Science Signaling

Severe stress can trigger complex behavioural changes such as high anxiety (1). Inhibitory GABA-ergic interneurons in the lateral division of the central amygdala (CEl) control anxiety through feedforward inhibition of their target cells in the medial division (CEm) (2, 3). In particular, PKC{delta}-positive (PKC{delta}+) interneurons in CEl are critical elements of the neuronal circuitry of fear and anxiety (3-5), but the molecular mechanisms they employ are poorly understood. Here, we show that, during stress, GABA-ergic synapses of amygdala PKC{delta}+ interneurons are regulated by a serine protease plasmin. On stress, plasmin cleaves the extracellular portion of the tyrosine kinase receptor EphA4 triggering its dissociation from gephyrin, a postsynaptic GABA-receptor anchoring protein. Dynamic EphA4/gephyrin interaction leads to modification of dendritic spine morphology and synaptic GABA-receptor expression profile. Consistent with the critical role for the plasmin/EphA4/gephyrin signalling axis in anxiogenesis, viral delivery of plasmin-resistant (prEphA4) form of EphA4 into the central amygdala prevents the development of stress-induced anxiety in mice, while the delivery of plasmin-truncated EphA4 (tEphA4) dramatically enhances this effect. Thus, our studies identify a novel, critical molecular cascade regulating GABA-ergic signalling in the central amygdala synapses that allows bidirectional switching of animal behaviour from high to low anxiety states.

Neutrophils are key players in the hyperinflammatory response upon SARS-CoV-2 infection. We have previously described that cytosolic proliferating cell nuclear antigen (PCNA) controls neutrophil survival and NADPH oxidase-dependent ROS production. We here show that both PCNA and S100A8 expression and interaction were elevated in neutrophils from patients with COVID-19 compared to healthy donors and this was correlated with disease severity. Increased PCNA expression was accompanied by a decreased apoptosis and increased NADPH-oxidase activity in neutrophils from COVID-19 patients compared to healthy donors. These effects, as well as the interaction between PCNA and S100A8, were potently counteracted by T2 amino alcohol (T2AA), a PCNA inhibitor, demonstrating that the PCNA scaffold orchestrated neutrophil activation. Notably, the interaction between PCNA-S100A8 was more intense in the CD16high-CD62Llow activated neutrophil subset. We propose that PCNA-S100A8 complex acts as potential driver for neutrophil dysregulation in COVID-19 and show for the first time that the PCNA scaffold is a decisive component of both neutrophil activation and heterogeneity.

Toll-like receptors (TLRs) are transmembrane proteins required for recognizing microbial components or cellular danger signals to activate intracellular signaling pathways leading to induction of anti-microbial and inflammatory genes. Inactive TLRs require ligand-induced activation to recruit adaptor proteins, e.g., MyD88, to trigger the synthesis of cytokines and interferons. TLR9 is an endosomal membrane-bound protein, which binds to CpG-containing microbial DNA or endogenous signals from dead cells or tissue damage. We showed that TLR9 activation requires EGFR, a tyrosine (Tyr) kinase, which interacts with and phosphorylates the cytoplasmic domain of TLR9. Blocking EGFR activity pharmacologically, or knocking out EGFR gene in myeloid cells, suppressed lethal TLR9-induced hepatotoxicity. Here, we reveal that TLR9 required two Src family of kinases, Syk and Lyn, which, together with EGFR, led to phosphorylation and activation of TLR9. Lack of either of these kinases inhibited TLR9-MyD88 interaction, thereby inhibiting TLR9-mediated gene induction. Unlike EGFR, which constitutively binds TLR9, activated Syk interacted with TLR9 in a CpG-dependent manner. Activated Syk interacted with TLR9 and was critical for activating TLR9-bound EGFR. Quantitative mass spectrometric analyses revealed TLR9 was phosphorylated, sequentially, on Tyr870 and Tyr980 by Syk and EGFR, respectively. Mutation of either of these tyrosines led to complete loss of TLR9-induced cytokine production. For activation, Syk was phosphorylated by Lyn, which was activated by CpG-mediated scavenger-receptor A, and surprisingly, independent of TLR9. In summary, our results uncovered the molecular details of TLR9 activation by its Tyr-phosphorylation, which is critical for TLR9-mediated intracellular signaling.

