Platelets promote human macrophages-mediated macropinocytosis of Clostridioides difficile

Clostridioides difficile is the main causative agent of hospital-acquired diarrhea and the potentially lethal disease, C. difficile infection. The cornerstone of the current therapy is the use of antibiotics, which is not fully effective. The molecular mechanisms, inflammatory conditions and host-immune responses that could benefit the persistence or elimination of C. difficile remain unclear. Macrophages perform different ways of endocytosis as part of their immune surveillance functions and platelets, classically known for their coagulatory role, are also important modulators of the immune system. The aim of this study was to evaluate the endocytosis of vegetative C. difficile by human macrophages and the involvement of platelets in this process. Our results showed that both macrophages and platelets interact with live and heat-killed C. difficile. Furthermore, platelets form complexes with human monocytes in healthy donor's fresh blood and the presence of C. difficile increased these cell-cell interactions. Using flow cytometry and confocal microscopy, we show that macrophages can internalize C. difficile and that platelets improve this uptake. By using inhibitors of different endocytic pathways, we demonstrate that macropinocytosis is the route of entry of C. difficile into the cell. Taken together, our findings are the first evidence for the internalization of vegetative non-toxigenic and hypervirulent C. difficile by human macrophages and highlight the role of platelets in innate immunity during C. difficile infection. Deciphering the crosstalk of C. difficile with immune cells could provide new tools for understanding the pathogenesis of C. difficile infection and for the development of host-directed therapies.

Styrylpyrone Derivative (SPD) Extracted from Goniothalamus umbrosus Binds to Dengue Virus Serotype-2 Envelope Protein and Inhibits Early Stage of Virus Replication

A study was conducted to investigate the anti-viral effect of a styrylpyrone derivative (SPD) called goniothalamin and the effects on the dengue virus serotype 2 (DENV-2) replication cycle. The SPD was prepared from the root of Goniothalamus umbrosus after purification with petroleum ether. The isolated SPD was then subjected to gas chromatography–mass spectrometry (GC-MS) and nuclear magnetic resonance (NMR) analyses for structure validation. The cytotoxicity of the SPD was evaluated using a cell viability assay, while the anti-viral activity of the SPD towards DENV-2 was confirmed by conducting a foci reduction assay which involved virus yield reduction, time-of-addition, and time removal assays. Transcriptomic analysis via quantitative real-time polymerase chain reaction (qRT-PCR) using the DENV-2 E gene was conducted to investigate the level of gene transcript. Immunocytochemistry analysis was used to investigate the effects of SPD treatment on protein E expression. Finally, software molecular docking of the SPD and E protein was also performed. The cytotoxicity assay confirmed that the SPD was not toxic to Vero cells, even at the highest concentration tested. In the time-of-addition assay, more than 80% foci reduction was observed when SPDs were administered at 2 h post-infection (hpi), and the reduction percentage then dropped with the delay of the treatment time, suggesting the inhibition of the early replication cycle. However, the time removal assay showed that more than 80% reduction could only be observed after 96 h post-treatment with the SPD. Treatment with the SPD reduced the progeny infectivity when treated for 24 h and was dose-dependent. The result showed that transcript level of the E gene in infected cells treated with the SPD was reduced compared to infected cells without treatment. In immunocytochemistry analysis, the DENV-2 E protein exhibited similar expression trends, shown by the gene transcription level. Molecular docking showed that the SPD can interact with E protein through hydrogen bonds and other interactions. Overall, this study showed that SPDs have the potential to be anti-DENV-2 via a reduction in viral progeny infectivity and a reduction in the expression of the DENV-2 E gene and protein at different phases of viral replication. SPDs should be further researched to be developed into an effective anti-viral treatment, particularly for early-phase dengue viral infection.

Decoding the Clinical and Laboratory Parameters of COVID-19 and Dengue Co-infection

Severe acute respiratory diseases caused by coronavirus disease 2019 (COVID-19) have caused infections around the world, and this disease has been declared a global pandemic by the World Health Organization. COVID-19 has severely impacted the world economy, and as it has multiple unnoticeable transmission routes, it can derail the health care system for a long time. Most states in India are affected by the COVID pandemic. As India is known for its seasonal infections such as dengue, leptospirosis, influenza, malaria, and enteric fever, it is expected that these infections may co-exist. Coinfection of these two viral infections causes challenges in diagnosis and treatment, especially in places with limited resources. Antibody-mediated enhancement of the immune response is a cause for concern in co-infection of COVID-19 and dengue. The present article discusses the clinical features, serological cross reactions, and antibody-dependent enhancement of COVID-19 coinfection with dengue infection.

