Phase 1 studies of Walter Reed Army Institute of Research candidate attenuated dengue vaccines: selection of safe and immunogenic monovalent vaccines

We describe the results of initial safety testing of 10 live-attenuated dengue virus (DENV) vaccine candidates modified by serial passage in primary dog kidney (PDK) cells at the Walter Reed Army Institute of Research. The Phase 1 studies, conducted in 65 volunteers, were designed to select an attenuated vaccine candidate for each DENV serotype. No recipient of the DENV candidate vaccines sustained serious injury or required treatment. Three vaccine candidates were associated with transient idiosyncratic reactions in one volunteer each, resulting in their withdrawal from further clinical development. Increasing PDK cell passage of DENV-1, DENV-2, and DENV-3 candidate vaccines increased attenuation for volunteers, yet also decreased infectivity and immunogenicity. This effect was less clear for DENV-4 candidate vaccines following 15 and 20 PDK cell passages. Only one passage level each of the tested DENV-2, -3, and -4 vaccine candidates was judged acceptably reactogenic and suitable for expanded clinical study. Subsequent studies with more recipients will further establish safety and immunogenicity of the four selected vaccine candidates: DENV-1 45AZ5 PDK 20, DENV-2 S16803 PDK 50, DENV-3 CH53489 PDK 20, and DENV-4 341750 PDK 20.

Primary humoral immune responses to formalin inactivated hemorrhagic fever with renal syndrome vaccine (Hantavax): consideration of active immunization in South Korea

The efficacy of a formalin-inactivated hemorrhagic fever with renal syndrome (HFRS) vaccine and the effectiveness of a related vaccination program have not been previously evaluated. We measured the primary immune responses to Hantavax by plaque reduction neutralizing antibody test (PRNT), hemagglutination inhibition test (HAI), ELISA and high density particle agglutination test (HDPA) in order to confirm a possible biological efficacy through independent substantiation of experimental results and to compare the results with previous studies. Following two doses of primary vaccination, the seroconversion rate of PRNT and HAI antibody was 33.3% (10/30) [95% C.I. 17.3-52.5%] and 26.7% (8/30) [95% C.I. 12.3-45.9%], respectively. The correlation between PRNT and HAI antibody showed a statistical significance (r=0.58, p<0.01). The seroconversion rate of HDPA and ELISA were both 76.7% (23/30) [95% C.I. 57.7-90.1%], which correlated well with each other (r=0.58, p<0.01). In our study, Hantavax elicited low neutralizing antibody responses, at least in the volunteers samples that we tested. The vaccination program, including the vaccine itself, that has been adopted by the national immunization program to protect against HFRS in Korea should be re-evaluated and re-formulated to produce a higher protective immune response rate.

Clinical evaluation of a vaccinia-vectored Hantaan virus vaccine

We evaluated a vaccinia-vectored vaccine for hemorrhagic fever with renal syndrome in clinical trials. A Phase I dose-escalation study in 16 volunteers divided into four groups demonstrated that subcutaneous inoculation of approximately 10(7) plaque-forming units of the recombinant virus was safe and immunogenic. Vaccination of a fifth group of 12 volunteers indicated that neutralizing antibody titers to both vaccinia virus and Hantaan virus were enhanced after a second inoculation. Comparing two routes of vaccination showed that scarification effectively induced neutralizing antibodies in vaccinia virus-naive volunteers but that subcutaneous inoculation was superior to scarification in vaccinia virus-immune individuals. A Phase II, double-blinded, placebo-controlled clinical trial was conducted among 142 volunteers. Two subcutaneous vaccinations were administered at 4-week intervals. Neutralizing antibodies to Hantaan virus or to vaccinia virus were detected in 72% or 98% of vaccinia virus-naive volunteers, respectively. In contrast, only 26% of the vaccinia virus-immune volunteers developed neutralizing antibody responses to Hantaan virus. J. Med. Virol. 60:77-85, 2000. Published 2000 Wiley-Liss, Inc.

Experimental infection of nonhuman primates with sandfly fever virus

Due to the lack of an animal model, previous studies of sandfly fever have relied upon human challenge trials. We examined the infectivity and potential pathogenicity of sandfly fever virus in cynomolgus monkeys (Macaca fascicularis). Three different preparations of sandfly fever virus. Sicilian strain, and a placebo were compared by different routes of administration. The most notable postchallenge clinical event was a decrease in lymphocytes in the intramuscularly challenged monkeys. Plaque-reduction neutralization responses peaked earlier in animals challenged intravenously as compared with those in animals challenged intramuscularly. There was no evidence for neurotropism or meningeal inflammation. Sandfly fever virus was infectious for cynomolgus monkeys, but produced no detectable clinical disease that might serve as a marker for animal modeling studies. On the other hand, the preclinical data provide supportive evidence for safe parenteral administration of a Sicilian strain of sandfly fever virus inoculum to humans as a challenge model for sandfly fever disease.

