Inhibition of anti-viral stress granule formation by coronavirus endoribonuclease nsp15 ensures efficient virus replication

Cytoplasmic stress granules (SGs) are generally triggered by stress-induced translation arrest for storing mRNAs. Recently, it has been shown that SGs exert anti-viral functions due to their involvement in protein synthesis shut off and recruitment of innate immune signaling intermediates. The largest RNA viruses, coronaviruses, impose great threat to public safety and animal health; however, the significance of SGs in coronavirus infection is largely unknown. Infectious Bronchitis Virus (IBV) is the first identified coronavirus in 1930s and has been prevalent in poultry farm for many years. In this study, we provided evidence that IBV overcomes the host antiviral response by inhibiting SGs formation via the virus-encoded endoribonuclease nsp15. By immunofluorescence analysis, we observed that IBV infection not only did not trigger SGs formation in approximately 80% of the infected cells, but also impaired the formation of SGs triggered by heat shock, sodium arsenite, or NaCl stimuli. We further demonstrated that the intrinsic endoribonuclease activity of nsp15 was responsible for the interference of SGs formation. In fact, nsp15-defective recombinant IBV (rIBV-nsp15-H238A) greatly induced the formation of SGs, along with accumulation of dsRNA and activation of PKR, whereas wild type IBV failed to do so. Consequently, infection with rIBV-nsp15-H238A strongly triggered transcription of IFN-β which in turn greatly affected rIBV-nsp15-H238A replication. Further analysis showed that SGs function as an antiviral hub, as demonstrated by the attenuated IRF3-IFN response and increased production of IBV in SG-defective cells. Additional evidence includes the aggregation of pattern recognition receptors (PRRs) and signaling intermediates to the IBV-induced SGs. Collectively, our data demonstrate that the endoribonuclease nsp15 of IBV interferes with the formation of antiviral hub SGs by regulating the accumulation of viral dsRNA and by antagonizing the activation of PKR, eventually ensuring productive virus replication. We further demonstrated that nsp15s from PEDV, TGEV, SARS-CoV, and SARS-CoV-2 harbor the conserved function to interfere with the formation of chemically-induced SGs. Thus, we speculate that coronaviruses employ similar nsp15-mediated mechanisms to antagonize the host anti-viral SGs formation to ensure efficient virus replication.

Coronavirus encodes the conserved endoribonuclease nsp15, which has been reported to antagonize IFN responses by mediating evasion of recognition by dsRNA sensors. SGs are part of the host cell anti-viral response; not surprisingly, viruses in turn produce an array of antagonists to counteract such host response. Here, we show that IBV prevents the formation of SGs via nsp15, by reducing the accumulation of viral dsRNA, thereby evading the activation of PKR, phosphorylation of eIF2α, and formation of SGs. Depletion of SG scaffold proteins G3BP1/2 decreases IRF3-IFN response and increases the production of IBV. When overexpressed alone, nsp15s from different coronaviruses (IBV, PEDV, TGEV, SARS-CoV, and SARS-CoV-2) interferes with chemically- and physically-induced SGs, probably by targeting essential SGs assembly factors. In this way, coronaviruses antagonize the formation of SGs by nsp15, via reducing the viral dsRNA accumulation and sequestering/depleting critical component of SGs. To our knowledge, this is the first report describing the role of coronavirus nsp15 in the suppression of integral stress response, in crosstalk with anti-innate immune response.

Dance with the Devil: Stress Granules and Signaling in Antiviral Responses

Cells have evolved highly specialized sentinels that detect viral infection and elicit an antiviral response. Among these, the stress-sensing protein kinase R, which is activated by double-stranded RNA, mediates suppression of the host translation machinery as a strategy to limit viral replication. Non-translating mRNAs rapidly condensate by phase separation into cytosolic stress granules, together with numerous RNA-binding proteins and components of signal transduction pathways. Growing evidence suggests that the integrated stress response, and stress granules in particular, contribute to antiviral defense. This review summarizes the current understanding of how stress and innate immune signaling act in concert to mount an effective response against virus infection, with a particular focus on the potential role of stress granules in the coordination of antiviral signaling cascades.

The Viral Macrodomain Counters Host Antiviral ADP-Ribosylation

Macrodomains, enzymes that remove ADP-ribose from proteins, are encoded by several families of RNA viruses and have recently been shown to counter innate immune responses to virus infection. ADP-ribose is covalently attached to target proteins by poly-ADP-ribose polymerases (PARPs), using nicotinamide adenine dinucleotide (NAD+) as a substrate. This modification can have a wide variety of effects on proteins including alteration of enzyme activity, protein-protein interactions, and protein stability. Several PARPs are induced by interferon (IFN) and are known to have antiviral properties, implicating ADP-ribosylation in the host defense response and suggesting that viral macrodomains may counter this response. Recent studies have demonstrated that viral macrodomains do counter the innate immune response by interfering with PARP-mediated antiviral defenses, stress granule formation, and pro-inflammatory cytokine production. Here, we will describe the known functions of the viral macrodomains and review recent literature demonstrating their roles in countering PARP-mediated antiviral responses.

Programmed 'disarming' of the neutrophil proteome reduces the magnitude of inflammation

The antimicrobial functions of neutrophils are facilitated by a defensive armamentarium of proteins stored in granules, and by the formation of neutrophil extracellular traps (NETs). However, the toxic nature of these structures poses a threat to highly vascularized tissues, such as the lungs. Here, we identified a cell-intrinsic program that modified the neutrophil proteome in the circulation and caused the progressive loss of granule content and reduction of the NET-forming capacity. This program was driven by the receptor CXCR2 and by regulators of circadian cycles. As a consequence, lungs were protected from inflammatory injury at times of day or in mouse mutants in which granule content was low. Changes in the proteome, granule content and NET formation also occurred in human neutrophils, and correlated with the incidence and severity of respiratory distress in pneumonia patients. Our findings unveil a 'disarming' strategy of neutrophils that depletes protein stores to reduce the magnitude of inflammation.

Neutrophil Degranulation of Azurophil and Specific Granules

Neutrophils play a pivotal role in innate immunity and in the inflammatory reactions. Upon activation, neutrophils release several toxic molecules directed against microbial pathogens into the phagosome. These molecules include reactive oxygen species (ROS), myeloperoxidase, glucosidases, proteases, and antibacterial peptides. In resting cells these proteins and the enzyme responsible for ROS production (NOX2) are stored inside or at the membranes of different granules called azurophil or primary, specific or secondary, gelatinase or tertiary, and the secretory vesicles. Each granule has a specific density, content, and markers. Myeloperoxidase (MPO) is the azurophil granule marker, and the neutrophil-gelatinase-associated lipocalin (NGAL) is the specific granule marker. After cell activation by different stimuli, granule contents are released into the phagosome or in the extracellular space through a process called degranulation. Also during this process, membrane granules fuse with the phagosome and plasma membrane allowing expression of new markers at the cell surface. The degranulation can be assessed by measuring either the release of different proteins by neutrophils or the expression of granule markers at the plasma membrane. In this chapter, we describe the techniques used to measure degranulation of azurophil and specific neutrophil granules using different approaches such as measurement of MPO enzymatic activity and detection of MPO and NGAL proteins by SDS-PAGE and Western blot.

Antiviral Immune Response as a Trigger of FUS Proteinopathy in Amyotrophic Lateral Sclerosis

Mutations in the FUS gene cause familial amyotrophic lateral sclerosis (ALS-FUS). In ALS-FUS, FUS-positive inclusions are detected in the cytoplasm of neurons and glia, a condition known as FUS proteinopathy. Mutant FUS incorporates into stress granules (SGs) and can spontaneously form cytoplasmic RNA granules in cultured cells. However, it is unclear what can trigger the persistence of mutant FUS assemblies and lead to inclusion formation. Using CRISPR/Cas9 cell lines and patient fibroblasts, we find that the viral mimic dsRNA poly(I:C) or a SG-inducing virus causes the sustained presence of mutant FUS assemblies. These assemblies sequester the autophagy receptor optineurin and nucleocytoplasmic transport factors. Furthermore, an integral component of the antiviral immune response, type I interferon, promotes FUS protein accumulation by increasing FUS mRNA stability. Finally, mutant FUS-expressing cells are hypersensitive to dsRNA toxicity. Our data suggest that the antiviral immune response is a plausible second hit for FUS proteinopathy.

