ß-Coronaviruses Use Lysosomes for Egress Instead of the Biosynthetic Secretory Pathway.

ß-Coronaviruses are a family of positive-strand enveloped RNA viruses that includes the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Much is known regarding their cellular entry and replication pathways, but their mode of egress remains uncertain. Using imaging methodologies and virus-specific reporters, we demonstrate that ß-coronaviruses utilize lysosomal trafficking for egress rather than the biosynthetic secretory pathway more commonly used by other enveloped viruses. This unconventional egress is regulated by the Arf-like small GTPase Arl8b and can be blocked by the Rab7 GTPase competitive inhibitor CID1067700. Such non-lytic release of ß-coronaviruses results in lysosome deacidification, inactivation of lysosomal degradation enzymes, and disruption of antigen presentation pathways. ß-Coronavirus-induced exploitation of lysosomal organelles for egress provides insights into the cellular and immunological abnormalities observed in patients and suggests new therapeutic modalities.

Inflammasome Inhibition Links IRGM to Innate Immunity.

IRGM is a risk factor for several inflammatory diseases, yet no direct link to immune regulation had been shown. In this issue of Molecular Cell, Mehto et al. (2019) report that IRGM limits NLRP3 inflammasome activation-by both direct inhibition of NLRP3/ASC oligomerization and selective autophagic destruction of NLRP3/ASC.

The HIV-1 envelope protein gp120 impairs B cell proliferation by inducing TGF-ß1 production and FcRL4 expression.

The humoral immune response after acute infection with HIV-1 is delayed and ineffective. The HIV-1 envelope protein gp120 binds to and signals through integrin α4ß7 on T cells. We found that gp120 also bound to and signaled through α4ß7 on naive B cells, which resulted in an abortive proliferative response. In primary B cells, signaling by gp120 through α4ß7 resulted in increased expression of the immunosuppressive cytokine TGF-ß1 and FcRL4, an inhibitory receptor expressed on B cells. Coculture of B cells with HIV-1-infected autologous CD4(+) T cells also increased the expression of FcRL4 by B cells. Our findings indicated that in addition to mediating chronic activation of the immune system, viral proteins contributed directly to HIV-1-associated B cell dysfunction. Our studies identify a mechanism whereby the virus may subvert the early HIV-1-specific humoral immune response.

Activation of autophagy by inflammatory signals limits IL-1ß production by targeting ubiquitinated inflammasomes for destruction.

Autophagosomes delivers cytoplasmic constituents to lysosomes for degradation, whereas inflammasomes are molecular platforms activated by infection or stress that regulate the activity of caspase-1 and the maturation of interleukin 1ß (IL-1ß) and IL-18. Here we show that the induction of AIM2 or NLRP3 inflammasomes in macrophages triggered activation of the G protein RalB and autophagosome formation. The induction of autophagy did not depend on the adaptor ASC or capase-1 but was dependent on the presence of the inflammasome sensor. Blocking autophagy potentiated inflammasome activity, whereas stimulating autophagy limited it. Assembled inflammasomes underwent ubiquitination and recruited the autophagic adaptor p62, which assisted their delivery to autophagosomes. Our data indicate that autophagy accompanies inflammasome activation to temper inflammation by eliminating active inflammasomes.

AKT Regulates NLRP3 Inflammasome Activation by Phosphorylating NLRP3 Serine 5.

The cytosolic pattern recognition receptor NLRP3 senses host-derived danger signals and certain microbe-derived products in both humans and rodents. NLRP3 activation assembles an inflammasome complex that contains the adapter proteins ASC and caspase-1, whose activation triggers the maturation and release of the proinflammatory cytokines IL-1ß and IL-18. S5 phosphorylation of NLRP3 prevents its oligomerization and activation, whereas dephosphorylation of this residue by the phosphatase PP2A allows NLRP3 activation. However, the protein kinase that mediates NLRP3 S5 phosphorylation is unknown. In this study, we show that AKT associates with NLRP3 and phosphorylates it on S5, limiting NLRP3 oligomerization. This phosphorylation event also stabilizes NLRP3 by reducing its ubiquitination on lysine 496, which inhibits its proteasome-mediated degradation by the E3 ligase Trim31. Pharmacologic manipulation of AKT kinase activity reciprocally modulates NLRP3 inflammasome-mediated IL-1ß production. Inhibition of AKT reduced IL-1ß production following the i.p. injection of LPS into mice. We propose that AKT, Trim31, and PP2A together modulate NLRP3 protein levels and the tendency to oligomerize, thereby setting a tightly regulated threshold for NLRP3 activation.

