Replication of dengue virus type 2 in Aedes albopictus cell culture

DENV and ZIKV infection: Species specificity and broad cell tropism

Nearly one-third of countries worldwide have reported cases of Dengue virus (DENV) and Zika virus (ZIKV) infections, highlighting the significant threat these viruses pose to global public health. As members of the Flavivirus genus within the Flaviviridae family, DENV and ZIKV have demonstrated the ability to infect a wide range of cell lines from multiple species in vitro. However, the range of susceptible animal models is notably limited, and field studies indicate that their capacity to infect host organisms is highly restricted, with a very narrow range of target cells in vivo. The virus's ability to hijack host cellular machinery plays a crucial role in determining its cellular and species specificity. In this review, we examine how DENV and ZIKV exploit host cells to facilitate their replication, offering new insights that could inform the development of antiviral drugs and therapeutic targets.

Membrane feeding of dengue patient's blood as a substitute for direct skin feeding in studying Aedes-dengue virus interaction

Background

Understanding the interaction between Aedes vectors and dengue viruses (DENV) has significant implications in determining the transmission dynamics of dengue. The absence of an animal model and ethical concerns regarding direct feeding of mosquitoes on patients has resulted in most infection studies using blood meals spiked with laboratory-cultured DENV. Data obtained from such studies may not reflect the natural human-mosquito transmission scenario. This study explored the potential of using membrane feeding of dengue patient’s blood as a substitute for direct skin feeding.

Methods

Four to six-day old female Ae. aegypti were provided the opportunity to feed via direct exposure to a patient’s forearm for 15 min or via exposure to EDTA-treated blood from the same patient through an artificial membrane for 30 min. Mosquitoes from both feeding methods were incubated inside environmental chambers. Mosquitoes were sampled at day 13 post-feeding. Midgut and salivary glands of each mosquito were dissected to determine DENV infection by RT-qPCR and viral titration, respectively.

Results

Feeding rates: Direct skin feeding assay (DSFA) consistently showed higher mosquito feeding rates (93.3–100 %) when compared with the membrane feeding assay (MFA) (48–98.2 %). Midgut infection: Pair-wise comparison between methods showed no significant difference in midgut infection rates between mosquitoes exposed via each method and a strong correlation was observed in midgut infection rates for both feeding methods (r = 0.89, P < 0.0001). Overall midgut viral titers (n = 20) obtained by both methods were comparable (P ≥ 0.06). Salivary gland infection: Pair-wise comparison between both methods revealed no significant difference in salivary gland infection rate. Strong correlation in salivary gland infection was observed between DSFA and MFA (r = 0.81, P < 0.0001). In general, mosquitoes fed directly on dengue patients and those on patients’ blood (n = 11) had comparable virus titer (P ≥ 0.09).

Conclusion

DENV midgut and salivary gland infection rates showed good concordance between DSFA and MFA blood meal exposure methods. Freshly-obtained venous blood in EDTA from dengue patients for MFA can be used as a substitute to DSFA, especially in circumstances where bioethics approval or patient recruitment is difficult to obtain for vector competence studies. Nevertheless, mosquito numbers will need to be increased to compensate for lower feeding rate in MFA.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-016-1469-6) contains supplementary material, which is available to authorized users.

Novel, anionic, antiviral septapeptides from mosquito cells also protect monkey cells against dengue virus

We have shown previously that ultrafiltrates (5 kDa cutoff) of cell-free medium from mosquito cell cultures persistently infected with DENV serotype 2 (DENV-2) contained a novel antiviral agent (called viprolaxikine) that could protect pre-treated, naïve mosquito cells from DENV infection. Here, we show that viprolaxikine also reduced DENV-2 titers by almost 4 logs (>99.9%) when compared to Vero cells mock-treated with ultrafiltrates from cultures of uninfected mosquito cells. Protease treatment removed the anti-DENV-2 activity. Pre-incubation for 48-h was required to obtain the maximum, dose-dependent protection against DENV-2, indicating that the antiviral activity was based on the interaction between Vero cells and viprolaxikine rather than direct action of viprolaxikine on DENV-2. Activity was highest against DENV-2, but there was also significant activity against the 3 other DENV serotypes. LC-MS-MS analysis revealed that the active viprolaxikine fraction contained anionic, antiviral peptides, each comprised of 7 amino acids (DDHELQD, DETELQD and DEVMLQD or DEVLMQD) and with a common sequence motif of D-D/E-X-X-X-Q-D. These sequences do not occur in the dengue virus genome, suggesting that the peptides are produced by the host insect cells when persistently infected with DENV-2. These peptides represent a new class of anionic, insect-derived, antiviral peptides with activity against a flavivirus in both mammalian and insect cells.