Alpha-protein kinase 1 (ALPK1) is a pathogen recognition receptor that detects ADP-heptose (ADPH), a lipopolysaccharide biosynthesis intermediate, recently described as a pathogen-associated molecular pattern in Gram-negative bacteria. ADPH binding to ALPK1 activates its kinase domain and triggers TIFA phosphorylation on threonine 9. This leads to the assembly of large TIFA oligomers called TIFAsomes, activation of NF-{kappa}B and pro-inflammatory gene expression. Furthermore, mutations in ALPK1 are associated with inflammatory syndromes and cancers. While this kinase is of growing medical interest, only few tools are available to characterize its activity. Here, we describe a versatile non-radioactive ALPK1 in vitro kinase assay based on protein thiophosphorylation. We confirm that ALPK1 phosphorylates TIFA T9 and show that T2, T12 and T19 are also weakly phosphorylated by ALPK1. Interestingly, we find that ALPK1 itself is phosphorylated in response to ADPH recognition during Shigella flexneri and Helicobacter pylori infection and that disease-associated ALPK1 mutants exhibit altered kinase activity. In particular, T237M and V1092A mutations associated with ROSAH syndrome and spiradenoma/spiradenocarcinoma respectively, exhibit enhanced ADPH-induced kinase activity and constitutive assembly of TIFAsomes. Altogether, this study provides new insights into the ADPH sensing pathway and a new tool for measuring the activity of ALPK1.

New agents are needed that selectively kill cancer cells without harming normal tissues. The TRAIL ligand and its receptors, DR5 and DR4, exhibit cancer-selective toxicity, but TRAIL analogs or agonistic antibodies targeting these receptors have not received FDA approval for cancer therapy. Small molecules for activating DR5 or DR4 independently of protein ligands may bypass some of the pharmacological limitations of these protein drugs. Previously described Disulfide bond Disrupting Agents (DDAs) activate DR5 by altering its disulfide bonding through inhibition of the Protein Disulfide Isomerases (PDIs) ERp44, AGR2, and PDIA1. Work presented here extends these findings by showing that disruption of single DR5 disulfide bonds causes high-level DR5 expression, disulfide-mediated clustering, and activation of Caspase 8-Caspase 3 mediated pro-apoptotic signaling. Recognition of the extracellular domain of DR5 by various antibodies is strongly influenced by the pattern of DR5 disulfide bonding, which has important implications for the use of agonistic DR5 antibodies for cancer therapy. Disulfide-defective DR5 mutants do not activate the ER stress response or stimulate autophagy, indicating that these DDA-mediated responses are separable from DR5 activation and pro-apoptotic signaling. Importantly, other ER stressors, including Thapsigargin and Tunicamycin also alter DR5 disulfide bonding in various cancer cell lines and in some instances, DR5 mis-disulfide bonding is potentiated by overriding the Integrated Stress Response (ISR) with inhibitors of the PERK kinase or the ISR inhibitor ISRIB. These observations indicate that the pattern of DR5 disulfide bonding functions as a sensor of ER stress and serves as an effector of proteotoxic stress by driving extrinsic apoptosis independently of extracellular ligands.

Although transmembrane receptors engage multiple signaling pathways simultaneously, each pathway is typically investigated individually. How receptors juggle their interactions with different downstream effectors and the conditions under which distinct pathways are prioritized remains poorly understood. Here we show that the outcome of death receptor p75NTR signaling is determined through competition of effectors for interaction with its intracellular domain, in turn dictated by the nature of the incoming ligand. While NGF induces release of RhoGDI through recruitment of RIP2, thus decreasing RhoA activity in favor of NFkB signaling, MAG induces PKC-mediated phosphorylation of the RhoGDI N-terminus, thus promoting its interaction with the juxtamembrane domain of p75NTR, disengaging RIP2, and enhancing RhoA activity in detriment of NF-kB. This results in growth cone collapse, stunted neurite outgrowth and apoptosis in cerebellar granule neurons. If presented simultaneously, MAG prevails over NGF. The NMR solution structure of the complex between the RhoGDI N-terminus and p75NTR juxtamembrane domain reveals previously unknown structures of these proteins and clarifies the mechanism of p75NTR activation. These results show how ligand-directed competition between RIP2 and RhoGDI for p75NTR engagement determine axon growth and neuron survival. Similar principles are likely at work in other receptors engaging multiple effectors and signaling pathways.