Cytotoxic Granule Exocytosis From Human Cytotoxic T Lymphocytes Is Mediated by VAMP7

Cytotoxic T lymphocytes kill infected or malignant cells through the directed release of cytotoxic substances at the site of target cell contact, the immunological synapse. While genetic association studies of genes predisposing to early-onset life-threatening hemophagocytic lymphohistiocytosis has identified components of the plasma membrane fusion machinery, the identity of the vesicular components remain enigmatic. Here, we identify VAMP7 as an essential component of the vesicular fusion machinery of primary, human T cells. VAMP7 co-localizes with granule markers throughout all stages of T cell maturation and simultaneously fuses with granule markers at the IS. Knock-down of VAMP7 expression significantly decreased the killing efficiency of T cells, without diminishing early T cell receptor signaling. VAMP7 exerts its function in a SNARE complex with Syntaxin11 and SNAP-23 on the plasma membrane. The identification of the minimal fusion machinery in T cells provides a starting point for the development of potential drugs in immunotherapy.

CD107a Degranulation Assay to Evaluate Immune Cell Antitumor Activity

Cancer development is under surveillance by the immune system of the host. Tumor cells can be recognized and killed by cytotoxic lymphocytes- such as CD8+ T lymphocytes and natural killer (NK) cells-mainly through the immune secretion of lytic granules that kill target cells. This process involves the fusion of the granule membrane with the cytoplasmic membrane of the immune effector cell, resulting in surface exposure of lysosomal-associated proteins that are typically present on the lipid bilayer surrounding lytic granules, such as CD107a. Therefore, membrane expression of CD107a constitutes a marker of immune cell activation and cytotoxic degranulation. In this chapter, we detail the steps required to isolate peripheral blood mononuclear cells (PBMCs), coculture them with target tumor cell lines, and evaluate the cytotoxic immune function by means of flow cytometry evaluation of CD107a expression on the surface of NK cells.

Intestinal granular cells of a cartilaginous fish, thornback ray Raja clavata: Morphological characterization and expression of different molecules

This investigation aims to fill gaps in our understanding of the intestinal immune cells of elasmobranchs. Whole digestive tracts of fifteen thornback ray Raja clavata were provided by a trawl fleet from the Gulf of Asinara (Sardinia, western Mediterranean Sea). Histochemical, immunohistochemical and ultrastructural observations were conducted on the spiral intestine. Three types of granular cells were identified; type I in epithelium, types II and III in lamina propria-submucosa, with each of them containing cytoplasmic granules with different ultrastructural characteristics. Data on size and density of each granular cell type are provided. Immunostaining of intestinal sections showed the reactivity of the granular cells: type I cells were positive for lysozyme, mast cell tryptase and tumor necrosis factor-ɑ based on antibody staining; type III cells were immune-reactive to anti-interleukin 6 antibody, whilst type II cells were negative to all the antibodies used. Comparison of each granular cell type with immune cells of teleosts or mammals and an hypothesis on their nature and function are reported. A potential role for granular cells in intestinal cellular immunity is also discussed with respect to type I and type III cells having similarities to Paneth cells and neutrophils, respectively.

OASL1 Traps Viral RNAs in Stress Granules to Promote Antiviral Responses

Oligoadenylate synthetase (OAS) protein family is the major interferon (IFN)-stimulated genes responsible for the activation of RNase L pathway upon viral infection. OAS-like (OASL) is also required for inhibition of viral growth in human cells, but the loss of one of its mouse homolog, OASL1, causes a severe defect in termination of type I interferon production. To further investigate the antiviral activity of OASL1, we examined its subcellular localization and regulatory roles in IFN production in the early and late stages of viral infection. We found OASL1, but not OASL2, formed stress granules trapping viral RNAs and promoted efficient RLR signaling in early stages of infection. Stress granule formation was dependent on RNA binding activity of OASL1. But in the late stages of infection, OASL1 interacted with IRF7 transcripts to inhibit translation resulting in down regulation of IFN production. These results implicate that OASL1 plays context dependent functions in the antiviral response for the clearance and resolution of viral infections.

Syntaxin 4 mediates endosome recycling for lytic granule exocytosis in cytotoxic T-lymphocytes

Adaptive and innate immunity utilize the perforin-killing pathway to eliminate virus-infected or cancer cells. Cytotoxic T-lymphocytes (CTLs) and natural killer cells mediate this process by releasing toxic proteins at the contact area with target cells known as immunological synapse (IS). Formation of a stable IS and exocytosis of toxic proteins requires persistent fusion of Rab11a recycling endosomes with the plasma membrane (PM) that may assure the delivery of key effector proteins. Despite the importance of the recycling endosomal compartment, the membrane fusion proteins that control this process at the IS remain elusive. Here, by performing knockdown experiments we found that syntaxin 4 (STX4) is necessary for cytotoxic activity and CD107a degranulation against target cells in a similar fashion to syntaxin 11, which is involved in lytic granule (LG) exocytosis and immunodeficiency when it is mutated. Using total internal reflection fluorescent microscopy we identified that STX4 mediates fusion of EGFP-Rab11a vesicles at the IS. Immunoprecipitation experiments in lysates of activated CTLs indicate that endogenous STX4 may drive this fusion step by interacting with cognate proteins: Munc18-3/SNAP23/VAMP7 and/or VAMP8. These results reveal the role of STX4 in mediating fusion of Rab11a endosomes upstream of lytic granules (LGs) exocytosis and further demonstrate the importance of this pathway in controlling CTL-mediated cytotoxicity.

Syntaxin 8 is required for efficient lytic granule trafficking in cytotoxic T lymphocytes

Cytotoxic T lymphocytes (CTL) eliminate pathogen-infected and cancerous cells mainly by polarized secretion of lytic granules (LG, containing cytotoxic molecules like perforin and granzymes) at the immunological synapse (IS). Members of the SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) family are involved in trafficking (generation, transport and fusion) of vesicles at the IS. Syntaxin 8 (Stx8) is expressed in LG and colocalizes with the T cell receptor (TCR) upon IS formation. Here, we report the significance of Stx8 for human CTL cytotoxicity. We found that Stx8 mostly localized in late, recycling endosomal and lysosomal compartments with little expression in early endosomal compartments. Down-regulation of Stx8 by siRNA resulted in reduced cytotoxicity. We found that following perforin release of the pre-existing pool upon target cell contact, Stx8 down-regulated CTL regenerate perforin pools less efficiently and thus release less perforin compared to control CTL. CD107a degranulation, real-time and end-point population cytotoxicity assays, and high resolution microscopy support our conclusion that Stx8 is required for proper and timely sorting and trafficking of cytotoxic molecules to functional LG through the endosomal pathway in human CTL.

Super-resolution microscopy as a potential approach to diagnosis of platelet granule disorders

BACKGROUND

Many platelet functions are dependent on bioactive molecules released from their granules. Deficiencies of these granules in number, shape or content are associated with bleeding. The small size of these granules is such that imaging them for diagnosis has traditionally required electron microscopy. However, recently developed super-resolution microscopes provide sufficient spatial resolution to effectively image platelet granules. When combined with automated image analysis, these methods provide a quantitative, unbiased, rapidly acquired dataset that can readily and reliably reveal differences in platelet granules between individuals.

OBJECTIVE

To demonstrate the ability of structured illumination microscopy (SIM) to efficiently differentiate between healthy volunteers and three patients with Hermansky-Pudlak syndrome.

METHODS

Blood samples were taken from three patients with Hermansky-Pudlak syndrome and seven controls. Patients 1-3 have gene defects in HPS1, HPS6 and HPS5, respectively; all controls were healthy volunteers. Platelet-rich plasma was isolated from blood and the platelets fixed, stained for CD63 and processed for analysis by immunofluorescence microscopy, using a custom-built SIM microscope.