Mosquito cells accommodate balanced, persistent co-infections with a densovirus and Dengue virus

To study persistent viral co-infections in arthropods, we first produced stable, persistently infected C6/36 mosquito cell cultures by serial passage of exponentially growing whole cells infected with either a densovirus (AalDNV) or Dengue virus (DEN-2). We then obtained stable, persistent co-infections by reciprocal super-challenge and similar passaging. Persistently infected cultures did not differ from naïve-cell cultures in growth rate and cell morphology. Nor did they differ in high production of both viruses with high infection rates for naïve C6/36 cells. Immunocytochemistry revealed that 99-100% of the cells were coinfected but that super-infection order had some effect on antigen distribution for the two viruses. Our results combined with existing field information and previously published experimental work suggest that the capacity to support stable, viral co-infections may be a general phenomenon for arthropod cells, and that they may be achieved easily and rapidly by serial passaging of whole cultured cells. Such persistent infections would facilitate studies on interactions between co-infecting viruses.

S-phase-dependent enhancement of dengue virus 2 replication in mosquito cells, but not in human cells

Dengue virus (DEN) is the most prevalent cause of arthropod-borne viral illness in humans. We determined the influence of cellular growth state on DEN type 2 (DEN2) replication in mosquito and human cells, based on the hypothesis that manipulation of cellular growth state will facilitate identification of viral and cellular determinants of productive infection. Comparison of density-arrested and cycling C6/36 Aedes albopictus cells infected with a low-passage DEN2 isolate revealed that cycling cells generated higher virus titers per cell. When C6/36 cells were stalled in S-phase via a thymidine (THY) block, titers of low-passage DEN2 isolates and a high-passage strain, 16681, were increased approximately 30-fold and 10-fold, respectively. Moreover, virus release was earlier in THY-treated cells than in asynchronously cycling cells. Adsorption, entry, genome uncoating, and translation were not responsible for increased titers of virus from S-phase C6/36 cells. In contrast to the 30-fold increase in virus titers, intracellular levels of viral RNA were increased approximately 2-fold, suggesting that the S-phase-responsive step is late in the DEN2 replication cycle. Analysis of viral RNA and protein released from the cells indicated that enhanced DEN2 assembly is largely responsible for increased virus titers produced during S-phase. In contrast to C6/36 cells, DEN2 titers from S-phase human hepatoma cells or primary human fibroblasts were not increased. These results demonstrate a differential response of DEN2 to the mosquito and human cell cycle and provide a framework for detailed studies into the mechanisms mediating virus assembly.

An enzyme-linked immunosorbent assay using a chaotropic agent (sodium thiocyanate) for serotype specific reaction between crude dengue viral antigen and anti-dengue mouse antibody

An enzyme-linked immunosorbent assay (ELISA) has been developed to detect serotype specific reaction between crude dengue viral antigen and anti-dengue mouse hyperimmunized antibody under the stringent condition in the presence of a Chaotropic agent, sodium thiocyanate (NaSCN), in the reaction mixture of antigen and antibody. Rapidly sedimenting hemagglutinin (RHA) derived from type 2 dengue virus-infected mosquito cell culture fluid reacted to the antibody for both type 2 and type 3 dengue viruses in the ELISA. In contrast, its reactivity was reduced after the addition of NaSCN in the ELISA. Soluble complement-fixing antigen (SCF) derived from type 2 dengue virus-infected mosquito cell culture fluid reacted serotype specifically to anti-dengue type 2 antibody, and was relatively stable for the NaSCN treatment in the ELISA. Anti-type 2 RHA mouse antibody reacted to both type 1 and type 2 dengue viral antigens and its reactivity was reduced after the addition of NaSCN in the ELISA. Anti-type 2 SCF antibody reacted serotype specifically to type 2 dengue viral antigen with and without NaSCN in the ELISA.