Endocannabinoid (eCB) signalling gates many aspects of the stress response, including the hypothalamic-pituitary-adrenal (HPA) axis. The HPA axis is controlled by corticotropin releasing hormone (CRH) producing neurons in the paraventricular nucleus of the hypothalamus (PVN). Disruption of eCB signalling increases drive to the HPA axis, but the mechanisms subserving this process are poorly understood. Using an array of cellular, endocrine and behavioral readouts associated with activation of CRH neurons in the PVN, we evaluated the contributions of tonic eCB signaling to the generation of a stress response. The CB1 receptor antagonist/inverse agonist AM251, neutral antagonist NESS243, and NAPE PLD inhibitor LEI401 all uniformly increased c-fos in the PVN, unmasked stress-linked behaviors, such as grooming, and increased circulating CORT, recapitulating the effects of stress. Similar effects were also seen after direct administration of AM251 into the PVN, while optogenetic inhibition of PVN CRH neurons ameliorated stress-like behavioral changes produced by disruption of eCB signaling. These data indicate that under resting conditions, constitutive eCB signaling restricts activation of the HPA axis through local regulation of CRH neurons in the PVN.

V-domain immunoglobulin suppressor of T-cell activation (VISTA) has emerged as a unique immunoregulatory receptor on cells of the myeloid lineage. Agonizing VISTA on myeloid cells has recently been demonstrated to have a profound effect on dampening inflammatory responses. VISTA has been proposed to function both as a ligand and as a receptor. In this context, the role of VISTA as a ligand has been largely ignored. Using a high-avidity agonist of the VISTA receptor (VISTA-COMP), we investigated the effect of exogenous VISTA, as a ligand, on macrophages and neutrophil cellular pathways in an acute inflammatory setting. RNA sequencing analysis demonstrated that VISTA-COMP downregulates pro-inflammatory cytokines and chemokines and upregulates immunoregulatory genes in both LPS-stimulated macrophages and neutrophils ex vivo. Interestingly, unlike VISTA itself, the receptor is only expressed following LPS stimulation of these cell populations. Furthermore, the administration of VISTA-COMP attenuated the rise in circulating TNF levels in LPS-treated mice in vivo. These results suggest that VISTA serves a redundant role on macrophages and neutrophils acting as both a ligand and a receptor in the context of an acute inflammatory event.

The chemokine receptor CCR9 coordinates immune cell migration from the thymus to the small intestine along gradients of CCL25. Receptor dysregulation is associated with a variety of inflammatory bowel diseases such as Crohns and ulcerative colitis, while aberrant CCR9 overexpression correlates with tumor metastasis. Despite being an attractive therapeutic target, attempts to clinically antagonize CCR9 have been unsuccessful. This highlights the need for a deeper understanding of its specific regulatory mechanisms and signaling pathways. CCR9 is a G protein-coupled receptor (GPCR) and activates Gi and Gq pathways. Unexpectedly, live-cell BRET assays reveal only limited G protein activation and signaling is rapidly terminated. Truncating the receptor C-terminus significantly enhanced G protein coupling, highlighting the regulatory role of this domain. Signal suppression was not due to canonical arrestin-coordinated desensitization. Rather, removal of GPCR kinase (GRK) phosphorylation led to sustained and robust G protein activation by CCR9. Using site-directed mutagenesis, we identified specific phosphorylation patterns that attenuate G protein coupling. Receptor internalization does not correlate with G protein activation capabilities. Instead, CCR9 phosphorylation appeared to directly destabilize the interaction of G protein heterotrimers with the receptor. This interference could lead to rapid loss of productive coupling and downstream signaling as phosphorylation would effectively render the receptor incapable of G protein coupling. An arrestin-independent, phosphorylation-driven deactivation mechanism could complement arrestin-dependent regulation of other GPCRs and have consequences for therapeutically targeting these receptors.