Immunologic responses to vaccinia vaccines administered by different parenteral routes

To develop a less reactogenic but equally immunogenic vaccine, this study of 91 human volunteers compared the safety and immunogenic potency of a new, cell culture-derived vaccinia virus vaccine administered intradermally and intramuscularly with the licensed vaccinia vaccine administered by scarification. Cutaneous pox lesions developed in a higher proportion of scarification vaccinees. Scarification and intradermal vaccine recipients who developed cutaneous pox lesions had more local reactions but also achieved significantly higher cell-mediated and neutralizing antibody responses than those who did not develop pox lesions. Although less reactogenic, intradermal or intramuscular administration of vaccinia vaccine without the concomitant development of a cutaneous pox lesion induced lower immune responses.

Zebra schlieren optics for leak detection

A modification of schlieren optics was explored as a technique for industrial gas-leak detection. A high-contrast pattern on thin reflecting material was imaged with a zoom lens onto a negative of the same pattern as a method of eliminating the ordinary rays. The geometry of the industrial setting determines the useful spatial frequency of the pattern.

A live attenuated dengue-1 vaccine candidate (45AZ5) passaged in primary dog kidney cell culture is attenuated and immunogenic for humans

A dengue-1 candidate vaccine (45AZ5), previously found to be underattenuated in 2 volunteers, was further attenuated by passage in primary dog kidney (PDK) cell cultures. New candidate vaccines prepared from three levels of PDK-passaged virus, PDK-10, PDK-20, and PDK-27, were each injected into 9 or 10 volunteers. There was a significant, progressive decline in viremia, clinical illness, and hematologic changes from low to high PDK cell passage level. PDK-20 infected all 10 vaccinees and induced viremia in 5, transient fever in 3, symptoms that resulted in curtailed activities for < or = 1 day in 4, and neutralizing antibody in all 10, which persisted for > or = 1 year in 5 of 8 vaccinees tested. Progressive passage in PDK cell culture progressively attenuates vaccine candidate strain 45AZ5 for humans. Because passage level PDK-20 may be suitable for healthy adults at high risk of dengue fever, additional clinical trials of this strain are warranted.

Immunization of monkeys with baculovirus-dengue type-4 recombinants containing envelope and nonstructural proteins: evidence of priming and partial protection

Groups of rhesus monkeys were immunized with baculovirus-dengue type-4 (DEN-4) recombinant-infected cell extracts. One recombinant contained all of the DEN-4 structural proteins and two nonstructural (NS) proteins (C-M-E-NS1-NS2a), while the other was a fusion protein containing a portion of the respiratory syncytial virus G glycoprotein and DEN-4 envelope glycoprotein (RSVG-E). Both preparations were immunogenic; all monkeys receiving either immunogen responded with the production of antivirion antibodies in enzyme immunoassays. All except one monkey receiving the recombinant b(C-M-E-NS1-NS2a) made antibodies to NS1. One monkey that received b(RSVG-E) showed the production of low levels of neutralizing antibodies. Following challenge with unmodified DEN-4 virus, seven of nine monkeys in the immunized group became infected and were viremic for a mean of 4.1 days. The control, sham-inoculated monkeys were also viremic; the mean number of days of viremia in this group was 4.7 days. The remaining monkeys in the immunized group (n = 7), although not protected, had evidence of priming. Hemagglutination inhibition antibody responses following challenge indicated an anamnestic response in this group of animals. Based on these results, it was concluded that future immunization schedules should be altered to optimize immune responses and that immunization with more potent and purified immunogens would probably result in higher seroconversion rates and antibody levels in monkeys.

Solid-phase antibody capture hemadsorption assay for detection of hepatitis A virus immunoglobulin M antibodies

A solid-phase antibody capture hemadsorption (SPACH) assay was developed to detect hepatitis A virus (HAV)-specific immunoglobulin M (IgM) antibodies in sera from humans recently infected with hepatitis. The assay is performed with microtiter plates coated with anti-human IgM antibodies to capture IgM antibodies from the test sera. HAV-specific IgM antibody is detected by the addition of HAV hemagglutinating antigen and goose erythrocytes. Hemadsorption of erythrocytes to antigen-antibody complexes attached to the solid phase indicate the presence of IgM antibodies. The SPACH assay was compared to a commercial radioimmunoassay and was found to be equally or more sensitive and specific for the detection of HAV IgM antibodies. The SPACH assay is an alternative, rapid assay that doesn't require hazardous substrates or radioactivity for the detection of HAV-specific antibodies.

Detection of West Nile virus by the polymerase chain reaction and analysis of nucleotide sequence variation

A polymerase chain reaction (PCR) assay was developed to rapidly detect and identify West Nile (WN) virus. The RNA from seven isolates of WN virus from six countries and four other flaviviruses (Kunjin, Japanese encephalitis, St. Louis encephalitis, and yellow fever viruses) was reverse-transcribed (RT) and amplified by PCR. The nucleotide sequences of the amplified products were determined by a rapid, automated DNA sequencing method. The WN virus RT/PCR assay detected the target gene segment of sequencing method. The WN virus RT/PCR assay detected the target gene segment of isolates from both the African-Middle Eastern group and the Indian group with a sensitivity of approximately 0.05 pg of viral RNA. Kunjin virus was the only other flavivirus tested that produced a band of the appropriate size. Five of seven WN virus isolates showed 92-98% homology in the nucleotide sequence of their PCR products. The sequence of one isolate was virtually identical to the published sequence of the Nigerian isolate (99.5% homology). No correlation was established between the degree of nucleotide homology, geographic location, time of isolation, or source of the isolates.