The dengue viruses

Dengue, a major public health problem throughout subtropical and tropical regions, is an acute infectious disease characterized by biphasic fever, headache, pain in various parts of the body, prostration, rash, lymphadenopathy, and leukopenia. In more severe or complicated dengue, patients present with a severe febrile illness characterized by abnormalities of hemostasis and increased vascular permeability, which in some instances results in a hypovolemic shock. Four distinct serotypes of the dengue virus (dengue-1, dengue-2, dengue-3, and dengue-4) exist, with numerous virus strains found worldwide. Molecular cloning methods have led to a greater understanding of the structure of the RNA genome and definition of virus-specific structural and nonstructural proteins. Progress towards producing safe, effective dengue virus vaccines, a goal for over 45 years, has been made.

Flavivirus entry into cultured mosquito cells and human peripheral blood monocytes

The entry modes of Japanese encephalitis (JE) and dengue-2 (DEN-2) viruses into C6/36 mosquito cells and of DEN-2 virus into human peripheral blood monocytes in vitro were studied. Inoculation of either JE or DEN-2 virions into C6/36 cells resulted in direct penetration of the virions into the cytoplasm at the cell surface in 3 stages. At stage 1, virions attached to the plasma membrane of host cells by their envelope spikes; at stage 2, the virion envelopes approximated to and eventually overlapped the host plasma membrane, and in the process the plasma membrane at the attachment sites dissolved; and, at stage 3, virions penetrated into the cytoplasm through the plasma-membrane disruptions created at the adsorption sites. Virions themselves apparently disintegrated at or near the penetration sites, for no virions were seen in the deeper cytoplasm. Coated pits did not form at the virion attachment sites, and virion-containing vesicles were not found in the cytoplasm. In the entry of DEN-2 virus into human peripheral blood monocytes, virions were found, adsorbed onto the external surface of the plasma membrane and attached to the luminal surface of macropinocytic vacuolar membranes. The latter apparently occurred as the result of ruffling and macropinocytic activities of the cells. At both sites virions penetrated into the cytoplasm through the plasma or vacuolar membrane in the same manner as they did through the plasma membrane of C6/36 cells. No evidence of viral entry by receptor-mediated endocytosis was observed. Implications of the entry mode of the mosquito cell-generated DEN-2 virus into human peripheral blood monocytes to an early process of natural, mosquito-transmitted infection is discussed.

Site of suppression of Banzi viral replication by an antiviral factor released from Aedes albopictus cells persistently infected with Banzi virus

The ability of the antiviral factor present in culture medium of Aedes albopictus cells persistently infected with the flavivirus, Banzi virus, to inhibit the replication of Banzi virus was examined. The anti-Banzi viral factor did not inhibit the uncoating of the virion. Levels of viral RNA were markedly reduced in mosquito cells treated with the antiviral factor. Syntheses of negative-strand and of positive-strand viral RNA species were inhibited to approximately the same extent. This inhibition was virus-specific in that the anti-Banzi viral factor had no effect on the synthesis of viral RNA in mosquito cells infected with either Japanese encephalitis or Eastern equine encephalitis viruses. The anti-Banzi viral factor inhibited the in vitro Banzi viral RNA synthesis but not that of Eastern equine encephalitis virus or of Japanese encephalitis virus.