This study tests the hypothesis that activation of mitogen-activated protein kinase (MAPK) by physiologically-relevant concentrations of interleukin-33 (IL-33) contributes to enhanced cytokine expression by IL-12 stimulated human natural killer (NK) cells. While IL-33 canonically triggers type 2 cytokine responses, this cytokine can also synergize with type 1 cytokines like IL-12 to provoke interferon-gamma (IFN-{gamma}). We show that picogram concentrations of IL-12 and IL-33 are sufficient to promote robust secretion of IFN-{gamma} by human NK cells that greatly exceeds responses to either cytokine alone. Nanogram doses of IL-33, potentially consistent with levels in tissue microenvironments, synergize with IL-12 to induce secretion of additional cytokines, including tumor necrosis factor (TNF) and granulocyte-macrophage colony-stimulating factor (GM-CSF). IL-33-induced activation of the p38 MAPK pathway in human NK cells is crucial for enhanced release of IFN-{gamma} and TNF in response to IL-12. Mechanistically, IL-33-induced p38 MAPK signaling enhances stability of IFNG transcripts and triggers ADAM17-mediated cleavage of TNF from the cell surface. These data support our hypothesis and suggest that altered sensitivity of NK cells to IL-12 in the presence of IL-33 may have important consequences in diseases associated with mixed cytokine milieus, like asthma and chronic obstructive pulmonary disease. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=190 SRC="FIGDIR/small/777847v2_ufig1.gif" ALT="Figure 1"> View larger version (40K): org.highwire.dtl.DTLVardef@c1f7c9org.highwire.dtl.DTLVardef@72d47dorg.highwire.dtl.DTLVardef@dc84d4org.highwire.dtl.DTLVardef@194b42f_HPS_FORMAT_FIGEXP M_FIG C_FIG

Lymphocyte-activation gene 3 (LAG-3) is known to suppress T cell receptor (TCR) signaling by disrupting CD4/CD8 coreceptor and Lck association in the absence of ligand binding. However, the impact of Fibrinogen-like protein 1 (FGL-1) binding in modulating LAG-3 mediated inhibition and its specific impact on CD28 costimulatory signaling remains unclear. This study investigates the role of LAG-3/FGL-1 interaction in modulating T cell activation. Using phosphoproteomics and confocal microscopy in LAG-3+ Jurkat cells, we demonstrated that LAG-3/FGL-1 interaction suppresses T cell activation by disrupting CD28-mediated recruitment of Lck, impairing its colocalization with CD28. Importantly, this inhibition occurs independently of CD4-bound Lck, indicating that LAG-3 ligand engagement targets both free and membrane-associated Lck pools. Consequently, this leads to suppressed phosphorylation of CD28 and TCR/CD3 signaling components, broadly attenuating downstream tyrosine phosphorylation and T cell effector functions. Our findings identify the CD28-Lck axis as a critical target of LAG-3/FGL-1 inhibitory pathway and provide a rationale for therapeutic strategies that reinvigorate CD28-Lck signaling to enhance anti-tumor immunity.

G protein-coupled receptors rely on dynamic conformational changes to coordinate G protein activation and recruitment of regulatory transducers such as G protein-coupled receptor kinases and {beta}-arrestins. The chemotactic receptor GPR183 has been implicated in a context-dependent role in hematological malignancies. Here, we investigated the impact of A338V mutation located within the C-terminal tail of GPR183. This mutation is associated with acute myeloid leukaemia. Using bioluminescence resonance energy transfer-based assays in HEK293A cells, we assessed receptor-proximal signaling events. The A338V variant displayed preserved agonist potency and comparable agonist-induced Gi activation relative to wild type, although constitutive activity towards Gi was modestly reduced. In contrast, recruitment of GRK2 and {beta}-arrestin2 was consistently impaired across multiple assay configurations. These differences were not attributable to altered receptor abundance, as the C-tail untagged mutant exhibited increased plasma membrane expression despite reduced regulatory transducer engagement. While intramolecular conformational biosensor measurements revealed subtle differences in global receptor conformation between WT and A338V, extensive molecular dynamics simulations supported the altered conformational sampling of the C-terminal tail in the A338V variant. Together, these data support a model in which the A338V substitution selectively alters C-terminal structural dynamics, impairing GRK2 and {beta}-arrestin2 recruitment while preserving G protein activation.