Platelets in dengue infection: more than a numbers game

Dengue virus (DENV) infection is responsible for the development of dengue illness, which can be either asymptomatic, present mild manifestations or evolve to severe dengue. Thrombocytopenia is an important characteristic during DENV infection, being observed both in mild and severe dengue, although the lowest platelet counts are encountered during severe cases. This review gathers information regarding several mechanisms that have been related to alterations in platelet number and function, leading to thrombocytopenia but also platelet-mediated immune and inflammatory response. On this regard, we highlight that the decrease in platelet counts may be due to bone marrow suppression or consumption of platelets at the periphery. We discuss the infection of hematopoietic progenitors and stromal cells as mechanisms involved in bone marrow suppression. Concerning peripheral consumption of platelets, we addressed the direct infection of platelets by DENV, adhesion of platelets to leukocytes and vascular endothelium and platelet clearance mediated by anti-platelet antibodies. We also focused on platelet involvement on the dengue immunity and pathogenesis through translation and secretion of viral and host factors and through platelet-leukocyte aggregates formation. Hence, the present review highlights important findings related to platelet activation and thrombocytopenia during dengue infection, and also exhibits different mechanisms associated with decreased platelet counts.Graphical abstract:Schematic mechanistic representation of platelet-mediated immune responses and thrombocytopenia during dengue infection. (A) DENV-infected platelets secrete cytokines and chemokines and also adhere to activated vascular endothelium. Platelets aggregate with leukocytes, inducing the secretion of NETs and inflammatory mediators by neutrophils and monocytes, respectively. (B) DENV directly infects stromal cells and hematopoietic precursors, including megakaryocytes, which compromises megakaryopoiesis. Both central and peripheric mechanisms contribute to DENV-associated thrombocytopenia.

Covid-19 and dengue: Double punches for dengue-endemic countries in Asia

The coronavirus disease 2019 (Covid-19) pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), is an international public health crisis with devastating effects. In particular, this pandemic has further exacerbated the burden in tropical and subtropical regions of the world, where dengue fever, caused by dengue virus (DENV), is already endemic to the population. The similar clinical manifestations shared by Covid-19 and dengue fever have raised concerns, especially in dengue-endemic countries with limited resources, leading to diagnostic challenges. In addition, cross-reactivity of the immune responses in these infections is an emerging concern, as pre-existing DENV-antibodies might potentially affect Covid-19 through antibody-dependent enhancement. In this review article, we aimed to raise the issue of Covid-19 and dengue fever misdiagnosis, not only in a clinical setting but also with regards to cross-reactivity between SARS-CoV-2 and DENV antibodies. We also have discussed the potential consequences of overlapping immunological cascades between dengue and Covid-19 on disease severity and vaccine development.

Suppressed humoral immunity is associated with dengue nonstructural protein NS1-elicited anti-death receptor antibody fractions in mice

Dengue virus (DENV) infections may cause life-threatening dengue hemorrhagic fever (DHF). Suppressed protective immunity was shown in these patients. Although several hypotheses have been formulated, the mechanism of DENV-induced immunosuppression remains unclear. Previously, we found that cross-reactive antibodies against tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) receptor 1 (death receptor 4 [DR4]) were elicited in DHF patients, and that anti-DR4 autoantibody fractions were elicited by nonstructural protein 1 (NS1) immunizations in experimental mice. In this study, we found that anti-DR4 antibodies could suppress B lymphocyte function in vitro and in vivo. Treatment with the anti-DR4 immunoglobulin (Ig) induced caspase-dependent cell death in immortalized B lymphocyte Raji cells in vitro. Anti-DR4 Igs elicited by NS1 and DR4 immunizations markedly suppressed mouse spleen transitional T2 B (IgM⁺IgD⁺), bone marrow pre-pro-B (B220⁺CD43⁺), pre-B (B220⁺CD43^-), and mature B cell (B220⁺IgD⁺) subsets in mice. Furthermore, functional analysis revealed that the pre-elicitation of anti-NS1 and anti-DR4 Ig titers suppressed subsequently neutralizing antibody production by immunization with DENV envelop protein. Our data suggest that the elicitation of anti-DR4 titers through DENV NS1 immunization plays a suppressive role in humoral immunity in mice.