Increased immunogenicity and protective efficacy in outbred and inbred mice by strategic carboxyl-terminal truncation of Japanese encephalitis virus envelope glycoprotein

We constructed recombinant vaccinia viruses expressing the full-length envelope (E) glycoprotein of Japanese encephalitis virus (JEV) or a strategically truncated E glycoprotein, approximately 80% of the N-terminal sequence, and compared their antigenic structure and protective immunity in mice. The truncation site in the JEV E glycoprotein sequence corresponds to the position that had been shown to increase the immunogenicity of dengue type 4 or type 2 virus E glycoprotein. Analysis of the JEV E glycoprotein in recombinant virus-infected cells showed that C-terminally truncated E retains an antigenic structure similar to that of the full-length E glycoprotein. The full-length JEV E glycoprotein was detected predominantly intracellularly, while a small fraction (< 2%) was present on the cell surface. On the other hand, the truncated 80% E glycoprotein exhibited an alteration in the intracellular transport pathway resulting in increased accumulation (10-25%) on the cell surface and secretion (6-10%) into the medium. The C-terminally truncated E glycoprotein induced a greater antibody response and a higher level of protective immunity than did the full-length E glycoprotein in outbred CD-1 mice as well as in two strains of inbred mice that differ in their resistance to intraperitoneal (ip) JEV infection. In the case of outbred CD-1 and inbred C57/Bl mice, which possess a dominant autosomal genetic locus that controls resistance to a high dose of ip infection of JEV or the capacity to acquire resistance to intracerebral JEV infection, truncated E glycoprotein induced a higher titer of JEV neutralizing antibodies.

Comparison of replication rates and pathogenicities between the SA14 parent and SA14-14-2 vaccine strains of Japanese encephalitis virus in mouse brain neurons

The replication rates and pathogenicities of the SA 14 parent and SA 14-14-2 vaccine strains of Japanese encephalitis (JE) virus in neurons of the mouse brain following intracerebral inoculation were compared. All the mice inoculated with the SA 14 parent strain died within one week postinoculation (p.i.), whereas all the mice inoculated with the SA 14-14-2 vaccine strains survived without showing any signs of central nervous system (CNS) involvement. The virus titers of the mouse brains inoculated with the SA 14 strain reached progressively higher levels until day 5 when the animals died. On the other hand, the virus titers of the mouse brains inoculated with the SA 14-14-2 strain persisted at low levels for several days and could not be detected after 10 days. In the routine electron microscopical study, a majority of neurons in the mouse brains inoculated with the SA 14 strain contained virions and showed characteristic cytopathological changes in connection with viral replication. In the brains inoculated with the SA 14-14-2 strain, however, we failed to find neurons containing virions or showing characteristic cytopathological changes. In the alkaline phosphatase immunostaining of paraffin-embedded sections, a majority of neurons in the brains of mice inoculated with the SA 14 strain stained positively on day 5 p.i., but only a small number of neurons in scattered small foci stained positively in the brains inoculated with the SA 14-14-2 strain. The immunogold staining of Vibratome sections also revealed the identical patterns; moreover, electron microscopical examination of the immunopositive foci of the brain inoculated with the vaccine strain revealed neurons that contained virions in dilated cisternae of rough endoplasmic reticulum (RER), indicating that the SA 14-14-2 strain also replicated, albeit poorly, in neurons. The present results showed that upon intracerebral inoculation into mice the SA 14 parent strain of JE virus grew vigorously in a large number of neurons, killing the animals, while the SA 14-14-2 vaccine strain grew poorly only in a small number of neurons without causing mortality. Possible mechanisms involved in the alteration of pathogenicity between the SA 14 parent virus and the SA 14-14-2 vaccine virus are discussed.

Cytopathology of PC12 cells infected with Japanese encephalitis virus

Infection of a clonal rat pheochromocytoma cell line, PC12, with Japanese encephalitis (JE) virus produced successively higher titers of virus in the culture fluid during the 72-h experimental period. In electron microscopical observation, JE virus entered PC12 cells by direct penetration through the plasma membrane at 2 min postinoculation (p.i.) and caused marked cellular hypertrophy and extensive proliferation of the cellular secretory system including rough endoplasmic reticulum (RER) and Golgi complexes starting 24 h p.i. The proliferating RER of the virally infected cells contained progeny virions and characteristic endoplasmic reticulum vesicles in its cisternae, and the proliferating Golgi complexes contained virions in their saccules. These findings indicated that the proliferation of the cellular secretory system occurred in association with viral replication and maturation in the system. Seventy-two hours p.i., the cellular secretory system of infected PC12 cells showed degenerative changes with vesiculation, disorganization, and dispersion of the Golgi complexes and fragmentation, focal cystic dilation, and dissolution of the RER in the same manner as those seen in the secretory system of JE-virus-infected neurons in the mouse brain. Thus, JE-virus-infected PC12 cells seem to be a suitable neurogenic cell line for the study of the pathogenic mechanism of JE virus. At the same time, the virally infected cells seem to offer an interesting cell model for the study of the morphogenesis of the cellular secretory system.