Lipid transport plays a critical role in the distribution of lipids across subcellular compartments. This is pivotal during infection and other stress stimuli that increase metabolic demands. While lipid biosynthesis is regulated by immune stimuli, whether immune signalling also influences lipid transport mechanisms remains unexplored. Here, we demonstrate that TLR signalling impacts the gene expression of lipid transport proteins in human monocytic THP-1 cell line and compare it to the expression in primary bone marrow-derived mouse macrophages. Our data demonstrates that TLR4 signalling selectively modulates the expression of oxysterol-binding protein-related proteins (ORPs), a key family of proteins that transport lipids between organelles. Remarkably, TLR4 activation led to the downregulation of several ORP family members in human THP1-derived macrophages. However, this response was less profound in mouse macrophages. In contrast, the expression of steroidogenic acute regulatory domain (STARD) proteins, many of which transport lipids between mitochondria and other compartments, remained broadly unaffected. Moreover, IFNG, a cytokine that plays a key role in the immune response, did not considerably impact human ORP or STARD expression levels, either alone or in combination with LPS. Together, these results reveal that TLR signalling exerts selective and critical control over lipid trafficking pathways with important species-specific differences. These findings provide new insights into the crosstalk between immune signalling and lipid metabolism, which may offer novel targets to treat diseases characterized by dysregulated lipid pathways.

Many intracellular pathogens stimulate host cell stress by directly or indirectly causing an imbalance in host nutrients; depletion of amino acid pools in particular can act as a danger signal to infected cells. Using a restrictive host model of Salmonella enterica serovar Typhi (S. Typhi) infection, we identify early induction the integrated stress response (ISR) by viable bacteria, but not heat-killed bacteria. Genetic deletion of the amino acid sensing ISR kinase GCN2 (also known as EIF2AK4) prevented early ISR activation during S. Typhi infection, and murine macrophages lacking GCN2 show impaired bacterial clearance and decreased cytokine output. Supplementation of wildtype C57BL/6 murine macrophages with only the non-essential amino acid asparagine was sufficient to suppress S. Typhi-induced ISR activation and deletion of S. Typhi ansB, encoding an asparaginase, prevented ISR activation during infection. Pharmacological inhibition of mammalian target of rapamycin (mTOR), the other major amino acid sensing pathway in eukaryotic cells, prevented GCN2 activation and ISR induction in murine macrophages, indicating an upstream role for mTOR in signaling to GCN2. These findings suggest a role for the ISR in macrophage innate immune responses to S. Typhi infection and highlight a potential difference in nutrient-dependent signaling between the S. Typhi-susceptible human host and the restrictive murine host centered around asparagine, mTOR, and GCN2.

Acute stress triggers plasticity of forebrain synapses as well as behavioral changes. Here we reveal that Tumor Necrosis Factor (TNF) is a required downstream mediator of the stress response in mice, necessary for stress-induced synaptic potentiation in the ventral hippocampus and for an increase in anxiety-like behaviour. Acute stress is sufficient to activate microglia, triggering the long-term release TNF. Critically, on-going TNF signaling in the ventral hippocampus is necessary to sustain both the stress-induced synaptic and behavioral changes, as these could be reversed hours after induction by antagonizing TNF signaling. This demonstrates that TNF maintains the synaptic and behavioral stress response in vivo, making TNF a potential novel therapeutic target for stress disorders.