Characterization of west nile virus persistent infections in genetically resistant and susceptible mouse cells. II. Generation of temperature-sensitive mutants

Long-term persistent infections were established with the flavivirus, West Nile virus (WNV), strain E101, in embryofibroblast cultures derived from susceptible C3H/HE and congenic-resistant C3H/RV mice. Cultures were initially maintained by weekly subculture at 37 degrees, but at passage 6 sister cultures were shifted to 32 degrees. Virus progeny titers were observed to increase after the shift to 32 degrees indicating the possible presence of temperature-sensitive mutants. Temperature-sensitive mutants were found to arise in cultures of both susceptible and resistant cells. However, only in the resistant cultures did temperature-sensitive virus become the majority population. Temperature-sensitive mutants did not appear to be essential for either initiation or maintenance of WNV-persistant infections. The resistant cells appear to provide an environment which is advantageous for the amplification of temperature-sensitive mutants.

Development of dengue virus plaques under serum-free overlay medium

An improved plaque assay for dengue virus was developed utilizing baby hamster kidney (BHK-21) cells initially grown in shaker culture. Different media preparations were tested for uniform and fast formation of BHK-21 cell sheets. Several overlay formulas were tested to develop a rapid plaque assay in 6- and 24-well plastic plates. The best results were obtained utilizing Eagle minimal essential medium (pH 7.2 to 7.4) supplemented with 1 mg of NaHCO3 per ml and 5% newborn calf serum for the formation of cell monolayers after 8 to 24 h of incubation at 37 degrees C. Serum-free Eagle minimal essential medium supplemented with 1% methylcellulose and buffered with 10 mM N-2-hydroxyethyl piperazine-N'-2-ethanesulfonic acid (pH 7.4 to 7.6) was used as an overlay medium. This system allowed for plaque formation after 3 days of incubation of dengue type 2 virus and after 4 days for dengue type 1 and 4 viruses.

The susceptibility of cell lines of Aedes aegypti (Linn.), Aedes albopictus (Skuse) and Aedes pseudoscutellaris (Therobald) to infection with blutongue virus

Bluetongue virus multiplied in cell lines derived from Aedes albopictus and Aedes pseudoscutellaris cells. Virus reached a maximum titre in the Ae. pseudoscutellaris cells three days post inoculation, and in Ae. albopictus cells six days p.i. Virus growth was demonstrated in both cell lines at 27 degrees C and 37 degrees C. Significant titres of virus were still present in the Ae. albopictus cells after five subcultures at 27 degrees C over a period of six weeks. No cytopathic effect was observed in either cell line. A third cell line derived from the mosquito Ae. aegypti did not support the growth of Bluetongue virus.

Cellular resistance to arbovirus infection

When an arbovirus enters an arthropod in an infected blood meal, several mechanisms may interact to affect its life cycle and ultimate transmissibility. Intrinsic absolute failure in the establishment of infection must be contrasted with infection that is successfully established but is variably modulated in its viral yield throughout the vector's life-span. Degrees of vertebrate host resistance make this modulation a central factor in determining whether an arthropod is an important vector in nature; moreover, human intervention that affects modulating mechanisms may become a basis for disease control. In the absence of evidence of real immune resistance to arbovirus infections in arthropods, other more primitive modulating mechanisms must be considered: interferonlike substances may be formed in arthropod cells; arthropod cells may "cure" themselves by a unique endophagocytic digestion of their virus burden; homologous interference with viral replicative processes may be mediated via wild or mutant viral RNA species acting to shut down further RNA synthesis; and homologous interference may be mediated by RNA of defective-interfering virus formed earlier in infection.

Temperature-sensitive virus from Aedes albopictus cells chronically infected with Sindbis virus

Cultures of Aedes albopictus cells persistently infected with wild-type Sindbis virus (SV-W) give rise to small plaque-forming mutants which are also temperature sensitive. These mutants, designated SV-C, are neutralized by antiserum produced against SV-W. Mutant ts clones were isolated from SV-C by plaque purification. After serial undiluted passage in BHK or mosquito cells, each of the clones gave rise to ts(+) revertants which, however, remained mutant with respect to plaque morphology. Nineteen of 20 clones derived from SV-C were RNA(+), and one was RNA(-) (SV-C-2). The RNA synthesizing activity, once induced in infected cells by SV-C-2, was stable at the nonpermissive temperature (39.5 C). All clones derived from SV-C were inactivated at 60 C much more quickly than was SV-W. It was not possible to demonstrate complementation between any of the SV-C clones.