Human megakaryocytes possess intrinsic antiviral immunity through regulated induction of IFITM3

Evolving evidence indicates that platelets and megakaryocytes (MKs) have unexpected activities in inflammation and infection; whether viral infections upregulate biologically active, antiviral immune genes in platelets and MKs is unknown, however. We examined antiviral immune genes in these cells in dengue and influenza infections, viruses that are global public health threats. Using complementary biochemical, pharmacological, and genetic approaches, we examined the regulation and function of interferon-induced transmembrane protein 3 (IFITM3), an antiviral immune effector gene not previously studied in human platelets and MKs. IFITM3 was markedly upregulated in platelets isolated from patients during clinical influenza and dengue virus (DENV) infections. Lower IFITM3 expression in platelets correlated with increased illness severity and mortality in patients. Administering a live, attenuated DENV vaccine to healthy subjects significantly increased platelet IFITM3 expression. Infecting human MKs with DENV selectively increased type I interferons and IFITM3. Overexpression of IFITM3 in MKs was sufficient to prevent DENV infection. In naturally occurring, genetic loss-of-function studies, MKs from healthy subjects harboring a homozygous mutation in IFITM3 (rs12252-C, a common single-nucleotide polymorphism in areas of the world where DENV is endemic) were significantly more susceptible to DENV infection. DENV-induced MK secretion of interferons prevented infection of bystander MKs and hematopoietic stem cells. Thus, viral infections upregulate IFITM3 in human platelets and MKs, and IFITM3 expression is associated with adverse clinical outcomes. These observations establish, for the first time, that human MKs possess antiviral functions, preventing DENV infection of MKs and hematopoietic stem cells after local immune signaling.

Evaluation of anti-dengue activity of Carica papaya aqueous leaf extract and its role in platelet augmentation

Dengue disease is characterized by a marked decrease in platelet count, which is life threatening. In the present study, we investigated the antiviral activity of an aqueous extract of Carica papaya leaves (PLE) against dengue virus (DENV) and its effect on platelet augmentation. The anti-dengue activity of PLE in DENV-infected THP-1 cells was examined by immunoblotting and flow cytometry. The effect of PLE on erythrocyte damage was investigated using hemolytic and anti-hemolytic assays. Virus-infected THP-1 cells were assayed for IFN-α secretion. The effect of PLE on platelet augmentation in rats with cyclophosphamide-induced thrombocytopenia was also investigated. The platelet count of blood from the retro-orbital plexus of rats was determined on the 1^st, 4^th, 7^th, 11^th and 14^th day of study. On the 14^th day, the rats were sacrificed for histopathological examination of the liver, kidney and spleen. Plasma of thrombocytopenic rats was tested for thrombopoietin (TPO) and IL-6 secretion. The data suggest that PLE significantly decreases the expression of the envelope and NS1 proteins in DENV-infected THP-1 cells. A marked decrease in intracellular viral load upon PLE treatment confirmed its antiviral activity. This also resulted in a significant decrease in erythrocyte damage and hydrogen-peroxide-induced lipid peroxidation. A significant increase in the number of platelets was found in thrombocytopenic rats treated with PLE, along with an increase in IL-6 and TPO levels. These findings suggest that PLE can potentially be used as an antiviral agent, as it helps in platelet augmentation and exhibits antiviral activity against DENV.

Preventive and therapeutic challenges in combating Zika virus infection: are we getting any closer?

The neuroteratogenic nature of Zika Virus (ZIKV) infection has converted what would have been a tropical disease into a global threat. Zika is transmitted vertically via infected placental cells especially in the first and second trimesters. In the developing central nervous system (CNS), ZIKV can infect and induce apoptosis of neural progenitor cells subsequently causing microcephaly as well as other neuronal complications in infants. Its ability to infect multiple cell types (placental, dermal, and neural) and increased environmental stability as compared to other flaviviruses (FVs) has broadened the transmission routes for ZIKV infection from vector-mediated to transmitted via body fluids. To further complicate the matters, it is genetically similar (about 40%) with the four serotypes of dengue virus (DENV), so much so that it can almost be called a fifth DENV serotype. This homology poses the risk of causing cross-reactive immune responses and subsequent antibody-dependent enhancement (ADE) of infection in case of secondary infections or for immunized individuals. All of these factors complicate the development of a single preventive vaccine candidate or a pharmacological intervention that will completely eliminate or cure ZIKV infection. We discuss all of these factors in detail in this review and conclude that a combinatorial approach including immunization and treatment might prove to be the winning strategy.