Crosstalk across two major receptor families involved in signal transduction, namely receptor tyrosine kinases (RTKs) and G protein-coupled receptors (GPCRs), have been observed at different levels of their signaling cascades. Using newly developed enhanced bystander bioluminescence resonance energy transfer (ebBRET)-based biosensors that monitor the recruitment of SH2 domains to activated RTKs, we assessed the ability of GPCRs to modulate cellular localization of SH2 domains. Receptor-mediated activation of either Gq/11 or G12/13 but not Gs or Gi/o (e.g., thromboxane A2 receptor, TP, and type-2 protease activated receptor, PAR2) resulted in the plasma membrane (PM) dissociation of SH2 domains derived from RTKs effectors such as GRB2, STAT5 and PLC{gamma}1. The role of Gq/11, G12/13, Rho and downstream kinases in the subcellular SH2 domain redistribution was further confirmed using both pharmacological and genetic approaches. BRET imaging and spectrometric analyses showed that the dissociation of SH2 domains from the PM was accompanied by their accumulation in the nucleus and a reduction in RTK signaling activity, as determined using a STAT5 transcriptional assay. The effect of Gq/11 and G12/13 activation on STAT5 transcriptional activity was observed both in engineered systems and in HeLa cells endogenously expressing all the components of the regulatory mechanism. The Gq/11 / G12/13-mediated redistribution of SH2 domain-containing proteins represents an undescribed mechanism through which GPCRs regulate RTKs activity. Significance StatementThis study reveals a novel crosstalk mechanism between G protein coupled receptors and receptor tyrosine kinases showing that Gq/11 and G12/13 activation triggers Rho-dependent translocation of SH2-containing effector proteins, such as GRB2, PLC{gamma}1 and STAT5. This process causes compartmentalization inside the nucleus and thus reduces their availability at the plasma membrane, leading to attenuated RTK responses.

The understanding of pyroptotic cell death and its mechanistic insights in neutrophils remains less explored and ambiguous. This study analyzes neutrophil pyroptotic responses under different conditions and dissects distinct pyroptosis-associated events, including IL-1{beta} release and cell death. We decipher the signalling pathways underlying these responses and distinguish them from other cell death mechanisms. Interestingly, we observe IL-1{beta} release and enhanced survival in neutrophils in response to LPS-primed ATP-treatment, in contrast to a combination of IL-1{beta} release with cell death in macrophages. While LPS-primed nigericin-treated neutrophils exhibit IL-1{beta} release and cell death, characterized by nuclear rounding, cell swelling/ ballooning, plasma membrane pore formation, and subsequent cell rupture, confirming the occurrence of pyroptotic cell death. While nigericin alone triggers cytokine uncoupled pyroptosis. Intriguingly, these phenomena do not induce under other death programs, including apoptosis, NETosis. This provides opportunity to unravel the regulation of these pyroptotic responses in neutrophils. Data observed reveal the role of NLRP3 and context-dependent caspase -1 & 11 in IL-1{beta} secretion. While, DNA damage and caspase-7 & 9 regulate pyroptotic death. Intriguingly, forced DNA damage by ATM kinase inhibitor mitigated inflammasome activation and IL-1{beta} release, while spur death. Furthermore, this study identifies perinuclear actomyosin forces driving nuclear rounding and pyroptotic death. Both murine neutrophils exposed to bacteria and human neutrophils treated with nigericin exhibit these pyroptotic processes, highlighting their broad relevance. This study depicts decision whether to secrete cytokines or undergo pyroptotic cell death emphasizing the regulatory role of DNA damage-cytoskeletal axis beyond inflammasome activation. Moreover, LPS and bacteria induced acute lung injury in mice displays nuclear rounding presenting pyroptotic neutrophils and enhanced NLRP3, Caspase-11 activation, and IL-1{beta} release. Together, this study defines intriguing crosstalk of NLRP3, caspases, DNA damage, and actomyosin forces that orchestrate divergent inflammatory fates, potentially leading to opportunities for targeted strategies for combating inflammation. HIGHLIGHTSO_LINeutrophils undergo pyroptotic cell death with nigericin and bacteria, but not with ATP. C_LIO_LILPS priming is essential for IL-1{beta} secretion, while dispensable for cell swelling/ballooning and death. C_LIO_LINLRP3 and Caspase 1/11 regulate IL-1{beta} secretion, while DNA damage and Caspase-7/9 control pyroptotic cell death. C_LIO_LIPerinuclear actomyosin signalling regulates nuclear rounding during pyroptotic cell death. C_LI GRAPHICAL ABSTRACT O_FIG O_LINKSMALLFIG WIDTH=174 HEIGHT=200 SRC="FIGDIR/small/673694v1_ufig1.gif" ALT="Figure 1"> View larger version (43K): org.highwire.dtl.DTLVardef@102da3forg.highwire.dtl.DTLVardef@46838borg.highwire.dtl.DTLVardef@16648faorg.highwire.dtl.DTLVardef@413bc2_HPS_FORMAT_FIGEXP M_FIG C_FIG

