Heterologous prime-boost immunization induces protection against dengue virus infection in cynomolgus macaques

IMPORTANCE

Currently licensed dengue vaccines do not induce long-term protection in children without previous exposure to dengue viruses in nature. These vaccines are based on selected attenuated strains of the four dengue serotypes and employed in combination for two or three consecutive doses. In our search for a better dengue vaccine candidate, live attenuated strains were followed by non-infectious virus-like particles or the plasmids that generate these particles upon injection into the body. This heterologous prime-boost immunization induced elevated levels of virus-specific antibodies and helped to prevent dengue virus infection in a high proportion of vaccinated macaques. In macaques that remained susceptible to dengue virus, distinct mechanisms were found to account for the immunization failures, providing a better understanding of vaccine actions. Additional studies in humans in the future may help to establish whether this combination approach represents a more effective means of preventing dengue by vaccination.

Blockade-of-Binding Activities toward Envelope-Associated, Type-Specific Epitopes as a Correlative Marker for Dengue Virus-Neutralizing Antibody

Humans infected with dengue virus (DENV) acquire long-term protection against the infecting serotype, whereas cross-protection against other serotypes is short-lived. Long-term protection induced by low levels of type-specific neutralizing antibodies can be assessed using the virus-neutralizing antibody test. However, this test is laborious and time-consuming. In this study, a blockade-of-binding enzyme-linked immunoassay was developed to assess antibody activity by using a set of neutralizing anti-E monoclonal antibodies and blood samples from dengue virus-infected or -immunized macaques. Diluted blood samples were incubated with plate-bound dengue virus particles before the addition of an enzyme-conjugated antibody specific to the epitope of interest. Based on blocking reference curves constructed using autologous purified antibodies, sample blocking activity was determined as the relative concentration of unconjugated antibody that resulted in the same percent signal reduction. In separate DENV-1-, -2-, -3-, and -4-related sets of samples, moderate to strong correlations of the blocking activity with neutralizing antibody titers were found with the four type-specific antibodies 1F4, 3H5, 8A1, and 5H2, respectively. Significant correlations were observed for single samples taken 1 month after infection as well as samples drawn before and at various time points after infection/immunization. Similar testing using a cross-reactive EDE-1 antibody revealed a moderate correlation between the blocking activity and the neutralizing antibody titer only for the DENV-2-related set. The potential usefulness of the blockade-of-binding activity as a correlative marker of neutralizing antibodies against dengue viruses needs to be validated in humans. IMPORTANCE This study describes a blockade-of-binding assay for the determination of antibodies that recognize a selected set of serotype-specific or group-reactive epitopes in the envelope of dengue virus. By employing blood samples collected from dengue virus-infected or -immunized macaques, moderate to strong correlations of the epitope-blocking activities with the virus-neutralizing antibody titers were observed with serotype-specific blocking activities for each of the four dengue serotypes. This simple, rapid, and less laborious method should be useful for the evaluation of antibody responses to dengue virus infection and may serve as, or be a component of, an in vitro correlate of protection against dengue in the future.

Detection of antibodies to duck tembusu virus in human population with or without the history of contact with ducks

Duck tembusu virus (DTMUV) is an emerging duck pathogen in China and other Asian countries. It is unclear whether this emerging zoonotic infection poses a threat to humans. A previous study in 2012 showed surprisingly high rates of seropositivity and positive viral detection by RT-PCR in duck farm workers in China. To understand the nature of the threat of this emerging virus, we studied the neutralizing antibody response to a local isolate of DTMUV in an at-risk population, who were workers in duck farms and residents around farming areas in Central Thailand where DTMUV had been previously detected, and in a not-at-risk population, who were people living in the same or neighbouring province, but at a distance from the farms and who had no contact with ducks. The sera from the at-risk population showed higher anti-DTMUV neutralizing antibody titres as compared with those of the not-at-risk population. However, within the at-risk population, workers with direct contact with ducks did not show higher neutralizing titres than those without direct contact. Interestingly, some people in the not-at-risk group also displayed high neutralizing antibody titres to DTMUV. These sera were tested against other endemic Flaviviruses and showed no or low cross-reactivity suggesting the specificity of the neutralizing activity against DTMUV. These data raise a possibility of DTMUV as a potential zoonotic pathogen but the mode of transmission of the virus from ducks or other possible hosts to humans should be explored further.

Small plaque size variant of chikungunya primary isolate showed reduced virulence in mice

BACKGROUND

Plaque size is a common feature of viral characterization. Small plaque size is used as a marker of attenuation for live-attenuated vaccine development.

OBJECTIVE

To investigate whether the naturally occurring plaque size variation reflects virulence of the variants of chikungunya virus (CHIKV).