Serum Metabolomics Reveals Serotonin as a Predictor of Severe Dengue in the Early Phase of Dengue Fever

Effective triage of dengue patients early in the disease course for in- or out-patient management would be useful for optimal healthcare resource utilization while minimizing poor clinical outcome due to delayed intervention. Yet, early prognosis of severe dengue is hampered by the heterogeneity in clinical presentation and routine hematological and biochemical measurements in dengue patients that collectively correlates poorly with eventual clinical outcome. Herein, untargeted liquid-chromatography mass spectrometry metabolomics of serum from patients with dengue fever (DF) and dengue hemorrhagic fever (DHF) in the febrile phase (<96 h) was used to globally probe the serum metabolome to uncover early prognostic biomarkers of DHF. We identified 20 metabolites that are differentially enriched (p<0.05, fold change >1.5) in the serum, among which are two products of tryptophan metabolism-serotonin and kynurenine. Serotonin, involved in platelet aggregation and activation decreased significantly, whereas kynurenine, an immunomodulator, increased significantly in patients with DHF, consistent with thrombocytopenia and immunopathology in severe dengue. To sensitively and accurately evaluate serotonin levels as prognostic biomarkers, we implemented stable-isotope dilution mass spectrometry and used convalescence samples as their own controls. DHF serotonin was significantly 1.98 fold lower in febrile compared to convalescence phase, and significantly 1.76 fold lower compared to DF in the febrile phase of illness. Thus, serotonin alone provided good prognostic utility (Area Under Curve, AUC of serotonin = 0.8). Additionally, immune mediators associated with DHF may further increase the predictive ability than just serotonin alone. Nine cytokines, including IFN-γ, IL-1β, IL-4, IL-8, G-CSF, MIP-1β, FGF basic, TNFα and RANTES were significantly different between DF and DHF, among which IFN-γ ranked top by multivariate statistics. Combining serotonin and IFN-γ improved the prognosis performance (AUC = 0.92, sensitivity = 77.8%, specificity = 95.8%), suggesting this duplex panel as accurate metrics for the early prognosis of DHF.

Serological and Entomological Study of Dengue in Dang and Chitwan Districts of Nepal

A descriptive cross-sectional study was carried out among 264 suspected dengue patients in two districts (Dang and Chitwan) of Nepal from June 2013 to November 2013. The anti-dengue IgM positivity was found to be (51/264)19.31% by capture ELISA, of which 21 (41.2%) were male and 30 (58.8%) were female. Symptoms of seropositive cases were fever, anorexia, nausea, headache, retro-orbital pain, skin rashes, and myalgia. Hematological features like thrombocytopenia and leucopenia were found to be significantly associated with the dengue fever (DF). Discarded tires were found as the commonest breeding habitats for the dengue vectors. Higher sero-positivity was recorded from the area having higher Breteau index (BI). The pH, chloride ion concentration and the salinity of the water from breeding habitats were found to be ranging from 6.9±0.82 to 8, 103.33±17.52 mg/L to 140.65 mg/L, and 0.19±0.032 ppt to 0.25 ppt respectively. This study may be helpful for the health authorities and public health workers for early diagnosis of DF and for the improved preventive measures to be adopted in the epidemic and possible epidemic areas.

Thrombocytopenia in Dengue: Interrelationship between Virus and the Imbalance between Coagulation and Fibrinolysis and Inflammatory Mediators

Dengue is an infectious disease caused by dengue virus (DENV). In general, dengue is a self-limiting acute febrile illness followed by a phase of critical defervescence, in which patients may improve or progress to a severe form. Severe illness is characterized by hemodynamic disturbances, increased vascular permeability, hypovolemia, hypotension, and shock. Thrombocytopenia and platelet dysfunction are common in both cases and are related to the clinical outcome. Different mechanisms have been hypothesized to explain DENV-associated thrombocytopenia, including the suppression of bone marrow and the peripheral destruction of platelets. Studies have shown DENV-infected hematopoietic progenitors or bone marrow stromal cells. Moreover, anti-platelet antibodies would be involved in peripheral platelet destruction as platelets interact with endothelial cells, immune cells, and/or DENV. It is not yet clear whether platelets play a role in the viral spread. Here, we focus on the mechanisms of thrombocytopenia and platelet dysfunction in DENV infection. Because platelets participate in the inflammatory and immune response by promoting cytokine, chemokine, and inflammatory mediator secretion, their relevance as "immune-like effector cells" will be discussed. Finally, an implication for platelets in plasma leakage will be also regarded, as thrombocytopenia is associated with clinical outcome and higher mortality.

Thrombocytopenia in Plasmodium vivax malaria is related to platelets phagocytosis

Background

Although thrombocytopenia is a hematological disorder commonly reported in malarial patients, its mechanisms are still poorly understood, with only a few studies focusing on the role of platelets phagocytosis.

