Search

SUMMARY Receptor-interacting protein kinase 1 (RIPK1) and 3 (RIPK3) are well known for their capacity to drive necroptosis via mixed-lineage kinase-like domain (MLKL). Recently, RIPK1/3 kinase activity has been shown to drive inflammation via activation of MAPK signaling. However, the regulatory mechanisms underlying this kinase-dependent cytokine production remain poorly understood. In the present study, we establish that the kinase activity of RIPK1/3 regulates cytokine translation in mouse and human macrophages. Furthermore, we show that this inflammatory response is downregulated by type I interferon (IFN) signaling, independent of type I IFN-promoted cell death. Specifically, low-level constitutive IFN signaling attenuates RIPK-driven activation of cap-dependent translation initiation pathway components AKT, mTORC1, 4E-BP and eIF4E, while promoting RIPK-dependent cell death. Altogether, these data characterize constitutive IFN signaling as a regulator of RIPK-dependent inflammation and establish cap-dependent translation as a crucial checkpoint in the regulation of cytokine production., In Brief Balancing inflammatory responses is critical for host survival. Muendlein et al. show that constitutive type I IFN signaling inhibits translation machinery activated downstream of the kinase activity of RIPK1/3, preventing the production of a subset of inflammatory cytokines. This work identifies cap-dependent translation as a checkpoint in regulation of RIPK1/3-kinase-dependent inflammation., Graphical Abstract

The stimulator of interferon genes (STING) protein senses cyclic di-nucleotides released in response to double stranded DNA, and functions as an adaptor molecule for type I interferon (IFN-I) signaling by activating IFN-I stimulated genes (ISG). We found impaired T cell infiltration into the peritoneum in response to TNF-α in global and EC-specific STING-/- mice and discovered that T cell transendothelial migration (TEM) across mouse and human endothelial cells (EC) deficient in STING was strikingly reduced compared to control EC, whereas T cells adhesion was not impaired. STING-/- T cells showed no defect in TEM or adhesion to EC, or immobilized endothelial cell expressed molecules ICAM-1 and VCAM-1 compared to WT T cells. Mechanistically, CXCL10, an ISG and a chemoattractant for T cells, was dramatically reduced in TNF-α-stimulated STING-/- EC and genetic loss or pharmacologic antagonism of IFN-type I interferon receptor (IFNAR) pathway reduced T cell TEM. Our data demonstrate a central role for EC STING during T cell TEM that is dependent on the ISG CXCL10 and on IFN-I-IFNAR signaling.

Inflammation and cell death are closely linked arms of the host immune response to infection, which when carefully balanced ensure host survival. One example of this balance is the tightly regulated transition from TNFR1-associated pro-inflammatory complex I to pro-death complex II. By contrast, here we show that a TRIF-dependent complex containing FADD, RIPK1 and caspase-8 (that we have termed the TRIFosome) mediates cell death in response to Yersinia pseudotuberculosis and LPS. Furthermore, we show that constitutive binding between ZBP1 and RIPK1 is essential for the initiation of TRIFosome interactions, caspase-8-mediated cell death and inflammasome activation, thus positioning ZBP1 as an effector of cell death in the context of bacterial blockade of pro-inflammatory signaling. Additionally, our findings offer an alternative to the TNFR1-dependent model of complex II assembly, by demonstrating pro-death complex formation reliant on TRIF signaling., Yersiania YopJ protein has been shown to drive caspase-8-mediated pyroptosis. Here the authors show a precise mechanism of this non-canonical cell death pathway that is controlled by a TRIF-dependent complex of FADD, RIPK1, caspase-8 and ZBP1.

The activity and localization of enzymes in blood leukocytes were examined in dark brown (+/+), silver-blue (p/p) and sapphire (a/a p/p) minks during several periods of postnatal development. This study was performed by morphometric and cytochemical methods using light microscopy and image analysis software “VidеoTest». Significant genotypic differences were determined in the distribution of peroxidase, naphthol AS-D chloroacetate esterase (NASDCE) and alkaline phosphatase (AP) in leukocytes. Sapphire mink (a/a) are characterized by the lowest reproductive potential and increased postnatal mortality among the studied genotypes. Leukocytes of sapphire mink showed a significant increase in the size of and a change in the number of peroxidase- and esterase-positive structures, as well as the presence of phosphatase-negative areas in the cytoplasm. Apparently, these structures are abnormal granules and indicate the disturbance of granulogenesis. Mink of all the three genotypes demonstrated similar age-related changes of peroxidase and alkaline phosphatase activities. The lowest peroxidase activity was observed in early postnatal development, whereas the highest – at the age of 120 days. The increase in alkaline phosphatase activity occurred during the phase of active growth – on the 60th day of postnatal ontogeny. The pattern of age-related changes in NASDCE activity depended on the mink’s genotype. Sapphire mink had the highest level of NASDCE activity at the age of 4 days and the lowest – at 180 days, whereas both dark brown and silver-blue mink at the same age (180 days) had the highest values of NASDCE activity.The postnatal dynamics of the enzymes’ activity in the three genotypes apparently reflects the specific features of metabolism in leukocytes in different periods. The cytochemicalfeatures of peripheral blood leukocytes in sapphire mink suggest the presence of a morphofunctional cellular defect, which is the cause of the animals’ low viability.