METHODS

We selected and purified a variant with small plaque size from the primary isolate. The viral variant was tested for the plaque morphology, in vitro growth kinetics and mouse neurovirulence in comparison with the parental wild type.

RESULTS

The small plaque size variant showed stable homogenous small plaques after 4 plaque purifications. The small plaque virus grew slower and to the lower titer when compared with wild type virus. After 21 days of infection, mice that received small plaque virus showed 98% survival rate while 74% of mice survived after infected with wild type virus.

CONCLUSION

The small plaque size variant of CHIKV can be obtained by plaque purification and the virus displays decreased virulence.

The synergistic effect of nsP2-L618, nsP3-R117, and E2-K187 on the large plaque phenotype of chikungunya virus

Chikungunya virus (CHIKV), a mosquito-borne Alphavirus, is the etiological agent of chikungunya fever. CHIKV re-emerged from 2004 onwards, and subsequently caused major outbreaks in many parts of the world including the Indian Ocean islands, Asia, and the Americas. In this study, a large plaque variant of CHIKV isolated from patient in Thailand was subjected to repeated cycles of plaque-purification in Vero cells. The resulting virus produced homogenous large plaques and showed a more pathogenic phenotype than the parental wild-type CHIKV. Whole genome analysis of the large plaque virus in comparison to parental isolate revealed a number of mutations, leading to the following amino acid changes: nsP2 (P⁶¹⁸→L), nsP3 (G¹¹⁷→R), and E2 (N¹⁸⁷→K). Eight recombinant CHIKVs were constructed to determine which amino acids mediated the large plaque phenotype. The results showed the recombinant virus which contains all three mutations, rCHK-L, produced significantly larger plaques than the other recombinant viruses (p < 0.01). Moreover, the plaque size of the other recombinant virus tended to be smaller if they contained only one or two of the large plaque associated mutations in the viral genome. In conclusion, the combination of all three residues (nsP2-L⁶¹⁸, nsP3-R¹¹⁷, and E2-K¹⁸⁷) is required to produce the large plaque phenotype of CHIKV.

Generation and preclinical immunogenicity study of dengue type 2 virus-like particles derived from stably transfected mosquito cells

Recent phase IIb/III trials of a tetravalent live attenuated vaccine candidate revealed a need for improvement in the stimulation of protective immunity against diseases caused by dengue type 2 virus (DENV-2). Our attempts to develop particulate antigens for possibly supplementing live attenuated virus preparation involve generation and purification of recombinant DENV-2 virus-like particles (VLPs) derived from stably (prM+E)-expressing mosquito cells. Two VLP preparations generated with either negligible or enhanced prM cleavage exhibited different proportions of spherical particles and tubular particles of variable lengths. In BALB/c mice, VLPs were moderately immunogenic, requiring adjuvants for the induction of strong virus neutralizing antibody responses. VLPs with enhanced prM cleavage induced higher levels of neutralizing antibody than those without, but the stimulatory activity of both VLPs was similar in the presence of adjuvants. Comparison of EDIII-binding antibodies in mice following two adjuvanted doses of these VLPs revealed subtle differences in the stimulation of anti-EDIII binding antibodies. In cynomolgus macaques, VLPs with enhanced prM cleavage augmented strongly neutralizing antibody and EDIII-binding antibody responses in live attenuated virus-primed recipients, suggesting that these DENV-2 VLPs may be useful as the boosting antigen in prime-boost immunization. As the levels of neutralizing antibody induced in macaques with the prime-boost immunization were comparable to those infected with wild type virus, this virus-prime VLP-boost regimen may provide an immunization platform in which a need for robust neutralizing antibody response in the protection against DENV-2-associated illnesses could be tested.

Identification of prohibitin as a Chikungunya virus receptor protein

Chikungunya virus (CHIKV) has recently re-emerged causing millions of infections in countries around the Indian Ocean. While CHIKV has a broad host cell range and productively infects a number of different cell types, macrophages have been identified as a potential viral reservoir serving to increase the duration of symptoms. To date no CHIKV interacting protein has been characterized and this study sought to identify CHIKV binding proteins expressed on target cell membranes. Two-dimensional virus overlay identified prohibitin (PHB) as a microglial cell expressed CHIKV binding protein. Co-localization, co-immunoprecipitation as well as antibody and siRNA mediated infection inhibition studies all confirmed a role for PHB in mediating internalization of CHIKV into microglial cells. PHB is the first identified CHIKV receptor protein, and this study is evidence that PHB may play a role in the internalization of multiple viruses.