Biased S1PR1 Signaling in B Cells Subverts Responses to Homeostatic Chemokines, Severely Disorganizing Lymphoid Organ Architecture.

Ligand-engaged chemoattractant receptors trigger Gαi subunit nucleotide exchange, stimulating the activation of downstream effector molecules. Activated chemoattractant receptors also dock G protein-coupled receptor kinases (GRKs) that help mediate receptor desensitization. In this study, we show that the B cell-specific loss of GRK2 severely disrupts B cell trafficking and immune cell homeostasis. The GRK2 deficiency in developing murine B cells leads to a severe immune phenotype, including a major reduction of bone marrow IgD+ cells, splenomegaly with a loss of white pulp and grossly expanded red pulp, a deficit of Peyer patches, and small lymph nodes with marked reductions in B cell numbers. The major phenotypes in these mice arise from excessive S1PR1 signaling combined with inadequate homeostatic chemokine receptor signaling. CXCL13 signaling is the most severely compromised. In B cells, our data also indicate that S1PR1 signals constitutively, as blocking S1PR1 signaling with an S1PR1 antagonist enhanced CXCL13-triggered wild-type B cell migration. Furthermore, blocking S1PR1 signaling in the GRK2-deficient B cells partially corrected their poor response to chemokines. Treating mice lacking GRK2 expression in their B cells with an S1PR1 antagonist partially normalized B cell trafficking into lymph node and splenic follicles. These findings reveal the critical interdependence of Gαi-linked signaling pathways in controlling B lymphocyte trafficking.

Gαi2 Signaling Regulates Inflammasome Priming and Cytokine Production by Biasing Macrophage Phenotype Determination.

Macrophages exist as innate immune subsets that exhibit phenotypic heterogeneity and functional plasticity. Their phenotypes are dictated by inputs from the tissue microenvironment. G-protein-coupled receptors are essential in transducing signals from the microenvironment, and heterotrimeric Gα signaling links these receptors to downstream effectors. Several Gαi-coupled G-protein-coupled receptors have been implicated in macrophage polarization. In this study, we use genetically modified mice to investigate the role of Gαi2 on inflammasome activity and macrophage polarization. We report that Gαi2 in murine bone marrow-derived macrophages (BMDMs) regulates IL-1ß release after activation of the NLRP3, AIM2, and NLRC4 inflammasomes. We show this regulation stems from the biased polarity of Gαi2 deficient (Gnai2 -/-) and RGS-insensitive Gαi2 (Gnai2 G184S/G184S) BMDMs. We determined that although Gnai2 G184S/G184S BMDMs (excess Gαi2 signaling) have a tendency toward classically activated proinflammatory (M1) phenotype, Gnai2-/- BMDMs (Gαi2 deficient) are biased toward alternatively activated anti-inflammatory (M2) phenotype. Finally, we find that Gαi2-deficient macrophages have increased Akt activation and IFN-ß production but defects in ERK1/2 and STAT3 activation after LPS stimulation. Gαi2-deficient macrophages also exhibit increased STAT6 activation after IL-4 stimulation. In summary, our data indicates that excess Gαi2 signaling promotes an M1 macrophage phenotype, whereas Gαi2 signaling deficiency promotes an M2 phenotype. Understanding Gαi2-mediated effects on macrophage polarization may bring to light insights regarding disease pathogenesis and the reprogramming of macrophages for the development of novel therapeutics.

Normal Thymocyte Egress, T Cell Trafficking, and CD4+ T Cell Homeostasis Require Interactions between RGS Proteins and Gαi2.