RESULTS

SIM can successfully resolve CD63-positive structures in fixed platelets. A determination of the number of CD63-positive structures per platelet allowed us to conclude that each patient was significantly different from all of the controls with 99% confidence.

CONCLUSIONS

A super-resolution imaging approach is effective and rapid in objectively differentiating between patients with a platelet bleeding disorder and healthy volunteers. CD63 is a useful marker for predicting Hermansky-Pudlak syndrome and could be used in the diagnosis of patients suspected of other platelet granule disorders.

Regulation of antiviral innate immune signaling by stress-induced RNA granules

Activation of antiviral innate immunity is triggered by cellular pattern recognition receptors. Retinoic acid inducible gene-I (RIG-I)-like receptors (RLRs) detect viral non-self RNA in cytoplasm of virus-infected cells and play a critical role in the clearance of the invaded viruses through production of antiviral cytokines. Among the three known RLRs, RIG-I and melanoma differentiation-associated gene 5 recognize distinct non-self signatures of viral RNA and activate antiviral signaling. Recent reports have clearly described the molecular machinery underlying the activation of RLRs and interactions with the downstream adaptor, mitochondrial antiviral signaling protein (MAVS). RLRs and MAVS are thought to form large multimeric filaments around cytoplasmic organelles depending on the presence of Lys63-linked ubiquitin chains. Furthermore, RLRs have been shown to localize to stress-induced ribonucleoprotein aggregate known as stress granules and utilize them as a platform for recognition/activation of signaling. In this review, we will focus on the current understanding of RLR-mediated signal activation and the interactions with stress-induced RNA granules.

Yellow head virus infection in black tiger shrimp reveals specific interaction with granule-containing hemocytes and crustinPm1 as a responsive protein

Yellow head virus (YHV) causes acute infections and mass mortality in black tiger shrimp culture. Our study aims to investigate molecular interaction between YHV and circulating hemocytes of Penaeus monodon at early infection. Total shrimp hemocytes were isolated by Percoll gradient centrifugation and identified by flow cytometric analysis. At least three types of hemocyte cells were identified as hyaline, semi-granular, and granular hemocytes. Experimental infection of YHV in shrimp culture demonstrated drastic changes in total and each hemocyte cell counts. Immunohistochemistry analysis demonstrated interaction and replication of YHV mainly with the granule-containing hemocytes and little to none in hyaline cell. These granule-containing hemocytes are proposed to be YHV targets providing the first line of defense to viral infection. Protein expression profiling of granule-containing hemocytes revealed several immune-responsive proteins including antimicrobial protein crustins (crustinPm1 and crustinPm4), alpha-2-macroglobulin, and kazal-type serine proteinase inhibitor. During an early phase of YHV infection at 6 hpi crustinPm1 illustrated a significant increase of mRNA and protein expression level in plasma. The results suggest that an antimicrobial crustinPm1 may participate in shrimp defense mechanism against YHV, especially on the granule-containing hemocytes.

Behavior and Properties of Mature Lytic Granules at the Immunological Synapse of Human Cytotoxic T Lymphocytes

Killing of virally infected cells or tumor cells by cytotoxic T lymphocytes requires targeting of lytic granules to the junction between the CTL and its target. We used whole-cell patch clamp to measure the cell capacitance at fixed intracellular [Ca2+] to study fusion of lytic granules in human CTLs. Expression of a fluorescently labeled human granzyme B construct allowed identification of lytic granule fusion using total internal reflection fluorescence microscopy. In this way capacitance steps due to lytic granule fusion were identified. Our goal was to determine the size of fusing lytic granules and to describe their behavior at the plasma membrane. On average, 5.02 ± 3.09 (mean ± s.d.) lytic granules were released per CTL. The amplitude of lytic granule fusion events was ~ 3.3 fF consistent with a diameter of about 325 nm. Fusion latency was biphasic with time constants of 15.9 and 106 seconds. The dwell time of fusing lytic granules was exponentially distributed with a mean dwell time of 28.5 seconds. Fusion ended in spite of the continued presence of granules at the immune synapse. The mobility of fusing granules at the membrane was indistinguishable from that of lytic granules which failed to fuse. While dwelling at the plasma membrane lytic granules exhibit mobility consistent with docking interspersed with short periods of greater mobility. The failure of lytic granules to fuse when visible in TIRF at the membrane may indicate that a membrane-confined reaction is rate limiting.

Characterization of the circulating hemocytes in mud crab (Scylla olivacea) revealed phenoloxidase activity

This study focused on an isolation and characterization of the circulating hemocytes in mud crab, Scylla olivacea. Isolation of specific cell types of hemocytes from crab hemolymph was accomplished by using 60% Percoll density gradient centrifugation. Four separated bands of the hemocytes were successfully obtained. Characterization of these isolated hemocytes by light microscope using trypan blue-rose bengal staining, rose bengal-hematoxilin staining, and phase contrast revealed four distinct types of hemocyte cells. Using their specific morphology and granularity, they were identified as hyaline cell (HC), small granular cell (SGC), large granular cell (LGC) and mixed granular cell (MGC). Transmission electron microscopy (TEM) revealed more details on specific cell size, size of cytoplasmic granule, and nuclear to cytoplasmic ratio, and confirmed the classification. Relative abundance of these cells types in the hemolymph of an adult crab were 15.50±8.22% for HC, 55.50±7.15% for SGC, 13.50±5.28% for LGC, and 15.50±3.50% for MGC. Proteomic analysis of protein expression for each specific cell types by two-dimensional electrophoresis identified two highly abundant proteins, prophenoloxidase (ProPO) and peroxinectin in LGC. Determination of phenoloxidase (PO) activity in each isolated cell types using in vitro and in situ chemical assays confirmed the presence of PO activity only in LGC. Based on an increased PO activity of crab hemolymph during the course of White Spot Syndrome Virus (WSSV) infection, these results suggest that prophenoloxidase pathway was employed for host defense mechanism against WSSV and it may link to the role of large granular hemocyte.

DHX36 enhances RIG-I signaling by facilitating PKR-mediated antiviral stress granule formation

RIG-I is a DExD/H-box RNA helicase and functions as a critical cytoplasmic sensor for RNA viruses to initiate antiviral interferon (IFN) responses. Here we demonstrate that another DExD/H-box RNA helicase DHX36 is a key molecule for RIG-I signaling by regulating double-stranded RNA (dsRNA)-dependent protein kinase (PKR) activation, which has been shown to be essential for the formation of antiviral stress granule (avSG). We found that DHX36 and PKR form a complex in a dsRNA-dependent manner. By forming this complex, DHX36 facilitates dsRNA binding and phosphorylation of PKR through its ATPase/helicase activity. Using DHX36 KO-inducible MEF cells, we demonstrated that DHX36 deficient cells showed defect in IFN production and higher susceptibility in RNA virus infection, indicating the physiological importance of this complex in host defense. In summary, we identify a novel function of DHX36 as a critical regulator of PKR-dependent avSG to facilitate viral RNA recognition by RIG-I-like receptor (RLR).

Nuclear receptor 4a3 (nr4a3) regulates murine mast cell responses and granule content

Nuclear receptor 4a3 (Nr4a3) is a transcription factor implicated in various settings such as vascular biology and inflammation. We have recently shown that mast cells dramatically upregulate Nuclear receptor 4a3 upon activation, and here we investigated the functional impact of Nuclear receptor 4a3 on mast cell responses. We show that Nuclear receptor 4a3 is involved in the regulation of cytokine/chemokine secretion in mast cells following activation via the high affinity IgE receptor. Moreover, Nuclear receptor 4a3 negatively affects the transcript and protein levels of mast cell tryptase as well as the mast cell's responsiveness to allergen. Together, these findings identify Nuclear receptor 4a3 as a novel regulator of mast cell function.