Comparative study of mouse brains infected with Japanese encephalitis virus by intracerebral or intraperitoneal inoculation

The brains of mice infected with Japanese encephalitis (JE) virus by intracerebral inoculation (IC), intraperitoneal inoculation with sham intracerebral inoculation (IP+sIC), and intraperitoneal inoculation (IP) were studied by light and electron microscopy. The mortality rates and mean survival days were 100% and 4.8 days for the IC group, 92% and 9.0 days for the IP+sIC group, and 58% and 13.4 days for the IP group. Accordingly, the brain samples of sick mice were examined by light and electron microscopy 4 days post-inoculation (p.i.) for the IC group, 7 days p.i. for the IP+sIC group and 12 days p.i. for the IP group. In light microscopy, the mouse brains in the IC group showed little inflammatory change with only mild generalized glial-cell proliferation and mononuclear cell infiltration. In electron microscopy, however, a majority of neurons in the brain were seen to be infected with virus that replicated exclusively in the neuronal secretory system, including rough endoplasmic reticulum (RER) and the Golgi apparatus. In contrast, light microscopic observation of the brains from the IP+sIC and the IP groups showed prominent inflammatory changes with leucocytic infiltration and perivascular cuffing. Neuronal degeneration and neuronophagia were also prominent. In electron microscopy, neurons were infected in the same manner as in the IC group, but showed more advanced degenerative changes with marked cytoplasmic rarefaction and frequent neuronal disintegration. Mononuclear cells were frequently found in direct contact with degenerating and disintegrating neurons. The results showed that (a) the basic process of JE virus replication in brain neurons was present in the three groups of mice, (b) in the peripherally inoculated mice the process was accompanied by inflammatory reaction with resultant neuronal destruction, and (c) breach in the blood-brain barrier at the time of peripheral viral inoculation played an important role in the viral invasion of the CNS.

Entry and replication of Japanese encephalitis virus in cultured neurogenic cells

The entry mode and growth pattern of Japanese encephalitis (JE) virus in mouse neuroblastoma N18TG2 cells and mouse neuroblastoma x rat glioma NG108-15 hybrid cells were studied by electron microscopy. At two minutes after inoculation, JE virions adsorbed onto and directly penetrated through the plasma membrane of the hybrid cells, whereas virions did not adsorb nor entered the neuroblastoma cells. Correspondingly, the hybrid cells showed assembling progeny JE virions in the cisternae of rough endoplasmic reticulum (RER) 1 day postinoculation (p.i.) although virions were rarely found on the following days during the experiment. On the other hand, progeny virions did not assemble in the RER cisternae of the neuroblastoma cells throughout the experiment. The morphologic observations, therefore, suggest that (a) the hybrid cells express JE-virus receptors which facilitate the viral attachment onto and entry into the cells, while the neuroblastoma cells do not and (b) JE virus replicates very poorly after the entry into the hybrid cells while it does not replicate at all in the neuroblastoma cells. The virus titrations of the media of the neuroblastoma and hybrid cell cultures showed only titers indicative of residual virus of the inoculum that progressively decreased during the experiment. The present results show therefore that of the two neurogenic cell culture lines studied only the hybrid cell line can be used for the study of viral entry and replication, although it is not suited for virus production. Possible reasons for the poor replication of JE virus in the hybrid cells are discussed.

Expression of the envelope antigen of dengue virus in vaccine strains of Salmonella

The envelope gene of dengue 4 virus (DEN) was cloned in a plasmid under the control of Escherichia coli expression signals. A clone that expressed 93% of the gene was found to be detrimental to the bacterial host. Another clone which carried only 76% of the E gene was found to be quite stable in vitro as well as in vivo. The killed recombinant bacteria induced antibodies in mice which recognized native DEN virus. Attenuated Salmonella typhimurium (SAL) strains carrying the DEN-E plasmid were tested for their efficacy as orally administered live vaccines. Protective immunization was assessed in a mouse model by immunizing three-week old BALB/c mice followed by challenge with DEN virus. It was found that these young mice were highly susceptible to the carrier SAL strains (M206 and aroA SL3261). Moreover, the SAL-infected mice were more susceptible to DEN virus challenge than control mice, suggesting that the SAL infection caused immunosuppression in these young mice.