Adaptive immunity depends on mature thymocytes leaving the thymus to enter the bloodstream and the trafficking of T cells through lymphoid organs. Both of these require heterotrimeric Gαi protein signaling, whose intensity and duration are controlled by the regulator of G protein signaling (RGS) proteins. In this study, we show that RGS protein/Gαi2 interactions are essential for normal thymocyte egress, T cell trafficking, and homeostasis. Mature thymocytes with a Gαi2 mutation that disables RGS protein binding accumulated in the perivascular channels of thymic corticomedullary venules. Severe reductions in peripheral naive CD4+ T cells and regulatory T cells occurred. The mutant CD4+ T cells adhered poorly to high endothelial venules and exhibited defects in lymph node entrance and egress. The kinetics of chemokine receptor signaling were disturbed, including chemokine- induced integrin activation. Despite the thymic and lymph node egress defects, sphingosine-1-phosphate signaling was not obviously perturbed. This study reveals how RGS proteins modulate Gαi2 signaling to facilitate thymocyte egress and T cell trafficking.

B lymphocytes exit lymph nodes through cortical lymphatic sinusoids by a mechanism independent of sphingosine-1-phosphate-mediated chemotaxis.

Sphingosine-1-phosphate (S1P) helps mediate lymphocyte egress from lymph nodes, yet many mechanistic questions remain. Here, we show the presence of B lymphocyte egress sites located in the lymph node cortex close to lymph node follicles. B cells exited lymph nodes by squeezing through apparent portals in the lymphatic endothelium of these sinusoids. Treatment with the S1P receptor agonist FTY720 emptied the cortical sinusoids of lymphocytes, blocked lymphatic endothelial penetration, and displaced B lymphocytes into the T cell zone. S1pr3(-/-) B cells, which lack chemoattractant responses to S1P, transited lymph nodes normally, whereas Gnai2(-/-) B cells, which have impaired responses to chemokines and S1P, transited more rapidly than did wild-type cells. This study identifies a major site of B lymphocyte lymph node egress, shows that FTY720 treatment blocks passage through the cortical lymphatic endothelium, and argues against a functional role for S1P chemotaxis in B lymphocyte egress.

Activator of G-Protein Signaling 3-Induced Lysosomal Biogenesis Limits Macrophage Intracellular Bacterial Infection.

Many intracellular pathogens cause disease by subverting macrophage innate immune defense mechanisms. Intracellular pathogens actively avoid delivery to or directly target lysosomes, the major intracellular degradative organelle. In this article, we demonstrate that activator of G-protein signaling 3 (AGS3), an LPS-inducible protein in macrophages, affects both lysosomal biogenesis and activity. AGS3 binds the Gi family of G proteins via its G-protein regulatory (GoLoco) motif, stabilizing the Gα subunit in its GDP-bound conformation. Elevated AGS3 levels in macrophages limited the activity of the mammalian target of rapamycin pathway, a sensor of cellular nutritional status. This triggered the nuclear translocation of transcription factor EB, a known activator of lysosomal gene transcription. In contrast, AGS3-deficient macrophages had increased mammalian target of rapamycin activity, reduced transcription factor EB activity, and a lower lysosomal mass. High levels of AGS3 in macrophages enhanced their resistance to infection by Burkholderia cenocepacia J2315, Mycobacterium tuberculosis, and methicillin-resistant Staphylococcus aureus, whereas AGS3-deficient macrophages were more susceptible. We conclude that LPS priming increases AGS3 levels, which enhances lysosomal function and increases the capacity of macrophages to eliminate intracellular pathogens.

Neutrophil recruitment to lymph nodes limits local humoral response to Staphylococcus aureus.

