Systemic but not MDSC-specific IRF4 deficiency promotes an immunosuppressed tumor microenvironment in a murine pancreatic cancer model.

Pancreatic ductal adenocarcinoma is characterized by a strong immunosuppressive network with a dense infiltration of myeloid cells including myeloid-derived suppressor cells (MDSC). Two distinct populations of MDSC have been defined: polymorphonuclear MDSC (PMN-MDSC) and monocytic MDSC (M-MDSC). Several factors influence the development and function of MDSC including the transcription factor interferon regulatory factor 4 (IRF4). Here, we show that IRF4 deficiency accelerates tumor growth and reduces survival, accompanied with a dense tumor infiltration with PMN-MDSC and reduced numbers of CD8+ T cells. As IRF4 has been described to modulate myeloid cell development and function, particularly of PMN-MDSC, we analyzed its role using MDSC-specific IRF4 knockout mice with the Ly6G or LysM knock-in allele expressing Cre recombinase and Irf4flox. In GM-CSF-driven bone marrow cultures, IRF4 deficiency increased the frequency of MDSC-like cells with a strong T cell suppressive capacity. Myeloid (LysM)-specific depletion of IRF4 led to increased tumor weight and a moderate splenic M-MDSC expansion in tumor-bearing mice. PMN cell (Ly6G)-specific depletion of IRF4, however, did not influence tumor progression or MDSC accumulation in vivo in accordance with our finding that IRF4 is not expressed in PMN-MDSC. This study demonstrates a critical role of IRF4 in the generation of an immunosuppressive tumor microenvironment in pancreatic cancer, which is independent of IRF4 expression in PMN-MDSC.

OAS1/RNase L executes RIG-I ligand-dependent tumor cell apoptosis.

Cytoplasmic double-stranded RNA is sensed by RIG-I-like receptors (RLRs), leading to induction of type I interferons (IFN-Is), proinflammatory cytokines, and apoptosis. Here, we elucidate signaling mechanisms that lead to cytokine secretion and cell death induction upon stimulation with the bona fide RIG-I ligand 5'-triphosphate RNA (3p-RNA) in tumor cells. We show that both outcomes are mediated by dsRNA-receptor families with RLR being essential for cytokine production and IFN-I-mediated priming of effector pathways but not for apoptosis. Affinity purification followed by mass spectrometry and subsequent functional analysis revealed that 3p-RNA bound and activated oligoadenylate synthetase 1 and RNase L. RNase L-deficient cells were profoundly impaired in their ability to undergo apoptosis. Mechanistically, the concerted action of translational arrest triggered by RNase L and up-regulation of NOXA was needed to deplete the antiapoptotic MCL-1 to cause intrinsic apoptosis. Thus, 3p-RNA-induced apoptosis is a two-step process consisting of RIG-I-dependent priming and an RNase L-dependent effector phase.

Defective Interfering Genomes and the Full-Length Viral Genome Trigger RIG-I After Infection With Vesicular Stomatitis Virus in a Replication Dependent Manner.

Replication competent vesicular stomatitis virus (VSV) is the basis of a vaccine against Ebola and VSV strains are developed as oncolytic viruses. Both functions depend on the ability of VSV to induce adequate amounts of interferon-α/ß. It is therefore important to understand how VSV triggers interferon responses. VSV activates innate immunity via retinoic acid-inducible gene I (RIG-I), a sensor for viral RNA. Our results show that VSV needs to replicate for a robust interferon response. Analysis of RIG-I-associated RNA identified a copy-back defective-interfering (DI) genome and full-length viral genomes as main trigger of RIG-I. VSV stocks depleted of DI genomes lost most of their interferon-stimulating activity. The remaining full-length genome and leader-N-read-through sequences, however, still triggered RIG-I. Awareness for DI genomes as trigger of innate immune responses will help to standardize DI genome content and to purposefully deplete or use DI genomes as natural adjuvants in VSV-based therapeutics.

Heterozygous OAS1 gain-of-function variants cause an autoinflammatory immunodeficiency.