Comparative analysis of circulating hemocytes of the freshwater snail Pomacea canaliculata

Molluscs are invertebrates of great relevance for economy, environment and public health. The numerous studies on molluscan immunity and physiology registered an impressive variability of circulating hemocytes. This study is focused on the first characterization of the circulating hemocytes of the freshwater gastropod Pomacea canaliculata, a model for several eco-toxicological and parasitological researches. Flow cytometry analysis identified two populations of hemocytes on the basis of differences in size and internal organization. The first population contains small and agranular cells. The second one displays major size and a more articulated internal organization. Light microscopy evidenced two principal morphologies, categorized as Group I (small) and II (large) hemocytes. Group I hemocytes present the characteristics of blast-like cells, with an agranular and basophilic cytoplasm. Group I hemocytes can adhere onto a glass surface but seem unable to phagocytize heat-inactivated Escherichia coli. The majority of Group II hemocytes displays an agranular cytoplasm, while a minority presents numerous granules. Agranular cytoplasm may be basophilic or acidophilic. Granules are positive to neutral red staining and therefore acidic. Independently from their morphology, Group II hemocytes are able to adhere and to engulf heat-inactivated E. coli. Transmission electron microscopy analysis clearly distinguished between agranular and granular hemocytes and highlighted the electron dense content of the granules. After hemolymph collection, time-course analysis indicated that the Group II hemocytes are subjected to an evident dynamism with changes in the percentage of agranular and granular hemocytes. The ability of circulating hemocytes to quickly modify their morphology and stainability suggests that P. canaliculata is endowed with highly dynamic hemocyte populations able to cope with rapid environmental changes as well as fast growing pathogens.

Relationship of other cytoplasmic ribonucleoprotein bodies (cRNPB) to GW/P bodies

GW/P body components are involved in the post-transcriptional -processing of messenger RNA (mRNA) through the RNA interference and 5' → 3' mRNA degradation pathways, as well as functioning in mRNA transport and stabilization. It is currently thought that the relevant mRNA silencing and degrading factors are partitioned to these cytoplasmic microdomains thus effecting post-transcriptional regulation and the prevention of accidental degradation of functional mRNA. Although much attention has focused on GW/P bodies, a variety of other cytoplasmic RNP bodies (cRNPB) also have highly specialized functions and have been shown to interact or co-localize with components of GW/P bodies. These cRNPB include neuronal transport RNP granules, stress granules, RNP-rich cytoplasmic germline granules or chromatoid bodies, sponge bodies, cytoplasmic prion protein-induced RNP granules, U bodies and TAM bodies. Of clinical relevance, autoantibodies directed against protein and miRNA components of GW/P bodies have been associated with autoimmune diseases, neurological diseases and cancer. Understanding the molecular function of GW/P bodies and their interactions with other cRNPB may provide clues to the etiology or pathogenesis of diseases associated with autoantibodies directed to these structures. This chapter will focus on the similarities and differences of the various cRNPB as an approach to understanding their functional relationships to GW/P bodies.

Neutrophil extracellular traps: double-edged swords of innate immunity

Spectacular images of neutrophils ejecting nuclear chromatin and bactericidal proteins, in response to microbes, were first reported in 2004. As externalized chromatin could entangle bacteria, these structures were named neutrophil extracellular traps (NETs). Subsequent studies identified microorganisms and sterile conditions that stimulate NETs, as well as additional cell types that release extracellular chromatin. The release of NETs is the most dramatic stage in a cell death process called NETosis. Experimental evidence suggests that NETs participate in pathogenesis of autoimmune and inflammatory disorders, with proposed involvement in glomerulonephritis, chronic lung disease, sepsis, and vascular disorders. Exaggerated NETosis or diminished NET clearance likely increases risk of autoreactivity to NET components. The biological significance of NETs is just beginning to be explored. A more complete integration of NETosis within immunology and pathophysiology will require better understanding of NET properties associated with specific disease states and microbial infections. This may lead to the identification of important therapeutic targets.

Critical role of an antiviral stress granule containing RIG-I and PKR in viral detection and innate immunity

Retinoic acid inducible gene I (RIG-I)-like receptors (RLRs) function as cytoplasmic sensors for viral RNA to initiate antiviral responses including type I interferon (IFN) production. It has been unclear how RIG-I encounters and senses viral RNA. To address this issue, we examined intracellular localization of RIG-I in response to viral infection using newly generated anti-RIG-I antibody. Immunohistochemical analysis revealed that RLRs localized in virus-induced granules containing stress granule (SG) markers together with viral RNA and antiviral proteins. Because of similarity in morphology and components, we termed these aggregates antiviral stress granules (avSGs). Influenza A virus (IAV) deficient in non-structural protein 1 (NS1) efficiently generated avSGs as well as IFN, however IAV encoding NS1 produced little. Inhibition of avSGs formation by removal of either the SG component or double-stranded RNA (dsRNA)-dependent protein kinase (PKR) resulted in diminished IFN production and concomitant enhancement of viral replication. Furthermore, we observed that transfection of dsRNA resulted in IFN production in an avSGs-dependent manner. These results strongly suggest that the avSG is the locus for non-self RNA sensing and the orchestration of multiple proteins is critical in the triggering of antiviral responses.

Lytic granule polarization, rather than degranulation, is the preferred target of inhibitory receptors in NK cells

Natural cytotoxicity is achieved by polarized release of perforin and granzymes at the NK cell-target cell immunological synapse. Signals for granule polarization and degranulation can be uncoupled in NK cells, which raises the question of their respective sensitivity to inhibitory receptors. Expression of either HLA-C or HLA-E on the human cell line 721.221 blocked granule polarization, degranulation, and CD16-dependent MIP-1α secretion by NK cell clones that expressed inhibitory receptors of matching HLA specificity. To test inhibition of signals for polarization and degranulation separately, Drosophila S2 cells expressing ICAM-1 with either HLA-C or HLA-E were used. CD16-dependent degranulation and MIP-1α secretion were not fully inhibited, suggesting that other receptor-ligand interactions, which occur with 721.221 cells, contribute to inhibition. In contrast, HLA-C or HLA-E on S2 cells coexpressing ICAM-1 or ULBP1 were sufficient to block granule polarization induced by either LFA-1 or NKG2D, even during concomitant CD16-dependent degranulation. Similarly, expression of a ligand for NKR-P1A on S2 cells inhibited granule polarization but not CD16-induced degranulation. Therefore, granule polarization, rather than degranulation, is the preferred target of inhibitory receptors in NK cells.

The exocytosis of lytic granules is impaired in Vti1b- or Vamp8-deficient CTL leading to a reduced cytotoxic activity following antigen-specific activation

The exocytosis of cytotoxic proteins stored in lytic granules of activated CTL is a key event during killing of target cells. Membrane fusion events that are mediated by soluble N-ethylmaleimide-sensitive-factor attachment protein receptor (SNARE) proteins are crucial, as demonstrated by patients with familial hemophagocytic lymphohistocytosis type 4 who have mutations in the SNARE protein syntaxin-11 that result in an impaired degranulation of cytotoxic cells. We found an increased mRNA expression of the SNARE protein genes Vti1b and Vamp8 during Ag-specific activation of CTL from TCR-transgenic OT-I mice. Therefore, we investigated the cytolytic activity of CTL from TCR-transgenic Vti1b and Vamp8 knockout mice. At 3 d as well as at 4 d of Ag-specific stimulation, the degranulation of CTL was significantly reduced in Vti1b and Vamp8 knockout mice, as determined by cell surface expression of the degranulation marker CD107a. After 3 d of Ag-specific stimulation, the cytolytic activity of Vti1b- and Vamp8-deficient CTL was reduced to approximately 50% compared with heterozygous controls. However, 4 d after stimulation, the cytotoxic activity of Vti1b- as well as Vamp8-deficient CTL was not impaired anymore. The capacity of Vti1b- and Vamp8-deficient dendritic cells to process Ags and to stimulate the proliferation of CTL was not reduced, arguing against an indirect effect on the activation of CTL. These findings suggest a role of the SNARE proteins vti1b and vesicle-associated membrane protein 8 in the degranulation of CTL. However, a deficiency can apparently be compensated and affects only transiently the cytotoxic activity of CTL during their development to armed effector cells.