Methods and Findings

Thirty-five malaria vivax patients and eight healthy volunteers (HV) were enrolled in the study. Among vivax malaria patients, thrombocytopenia (<150,000 platelets/µL) was found in 62.9% (22/35). Mean platelet volume (MPV) was higher in thrombocytopenic patients as compared to non- thrombocytopenic patients (p = 0.017) and a negative correlation was found between platelet count and MPV (r = −0.483; p = 0.003). Platelets from HV or patients were labeled with 5-chloromethyl fluorescein diacetate (CMFDA), incubated with human monocytic cell line (THP-1) and platelet phagocytosis index was analyzed by flow cytometry. The phagocytosis index was higher in thrombocytopenic patients compared to non-thrombocytopenic patients (p = 0.042) and HV (p = 0.048). A negative correlation was observed between platelet count and phagocytosis index (r = −0.402; p = 0.016). Platelet activation was assessed measuring the expression of P-selectin (CD62-P) in platelets’ surface by flow cytometry. No significant difference was found in the expression of P-selectin between thrombocytopenic patients and HV (p = 0.092). After evaluating the cytokine profile (IL-2, IL-4, IL-6, IL-10, TNF-α, IFN-γ and IL-17) in the patients’ sera, levels of IL-6, IL-10 and IFN-γ were elevated in malaria patients compared to HV. Moreover, IL-6 and IL-10 values were higher in thrombocytopenic patients than non-thrombocytopenic ones (p = 0.044 and p = 0.017, respectively. In contrast, TNF-α levels were not different between the three groups, but a positive correlation was found between TNF-α and phagocytosis index (r = −0.305; p = 0.037).

Conclusion/Significance

Collectively, our findings indicate that platelet phagocytosis may contribute to thrombocytopenia found in vivax malaria. Finally, we believe that this study opens new avenues to explore the mechanisms involved in platelet dysfunction, commonly found in vivax malaria patients.

Dengue induces platelet activation, mitochondrial dysfunction and cell death through mechanisms that involve DC-SIGN and caspases

BACKGROUND

Worldwide, dengue is the most prevalent human arbovirus disease. Dengue infection may cause a range of clinical manifestations from self-limiting febrile illness through to a life-threatening syndrome accompanied by both bleeding and shock. Thrombocytopenia is frequently observed in mild and severe disease; however, the mechanisms involved in DENV-induced platelet activation and thrombocytopenia are incompletely understood.

PATIENTS AND METHODS

Freshly isolated platelets from patients with dengue were evaluated for markers of activation, mitochondrial alteration and activation of cell death pathways. In parallel, we examined direct DENV-induced activation and apoptosis of platelets obtained from healthy subjects.

RESULTS

We found that platelets from DENV-infected patients exhibited increased activation by comparison to control subjects. Moreover, platelets from DENV-infected patients exhibited classic signs of the intrinsic pathway of apoptosis that include increased surface phosphatidylserine exposure, mitochondrial depolarization and activation of caspase-9 and -3. Indeed, thrombocytopenia was shown to strongly associate with enhanced platelet activation and cell death in DENV-infected patients. Platelet activation, mitochondrial dysfunction and caspase-dependent phosphatidylserine exposure on platelets were also observed when platelets from healthy subjects were directly exposed to DENV in vitro. DENV-induced platelet activation was shown to occur through mechanisms largely dependent on DC-SIGN.

CONCLUSIONS

Together our results demonstrate that platelets from patients with dengue present signs of activation, mitochondrial dysfunction and activation of the apoptosis caspase cascade, which may contribute to the development of thrombocytopenia in patients with dengue. Our results also suggest the involvement of DC-SIGN as a critical receptor in DENV-dependent platelet activation.

The importance of hematopoietic progenitor cells in dengue

Scientific investigations designed to better understand and assess the distinguishing clinical characteristics pave the way to a successful treatment for a disease. Since the peripheral blood is obtained easily, the most frequent type of investigation performed on infectious agents focuses on the hematological components of blood drawn from patients. Bone marrow aspirates, although somewhat more difficult to obtain, should be evaluated more frequently because they provide additional information, giving us a glimpse into the development of the disease. Understanding the distinct and unique changes in hematological components of the bone marrow induced by a particular pathogen or corresponding to a specific illness may be a valuable asset for the diagnosis and prognosis of disease. A good example of a pathogen that could be better evaluated with greater knowledge of the bone marrow is dengue, one of the most important public vector-borne human diseases. Owing to the multitude of clinical manifestations and the dynamic alterations of various blood components over time, this disease is one of the most difficult to prevent and treat in humans. Although large amounts of data have been generated in the literature, there remains a large gap between this information and its relevance for the purpose of patient care. While evaluating the cellular components in the circulated blood from ill patients provides us with valuable information about the pathogenesis of various pathogens, there are other players participating in the progression to disease. The goal of this review is to emphasize the importance of bone marrow hematopoietic progenitor cells in disease and to inspire other researchers to incorporate them into their investigations on dengue pathogenesis. It is anticipated that the knowledge derived from these investigations not only elicit original concepts on the pathogenesis of dengue but also foster a new way of thinking in terms of vaccine or therapeutic development to prevent and treat dengue.