Analysis of autoinflammatory and immunodeficiency disorders elucidates human immunity and fosters the development of targeted therapies. Oligoadenylate synthetase 1 is a type I interferon-induced, intracellular double-stranded RNA (dsRNA) sensor that generates 2'-5'-oligoadenylate to activate ribonuclease L (RNase L) as a means of antiviral defense. We identified four de novo heterozygous OAS1 gain-of-function variants in six patients with a polymorphic autoinflammatory immunodeficiency characterized by recurrent fever, dermatitis, inflammatory bowel disease, pulmonary alveolar proteinosis, and hypogammaglobulinemia. To establish causality, we applied genetic, molecular dynamics simulation, biochemical, and cellular functional analyses in heterologous, autologous, and inducible pluripotent stem cell-derived macrophages and/or monocytes and B cells. We found that upon interferon-induced expression, OAS1 variant proteins displayed dsRNA-independent activity, which resulted in RNase L-mediated RNA cleavage, transcriptomic alteration, translational arrest, and dysfunction and apoptosis of monocytes, macrophages, and B cells. RNase L inhibition with curcumin modulated and allogeneic hematopoietic cell transplantation cured the disorder. Together, these data suggest that human OAS1 is a regulator of interferon-induced hyperinflammatory monocyte, macrophage, and B cell pathophysiology.

Blocking inflammation on the way: Rationale for CXCR2 antagonists for the treatment of COVID-19.

An exacerbated and unbalanced immune response may account for the severity of COVID-19, the disease caused by the novel severe acute respiratory syndrome (SARS) coronavirus 2, SARS-CoV-2. In this Viewpoint, we summarize recent evidence for the role of neutrophils in the pathogenesis of COVID-19 and propose CXCR2 inhibition as a promising treatment option to block neutrophil recruitment and activation.

RIG-I-based immunotherapy enhances survival in preclinical AML models and sensitizes AML cells to checkpoint blockade.

Retinoic acid-inducible gene-I (RIG-I) is a cytoplasmic immune receptor sensing viral RNA. It triggers the release of type I interferons (IFN) and proinflammatory cytokines inducing an adaptive cellular immune response. We investigated the therapeutic potential of systemic RIG-I activation by short 5'-triphosphate-modified RNA (ppp-RNA) for the treatment of acute myeloid leukemia (AML) in the syngeneic murine C1498 AML tumor model. ppp-RNA treatment significantly reduced tumor burden, delayed disease onset and led to complete remission including immunological memory formation in a substantial proportion of animals. Therapy-induced tumor rejection was dependent on CD4+ and CD8+ T cells, but not on NK or B cells, and relied on intact IFN and mitochondrial antiviral signaling protein (MAVS) signaling in the host. Interestingly, ppp-RNA treatment induced programmed death ligand 1 (PD-L1) expression on AML cells and established therapeutic sensitivity to anti-PD-1 checkpoint blockade in vivo. In immune-reconstituted humanized mice, ppp-RNA treatment reduced the number of patient-derived xenografted (PDX) AML cells in blood and bone marrow while concomitantly enhancing CD3+ T cell counts in the respective tissues. Due to its ability to establish a state of full remission and immunological memory, our findings show that ppp-RNA treatment is a promising strategy for the immunotherapy of AML.

A Novel Complete Autosomal-Recessive STAT1 LOF Variant Causes Immunodeficiency with Hemophagocytic Lymphohistiocytosis-Like Hyperinflammation.

BACKGROUND: Complete signal transducer and activator of transcription 1 (STAT1) deficiency causes a rare primary immunodeficiency that is characterized by defective IFN-dependent gene expression leading to life-threatening viral and mycobacterial infections early in life. OBJECTIVE: To characterize a novel STAT1 loss-of-function variant leading to pathological infection susceptibility and hyperinflammation. METHODS: Clinical, immunologic, and genetic characterization of a patient with severe infections and hemophagocytic lymphohistiocytosis-like hyperinflammation was investigated. RESULTS: We reported a child of consanguineous parents who presented with multiple severe viral infections that ultimately triggered hemophagocytic lymphohistiocytosis and liver failure. Despite intensified therapy with antivirals and cytomegalovirus-specific donor cells, the child died after hematopoietic stem cell transplantation because of cytomegalovirus reactivation with acute respiratory distress syndrome. Exome sequencing revealed a homozygous STAT1 variant (p.Val339ProfsTer18), leading to loss of STAT1 protein expression. Upon type I and type II IFN stimulation, immune and nonimmune cells showed defective upregulation of IFN-stimulated genes and increased susceptibility to viral infection in vitro. Increased viral infection rates were paralleled by hyperinflammatory ex vivo cytokine responses with increased production of TNF, IL-6, and IL-18. CONCLUSIONS: Complete STAT1 deficiency is a devastating disorder characterized by severe viral infections and ensuing hyperinflammatory responses. Early diagnosis can be made by exome sequencing and variant validation by functional testing of STAT1-dependent programmed cell death 1 ligand 1 surface expression on monocytes. Furthermore, high awareness for hyperinflammatory complications and potential targeted treatment strategies such as IL-18 binding protein could be considered. Hematopoietic stem cell transplantation is the only definitive treatment strategy but remains challenging.