Chikungunya in Southeast Asia: understanding the emergence and finding solutions

In the last few years, chikungunya has become a major problem in Southeast Asia, with large numbers of cases being reported in Singapore, Malaysia, and Thailand. Much of the current epidemic of chikungunya in Southeast Asia is being driven by the emergence of a strain of chikungunya virus that originated in Africa and spread to islands in the Indian Ocean, as well as to India and Sri Lanka, and then onwards to Southeast Asia. There is currently no specific treatment for chikungunya and no vaccine is available for this disease. This review seeks to provide a short update on the reemergence of chikungunya in Southeast Asia and the prospects for control of this disease.

Characteristics of dengue virus-infected peripheral blood mononuclear cell death that correlates with the severity of illness

The pathogenic mechanism of the severe form of dengue is complicated. Recent reports indicate that apoptotic death of various tissues or organs may be associated with vascular leakage, and ultimately leads to the death of DENV-infected patients. In the present study, we provide additional evidence supporting the detrimental role of apoptosis in DENV infection. A comparison of the rate of apoptosis in PBMCs isolated from patients suffering DF, a mild form of the disease, and the rate in patients with DHF, a life-threatening disease, revealed that PBMCs from DHF patients underwent apoptosis at a significantly higher rate than those suffering from DF alone. This suggests that the severity of natural DENV infection correlates with PBMC apoptosis. In addition, this cell death was induced not only by DENV itself, but also by the apoptotic activities of pro-inflammatory cytokines, such as TNF-alpha, and IL-1beta, that were upregulated in DHF patients. The death of these mononuclear cells that function in an innate immune system may explain the higher viral load in DHF patients than in DF patients. Interestingly, a gene expression profile pattern elucidated that apoptosis occurring during natural DENV infection involved mainly the extrinsic apoptosis pathway, which is mediated via both caspase-dependent and caspase-independent mechanisms. In conclusion, our data highlight the adverse effect of apoptosis induced by DENV and by pro-inflammatory cytokines during natural DENV infection.

Overexpression of tumor necrosis factor alpha by a recombinant rabies virus attenuates replication in neurons and prevents lethal infection in mice

The effect of tumor necrosis factor alpha (TNF-alpha) on rabies virus (RV) infection of the mouse central nervous system (CNS) was studied, using recombinant RV engineered to express either soluble TNF-alpha [SPBN-TNF-alpha+] or insoluble membrane-bound TNF-alpha [SPBN-TNF-alpha(MEM)]. Growth curves derived from infections of mouse neuroblastoma NA cells revealed significantly less spread and production of SPBN-TNF-alpha+ than of SPBN-TNF-alpha(MEM) or SPBN-TNF-alpha-, which carries an inactivated TNF-alpha gene. The expression of soluble or membrane-bound TNF-alpha was not associated with increased cell death or induction of alpha/beta interferons. Brains of mice infected intranasally with SPBN-TNF-alpha+ showed significantly less virus spread than did mouse brains after SPBN-TNF-alpha- infection, and none of the SPBN-TNF-alpha+-infected mice succumbed to RV infection, whereas 80% of SPBN-TNF-alpha- -infected mice died. Reduced virus spread in SPBN-TNF-alpha+-infected mouse brains was paralleled by enhanced CNS inflammation, including T-cell infiltration and microglial activation. These data suggest that TNF-alpha exerts its protective activity in the brain directly through an as yet unknown antiviral mechanism and indirectly through the induction of inflammatory processes in the CNS.

Human CD8+ cytotoxic T cell responses to adenovirus capsid proteins

Adenoviruses (Ads) cause fatal disease in allogeneic stem cell transplant recipients, but there is no established therapy. Ad-specific CD8+ T cells were detected in PBMC from healthy adults at a mean frequency of 77 per 10(5) CD8+ T cells (range 8-260) by interferon-gamma ELISPOT and cytokine flow cytometry assays. CD8+ T cell lines from 7 of 7 donors exhibited MHC-class-I-restricted killing of targets expressing the capsid protein hexon. In contrast, cytotoxicity against the capsid proteins fiber and penton base was weaker or not detected. Two HLA-A2-restricted hexon epitopes and one HLA-B-restricted epitope were identified, all of which are adjacent to or overlap an HLA-DP4-restricted epitope in the highly conserved C-terminus. Thus, hexon is the immunodominant T cell target among capsid proteins and contains multiple C-terminal epitopes conserved among serotypes. These data support evaluation of donor lymphocyte infusions for treatment of Ad disease post-transplant.