Role of CD61+ cells in thrombocytopenia of dengue patients

Although hematological disorders with salient features of thrombocytopenia have been well documented in dengue patients, the role of CD61-expressing platelets and the megakaryocytic cell lineage in the pathogenesis of dengue virus (DENV) infection remains largely unexplored. A prospective observational study was performed using blood samples and PBMCs from dengue-confirmed patients, as well as from rhesus monkeys (RM) experimentally infected with DENV. Immunohistochemical staining and FACS techniques were applied to evaluate the frequencies of CD61(+) cells that contained DENV antigen. Highly enriched population of CD61(+) cells was also isolated from acute DENV-infected RM and assayed for DENV RNA by quantitative RT-PCR. Results revealed that DENV antigen was found in small vesicles of varying size, and more frequently in anucleated cells associated with platelets in dengue patients. The DENV antigen-containing cells were CD61(+) and appeared to share characteristics of megakaryocytes. Kinetic profiles of CD61(+) cells from DENV-infected RM revealed a transient increase in CD61(+)CD62P(+) cells early after DENV infection. DENV RNA in a highly enriched population of CD61(+) cells from the infected RM was observed during acute stage. Our results indicate that virus containing CD61(+) cells may be directly linked to the platelet dysfunction and low platelet count characteristics of dengue patients.

Hemophagocytosis in dengue: comprehensive report of six cases

Hemophagocytic syndrome is a potentially fatal disorder. It is being increasingly reported but remained under-recognized in dengue. Most reported cases were in association with plasma leakage and shock but multi-organ impairment was also observed. We describe the time-lines of 6 cases of confirmed dengue with varying severities of hemophagocytosis. All had persistent fever, cytopenia and elevated transaminases with markedly elevated ferritin levels during and beyond the plasma leakage phase. Acute renal failure and central nervous system manifestation were observed in two patients. Morphological hemophagocytosis was demonstrated in three patients. All survivors showed clinical and biochemical resolution of hemophagocytosis indicating its transient nature. Persistence of fever and cytopenia together with multi-organ dysfunction, out of proportion to and beyond the plasma leakage phase should prompt clinicians to consider this phenomenon.

Interaction of the European genotype porcine reproductive and respiratory syndrome virus (PRRSV) with sialoadhesin (CD169/Siglec-1) inhibits alveolar macrophage phagocytosis

Porcine reproductive and respiratory syndrome virus (PRRSV) is an arterivirus that shows a restricted in vivo tropism for subsets of porcine macrophages, with alveolar macrophages being major target cells. The virus is associated with respiratory problems in pigs of all ages and is commonly isolated on farms with porcine respiratory disease complex (PRDC). Due to virus-induced macrophage death early in infection, PRRSV hampers the innate defence against pathogens in the lungs. In addition, the virus might also directly affect the antimicrobial functions of macrophages. This study examined whether interaction of European genotype PRRSV with primary alveolar macrophages (PAM) affects their phagocytic capacity. Inoculation of macrophages with both subtype I PRRSV (LV) and subtype III PRRSV (Lena) showed that the virus inhibits PAM phagocytosis. Similar results were obtained using inactivated PRRSV (LV), showing that initial interaction of the virion with the cell is sufficient to reduce phagocytosis, and that no productive infection is required. When macrophages were incubated with sialoadhesin- (Sn) or CD163-specific antibodies, two entry mediators of the virus, only Sn-specific antibodies downregulated the phagocytic capacity of PAM, indicating that interaction with Sn, but not CD163, mediates the inhibitory effect of PRRSV on phagocytosis. In conclusion, this study shows that European genotype PRRSV inhibits PAM phagocytosis in vitro, through the interaction with its internalization receptor Sn. If similar events occur in vivo, this interaction may be important in the development of PRDC, as often seen in the field.

Platelet apoptosis and apoptotic platelet clearance by macrophages in secondary dengue virus infections

BACKGROUND

The mechanisms of thrombocytopenia and platelet phagocytosis in dengue illness are not fully understood.