Opening the flood-gates: how neutrophil-endothelial interactions regulate permeability

Many diseases have an inflammatory component, where neutrophil interactions with the vascular endothelium lead to barrier dysfunction and increased permeability. Neutrophils increase permeability through secreted products such as the chemokines CXCL1, 2, 3, and 8, through adhesion-dependent processes involving beta(2) integrins interacting with endothelial ICAM-1, and through combinations where beta(2) integrin engagement leads to degranulation and secretion of heparin-binding protein. Some neutrophil products, such as arachidonic acid or the leukotriene LTA4, are further processed by endothelial enzymes via transcellular metabolism before the resulting products thromboxane A2 or LTC4 can activate their cognate receptors. Neutrophils also generate reactive oxygen species that induce vascular leakage. This review focuses on the mechanisms of neutrophil-mediated leakage.

Distinct profiles of cytotoxic granules in memory CD8 T cells correlate with function, differentiation stage, and antigen exposure

Cytotoxic CD8 T cells exert their antiviral and antitumor activity primarily through the secretion of cytotoxic granules. Degranulation activity and cytotoxic granules (perforin plus granzymes) generally define CD8 T cells with cytotoxic function. In this study, we have investigated the expression of granzyme K (GrmK) in comparison to that of GrmA, GrmB, and perforin. The expression of the cytotoxic granules was assessed in virus-specific CD8 T cells specific to influenza virus, Epstein-Barr virus (EBV), cytomegalovirus (CMV), or human immunodeficiency virus type 1 (HIV-1). We observed a dichotomy between GrmK and perforin expression in virus-specific CD8 T cells. The profile in influenza virus-specific CD8 T cells was perforin(-) GrmB(-) GrmA(+/-) GrmK(+); in CMV-specific cells, it was perforin(+) GrmB(+) GrmA(+) GrmK(-/+); and in EBV- and HIV-1-specific cells, it was perforin(-/+) GrmB(+) GrmA(+) GrmK(+). On the basis of the delineation of memory and effector CD8 T cells with CD45RA and CD127, the GrmK(+) profile was associated with early-stage memory CD8 T-cell differentiation, the perforin(+) GrmB(+) GrmA(+) profile with advanced-stage differentiation, and the GrmB(+) GrmA(+) Grmk(+) profile with intermediate-stage differentiation. Furthermore, perforin and GrmB but not GrmA and GrmK correlated with cytotoxic activity. Finally, changes in antigen exposure in vitro and in vivo during primary HIV-1 infection and vaccination modulated cytotoxic granule profiles. These results advance our understanding of the relationship between distinct profiles of cytotoxic granules in memory CD8 T cells and function, differentiation stage, and antigen exposure.

Lytic granule loading of CD8+ T cells is required for HIV-infected cell elimination associated with immune control

Virus-specific CD8+ T cells probably mediate control over HIV replication in rare individuals, termed long-term nonprogressors (LTNPs) or elite controllers. Despite extensive investigation, the mechanisms responsible for this control remain incompletely understood. We observed that HIV-specific CD8+ T cells of LTNPs persisted at higher frequencies than those of treated progressors with equally low amounts of HIV. Measured on a per-cell basis, HIV-specific CD8+ T cells of LTNPs efficiently eliminated primary autologous HIV-infected CD4+ T cells. This function required lytic granule loading of effectors and delivery of granzyme B to target cells. Defective cytotoxicity of progressor effectors could be restored after treatment with phorbol ester and calcium ionophore. These results establish an effector function and mechanism that clearly segregate with immunologic control of HIV. They also demonstrate that lytic granule contents of memory cells are a critical determinant of cytotoxicity that must be induced for maximal per-cell killing capacity.

Antigen-presenting cells, including Langerhans cells, veiled cells and interdigitating cells

The accessory functions attributed to macrophages, such as antigen presentation, are probably carried out by specialized, marrow-derived cells which always have Ia antigen on their surfaces. These cells are not actively phagocytic, but are mainly engaged in engulfing large volumes on fluid. They are found in the epidermis as sessile cells, the Langerhans cells, but some re-enter the dermis and appear in afferent lymph as actively moving, veiled cells. Here they are joined by other veiled cells which have differentiated in the dermis: both populations then enter the draining lymph node. A similar process of differentiation probably occurs in other specialized tissues leading to the formation of cells that enter the afferent lymph and become veiled cells. In the lymph node, veiled cells localize in the paracortex or T-dependent area and later differentiate into another sessile cell type, the interdigitating cell. The life-style of this family of cells appears to be well-adapted for the transport of antigen into the paracortex, an area from which free antigen is largely excluded, and it seems likely that T-cell activation is triggered by the arrival of veiled cells bearing a new antigen on their surfaces.

Myosin IIA is required for cytolytic granule exocytosis in human NK cells

Natural killer (NK) cell cytotoxicity involves the formation of an activating immunological synapse (IS) between the effector and target cell through which granzymes and perforin contained in lytic granules are delivered to the target cell via exocytosis. Inhibition of nonmuscle myosin II in human NK cells with blebbistatin or ML-9 impaired neither effector–target cell conjugation nor formation of a mature activating NK cell IS (NKIS; formation of an actin ring and polarization of the microtubule-organizing center and cytolytic granules to the center of the ring). However, membrane fusion of lytic granules, granzyme secretion, and NK cell cytotoxicity were all effectively blocked. Specific knockdown of the myosin IIA heavy chain by RNA interference impaired cytotoxicity, membrane fusion of lytic granules, and granzyme secretion. Thus, myosin IIA is required for a critical step between NKIS formation and granule exocytosis.

Protein kinase Cdelta regulates antigen receptor-induced lytic granule polarization in mouse CD8+ CTL

Lytic granule exocytosis is the major pathway used by CD8+ CTL to kill virally infected and tumor cells. Despite the obvious importance of this pathway in adaptive T cell immunity, the molecular identity of enzymes involved in the regulation of this process is poorly characterized. One signal known to be critical for the regulation of granule exocytosis-mediated cytotoxicity in CD8+ T cells is Ag receptor-induced activation of protein kinase C (PKC). However, it is not known which step of the process is regulated by PKC. In addition, it has not been determined to date which of the PKC family members is required for the regulation of lytic granule exocytosis. By combination of pharmacological inhibitors and use of mice with targeted gene deletions, we show that PKCdelta is required for granule exocytosis-mediated lytic function in mouse CD8+ T cells. Our studies demonstrate that PKCdelta is required for lytic granule exocytosis, but is dispensable for activation, cytokine production, and expression of cytolytic molecules in response to TCR stimulation. Importantly, defective lytic function in PKCdelta-deficient cytotoxic lymphocytes is reversed by ectopic expression of PKCdelta. Finally, we show that PKCdelta is not involved in target cell-induced reorientation of the microtubule-organizing center, but is required for the subsequent exocytosis step, i.e., lytic granule polarization. Thus, our studies identify PKCdelta as a novel and selective regulator of Ag receptor-induced lytic granule polarization in mouse CD8+ T cells.

TNF trafficking to human mast cell granules: mature chain-dependent endocytosis

Mast cells play a crucial role at the early stages of immune response against bacteria and parasites where their functionality is based on their capability of releasing highly bioactive compounds, among them TNF. Mast cells are considered the only cells storing preformed TNF, which allows for the immediate release of this cytokine upon contact with pathogens. We approached the question of mechanisms and amino acid motifs directing newly synthesized TNF for storage in cytoplasmic granules by analyzing the trafficking of a series of TNF-enhanced GFP fusion proteins in human mast cell lines HMC-1 and LAD2. Protein covering the full TNF sequence was successfully sorted into secretory granules in a process involving transient exposure on the outer membrane and re-endocytosis. In human cells, contrary to results previously obtained in a rodent model, TNF seems not to be glycosylated and, thus, trafficking is carbohydrate independent. In an effort to localize the amino acid motif responsible for granule targeting, we constructed additional fusion proteins and analyzed their trafficking, concluding that granule-targeting sequences are localized in the mature chain of TNF and that the cytoplasmic tail is expendable for endocytotic sorting of this cytokine, thus excluding direct interactions with intracellular adaptor proteins.