Neutrophils form the first line of host defense against bacterial pathogens. They are rapidly mobilized to sites of infection where they help marshal host defenses and remove bacteria by phagocytosis. While splenic neutrophils promote marginal zone B cell antibody production in response to administered T cell independent antigens, whether neutrophils shape humoral immunity in other lymphoid organs is controversial. Here we investigate the neutrophil influx following the local injection of Staphylococcus aureus adjacent to the inguinal lymph node and determine neutrophil impact on the lymph node humoral response. Using intravital microscopy we show that local immunization or infection recruits neutrophils from the blood to lymph nodes in waves. The second wave occurs temporally with neutrophils mobilized from the bone marrow. Within lymph nodes neutrophils infiltrate the medulla and interfollicular areas, but avoid crossing follicle borders. In vivo neutrophils form transient and long-lived interactions with B cells and plasma cells, and their depletion augments production of antigen-specific IgG and IgM in the lymph node. In vitro activated neutrophils establish synapse- and nanotube-like interactions with B cells and reduce B cell IgM production in a TGF-ß1 dependent manner. Our data reveal that neutrophils mobilized from the bone marrow in response to a local bacterial challenge dampen the early humoral response in the lymph node.

An essential role for RGS protein/Gαi2 interactions in B lymphocyte-directed cell migration and trafficking.

Chemokines engage B lymphocyte surface receptors, triggering heterotrimeric G protein Gαi subunit guanine nucleotide exchange. RGS proteins limit the duration that Gαi subunits remain GTP bound, and the loss of an individual RGS protein typically enhances chemokine receptor signaling. In this study, we show that B cells carrying a Gαi2 (G184S/G184S) mutation that disables all RGS protein/Gαi2 interactions exhibit an unexpectedly severe reduction in chemokine receptor signaling. The Gαi2 (G184S/G184S) B cells have markedly elevated basal calcium levels, but poor chemokine-induced increases, enhanced nonspecific migration, but extremely poor chemotaxis. In striking contrast, the Gαi2 (G184S/G184S) B cells exhibited enhanced sensitivity to sphingosine 1-phosphate (S1P). S1P elicited heightened intracellular calcium responses and enhanced S1P-triggered cell migration. Mice with the Gαi2 (G184S/G184S) mutation displayed excessive numbers of germinal center-like structures; abnormal serum Ig profiles; and aberrant B lymphocyte trafficking. These findings establish an essential role for RGS proteins in B cell chemoattractant signaling and for the proper position of B lymphocytes in lymphoid organs.

B Lymphocyte-Specific Loss of Ric-8A Results in a Gα Protein Deficit and Severe Humoral Immunodeficiency.

Resistance to inhibitors of cholinesterase 8A (Ric-8A) is a highly evolutionarily conserved cytosolic protein initially identified in Caenorhabditis elegans, where it was assigned a regulatory role in asymmetric cell divisions. It functions as a guanine nucleotide exchange factor for Gαi, Gαq, and Gα12/13 and as a molecular chaperone required for the initial association of nascent Gα subunits with cellular membranes in embryonic stem cell lines. To test its role in hematopoiesis and B lymphocytes specifically, we generated ric8 (fl/fl) vav1-cre and ric8 (fl/fl) mb1-cre mice. The major hematopoietic cell lineages developed in the ric8 (fl/fl) vav1-cre mice, notwithstanding severe reduction in Gαi2/3, Gαq, and Gα13 proteins. B lymphocyte-specific loss of Ric-8A did not compromise bone marrow B lymphopoiesis, but splenic marginal zone B cell development failed, and B cells underpopulated lymphoid organs. The ric8 (fl/fl) mb1-cre B cells exhibited poor responses to chemokines, abnormal trafficking, improper in situ positioning, and loss of polarity components during B cell differentiation. The ric8 (fl/fl) mb1-cre mice had a severely disrupted lymphoid architecture and poor primary and secondary Ab responses. In B lymphocytes, Ric-8A is essential for normal Gα protein levels and is required for B cell differentiation, trafficking, and Ab responses.

