Dengue virus multiplication in cultures of mouse peritoneal macrophages: effects of macrophage activators

Morphological Aspects and Viremia Analysis of BALB/c Murine Model Experimentally Infected with Dengue Virus Serotype 4

Ever since its brief introduction in the Brazilian territory in 1981, dengue virus serotype 4 (DENV-4) remained absent from the national epidemiological scenario for almost 25 years. The emergence of DENV-4 in 2010 resulted in epidemics in most Brazilian states. DENV-4, however, remains one of the least studied among the four DENV serotypes. Despite being known as a mild serotype, DENV-4 is associated with severe cases and deaths and deserves to be investigated; however, the lack of suitable experimental animal models is a limiting factor for pathogenesis studies. Here, we aimed to investigate the susceptibility and potential tropism of DENV-4 for liver, lung and heart of an immunocompetent mice model, and to evaluate and investigate the resulting morphological and ultrastructural alterations upon viral infection. BALB/c mice were inoculated intravenously with non-neuroadapted doses of DENV-4 isolated from a human case. The histopathological analysis of liver revealed typical alterations of DENV, such as microsteatosis, edema and vascular congestion, while in lung, widespread areas of hemorrhage and interstitial pneumonia were observed. While milder alterations were present in heart, characterized by limited hemorrhage and discrete presence of inflammatory infiltrate, the disorganization of the structure of the intercalated disc is of particular interest. DENV-4 RNA was detected in liver, lung, heart and serum of BALB/c mice through qRT-PCR, while the NS3 viral protein was observed in all of the aforementioned organs through immunohistochemistry. These findings indicate the susceptibility of the model to the serotype and further reinforce the usefulness of BALB/c mice in studying the many alterations caused by DENV.

Role of cognitive parameters in dengue hemorrhagic fever and dengue shock syndrome

Dengue is becoming recognized as one of the most important vector-borne human diseases. It is predominant in tropical and subtropical zones but its geographical distribution is progressively expanding, making it an escalating global health problem of today. Dengue presents with spectrum of clinical manifestations, ranging from asymptomatic, undifferentiated mild fever, dengue fever (DF), to dengue hemorrhagic fever (DHF) with or without shock (DSS), a life-threatening illness characterized by plasma leakage due to increased vascular permeability. Currently, there are no antiviral modalities or vaccines available to treat and prevent dengue. Supportive care with close monitoring is the standard clinical practice. The mechanisms leading to DHF/DSS remains poorly understood. Multiple factors have been attributed to the pathological mechanism, but only a couple of these hypotheses are popular in scientific circles. The current discussion focuses on underappreciated factors, temperature, natural IgM, and endotoxin, which may be critical components playing roles in dengue pathogenesis.

Challenges for the formulation of a universal vaccine against dengue

Dengue is rapidly becoming a disease of an escalating global public health concern. The disease is a vector-borne disease, transmitted by the bite of an Aedes spp. mosquito. Dynamic clinical manifestations, ranging from asymptomatic, flu-like febrile illness, dengue fever (DF) to dengue hemorrhagic fever (DHF) with or without dengue shock syndrome (DSS), make the disease one of the most challenging to diagnose and treat. DF is a self-limited illness, while DHF/DSS, characterized by plasma leakage resulting from an increased vascular permeability, can have severe consequences, including death. The pathogenesis of dengue virus infection remains poorly understood, mainly due to the lack of a suitable animal model that can recapitulate the cardinal features of human dengue diseases. Currently, there is no specific treatment or antiviral therapy available for dengue virus infection and supportive care with vigilant monitoring is the principle course of treatment. Since vector control programs have been largely unsuccessful in preventing outbreaks, vaccination seems to be the most viable option for prevention. There are four dengue viral serotypes and each one of them is capable of causing severe dengue. Although immunity induced by infection by one serotype is effective in protection against the homologous viral serotype, it only has a transient protective effect against infection with the other three serotypes. The meager cross protective immunity generated wanes over time and may even induce a harmful effect at the time of subsequent secondary infection. Thus, it is imperative to have a vaccine that can elicit equal and long-lasting immunity to all four serotypes simultaneously. Numerous tetravalent vaccines are currently either in the pipeline for clinical trials or under development. For those frontrunner tetravalent vaccines in clinical trials, despite good safety and immunogenicity profiles registered, issues such as imbalanced immune responses between serotypes and questions with regard to whether the optimum formulation have been identified remain unresolved. This review centers on these issues and offers strategies that may improve the tetravalent vaccine formulation.