Many immune responses depend upon activation of NF-{kappa}B, a key transcription factor in the elicitation of a cytokine response. Here we demonstrate that N4BP1 inhibited TLR-dependent activation of NF-{kappa}B by interacting with the NF-{kappa}B signaling essential modulator (NEMO, also known as I{kappa}B kinase {gamma}) to attenuate NEMO-NEMO dimerization or oligomerization. The UBA-like (ubiquitin associated-like) and CUE-like (ubiquitin conjugation to ER degradation) domains in N4BP1 mediated the interaction with the NEMO COZI domain. Both in vitro and in mice, N4bp1 deficiency specifically enhanced TRIF-independent (TLR2, TLR7, or TLR9-mediated), but not TRIF-dependent (TLR3 or TLR4-mediated), NF-{kappa}B activation leading to increased production of proinflammatory cytokines. In response to TLR4 or TLR3 activation, TRIF caused activation of caspase 8, which cleaved N4BP1 distal to residues D424 and D490 and abolished its inhibitory effect. N4bp1-/- mice also exhibited diminished numbers of T cells in the peripheral blood. Our work identifies N4BP1 as an inhibitory checkpoint protein that must be overcome to activate NF-{kappa}B, and a TRIF-initiated caspase 8-dependent mechanism by which this is accomplished.

The mammalian ovary is the dynamic end-organ of the hypothalamic pituitary ovarian axis. In this coordinated system, ovarian cells undergo continuous cycles of apoptosis, proliferation, and differentiation. These changes parallel fluctuations in ovarian hormones such as estradiol (E2); however, the ovarian immune microenvironment during high-E2 and low-E2 states is not fully understood. We induced a high-E2 state in the mouse ovary by stimulation via gonadotropin treatment. Single-cell RNA sequencing and flow cytometry analysis of ovarian leukocytes revealed abundant mature NK cells, B1 and B2 cells, CD8+ T cells, CD4+ T cells, mature CD4-CD8-T cells, T regulatory cells, and distinct myeloid subsets, including Trem2+ and Apoe+ macrophages. In vivo labeling of circulating cells determined that the vast majority of ovarian leukocytes were tissue-resident. Following gonadotropin treatment, the frequency of NK cells increased two-fold, while B1 cell frequency was reduced by half. Consistently, flow cytometric analysis revealed an increase in mature CD11b+ NK cells following gonadotropin treatment. Gonadotropin treatment also increased cell-cell signaling by myeloid cells at the expense of NK cells. Our findings reveal a diverse resident immune landscape in the ovary that responds robustly to hormonal changes. These findings have implications for a role for immune cells in ovarian physiology and functional dysregulation.

The innate immune protein human surfactant protein D (SP-D) recognises pathogens in the lungs via binding to carbohydrate surface structures. SP-D has been shown to target gram-negative bacterial lipopolysaccharide via calcium-dependent binding, preferentially to the inner core heptose (HepI). To further investigate this recognition, we have determined the high-resolution crystal structures of a trimeric recombinant fragment of human SP-D complexed with synthetic di- and trisaccharides, HepI-Kdo, HepIII-HepII-HepI, and HepII-HepI phosphorylated at either HepI or HepII, inner core motifs common to the lipopolysaccharide of many gram-negative bacteria. In contrast to acid-hydrolysed lipopolysaccharide used in several previous studies, these synthetic saccharides allow presentation of both the innermost Kdo in its natural pyranose form and heptose phosphorylation. The structures confirm the flexibility of SP-D to adopt an alternative binding mode when the preferred epitope is not available, reveal a preference for recognition of the reducing terminal heptose (HepI) via the glyceryl group, indicate that a single Kdo attached to HepI does not have a significant role in ligand recognition, and provide evidence that recognition of phosphorylated inner core diheptosyl ligands varies dependent on whether HepI or HepII is phosphorylated. The disaccharide with HepII O4 phosphorylation binds via the preferred HepI glyceryl-hydroxyls, while HepI O4 phosphorylation reveals HepII binding via the pyranose ring O3 and O4 hydroxyls. The ability of the HepII O4 phosphate to prevent preferred HepI recognition suggests a role for heptose phosphorylation in shielding the bacterial LPS inner core from immune recognition.