Preparation of an attenuated dengue 4 (341750 Carib) virus vaccine. II. Safety and immunogenicity in humans

To determine safety and immunogenicity, a single 0.5 ml dose of a monovalent live-attenuated dengue (DEN) 4 (341750 Carib) vaccine was given sc to 3 groups of flavivirus nonimmune volunteers in increasing concentrations. Two recipients received 10(3) plaque forming units (PFU)/dose (1:100 dilution of stock vaccine). One remained asymptomatic, but became viremic between days 12 and 15, experienced a mild elevation of temperature (37.4 degrees C), and developed DEN-4 specific antibody. Neither recipient of the 10(4) PFU became infected. Eight volunteers then received undiluted vaccine (10(5) PFU). Viremia and antibody (neutralizing, hemagglutination inhibition, and IgM) developed in 5 of the 8 (63%). These 5 volunteers also developed a scarcely noticeable macular, blanching rash and minimal temperature elevations (37.3, 38.1, 37, 37.9, and 37.9 degrees C). Clinically insignificant decreases in total white blood cell, lymphocyte, and polymorphonuclear cell counts and an elevation in mononuclear cell counts occurred in association with viremia. This vaccine is safe, reasonably immunogenic, and suitable for further evaluation.

Preparation of an attenuated dengue 4 (341750 Carib) virus vaccine. I. Pre-clinical studies

Dengue 4 (DEN-4) virus strain 341750 Carib was modified by serial passage in primary canine kidney (PCK) cell cultures. By the 15th PCK passage, this virus was less infectious for monkeys and resulted in a significantly reduced viremia as compared to the parent DEN-4 virus. The 30th PCK passage of DEN-4 341750 Carib was non-infectious for monkeys. A vaccine prepared at the 20th PCK passage in DBS-FRhL-2 cells stimulated the production of both neutralizing and hemagglutination inhibition antibodies in monkeys; these animals were also protected against challenge with the homologous strain as well as a heterologous strain of DEN-4. An ID50 titration in monkeys resulted in a titer of greater than 10(4) plaque-forming units (PFU) for the vaccine virus and 0.5 PFU for the parent virus. Reduced monkey infectivity of this magnitude has been correlated with human attenuation in previous dengue vaccine candidates. The DEN-4 strain 341750 Carib PCK-20/FRhL-4 vaccine has been characterized and sufficiently tested to be considered for safety and immunogenicity trials in humans.

Ultrastructural changes of mouse brain neurons infected with Japanese encephalitis virus

Ultrastructural changes of mouse brain neurons infected intracerebrally with Japanese encephalitis (JE) virus were studied. JE virus selectively infected the neurons, causing ultrastructural changes in association with viral replication in the cellular secretory system, principally involving rough endoplasmic reticulum (RER) and the Golgi apparatus. In the early phase of infection, RER of infected neurons showed hypertrophic changes, containing assembling virions within its dilated cisternae. In the later phase, the RER became cystic and degenerative and dissolved into the cytoplasm. The Golgi apparatus also contained in its saccules multiple virions, presumably transported from the RER cisternae, which were then released into the cytoplasm within coated vesicles for secretory-type exocytosis. In the process, the Golgi apparatus also fragmented and degenerated through vesiculation, vacuolation, and dispersion. Thus, the JE virus infection of neurons resulted in obliteration of RER and the Golgi apparatus, leaving behind the rarefied cytoplasm devoid of these organelles. However, destruction of the neurons themselves was not prominent and infected neurons in the later phase of infection showed some regenerative changes of these membranous organelles. The cause of death of infected animals, therefore, appeared to be extensive neuronal dysfunction rather than neuronal destruction in the CNS.

Preparation of noninfectious hepatitis A virus hemagglutinin for detecting hemagglutination inhibition antibodies

Hepatitis A virus (HAV) harvested from infected MRC-5 cells can hemagglutinate various species of erythrocytes at acid pH (Eckels et al., 1989). Further studies revealed that the majority of the hemagglutinin (HA) in MRC-5 and BS-C-1 cells was cell-associated. A simplified procedure for preparing HAV-HA consisted of collecting infected cells in phosphate-buffered saline followed by three cycles of freeze-thawing and sonication. The fluids were clarified and stored at 4 degrees C. The analysis of HA by rate-zonal sucrose gradient centrifugation indicated that the majority of HA co-migrated with infectious virus. Complete inactivation of infectious HAV with 0.03% beta-propiolactone (BPL) did not affect HA activity, while inactivation with 0.05% formalin caused a 16-fold reduction in titer. There was no difference in HAI antibody titers when BPL-treated HA was compared to untreated HA in the hemagglutination inhibition (HAI) test.

Monoclonal antibodies for dengue virus prM glycoprotein protect mice against lethal dengue infection

Five murine monoclonal antibodies (Mabs) reactive against the prM glycoproteins of DEN-3 and -4 were used to passively protect mice in vivo against lethal challenge with homologous and heterologous dengue virus serotypes. Four of the 5 prM-reactive monoclonals cross-protected mice against heterologous challenge, whereas 1 protected against challenge with only the homologous serotype. Although in vitro binding to virions was readily demonstrated, only 2 of the prM Mabs had detectable neutralizing activity. The neutralizing activity could not be enhanced by anti-mouse immunoglobulin or complement. However, 4 of the 5 prM Mabs fixed complement. This is the first report of prM-specific Mabs that are protective in mice.

Hepatitis A virus hemagglutination and a test for hemagglutination inhibition antibodies

Like enteroviruses, hepatitis A virus (HAV) hemagglutinated various species of erythrocytes under similar conditions. HAV-specific antibodies in both acute- and convalescent-phase sera were found to inhibit hemagglutination. The HAV hemagglutination inhibition test can be used for diagnosis, epidemiological surveillance, and vaccine assessment.