Susceptibility of mouse macrophage J774 to dengue virus infection

The aim of this study was to investigate whether the J774 mouse macrophage cell line could be used as an in vitro model for dengue virus infection (DENV). After 3 days, infection in J774 cells was assessed by detecting dengue virus non-structural protein 1 (NSP-1) production either by dot blot or indirect immunofluorescence assay (IFA) of saponine-permeabilized J774 cells and then confirmed by RT-PCR (171 bp product, corresponding to the DENV-2 core). Based on the presence of NSP-1 in infected but not in non-infected cells by both IFA and dot blot, as well as the amplification of a 171-bp DENV-2-specific RT-PCR product exclusively in the infected cells, the J774 cell line was found to be permissive for dengue virus infection. As far as we know, this is the first report that the J774 mouse macrophage cell line is infected with dengue virus and, thus, that it can be used as an alternative in vitro model for dengue virus infection studies. This finding could help to further elucidate the mechanisms involved in dengue virus infection and pathogenesis.

Liposomes as delivery systems in the prevention and treatment of infectious diseases

Research on the potential application of liposomes in the prevention and treatment of infectious diseases has focussed on improvement of the therapeutic index of antimicrobial drugs and immunomodulators and on stimulation of the immune response to otherwise weak antigens in vaccines composed of purified micro-organism subunits. In this review current approaches in this field are outlined. The improved therapeutic index of antimicrobial drugs after encapsulation in liposomes is a result of enhanced drug delivery to infected tissue or infected cells and/or a reduction of drug toxicity of potentially toxic antibiotics. Liposomal encapsulation of immunomodulators that activate macrophages aims at reducing the toxicity of these agents and targeting them to the cells of the mononuclear phagocyte system in order to increase the nonspecific resistance of the host against infections. Studies on the immunogenicity of liposomal antigens have demonstrated that liposomes can potentiate the humoral and cell mediated immunity to a variety of antigens.

Lectin-mediated enhancement of dengue virus infection in a mouse macrophage cell line Mk1

Treatment of a mouse macrophage cell line Mk1 with pokeweed mitogen (PWM) either before or during but not after virus inoculation resulted in an enhancement of dengue virus (DV) infection. The infection enhancement was primarily due to an increase in the number of DV-infected cells but not to increased virus production in a cell. These results suggested that PWM treatment mediated increased DV binding and/or penetration to Mk1 cells, thereby resulting in the infection enhancement. N-acetylglucosamine (GlucNAc) did not suppress PWM-mediated enhancement of DV infection when added to Mk1 cells after PWM treatment was done, although GlucNAc clearly suppressed the effect of PWM when added simultaneously with PWM. The results implied the possibility that the PWM-mediated increase in viral binding/penetration was not due to a cross-linking by PWM between DV and a cell-surface receptor, but due to another mechanism, presumably exposure of a masked DV receptor(s). The DV receptor, unidentified as yet, involved in the PWM-mediated infection enhancement appeared to have no relation with IgG Fc receptors that are known to be involved in antibody-mediated enhancement of DV infection.

Stimulation of macrophages by endotoxin results in the reactivation of a persistent herpes simplex virus infection

Previous work has shown that splenic macrophages derived from herpes simplex virus (HSV)-resistant C57BL/6 mice undergo a persistent HSV infection which is characterized by the continuous release of infectious virus particles from a small subpopulation of infected cells. Treatment of persistently infected macrophages for 2 weeks with lipopolysaccharide (LPS) resulted in an increase of HSV yield and in virus-induced cytopathic effects. HSV was also reactivated by treatment of macrophage cultures with lipid A or tumour necrosis factor (TNF). Like macrophages of C57BL/6 origin, cells from LPS-hyporesponsive C3H/HeJ mice could be persistently infected with HSV. These cells were resistant to LPS-induced virus reactivation. The results show that macrophages derived from C57BL/6 mice are rendered susceptible to lytic HSV infection by treatment with LPS or TNF. Thus, these substances may interfere with persistent HSV infection which can be established due to genetically controlled properties of the host.

The potential use of liposome-mediated antiviral therapy

The natural targeting of liposomes to cells of the reticuloendothelial system should be exploited to examine whether selective delivery of antiviral or immunomodulatory agents could be beneficial for the treatment of virus diseases. In this review we discuss the potential use of liposomes in the treatment of virus diseases, the targeting of liposome-encapsulated immunomodulators to macrophages in order to render these cells cytolytic for virus-infected cells, and the targeting of liposome-encapsulated antiviral drugs to macrophages to achieve direct suppression of virus replication with in these cells.

Enhancement of dengue virus infection in cultured mouse macrophages by lipophilic derivatives of muramyl peptides

Dengue virus multiplication in cultures of a murine myelomonocytic cell line (WEHI-3) as well as mouse peritoneal macrophages was enhanced by treatment of the cells with lipophilic derivatives of muramyl peptides for 2 or 3 days before virus inoculation, but not for 2 hr before virus inoculation or during the adsorption period. The infection-enhancing activity of the materials was dependent on their chemical structure, correlating with their immunoadjuvanticity. The infection enhancement in WEHI-3 cells was due primarily to an increase in the number of virus-infected cells which was accompanied by an increased cellular capacity to bind latex particles to their cell surfaces.