Impaired Mast Cell Maturation and Degranulation and Attenuated Allergic Responses inNdrg1-Deficient Mice

We have previously reported that N-myc downstream regulated gene-1 (NDRG1) is an early inducible protein during the maturation of mouse bone marrow-derived mast cells (BMMCs) toward a connective tissue mast cell-like phenotype. To clarify the function of NDRG1 in mast cells and allergic responses, we herein analyzed mast cell-associated phenotypes of mice lacking the Ndrg1 gene. Allergic responses including IgE-mediated passive systemic and cutaneous anaphylactic reactions were markedly attenuated in Ndrg1-deficient mice as compared with those in wild-type mice. In Ndrg1-deficient mice, dermal and peritoneal mast cells were decreased in number and morphologically abnormal with impaired degranulating ability. Ex vivo, Ndrg1-deficient BMMCs cocultured with Swiss 3T3 fibroblasts in the presence of stem cell factor, a condition that facilitates the maturation of BMMCs toward a CTMC-like phenotype, displayed less exocytosis than replicate wild-type cells after the cross-linking of FcepsilonRI or stimulation with compound 48/80, even though the exocytotic response of IL-3-maintained, immature BMMCs from both genotypes was comparable. Unlike degranulation, the production of leukotriene and cytokines by cocultured BMMCs was unaffected by NDRG1 deficiency. Taken together, the altered phenotypes of Ndrg1-deficient mast cells both in vivo and ex vivo suggest that NDRG1 has roles in the terminal maturation and effector function (degranulation) of mast cells.

Alpha E beta 7 integrin interaction with E-cadherin promotes antitumor CTL activity by triggering lytic granule polarization and exocytosis

Various T cell adhesion molecules and their cognate receptors on target cells promote T cell receptor (TCR)–mediated cell killing. In this report, we demonstrate that the interaction of epithelial cell marker E-cadherin with integrin α_E(CD103)β₇, often expressed by tumor-infiltrating lymphocytes (TILs), plays a major role in effective tumor cell lysis. Indeed, we found that although tumor-specific CD103⁺ TIL-derived cytotoxic T lymphocyte (CTL) clones are able to kill E-cadherin⁺/intercellular adhesion molecule 1⁻ autologous tumor cells, CD103⁻ peripheral blood lymphocyte (PBL)-derived counterparts are inefficient. This cell killing is abrogated after treatment of the TIL clones with a blocking anti-CD103 monoclonal antibody or after targeting E-cadherin in the tumor using ribonucleic acid interference. Confocal microscopy analysis also demonstrated that α_Eβ₇ is recruited at the immunological synapse and that its interaction with E-cadherin is required for cytolytic granule polarization and subsequent exocytosis. Moreover, we report that the CD103⁻ profile, frequently observed in PBL-derived CTL clones and associated with poor cytotoxicity against the cognate tumor, is up-regulated upon TCR engagement and transforming growth factor β1 treatment, resulting in strong potentiation of antitumor lytic function. Thus, CD8⁺/CD103⁺ tumor-reactive T lymphocytes infiltrating epithelial tumors most likely play a major role in antitumor cytotoxic response through α_Eβ₇–E-cadherin interactions.

Zebrafish (Danio rerio) whole kidney assays to measure neutrophil extracellular trap release and degranulation of primary granules

The zebrafish (Danio rerio) is an excellent model system for studies in developmental biology, genetics, and toxicology, and is increasingly gaining importance in the field of immunology. The use of whole zebrafish kidneys as source of neutrophils for degranulation assays and detection of neutrophil extracellular traps is described for the first time. Neutrophils from zebrafish kidneys released neutrophil extracellular traps (NETs) and myeloperoxidase (MPO) upon stimulation with calcium ionophore, phorbol myristate acetate, and beta-glucan. Immunocytochemical study of zebrafish kidney cells revealed that NETs are made of DNA fibers associated with neutrophil granular proteins, but not with cytoskeleton. Rapid, direct MPO and extracellular DNA detection assays were developed to quantify NET release and degranulation of neutrophil primary granules from whole zebrafish kidneys. The assays were used to measure the effects of acute crowding and handling stress on neutrophils, and to examine the potential for use of zebrafish whole kidney assays in evaluation of neutrophil function under different conditions in vivo. The whole kidney NET release and degranulation assays are quantitative, can rapidly measure a large number of samples, and are capable of detecting inhibition of neutrophil activity in stressed fish, overcoming the limitations that prevented use of zebrafish in the investigations of cellular innate immune function. The assays can be used as a new research model to study effects of stress, immunomodulators, toxicants, and diseases on fish neutrophil biology.

Ontogenesis of haemocytes in shrimp (Fenneropenaeus chinensis) studied with probes of monoclonal antibody

Monoclonal antibodies (Mabs) specifically against haemocytes of shrimp (Fenneropenaeus chinensis) were employed to study the ontogenesis of haemocytes by immunofluorescence assay technique (IFAT) with the aids of avidin-biotin complex method. Different developmental stages were identified by means of histochemical staining. Stages including blastula, gastrula, limb bud stage, embryonic-nauplius and nauplius were examined. With Mab 2A3 against both hyline, semi-granular and granular haemocytes, the first positive cells were detected at the limb bud stage of the embryos. Distinguishable positive haemocytes with a diameter of about 7 microm appeared in embryonic-nauplius. Whereas, with Mab 1H11 specific to cytoplasmic granulars in haemocytes, the first positive granular haemocytes were observed at the stage of embryonic-nauplius. Increased number of both total and granular haemocytes at the stage of nauplius impressed a possible enhancement of the immune system as the embryogenesis progressed.

Fish cast NETs: neutrophil extracellular traps are released from fish neutrophils

Neutrophil extracellular traps (NETs), which are extracellular DNA structures released from neutrophils, are described and characterized for the first time in fish using fluorescent confocal microscopy. Confocal images of fish neutrophil suspensions stained with 6'-diamino-2-phenylindole, dihydrochloride DNA fluorescent stain (DAPI) revealed the presence of NETs which appeared as fibrous structures connecting several cells. Co-localization of NETs with neutrophil granular proteins and actin was investigated using specific antibodies and probes. Double staining of neutrophils with SYTOX green and DAPI revealed that SYTOX stain applied to living cells stained extracellular DNA, but not nuclei. NETs are actively released from stimulated living cells, associated with granular proteins, but not with cytoskeleton, and are not a product of nuclear degradation seen in late apoptotic stages. Additionally, a fluorometric microtiter plate assay to quantify the release of NETs was adopted for use with fish neutrophils, and the effect of stress on NETs release was studied. This assay detected the inhibition of DNA release during stress conditions. In summary, NETs were released from living fish kidney neutrophils upon stimulation, characterized using fluorescence DNA-binding dyes, specific antibodies and probes, and quantified using a microtiter plate fluorometric assay that can rapidly measure a large number of samples. Detection of NETs can be used as an additional assay to an existing battery of functional tests, and as a new research model to study the effects of stress, immunomodulators, and diseases.

Upregulation of vitamin D binding protein (Gc-globulin) binding sites during neutrophil activation from a latent reservoir in azurophil granules

Vitamin D binding protein (DBP) is a multifunctional plasma transport protein that is also found on the surface of many cell types. Cell surface DBP significantly enhances chemotactic activity of complement (C) peptides C5a and C5a des Arg. However, both DBP binding and C5a chemotaxis enhancement can vary among neutrophil donors. To test if activation during cell purification is responsible for this variability, neutrophils were isolated using both standard and lipopolysaccharide (LPS)-free protocols. Cells isolated by the LPS-free method had no DBP-enhanced chemotaxis to C5a or DBP binding to plasma membranes. Moreover, neutrophils treated with LPS bound more avidity to immobilized DBP than sham-treated cells. Subcellular fractionation of neutrophils (standard protocol) revealed a heavy plasma membrane (HM) band that contained components of light plasma membranes and all three granules. The HM band possessed most of the DBP binding activity (58%), and activation of cells with ionomycin greatly increased DBP binding to HM. Azurophil granules contained 33% of the total DBP binding sites and there was a highly significant positive correlation (r=0.988) between release of the granule marker myeloperoxidase and DBP binding. These results indicate that fusion of granules with the plasma membrane forms HM that contains DBP binding sites.