Identification of viral genomic elements responsible for rabies virus neuroinvasiveness

Attenuated tissue culture-adapted and natural street rabies virus (RV) strains differ greatly in their neuroinvasiveness. To identify the elements responsible for the ability of an RV to enter the CNS from a peripheral site and to cause lethal neurological disease, we constructed a full-length cDNA clone of silver-haired bat-associated RV (SHBRV) strain 18 and exchanged the genes encoding RV proteins and genomic sequences of this highly neuroinvasive RV strain with those of a highly attenuated nonneuroinvasive RV vaccine strain (SN0). Analysis of the recombinant RV (SB0), which was recovered from SHBRV-18 cDNA, indicated that this RV is phenotypically indistinguishable from WT SHBRV-18. Characterization of the chimeric viruses revealed that in addition to the RV glycoprotein, which plays a predominant role in the ability of an RV to invade the CNS from a peripheral site, viral elements such as the trailer sequence, the RV polymerase, and the pseudogene contribute to RV neuroinvasiveness. Analyses also revealed that neuroinvasiveness of an RV correlates inversely with the time necessary for internalization of RV virions and with the capacity of the virus to grow in neuroblastoma cells.

Overexpression of the rabies virus glycoprotein results in enhancement of apoptosis and antiviral immune response

A recombinant rabies virus (RV) carrying two identical glycoprotein (G) genes (SPBNGA-GA) was constructed and used to determine the effect of RV G overexpression on cell viability and immunity. Immunoprecipitation analysis and flow cytometry showed that tissue culture cells infected with SPBNGA-GA produced, on average, twice as much RV G as cells infected with RV carrying only a single RV G gene (SPBNGA). The overexpression of RV G in SPBNGA-GA-infected NA cells was paralleled by a significant increase in caspase 3 activity followed by a marked decrease in mitochondrial respiration, neither of which was observed in SPBNGA-infected cells. Furthermore, fluorescence staining and confocal microscopy revealed an increased extent of apoptosis and markedly reduced neurofilament and F actin in SPBNGA-GA-infected primary neuron cultures compared with neuronal cells infected with SPBNGA, supporting the concept that RV G or motifs of the RV G gene trigger the apoptosis cascade. Mice immunized with SPBNGA-GA showed substantially higher antibody titers against the RV G and against the nucleoprotein than SPBNGA-immunized mice, suggesting that the speed or extent of apoptosis directly determines the magnitude of the antibody response.

Overexpression of cytochrome C by a recombinant rabies virus attenuates pathogenicity and enhances antiviral immunity

The pathogenicity of individual rabies virus strains appears to correlate inversely with the extent of apoptotic cell death they induce and with the expression of rabies virus glycoprotein, a major inducer of an antiviral immune response. To determine whether the induction of apoptosis by rabies virus contributes to a decreased pathogenicity by stimulating antiviral immunity, we have analyzed these parameters in tissue cultures and in mice infected with a recombinant rabies virus construct that expresses the proapoptotic protein cytochrome c. The extent of apoptosis was strongly increased in primary neuron cultures infected with the recombinant virus carrying the active cytochrome c gene [SPBN-Cyto c(+)], compared with cells infected with the recombinant virus containing the inactive cytochrome c gene [SPBN-Cyto c(-)]. Mortality in mice infected intranasally with SPBN-Cyto c(+) was substantially lower than in SPBN-Cyto c(-)-infected mice. Furthermore, virus-neutralizing antibody (VNA) titers were significantly higher in mice immunized with SPBN-Cyto c(+) at the same dose. The VNA titers induced by these recombinant viruses paralleled their protective activities against a lethal rabies virus challenge infection, with SPBN-Cyto c(+) revealing an effective dose 20 times lower than that of SPBN-Cyto c(-). The strong increase in immunogenicity, coupled with the marked reduction in pathogenicity, identifies the SPBN-Cyto c(+) construct as a candidate for a live rabies virus vaccine.

High level expression of a human rabies virus-neutralizing monoclonal antibody by a rhabdovirus-based vector

Humans exposed to rabies virus must be promptly treated by passive immunization with anti-rabies antibody and active immunization with rabies vaccine. Currently, antibody prepared from pooled human serum or from immunized horses is utilized. However, neither of these reagents are readily available, entirely safe, or consistent in their biological activity. An ideal reagent would consist of a panel of human monoclonal antibodies. Such antibodies are now available, their only drawback being the cost of production. Using recombinant technology, we constructed a rabies virus-based vector which expresses high levels (approximately 60 pg/cell) of rabies virus-neutralizing human monoclonal antibody. The vector is a modified vaccine strain of rabies virus in which the rabies virus glycoprotein has been replaced with a chimeric vesicular stomatitis virus glycoprotein, and both heavy and light chain genes encoding a human monoclonal antibody have been inserted. This recombinant virus can infect a variety of mammalian cell lines and is non-cytolytic, allowing the use of cell culture technology routinely employed to produce rabies vaccines.