The Transcription Factor EB Links Cellular Stress to the Immune Response

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The transcription factor EB (TFEB) is the master transcriptional regulator of autophagy and lysosome biogenesis. Recent advances have led to a paradigm shift in our understanding of lysosomes from a housekeeping cellular waste bin to a dynamically regulated pathway that is efficiently turned up or down based on cellular needs. TFEB coordinates the cellular response to nutrient deprivation and other forms of cell stress through the lysosome system, and regulates a myriad of cellular processes associated with this system including endocytosis, phagocytosis, autophagy, and lysosomal exocytosis. Autophagy and the endolysosomal system are critical to both the innate and adaptive arms of the immune system, with functions in effector cell priming and direct pathogen clearance. Recent studies have linked TFEB to the regulation of the immune response through the endolysosmal pathway and by direct transcriptional activation of immune related genes. In this review, we discuss the current understanding of TFEB's function and the molecular mechanisms behind TFEB activation. Finally, we discuss recent advances linking TFEB to the immune response that positions lysosomal signaling as a potential target for immune modulation.

SARS-coronavirus open reading frame-9b suppresses innate immunity by targeting mitochondria and the MAVS/TRAF3/TRAF6 signalosome.

Coronaviruses (CoV) have recently emerged as potentially serious pathogens that can cause significant human morbidity and death. The severe acute respiratory syndrome (SARS)-CoV was identified as the etiologic agent of the 2002-2003 international SARS outbreak. Yet, how SARS evades innate immune responses to cause human disease remains poorly understood. In this study, we show that a protein encoded by SARS-CoV designated as open reading frame-9b (ORF-9b) localizes to mitochondria and causes mitochondrial elongation by triggering ubiquitination and proteasomal degradation of dynamin-like protein 1, a host protein involved in mitochondrial fission. Also, acting on mitochondria, ORF-9b targets the mitochondrial-associated adaptor molecule MAVS signalosome by usurping PCBP2 and the HECT domain E3 ligase AIP4 to trigger the degradation of MAVS, TRAF3, and TRAF 6. This severely limits host cell IFN responses. Reducing either PCBP2 or AIP4 expression substantially reversed the ORF-9b-mediated reduction of MAVS and the suppression of antiviral transcriptional responses. Finally, transient ORF-9b expression led to a strong induction of autophagy in cells. The induction of autophagy depended upon ATG5, a critical autophagy regulator, but the inhibition of MAVS signaling did not. These results indicate that SARS-CoV ORF-9b manipulates host cell mitochondria and mitochondrial function to help evade host innate immunity. This study has uncovered an important clue to the pathogenesis of SARS-CoV infection and illustrates the havoc that a small ORF can cause in cells.

Defective chemokine signal integration in leukocytes lacking activator of G protein signaling 3 (AGS3).

Activator of G-protein signaling 3 (AGS3, gene name G-protein signaling modulator-1, Gpsm1), an accessory protein for G-protein signaling, has functional roles in the kidney and CNS. Here we show that AGS3 is expressed in spleen, thymus, and bone marrow-derived dendritic cells, and is up-regulated upon leukocyte activation. We explored the role of AGS3 in immune cell function by characterizing chemokine receptor signaling in leukocytes from mice lacking AGS3. No obvious differences in lymphocyte subsets were observed. Interestingly, however, AGS3-null B and T lymphocytes and bone marrow-derived dendritic cells exhibited significant chemotactic defects as well as reductions in chemokine-stimulated calcium mobilization and altered ERK and Akt activation. These studies indicate a role for AGS3 in the regulation of G-protein signaling in the immune system, providing unexpected venues for the potential development of therapeutic agents that modulate immune function by targeting these regulatory mechanisms.

Roles of autophagy in HIV infection.

Autophagy is a major cellular pathway, which at basal levels regulates and maintains the cytoplasmic environment through the capture, isolation and digestion of intracellular materials in a specialized structure called an autophagosome. The unique ability of autophagy to degrade large targets, such as damaged and surplus organelles, intracellular microbes and protein aggregates, has made it a prime focus in inflammation and microbial research. Indeed, autophagy has been shown to be involved in a number of infectious and inflammatory pathologies, by which it may confer protection against intracellular microbes, be targeted by microbes for evasion or be hijacked for microbe biogenesis. In addition, autophagy helps regulate the intracellular and global immune response to both extracellular and intracellular pathogens. Here we review the current literature on the interactions between autophagy and HIV among different immune cells and discuss new research that re-emphasizes the role of inflammation in HIV-mediated CD4(+) T cell death.