Flaviviruses can mediate fusion from without in Aedes albopictus mosquito cell cultures

Flavivirus-induced polykaryocytes were detected in monolayers of Aedes albopictus (clone C6/36) mosquito cells as early as 20 min after adsorbing virus to these cells. A high multiplicity of infection with dengue (DEN)-1, 2, 3, 4, Japanese encephalitis, and yellow fever viruses was required to demonstrate fusion from without (FFWO) with these flaviviruses. Optimal conditions for FFWO included exposure of adsorbed virus to pH 6.0 and an incubation temperature of 39 degrees C. DEN-2 monoclonal antibodies to the envelope E glycoprotein inhibited cell fusion, whereas monoclonal antibodies to the prM and NS1 proteins did not inhibit cell fusion. These results indicate that flaviviruses cause FFWO soon after adsorption to C6/36 mosquito cells and the process is most likely mediated by the virion envelope E glycoprotein.

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.

A comparative study of entry modes into C6/36 cells by Semliki Forest and Japanese encephalitis viruses

The entry modes of Semliki Forest virus and Japanese encephalitis virus into C6/36 cells were compared by electron microscopic observation. At physiological pH, the two viruses showed characteristically different entry modes. Following attachment to the plasma membrane, many SF virions appeared within plasma membrane invaginations and cytoplasmic vesicles; on the other hand, JE virions remained to be found exclusively at the cell surface, with no virions appearing within cytoplasmic vesicles. Electron microscopic observation, therefore, indicated that SF virus entered C6/36 cells by receptor-mediated endocytosis, while JE virus penetrated the cells at the surface and disintegrated at or near the adsorption sites. At pH 5.8, SF virus also entered C6/36 cells by direct penetration at the cell surface. On the basis of the present and other findings, the following working hypotheses are presented for future investigations: (a) at physiological pH, the fusion protein of SF virus is in an inactive state and needs to be activated by acidic pH within the endosome in order to act on the host-cell membrane, but that of JE virus is in an active state and is capable of dissolving the host plasma membrane at the cell surface immediately after the attachment; (b) the states of viral fusion proteins (inactive or active) at the time of viral attachment to the cell surface determine which of the two entry modes these viruses follow.

Maturation process of Japanese encephalitis virus in cultured mosquito cells in vitro and mouse brain cells in vivo

The maturation process of Japanese encephalitis (JE) virus in C6/36 cells in vitro and in mouse brain cells in vivo was studied by electron microscopy. In the C6/36 cell infection, 500 to 2250 virions per cell were released into the medium during the period of study; yet, no virus budding process was observed at the host cell membranes. JE virions at various maturation stages appeared within the cisternae of rough endoplasmic reticulum (RER) of infected cells at 24 hours p.i.; and, although C6/36 cells did not show a well-developed Golgi apparatus, the virions appeared to be carried to the cell surface within host-cell secretory vesicles for extracellular release as early as 24 hours p.i. The occurrence of a secretory-type intracellular transport of maturing JE virus particles was well recognizable in brain cells of infected mice, in which JE virus particles were found almost exclusively in the cisternae of RER, in the Golgi apparatus, and in various vesicles, including coated vesicles, in the vicinity of the Golgi apparatus. Our previous study of dengue-2 virus morphogenesis and our present study of JE virus morphogenesis differed substantially at various stages of maturation. Possible mechanisms which explain these differences were discussed.

An electron and immunoelectron microscopic study of dengue-2 virus infection of cultured mosquito cells: maturation events

The maturation process of dengue-2 virus in C6/36 mosquito cells was studied by electron microscopy at 12, 16, 24, 48, and 78 hours postinoculation (p.i.) and by immunoelectron microscopy at 48 and 78 hours p.i. Maturing virions appeared within cytoplasmic vacuoles and on the surface of infected cells from 24 hours p.i. onward in close topographical relationship to the dense particles that occurred concurrently in the cytoplasm. The dense particles measured 25 to 35 nm in diameter; the mature virions measured 50 to 55 nm in diameter, with a dense core measuring 30 to 35 nm in diameter covered by a 10 nm-thick membrane envelope. The morphological observations indicated that the dense particles were dengue nucleocapsids assembled in the cytoplasm and that they apparently budded into the vacuolar lumens and the extracellular space at the vacuolar and plasma membranes, acquiring membrane envelopes and becoming mature virions in the process. The virions that budded into the vacuolar lumens were released extracellularly by exocytosis. In the samples tested with dengue-2 polyclonal antibodies, intense immunostaining occurred at the sites of virus budding on the cell surface; host cell membrane and cytoplasm adjacent to the budding virions stained less intensely. In the samples tested with a dengue-2 monoclonal antibody specific for the envelope glycoprotein, budding virions stained rather exclusively, with no staining occurring in adjacent host membrane or cytoplasm.