Interferons (IFNs) are critical determinants in immune-competence and autoimmunity, and are endogenously regulated by a low-level constitutive feedback loop. However, little is known about the functions and origins of constitutive IFN. Recently, lipopolysaccharide (LPS)-induced IFN was implicated as a driver of necroptosis, a necrotic form of cell death downstream of receptor-interacting protein (RIP) kinase activation and executed by mixed lineage kinase like-domain (MLKL) protein. We found that the pre-established IFN status of the cell, instead of LPS-induced IFN, is critical for the early initiation of necroptosis in macrophages. This pre-established IFN signature stems from cytosolic DNA sensing via cGAS/STING, and maintains the expression of MLKL and one or more unknown effectors above a critical threshold to allow for MLKL oligomerization and cell death. Finally, we found that elevated IFN-signaling in systemic lupus erythematosus (SLE) augments necroptosis, providing a link between pathological IFN and tissue damage during autoimmunity.

cFLIP, an inhibitor of apoptosis, is a crucial regulator of cellular death by apoptosis and necroptosis; its importance in development is exemplified by the embryonic lethality in cFLIP-deficient animals. A homolog of caspase 8 (CASP8), cFLIP exists in two main isoforms: cFLIPL (long) and cFLIPR (short). Although both splice variants regulate death receptor (DR)-induced apoptosis by CASP8, the specific role of each isoform is poorly understood. Here, we report a previously unidentified model of resistance to Fas receptor-mediated liver failure in the wild-derived MSM strain, compared with susceptibility in C57BL/6 (B6) mice. Linkage analysis in F2 intercross (B6 x MSM) progeny identified several MSM loci controlling resistance to Fas-mediated death, including the caspase 8- and FADD-like apoptosis regulator (Cflar) locus encoding cFLIP. Furthermore, we identified a 21-bp insertion in the 3' UTR of the fifth exon of Cflar in MSM that influences differential splicing of cFLIP mRNA. Intriguingly, we observed that MSM liver cells predominantly express the FLIPL variant, in contrast to B6 liver cells, which have higher levels of cFLIPR. In keeping with this finding, genome-wide RNA sequencing revealed a relative abundance of FLIPL transcripts in MSM hepatocytes whereas B6 liver cells had significantly more FLIPR mRNA. Importantly, we show that, in the MSM liver, CASP8 is present exclusively as its cleaved p43 product, bound to cFLIPL. Because of partial enzymatic activity of the heterodimer, it might prevent necroptosis. On the other hand, it prevents cleavage of CASP8 to p10/20 necessary for cleavage of caspase 3 and, thus, apoptosis induction. Therefore, MSM hepatocytes are predisposed for protection from DR-mediated cell death.

With the stimulator of IFN genes (STING) C terminus being extensively studied, the role of the N-terminal domain (NTD) of STING remains an important subject of investigation. In this article, we identify novel mutations in NTD of Sting of the MOLF strain in response to HSV and Listeria monocytogenes both in vitro and in vivo. These mutations are responsible for low levels of IFN-β caused by failure of MOLF STING to translocate from the endoplasmic reticulum. These data provide evidence that the NTD of STING affects DNA responses via control of trafficking. They also show that the genetic diversity of wild-derived mice resembles the diversity observed in humans. Several human alleles of STING confer attenuated IFN-I production similar to what we observe with the MOLF Sting allele, a crucial functional difference not apparent in classical inbred mice. Thus, understanding the functional significance of polymorphisms in MOLF STING can provide basic mechanistic insights relevant to humans.

The cytoplasmic protein Stimulator of Interferon Genes (STING) plays an essential role in sensing cytoplasmic DNA of intracellular pathogens and initiating IFN type I responses in myeloid cells. It can also directly bind cyclic dinucleotides produced by intracellular bacteria or by the DNA-binding cyclic GMP-AMP (cGAMP) synthase. To date, the signaling mechanisms involving STING activation has been studied in macrophages and dendritic cells. Taking into consideration the fact that STING is highly expressed in the thymus and spleen, we decided to investigate its functional role in T cells. We revealed that STING can be activated in T cells by DMXAA (5,6-dimethylxanthenone-4-acetic acid), which binds to the same site in murine STING as cGAMP and triggers phosphorylation of TBK1 and IRF3 resulting in IFNβ production and increased expression of interferon stimulated genes. In contrast to T lymphocytes obtained from C57B6 WT mice, STING−/− T cells displayed no phosphorylation of TBK1/IRF3, and produced no detectable IFNβ upon stimulation. DMXAA treatment also resulted in IFNgamma secretion by WT cells, while DMXAA-treated knockouts produced only a small amount of IFNgamma comparable to unstimulated WT T cells. Both CD4+ and CD8+ subsets of WT T cells responded to DMXAA with phosphorylation of TBK1/IRF3 and IFNβ production, but only CD4+ cells produced IFNgamma. By performing RNA sequencing, numerous differences in gene expression (including interferon stimulated genes) between WT and STING−/− T cells before and after DMXAA stimulation were identified. The work was supported by the Russian Science Foundation (grant no. 115-15-00100 ).