A CD34(+) human cell line model of myeloid dendritic cell differentiation: evidence for a CD14(+)CD11b(+) Langerhans cell precursor

The study of early events in dendritic cell (DC) differentiation is hampered by the lack of homogeneous primary cell systems that allow the study of cytokine-driven, transitional DC differentiation steps. The CD34(+) acute myeloid leukemia cell line MUTZ-3 displays a unique ability to differentiate into interstitial DC (IDC) and Langerhans cells (LC) in a cytokine-dependent manner. Phenotypic characterization revealed MUTZ-3 to consist of three distinct subpopulations. Small CD34(+)CD14(-)CD11b(-) progenitors constitute the proliferative compartment of the cell line with the ability to differentiate through a CD34(-)CD14(-)CD11b(+) stage to ultimately give rise to a morphologically large, nonproliferating CD14(+)CD11b(hi) progeny. These CD14(+)CD11b(hi) cells were identified as common, immediate myeloid DC precursors with the ability to differentiate into LC and IDC, exhibiting characteristic and mutually exclusive expression of Langerin and DC-specific ICAM-grabbing nonintegrin, respectively. The identity of the MUTZ-3-derived LC subset was confirmed further by the presence of Birbeck granules. We conclude that the MUTZ-3 cell line provides a ready and continuous supply of common myeloid precursors, which should facilitate further study of the ontogeny of myeloid DC lineages.

Induction of cytotoxic granules in human memory CD8+ T cell subsets requires cell cycle progression

Memory CD8(+) T cell responses are thought to be more effective as a result of both a higher frequency of Ag-specific clones and more rapid execution of effector functions such as granule-mediated lysis. Murine models have indicated that memory CD8(+) T cells exhibit constitutive expression of perforin and can lyse targets directly ex vivo. However, the regulated expression of cytotoxic granules in human memory CD8(+) T cell subsets has been underexplored. Using intracellular flow cytometry, we observed that only a minor fraction of CD45RA(-)CD8(+) T cells, or of CD8(+) T cells reactive to EBV-HLA2 tetramer, expressed intracellular granzyme B (GrB). Induction of GrB-containing cytotoxic granules in both CD45RA(+) and CD45RA(-) cells was achieved by stimulation with anti-CD3/anti-CD28 mAb-coated beads, required at least 3 days, occurred after several rounds of cell division, and required cell cycle progression. The strongest GrB induction was seen in the CCR7(+) subpopulations, with poorest proliferation being observed in the CD45RA(-)CCR7(-) effector-memory pool. Our results indicate that, as with naive T cells, induction of cytotoxic granules in human Ag-experienced CD8(+) T cells requires time and cell division, arguing that the main numerical advantage of a memory T cell pool is a larger frequency of CTL precursors. The fact that granule induction can be achieved through TCR and CD28 ligation has implications for restoring lytic effector function in the context of antitumor immunity.

Défaut d’exocytose des granules lytiques

An in vivo disturbance of lymphocyte homeostasis occurs during the course of the hemophagocytic syndrome (HS). HS is a severe and often fatal syndrome resulting from potent and uncontrolled activation and proliferation of T-lymphocytes, mainly polyclonal CD8 lymphocytes, leading to excessive macrophage activation, high level of proinflammatory cytokine production and multiple deleterious effects. The onset of HS characterizes several inherited disorders in humans. In most of these conditions, the molecular defect impairs the granule-dependent cytotoxic activity of lymphocytes, thus highlighting the determinant role of this function in driving back the immune system to a state of equilibrium following infection. Several lines of evidence suggest that an increase in the expansion phase rather than a decrease in the contraction phase of the CD8+ T cells population characterizes the HS. Failure to kill antigen presenting cells through a transaction mechanism of cytotoxic cells should favor a sustained response, although the mechanism may be more complex than simple decrease of antigen load. Defect in the granule dependent cytotoxic function of lymphocytes result from perforin mutation in familial hemophagocytic lymphohistiocytosis type 2, from Munc13-4 (UNC13D) mutation in familial hemophagocytic lymphohistiocytosis type 3, from Rab27a mutation in Griscelli syndrome type 2, and from CHS/LYST mutation in Chediak-Higashi syndrome. The characterization of the molecular causes leading to these conditions identified Rab27a and Munc13-4 as two critical effectors of the exocytic machinery, required for the terminal transport/docking or priming of the cytotoxic granules, respectively. Different members of the Rab and Munc13 family of proteins are also used in neurotransmitter release at the neurological synapse, highlighting the similarity of the mechanisms regulating both secretory pathways. Future investigations regarding HS will continue to elucidate this exocytic pathway machinery and improve our understanding of how it finely regulates the immune response, an area that is likely to be useful for therapeutic intervention.

Activation of primary T lymphocytes results in lysosome development and polarized granule exocytosis in CD4+and CD8+subsets, whereas expression of lytic molecules confers cytotoxicity to CD8+T cells

Lytic granule exocytosis is the major cytotoxic mechanism used by CD8(+) cytotoxic lymphocytes. CD8(+) T cells acquire this effector function in the process characterized by lysosomal biogenesis, induction of expression of cytolytic molecules, and their selective sorting into the lysosomal vesicles. However, temporal relation of these differentiation stages during T cell activation has not been defined precisely. Also, although CD4(+) T cells typically do not express lytic molecules as a consequence of activation, and therefore, do not acquire granule exocytosis-mediated lytic function, it is not clear whether CD4(+) T cells are able to degranulate. By using in vitro TCR stimulation of primary mouse lymphocytes, we found that polyclonally activated CD4(+) T cells degranulate upon TCR ligation and polarize enlarged lysosomal granules in response to target cell recognition, despite the lack of granule exocytosis-mediated cytotoxicity. Upon TCR stimulation, resting CD8(+) T cells rapidly express lytic molecules and acquire potent lytic function early in activation. Maximal cytolytic potential, however, depends on enlargement of lysosomal granules during the subsequent activation stages. Thus, polyclonal TCR stimulation of resting T cells results in development of lysosomal granules and their release upon TCR engagement in CD4(+) and CD8(+) T cells, but only CD8(+) T cells acquire lytic function as a result of induction of expression of lytic molecules.

A novel flow cytometric assay focusing on perforin release mechanisms of cytotoxic T lymphocytes

CD8(hi+) cytotoxic T lymphocytes (CTL) are major players in immune defense. In addition, they contribute to the maintenance of immune homeostasis. We now describe a hitherto unavailable, but simple assay to determine ex vivo lytic granule-based cytotoxic functions of human CD8(hi+) CTL subgroups in a clinical setting, under target cell free conditions. Ficoll-isolated peripheral blood lymphocytes from 17 healthy volunteers were stimulated either by phorbol 12-myristate 13-acetate (PMA) in combination with ionomycin or by antibody mediated crosslinking of the CD3 molecule on the T cell surface. Using perforin as a marker for lytic granules, the reduction of CTL granules over time intervals up to 120 min was quantified by FACScan flow cytometry. The kinetics of perforin reduction were compared to the kinetics of NA-CBZ-L-lysine-thiobenzyl ester hydrochloride (BLT)-esterase release and of CD63 upregulation. The reduction in the perforin(+) portion of CD8(hi+) CTLs was correlated inversely with BLT-esterase release and CD63 upregulation. At 30 and 120 min after PMA/ionomycin stimulation, 55 +/- 14% and 42 +/- 14%, respectively, of CD8(hi+) CTLs still stained perforin(+) (time point 0 min = 100%). Perforin-granule release induced by CD3-crosslinking occurred as fast within 30 min (55 +/- 17%), but over the 120 min time interval it was not as complete when compared to PMA/ionomycin-stimulated perforin-reduction. Thus, the combination of an established degranulation assay with the power of immuno flow cytometry allows one to investigate the cytotoxic capability of CTL-subtypes and the kinetics of perforin-granule release. In addition, the assay may prove useful in the elucidation of intracellular signaling cascades governing the perforin-granule release process.