Monoclonal antibodies against dengue 2 virus E-glycoprotein protect mice against lethal dengue infection

A panel of 11 murine monoclonal antibodies directed against dengue type 2 was evaluated for antigen specificity by dot immunobinding assay and Western blot analysis and for in vitro and in vivo biological activities. Nine of the 11 monoclonal antibodies reacted with viral E-glycoprotein based on the Western blot analysis; one reacted with a 36 Kd protein present in dengue-infected C6/36 mosquito cells. The nine E-glycoprotein-reactive monoclonal antibodies also neutralized dengue 2 virus in a plaque reduction assay. Of the neutralizing monoclonal antibodies, five passively protected mice in vivo against lethal intracerebral dengue 2 challenge. The protective monoclonal antibodies were directed against viral determinants that fell into at least three spatially separate families of epitopes on E-glycoprotein, the antigenicities of which were preserved after heat/detergent denaturation.

Lack of attenuation of a candidate dengue 1 vaccine (45AZ5) in human volunteers

A dengue type 1, candidate live virus vaccine (45AZ5) was prepared by serial virus passage in fetal rhesus lung cells. Infected cells were treated with a mutagen, 5-azacytidine, to increase the likelihood of producing attenuated variants. The vaccine strain was selected by cloning virus that produced only small plaques in vitro and showed reduced replication at high temperatures (temperature sensitivity). Although other candidate live dengue virus vaccines selected for similar growth characteristics have been attenuated for humans, two recipients of the 45AZ5 virus developed unmodified acute dengue fever. Viremia was observed within 24 hr of inoculation and lasted 12 to 19 days. Virus isolates from the blood produced large plaques in cell culture and showed diminished temperature sensitivity. The 45AZ5 virus is unacceptable as a vaccine candidate. This experience points out the uncertain relationship between in vitro viral growth characteristics and virulence factors for humans.

Selection of attenuated dengue 4 viruses by serial passage in primary kidney cells. V. Human response to immunization with a candidate vaccine prepared in fetal rhesus lung cells

A dengue 4 (strain H241, PDK35-TD3 FRhL p3) vaccine attenuated by passage in primary dog kidney cells followed by passage and final vaccine preparation in DBS-FRhL-2 cells was tested in five yellow fever-immune volunteers. Only two volunteers seroconverted by producing hemagglutination-inhibiting and neutralizing antibodies. Mild illness, compatible with dengue infection was found only in the individuals who later developed antibodies. Both volunteers developed a rash by the 8th day following vaccination, coinciding with a slight elevation in temperature and leukopenia. Additionally, several serum enzymes were elevated during the observation period. Dengue 4 virus was isolated from the blood of the two infected volunteers starting as early as day 5 post vaccination. During the viremic period, which lasted 5 days, phenotypically-changed virus was recovered, indicating genetic instability of the vaccine virus. The clinical disease and immune response in the two infected individuals was probably related to replication of the variant virus. Further testing of this vaccine in its present form is not indicated.

Dengue virus type 2 vaccine: reactogenicity and immunogenicity in soldiers

A live dengue virus type 2 (dengue-2) vaccine (PR-159/S-1) was tested for reactogenicity and immunogenicity in a placebo-controlled, double-blind clinical trial involving 98 soldiers. Seroconversion rates based on the development of neutralizing antibody to dengue-2 were 90% in 70 recipients with immunity to yellow fever and 61% in 28 vaccinees without such immunity (P less than .01). Peak titers of neutralizing antibody were three times higher in recipients with antibody to yellow fever virus and persisted in most for at least 18 months. Individuals seroconverting to the vaccine virus more frequently experienced systemic symptoms than those who received placebo (P less than .02). Future users of this dengue-2 vaccine may wish to employ immunization schedules that include preliminary immunization against yellow fever and must be prepared to accept mild vaccine-related symptoms in some recipients.

Antibody response to dengue-2 vaccine measured by two different radioimmunoassay methods

Two different radioimmunoassays were used to detect virus-specific antibodies in sera from human volunteers inoculated with an attenuated dengue type 2 (DEN-2) vaccine (PR-159/S-1). An indirect radioimmunoassay required purified DEN-2 virions for optimal reactivity but was 10 to 500 times more sensitive than neutralization or hemagglutination inhibition tests. An antibody capture radioimmunoassay was able to utilize crude antigens from either DEN-infected mouse brains or Aedes albopictus cell culture supernatants. When the two radioimmunoassay techniques were compared, the indirect method appeared to be the best assay for immunoglobulin G (IgG), whereas the antibody capture method was more sensitive for IgM detection. Selected human sera were examined for IgG, IgM, and IgA responses by using both techniques at various intervals after immunization. Although there were differences in magnitude, yellow fever immune as well as flavivirus nonimmune volunteers responded to DEN-2 vaccination by demonstrating IgG, IgM, and IgA antibody responses. In the nonimmune group, the most prevalent immunoglobulin detected was IgM, whereas in the yellow fever immune group, the predominant post-DEN-2 vaccine immunoglobulin was IgG. The preponderance of DEN-2-specific neutralizing antibodies were associated with either IgM or IgG according to the immune status of the volunteer. All classes of immunoglobulins attained maximum levels between 21 and 60 days postvaccination. In the majority of volunteers, IgM responses were relatively transient and could not be detected 6 months after immunization, whereas IgG and IgA antibodies were still detectable after this period.