METHODS

A prospective hospital-based study was conducted to examine the relationships between platelet counts, serum thrombopoietin (TPO) levels, and platelet apoptosis and phagocytosis in 81 patients with secondary dengue virus (DV) infections and 38 healthy volunteers. The apoptosis and phagocytosis of cultured platelets after exposure to DV were also examined.

RESULTS

Platelet apoptosis, platelet phagocytosis, and serum TPO levels were increased significantly in patients during the acute and early convalescence phases compared with levels observed in patients during the convalescence phase and in healthy volunteers. A significant correlation between platelet apoptosis and platelet phagocytosis was also observed in these patients. Platelet phagocytosis was inhibited significantly by the D89E mutant, which carries a point mutation in the RGD motif of milk fat globule-epidermal growth factor 8, a phosphatidylserine-recognizing bridge molecule. DV-induced platelet apoptosis and increased phagocytosis of DV-induced apoptotic platelets was confirmed using in vitro assays.

CONCLUSIONS

Our data suggest an increased phagocytosis of DV-induced apoptotic platelets by macrophages via a phosphatidylserine-recognizing pathway in secondary DV infection. Accelerated platelet clearance, however, was overcome by TPO-induced enhanced thrombopoiesis in these patients.

CLINICAL TRIALS REGISTRATION

UMIN000004835.

Multi-faceted role of naturally occurring autoantibodies in fighting pathogens

Naturally occurring antibodies (NAbs) play a vital role in the first line of defense against bacterial and viral infections. Most studies in mice and man have attributed this role to NAbs of the IgM isotype. However, there is also a significant amount of data on the anti-infectious function of NAbs of the IgG isotype. Most of these observations are derived from studies using a privileged source of NAbs, the pooled human IgG for intravenous application, IVIG. In addition to its use as a replacement in humoral immunodeficiencies, IVIG is extensively used in autoimmune and inflammatory diseases. The properties of NAbs, the principal components of IVIG, are considered crucial for their immune-regulatory properties, owing to their ability to recognize self-antigens and even autoantibodies. By virtue of these specificities for several cellular antigens, including exposed proteins that act as receptors for a variety of pathogens, certain NAbs in IVIG have a therapeutic role in preventing or modulating infections. We summarize in this chapter several examples that highlight the importance of NAbs in the control of certain bacterial and viral infections.

Infection of bone marrow cells by dengue virus in vivo

Abnormal bone marrow (BM) suppression is one of the hallmarks of dengue virus (DENV) infection in patients. Although the etiology remains unclear, direct viral targeting of the BM has been reasoned to be a contributing factor. The present studies were carried out in an effort to determine the potential effect of DENV infection on the cellularity of BM using a previously established nonhuman primate model of DENV-induced coagulopathy. BM aspirates were collected at various times from the infected nonhuman primate and cells were phenotypically defined and isolated using standard flow cytometry (fluorescence-activated cell sorting). These isolated cells were subjected to detection of DENV utilizing quantitative real-time reverse transcription polymerase chain reaction, electron microscopy, and immunostaining techniques. DENV RNA was detectable by quantitative real-time reverse transcription polymerase chain reaction in BM specimens and the presence of DENV-like particles within platelet was confirmed by electron microscopy. Enumeration of BM cells revealed a transient surge in cellularity at day 1, followed by a gradual decline from days 2 to 10 post infection. Detailed phenotypic studies showed similar kinetics in the frequencies of CD41(+)CD61(+) cells, regardless of CD34 and CD45 expression. The CD61(+) cells were not only the predominant cells that stained for DENV antigen but fluorescence-activated cell sorting-assisted isolation of CD61(+) cells from the BM were shown to contain infectious DENV by coculture with Vero cells. These data support the view that intravenous infection of nonhuman primate with DENV leads to direct infection of the BM, which is likely to be a contributing factor for transient cell suppression in the peripheral blood characteristic of acute DENV infection.