Increased multiplication of dengue virus in mouse peritoneal macrophage cultures by treatment with extracts of Ascaris-Parascaris parasites

Methylcellulose-elicited peritoneal macrophages from BALB/c mice were cultivated in vitro and inoculated with dengue virus (DV). At intervals thereafter portions of the culture fluids were taken and titrated for viral infectivity. Extracts from Ascaris suum and Parascaris equorum, either crude or Sephadex G-100 fractionated, were examined for effects on the multiplication of DV. The macrophage cultures treated with the above substances produced larger amounts of DV compared with untreated control cultures. The enhancing effect of the substances depended on doses added and duration of treatment and was suppressed by co-treatment with carrageenan, a specific macrophage-inhibiting agent, but was not related to the viability of cultured cells. In fluorescent antibody (FA) as well as infectious center assay experiments, it was shown that the DV-infected cells were found more frequently in treated cultures than in untreated control cultures. In the treated cultures phagocytosis by cultured cells was also of a higher magnitude than that in untreated cultures. In cocultures of macrophages and splenocytes from the same line of mice, no additive effect of splenocytes was noted. The limulus amebocyte lysate clotting enzyme reaction (Limulus test) indicated that involvement of bacterial lipopolysaccharides in the enhancement phenomena was negligible. The data so far obtained suggest that the enhancing effect was due to direct action of the parasitic extracts on macrophages. Four Sephadex G-100 fractions from the crude extracts showed similar activities; however, the effects of fractions I and III appeared to be comparatively strong. Significance of the findings in relation to the pathogenesis of DV infection was discussed.

Dengue type 2 virus infection in human peripheral blood monocyte cultures

Dengue type 2 virus (D2V) infection in cultured human monocytes was studied. D2V permissiveness of the monocytes was enhanced when the cells were inoculated with D2V in the presence of either polyclonal or type-specific monoclonal anti-dengue antibody. The enhancement of D2V permissiveness mediated by the antibodies was more clearly demonstrated when the monocytes had been treated with trypsin before virus inoculation, though treatment of the cells with trypsin alone decreased D2V permissiveness. The enhancement of infection by type-specific neutralizing monoclonal antibody suggests that the D2V particles possess at least two antigenic determinants closely associated with virus infectivity. Infectious center assays revealed that the infection enhancement in the presence of the antibodies was due primarily to an increase in the number of D2V-infected cells, and that only a small proportion of the monocyte population supported D2V replication. The virus-permissive monocytes did not bear HLA-DR antigens on their cell surface. The presence of nonadherent lymphocytes in the monocyte cultures before D2V inoculation did not affect the D2V permissiveness of the monocytes. Treatment of cultured monocytes with the synthetic adjuvants N-acetylmuramyl-L-alanyl-D-isoglutamine (MDP) and its lipophilic derivative, [B30]-MDP, did not significantly affect the D2V permissiveness of the cells.

Enhancement of dengue virus type 2 replication in mouse macrophage cultures by bacterial cell walls, peptidoglycans, and a polymer of peptidoglycan subunits

The effects of bacterial cell walls, peptidoglycans, and a water-soluble polymer of peptidoglycan subunits on dengue virus type 2 replication in cultured mouse peritoneal macrophages were studied. Pretreatment of macrophage cultures with all of test cell walls isolated from seven bacterial species for 3 days significantly enhanced the virus production in the cultures. Peptidoglycans prepared from four of the above cell walls also exerted the virus production-enhancing effects in a similar manner as the walls. A water-soluble polymer of peptidoglycan subunits which was prepared by treatment of Staphylococcus epidermidis wall peptidoglycan with an interpeptide bridge-splitting enzyme (endopeptidase) also definitely enhanced the virus production in macrophage cultures, although its activity was weaker than that of the original wall and peptidoglycan. Macrophage cultures from athymic nude mice, when treated with cell walls and peptidoglycans of S. epidermidis and Lactobacillus plantarum for 3 days, also showed an increased ability to support dengue virus type 2 replication. The infectious center assay demonstrated that the virus replication enhancement by S. epidermidis cell wall and peptidoglycan was primarily due to an increase in the number of virus-infected cells. This finding did not seem to be in conflict with the observation that macrophages treated with the above cell wall or peptidoglycan phagocytized more latex particles than did untreated macrophages. The conclusions based on the above experiments are that the treatment of mouse peritoneal macrophage cultures with bacterial cell walls and their components increases the take of dengue virus type 2 by macrophages and thus raises the virus production in the macrophage cultures.