Dengue 2 vaccine: dose response in volunteers in relation to yellow fever immune status

A live dengue 2 vaccine was tested in 38 volunteers in an evaluation of the safety, infectivity, and immunogenicity of doses of 10(1.8)-10(5.5) plaque-forming units. Twenty yellow fever-immune and 18 yellow fever-nonimmune individuals received 0.5 ml of vaccine sc. Immunization was dose related in yellow fever-immune volunteers, with a 50% immunizing dose of 10(3.3) plaque-forming units. In the group not immune to yellow fever, some but not all recipients of each vaccine dilution were immunized, and no 50% immunizing dose could be estimated. Volunteers immune to yellow fever developed adequate titers of neutralizing antibody to dengue 2 virus and maintained them for at least three years; those not immune to yellow fever developed lower antibody titers that disappeared within six months in half of the cases. More than 40 isolates of dengue 2 virus from 12 volunteers retained the in vitro growth characteristics of the vaccine virus; this result affirmed the genetic stability of the virus. Common clinical signs in immunized individuals were leukopenia (55%), macular rash (15%), and fever (10%).

Temperature-sensitive events during the replication of the attenuated S-1 clone of dengue type 2 virus

Temperature-sensitive events occurring during the replication of the attenuated S-1 clone of dengue type 2 virus were examined. The S-1 clone was more thermolabile than the parent virus at the nonpermissive temperature of 38.5 degrees C. Adsorption experiments in fetal rhesus monkey lung cells revealed an inefficient adsorption of S-1 at 38.5 degrees C compared with the parent virus, suggesting an alteration in a thermolabile virion protein important in adsorption. The production of S-1 viral RNA and antigen occurred at the nonpermissive temperature, which indicated that early events in the replication cycle of S-1 were not affected. Release of infectious virus at 38.5 degrees C was not impaired; however, lower amounts of infectious virus in infected cells at the nonpermissive temperature indicated that maturation of the S-1 clone was suppressed.

Antibody-dependent cell-mediated antibacterial activity: K lymphocytes, monocytes, and granulocytes are effective against shigella

Normal human peripheral blood lymphocytes, monocytes, and granulocytes decreased the viability of Shigella flexneri in vitro in cooperation with heat-inactivated rabbit hyperimmune anti-shigella antisera. Purified K and T lymphocyte populations used before the removal of Fc receptor-bearing cells were effective in this antibody-dependent complement-free system. In contrast, B lymphocytes and T lymphocytes from which Fc receptor-bearing cells were removed exhibited no activity. In the absence of antisera, lymphocytes and monocytes were ineffective whereas granulocytes were only moderately effective even when cells from patients recently infected with shigella were used. Convalescent (but not pre-infection) sera from these patients, however, induced marked cell-mediated antibacterial activity against their homologous infecting shigella whether the cells were from uninfected or infected individuals. These data suggest that Fc receptor-bearing lymphocytes, monocytes, and granulocytes play a role in host immune defense against enteric pathogens such as shigella even in locations where complement may be functionally deficient such as among secretory mucosal surfaces.

Antibody-dependent cell-mediated antibacterial activity: K lymphocytes, monocytes, and granulocytes are effective against shigella

Normal human peripheral blood lymphocytes, monocytes, and granulocytes decreased the viability of Shigella flexneri in vitro in cooperation with heat-inactivated rabbit hyperimmune anti-shigella antisera. Purified K and T lymphocyte populations used before the removal of Fc receptor-bearing cells were effective in this antibody-dependent complement-free system. In contrast, B lymphocytes and T lymphocytes from which Fc receptor-bearing cells were removed exhibited no activity. In the absence of antisera, lymphocytes and monocytes were ineffective whereas granulocytes were only moderately effective even when cells from patients recently infected with shigella were used. Convalescent (but not pre-infection) sera from these patients, however, induced marked cell-mediated antibacterial activity against their homologous infecting shigella whether the cells were from uninfected or infected individuals. These data suggest that Fc receptor-bearing lymphocytes, monocytes, and granulocytes play a role in host immune defense against enteric pathogens such as shigella even in locations where complement may be functionally deficient such as among secretory mucosal surfaces.

Dengue-2 vaccine: preparation from a small-plaque virus clone

The S-1 clone of dengue type 2 virus was used for the preparation of a live-attenuated vaccine after passage in DBS-FRhL-2 cell culture. The vaccine virus had a relatively higher replicative capacity at superoptimal temperatures than its precursor virus, S-1, passaged in primary green monkey kidney cells (S-1 PGMK). There was also a tendency for the S-1 vaccine virus to exhibit leakiness at increased temperatures. Another in vitro marker, replication in monkey peripheral blood leukocytes, indicated less host restriction for the S-1 vaccine in comparative assays with S-1 PGMK virus. Mouse virulence appeared to remain stable on passage in DBS-FRhL-2 cells, whereas monkey immunogenicity decreased. Cautious trials of the dengue type 2 S-1 vaccine in humans are indicated.