Frequency alterations in key innate immune cell components in the peripheral blood of dengue patients detected by FACS analysis

Dengue is a timing disease, and frequently dengue patients do not seek help until 2-3 days after prodrome. Thus, the innate immune parameters in dengue patients remain largely unexplored. A comprehensive FACS profiling of key innate immune cells in the peripheral blood of dengue patients was performed. Twenty-seven dengue patients varying in days of fever onset were enrolled and the majority of them had primary infection based on serological results. FACS panels were used to evaluate NK cells, platelet-leukocyte aggregates (PLA), inflammatory monocytes, and plasmacytoid (pDC) and myeloid dendritic cells (mDC). FACS results showed the following findings: (i) NK cells significantly dropped on day 5 after the onset of fever and gradually returned to normal within 2 weeks of illness; (ii) biphasic PLA were observed, maximum levels were reached on days 6-8 and 11-16, and platelet-monocyte aggregates (PMA) were the most frequent event; (iii) inflammatory monocytes were consistently lower on days 5-8 and gradually returned to the normal level in the second week of illness, and (iv) pDC reached the maximum level on day 5 and gradually declined to the baseline level after 1 week of illness. In contrast, mDC fluctuated somewhat during the first week of illness and returned to the baseline level in the second week of illness. The most interesting finding was the biphasic PLA, in particular the PMA. These innate immunological parameters could be a crucial factor dictating the complicated pathogenesis of dengue disease.

Cells in dengue virus infection in vivo

Dengue has been recognized as one of the most important vector-borne emerging infectious diseases globally. Though dengue normally causes a self-limiting infection, some patients may develop a life-threatening illness, dengue hemorrhagic fever (DHF)/dengue shock syndrome (DSS). The reason why DHF/DSS occurs in certain individuals is unclear. Studies in the endemic regions suggest that the preexisting antibodies are a risk factor for DHF/DSS. Viremia and thrombocytopenia are the key clinical features of dengue virus infection in patients. The amounts of virus circulating in patients are highly correlated with severe dengue disease, DHF/DSS. Also, the disturbance, mainly a transient depression, of hematological cells is a critical clinical finding in acute dengue patients. However, the cells responsible for the dengue viremia are unresolved in spite of the intensive efforts been made. Dengue virus appears to replicate and proliferate in many adapted cell lines, but these in vitro properties are extremely difficult to be reproduced in primary cells or in vivo. This paper summarizes reports on the permissive cells in vitro and in vivo and suggests a hematological cell lineage for dengue virus infection in vivo, with the hope that a new focus will shed light on further understanding of the complexities of dengue disease.

A non mouse-adapted dengue virus strain as a new model of severe dengue infection in AG129 mice

The spread of dengue (DEN) worldwide combined with an increased severity of the DEN-associated clinical outcomes have made this mosquito-borne virus of great global public health importance. Progress in understanding DEN pathogenesis and in developing effective treatments has been hampered by the lack of a suitable small animal model. Most of the DEN clinical isolates and cell culture-passaged DEN virus strains reported so far require either host adaptation, inoculation with a high dose and/or intravenous administration to elicit a virulent phenotype in mice which results, at best, in a productive infection with no, few, or irrelevant disease manifestations, and with mice dying within few days at the peak of viremia. Here we describe a non-mouse-adapted DEN2 virus strain (D2Y98P) that is highly infectious in AG129 mice (lacking interferon-α/β and -γ receptors) upon intraperitoneal administration. Infection with a high dose of D2Y98P induced cytokine storm, massive organ damage, and severe vascular leakage, leading to haemorrhage and rapid death of the animals at the peak of viremia. In contrast, very interestingly and uniquely, infection with a low dose of D2Y98P led to asymptomatic viral dissemination and replication in relevant organs, followed by non-paralytic death of the animals few days after virus clearance, similar to the disease kinetic in humans. Spleen damage, liver dysfunction and increased vascular permeability, but no haemorrhage, were observed in moribund animals, suggesting intact vascular integrity, a cardinal feature in DEN shock syndrome. Infection with D2Y98P thus offers the opportunity to further decipher some of the aspects of dengue pathogenesis and provides a new platform for drug and vaccine testing.

The spread of dengue (DEN) worldwide combined with an increased severity of the DEN-associated clinical outcomes have made this mosquito-borne virus of great global public health importance. Infection with DEN virus can be asymptomatic or trigger a wide spectrum of clinical manifestations, ranging from mild acute febrile illness to classical dengue fever and to severe DEN hemorrhagic fever/DEN shock syndrome (DHF/DSS). Progress in understanding DEN disease and in developing effective treatments has been hampered by the lack of a suitable animal model that can reproduce all or part of the disease's clinical manifestations and outcome. Only a few of the DEN virus strains reported so far elicit a virulent phenotype in mice, which results at best in an acute infection where mice die within few days with no, few or irrelevant disease manifestations. Here we describe a DEN virus strain which is highly virulent in mice and reproduces some of the aspects of severe DEN in humans, including the disease kinetics, organ damage/dysfunction and increased vascular permeability. This DEN virus strain thus offers the opportunity to further decipher some of the mechanisms involved in DEN pathogenesis, and provides a new platform for drug and vaccine testing in the mouse model.