflammatory milieu characterized by NF-B/STAT3 activation, which mirrors some features of cytokine release syndrome (CRS) (57) (Figure 2). In particular, pSTAT3 but not pSTAT1 is increased in lymphocytes and myeloid cells circulating in the blood of patients with COVID-19 who have pneumonia (58) and remains constantly elevated throughout disease evolution (59), indicating a persistent activation of this pathway (likely by cytokines such as IL6, IL17 and IL10). High amounts of inflammation inducers (cytokine storm) and the presence of circulating immunoregulatory myeloid cells (immunosuppression) are the hallmarks of this severe stage of disease (Figure 2).…”

“…For instance, high frequencies of STAT3 + ARG1 + CD14 + immunosuppressive cells have been identified in patients with pancreatic ductal adenocarcinoma (67,68). In both SARS-CoV-2-infected hACE2 transgenic mice and autopsy samples from the lung of patients with COVID-19, cellular FLICE (FADD-like IL-1β-converting enzyme)-inhibitory protein (c-FLIP) is overexpressed in myeloid cells (59). c-FLIP is unique in reprogramming monocytes into immune regulatory elements that display immunosuppressive functions and become a source of pro-inflammatory cytokines via an NF-B pathway, which enhances aberrant STAT3 signaling (59,68).…”

“…In both SARS-CoV-2-infected hACE2 transgenic mice and autopsy samples from the lung of patients with COVID-19, cellular FLICE (FADD-like IL-1β-converting enzyme)-inhibitory protein (c-FLIP) is overexpressed in myeloid cells (59). c-FLIP is unique in reprogramming monocytes into immune regulatory elements that display immunosuppressive functions and become a source of pro-inflammatory cytokines via an NF-B pathway, which enhances aberrant STAT3 signaling (59,68). The unrestrained and persistent immunosuppressive and inflammatory milieu establishes the terminal phase of COVID-19, which is characterized by a fatal immune silence (Figure 2).…”

Coronavirus Infection-Associated Cell Death Signaling and Potential Therapeutic Targets

“…Because it operates as an anti-apoptotic factor by inhibiting procaspase-8 activation, viruses upregulate cFLIP expression as a strategy to prevent extrinsic apoptosis in order to continue their replication [140]. High amounts of cFLIP were found in the pulmonary myeloid cells of COVID-19 patients as well as during the start of SARS-CoV-2 infection [141]. As a result, the use of small molecules that act against cFLIP to activate caspase 8 and induce extrinsic apoptosis has been proposed as an important strategy for combating virus replication [128].…”