Lymph node B lymphocyte trafficking is constrained by anatomy and highly dependent upon chemoattractant desensitization.

B lymphocyte recirculation through lymph nodes (LNs) requires crossing endothelial barriers and chemoattractant-triggered cell migration. Here we show how LN anatomy and chemoattractant receptor signaling organize B lymphocyte LN trafficking. Blood-borne B cells predominately used CCR7 signaling to adhere to high endothelial venules (HEVs). New B cell emigrants slowly transited the HEV perivenule space, and thereafter localized nearby, avoiding the follicle. Eventually, the newly arrived B cells entered the basal portion of the follicle gradually populating it. In contrast, newly arriving activated B cells rapidly crossed HEVs and migrated toward the lymph node follicle. During their LN residency, recirculating B cells reacquired their sphingosine-1 phospate receptor 1 (S1P1) receptors and markedly attenuated their sensitivity to chemokines. Eventually, the B cells exited the LN follicle by entering the cortical lymphatics or returning to the paracortical cords. Upon entering the lymph, the B cells lost their polarity, down-regulated their S1P1 receptors, and subsequently strongly up-regulated their sensitivity to chemokines. These results are summarized in a model of homeostatic trafficking of B cells through LNs.

IL-7 induces expression and activation of integrin α4ß7 promoting naive T-cell homing to the intestinal mucosa.

Interleukin-7 (IL-7) is a nonredundant cytokine that plays a critical role in T-cell homeostasis and promotes immunologic reconstitution in lymphopenic hosts. Here, we show that IL-7, at doses that reflect suprahomeostatic concentrations achieved in lymphopenic hosts, is a potent and selective inducer of the gut-homing integrin α4ß7 in human T cells, as documented both ex vivo and in vivo in patients enrolled in a clinical trial of IL-7 treatment. Induction of α4ß7 by IL-7 occurs primarily in naive T cells and is associated with functional activation of the integrin, as indicated by increased binding activity for the specific α4ß7 ligand, MAdCAM-1. The physiologic relevance of these findings was validated by the preferential homing of IL-7-treated naive human T cells to the intestinal compartment in humanized NOD/SCID/IL-2 receptor-γ(null) (NSG) mice. We also show that IL-7 triggers a peculiar activation program in naive T cells, characterized by the acquisition of memory-like phenotypic features and proliferation uncoupled from expression of classic T-cell activation markers. These findings provide a mechanism for the transient in vivo depletion of circulating T cells after IL-7 administration and suggest that intestinal homing and memory-like conversion of naive T cells are critical steps in the IL-7-driven immunologic reconstitution of lymphopenic hosts.

Constitutively active ezrin increases membrane tension, slows migration, and impedes endothelial transmigration of lymphocytes in vivo in mice.

ERM (ezrin, radixin moesin) proteins in lymphocytes link cortical actin to plasma membrane, which is regulated in part by ERM protein phosphorylation. To assess whether phosphorylation of ERM proteins regulates lymphocyte migration and membrane tension, we generated transgenic mice whose T-lymphocytes express low levels of ezrin phosphomimetic protein (T567E). In these mice, T-cell number in lymph nodes was reduced by 27%. Lymphocyte migration rate in vitro and in vivo in lymph nodes decreased by 18% to 47%. Lymphocyte membrane tension increased by 71%. Investigations of other possible underlying mechanisms revealed impaired chemokine-induced shape change/lamellipod extension and increased integrin-mediated adhesion. Notably, lymphocyte homing to lymph nodes was decreased by 30%. Unlike most described homing defects, there was not impaired rolling or sticking to lymph node vascular endothelium but rather decreased migration across that endothelium. Moreover, decreased numbers of transgenic T cells in efferent lymph suggested defective egress. These studies confirm the critical role of ERM dephosphorylation in regulating lymphocyte migration and transmigration. Of particular note, they identify phospho-ERM as the first described regulator of lymphocyte membrane tension, whose increase probably contributes to the multiple defects observed in the ezrin T567E transgenic mice.