Macrophages Acquire Neutrophil Granules for Antimicrobial Activity against Intracellular Pathogens

A key target of many intracellular pathogens is the macrophage. Although macrophages can generate antimicrobial activity, neutrophils have been shown to have a key role in host defense, presumably by their preformed granules containing antimicrobial agents. Yet the mechanism by which neutrophils can mediate antimicrobial activity against intracellular pathogens such as Mycobacterium tuberculosis has been a long-standing enigma. We demonstrate that apoptotic neutrophils and purified granules inhibit the growth of extracellular mycobacteria. Phagocytosis of apoptotic neutrophils by macrophages results in decreased viability of intracellular M. tuberculosis. Concomitant with uptake of apoptotic neutrophils, granule contents traffic to early endosomes, and colocalize with mycobacteria. Uptake of purified granules alone decreased growth of intracellular mycobacteria. Therefore, the transfer of antimicrobial peptides from neutrophils to macrophages provides a cooperative defense strategy between innate immune cells against intracellular pathogens and may complement other pathways that involve delivery of antimicrobial peptides to macrophages.

Constitutively polarized granules prime KHYG-1 NK cells

The major mechanism for NK cell lysis of tumor cells is granule-mediated cytotoxicity. Polarization of granules is a prelude to the release of their cytotoxic contents in response to target-cell binding. We describe the novel observation of constitutive granule polarization in the cytotoxic NK cell line, KHYG-1. Continuous degranulation of KHYG-1 cells, however, does not occur and still requires target-cell contact. Disruption of microtubules with colcemid is sufficient to disperse the granules in KHYG-1 and significantly decreases cytotoxicity. A similar effect is not obtained by inhibiting extracellular signal-related kinase 2 (ERK2), the most distal kinase investigated in the cytolytic pathway. Disruption of microtubules significantly down-regulates activation receptors, NKp44 and NKG2D, implicating them as potential microtubule-trafficking receptors. Such changes in upstream receptor expression may have caused deactivation of ERK2, since NKG2D cross-linking also leads to receptor down-regulation and diminished ERK phosphorylation. Thus, a functional role for NKG2D in KHYG-1 cytotoxicity is demonstrated. Moreover, the novel primed state may contribute to the high cytotoxicity exhibited by KHYG-1.

Cytotoxic T lymphocyte-induced killing in the absence of granzymes A and B is unique and distinct from both apoptosis and perforin-dependent lysis

Cytotoxic T lymphocyte (CTL)–induced death triggered by the granule exocytosis pathway involves the perforin-dependent delivery of granzymes to the target cell. Gene targeting has shown that perforin is essential for this process; however, CTL deficient in the key granzymes A and B maintain the ability to kill their targets by granule exocytosis. It is not clear how granzyme AB^−/− CTLs kill their targets, although it has been proposed that this occurs through perforin-induced lysis. We found that purified granzyme B or CTLs from wild-type mice induced classic apoptotic cell death. Perforin-induced lysis was far more rapid and involved the formation of large plasma membrane protrusions. Cell death induced by granzyme AB^−/− CTLs shared similar kinetics and morphological characteristics to apoptosis but followed a distinct series of molecular events. Therefore, CTLs from granzyme AB^−/− mice induce target cell death by a unique mechanism that is distinct from both perforin lysis and apoptosis.

Serine protease inhibitor 6 protects cytotoxic T cells from self-inflicted injury by ensuring the integrity of cytotoxic granules

How cytotoxic T lymphocytes (CTLs) kill intracellular pathogens without killing themselves has been a recurring question ever since their discovery. By using mice deficient in Serine Protease Inhibitor 6 (Spi6), we show that by inhibiting granzyme B (GrB), Spi6 protects CTLs from self-inflicted injury. Infection with either Lymphocytic Choriomeningitis virus (LCMV) or Listeria monocytogenes (LM) revealed increased apoptosis and diminished survival of Spi6 knockout (KO) CTLs, which was cell autonomous and could be corrected by GrB deficiency. Spi6 KO mice in turn were impaired in their ability to clear LCMV infection. Spi6 KO CTLs revealed a breakdown in the integrity of cytotoxic granules, increased cytoplasmic GrB, and ensuing apoptosis. We conclude that Spi6 protects CTLs from suicide caused by GrB-mediated breakdown of cytotoxic granules.

Is there a role for TNF-alpha in anti-neutrophil cytoplasmic antibody-associated vasculitis? Lessons from other chronic inflammatory diseases

Anti-neutrophil cytoplasmic antibody (ANCA)-associated vasculitis is the most common cause of rapidly progressive glomerulonephritis and immune-mediated pulmonary renal syndrome. Now that the acute manifestations of the disease generally can be controlled with immunosuppressive drugs, ANCA-associated vasculitis has become a chronic and relapsing inflammatory disorder. The need to develop safer and more effective treatment has led to great interest in the mediators of chronic inflammation. There are many lessons to be learned from studies of other chronic inflammatory diseases, particularly rheumatoid arthritis (RA). The identification of a TNF-alpha-dependent cytokine cascade in the in vitro cultures of synovium in joints of patients with RA led to studies of TNF blockade in experimental models of arthritis and subsequently to clinical trials. These have culminated in the widespread introduction of anti-TNF therapy not only in RA but also in Crohn disease, ankylosing spondylitis, and several other chronic inflammatory disorders. Following a similar investigative pathway, studies that show the importance of TNF production by leukocytes and intrinsic renal cells in glomerulonephritis have been followed by the demonstration of the effectiveness of TNF blockade in several experimental models of glomerulonephritis and vasculitis. In experimental autoimmune vasculitis, improvement in disease was paralleled by a reduction in leukocyte transmigration, as demonstrated by intravital microscopy. The benefit of infliximab (a mAb to TNF) in ANCA-associated vasculitis was recently reported in a prospective open-label study. However, the use of etanercept (a soluble TNF receptor fusion protein) was not found to be of significant benefit in a randomized, controlled trial in patients with Wegener granulomatosis. Therefore, there is a need for further evaluation of the use of anti-TNF antibodies in patients with ANCA-associated glomerulonephritis.

Anti-neutrophil cytoplasm-associated glomerulonephritis

Wegener's granulomatosis, microscopic polyangiitis, and renal limited vasculitis are associated with circulating anti-neutrophil cytoplasm antibodies and are an important cause of rapidly progressive glomerulonephritis. This review gives an account of recent advances in the understanding of the pathogenesis underlying these conditions and how these may lead to future treatments. Consideration is given to recent clinical trials in the management of anti-neutrophil cytoplasm antibodies (ANCA)-associated vasculitides.

Pathogenesis of vascular inflammation by anti-neutrophil cytoplasmic antibodies

The reports of a newborn who developed glomerulonephritis and pulmonary hemorrhage after transplacental transfer of anti-neutrophil cytoplasmic antibody (ANCA) IgG with specificity for myeloperoxidase (MPO) is compelling clinical evidence that ANCA are pathogenic. In vitro studies indicate that ANCA activate cytokine-primed neutrophils and monocytes through both direct Fab'2 binding and Fc receptor engagement. Neutrophils that have been activated by ANCA release oxygen radicals, lytic enzymes, and inflammatory cytokines and adhere to and kill endothelial cells. A murine model caused by passive administration of mouse anti-mouse MPO IgG provides convincing evidence that ANCA IgG alone in the absence of antigen-specific T cells can cause necrotizing glomerulonephritis and vasculitis. This pathogenic process is enhanced by synergistic inflammatory factors, probably through priming of neutrophils. Immunization of rats with human MPO induces antibodies that cross-react with rat MPO and cause glomerulonephritis and vasculitis. These ANCA act in concert with chemokines to cause adherence of leukocytes to the walls of small vessels with subsequent injury. To date, animal models of disease that is induced by anti-proteinase 3 are less robust. Clinical and experimental data suggest but do not prove that the ANCA autoimmune response is initiated by an immune response to an antisense peptide of the ANCA antigen or its mimic that may be introduced into the body by an infectious pathogen. This antibody response elicits anti-idiotypic antibodies that cross-react with ANCA antigens. The pathogenesis of ANCA disease is multifactorial, with genetic and environmental factors influencing onset of the autoimmune response, the mediation of acute injury, and the induction of the chronic response to injury.