Protection of Mice Against Dengue 2 Virus Encephalitis by Immunization with the Dengue 2 Virus Non-structural Glycoprotein NS1

Transcription

1 J. gen. Viral. (1987), 68, Printed in Great Britain Key words: dengue virus/non-structural protein/vaccine 853 Protection of Mice Against Dengue 2 Virus Encephalitis by Immunization with the Dengue 2 Virus Non-structural Glycoprotein NS1 By JACOB J. SCHLESINGER,* MICHAEL W. BRANDRISS AND EDWARD E. WALSH Department of Medicine, the Rochester General Hospital and the University of Rochester School oj Medicine and Dentistry, Rochester, New York 14621, U.S.A. (Accepted 20 November 1986) SUMMARY Immunization of mice with the dengue 2 virus (DEN 2)-specified non-structural protein NS1 provided significant protection against intracerebral challenge with the virus in the absence of detectable neutralizing or other anti-virion antibody. NS1, purified from lysates of infected Vero cells by immunoaffinity chromatography, expressed an antigenic site(s) common to each of the four DEN serotypes, and hyperimmunization of rabbits with NS1 stimulated production of complement-fixing (CF) antibody with broad DEN serotype specificity. However, cross-protection was not observed: mice immunized with DEN 2 NS1 developed little or no heterologous CF antibody and were not protected against challenge with neurovirulent DEN 1. Induction of a protective immune response by NS1 suggests that it be considered for incorporation into possible synthetic or recombinant DNA DEN vaccines. Among the mosquito-borne flaviviruses of medical importance, the dengue viruses (DEN) are notable for their global distribution and the frequency of large-scale epidemics caused by them (Halstead, 1980). Attempts to produce a satisfactory DEN vaccine by conventional methods have met with limited success. Attenuated DEN vaccines have been prepared by intracerebral passage in mice (Sabin & Schlesinger, 1945: Schlesinger et al., 1956) but the undesirability of brain tissue in human vaccine preparations has restricted their use. More recently, tissue culture-derived attenuated DEN viruses have been considered as candidate vaccines but inconsistent neutralizing antibody responses as well as occasional genetic instability of the variants have raised doubts about their usefulness (Bancroft et al., 1984; Eckels et al., 1984). The chemical synthesis of viral polypeptides or their preparation by recombinant DNA methods may provide novel alternatives for the development of a satisfactory DEN vaccine. The choice of viral proteins for incorporation into such a vaccine will depend on their capacity to stimulate a protective immune response. Although immunity to flavivirus infection has generally been related to the presence of neutralizing antibodies against the virion envelope glycoprotein, we have recently shown that immunization with a yellow fever virus (YF) non-structural (NS) glycoprotein, gp48, protects mice and monkeys against lethal YF infection (Schlesinger et al., 1985, 1986). This protein, currently designated NS1 (Rice et al., 1985), is highly conserved among flaviviruses and is the soluble complement-fixing (CF) antigen which is expressed on the surface of DEN-infected cells (Cardiff &Lund, 1976; Smith & Wright, 1985). The present report describes antigenic characteristics of dengue 2 virus (DEN 2) NS1 and protection against intracerebral DEN 2 challenge in mice immunized with this protein. Immunoaffinity-purified NS1 was prepared from lysates of DEN 2 (New Guinea-C)-infected Vero cells by modification of a previously described column chromatography method (Schlesinger et al., 1985). Briefly, infected cells grown in roller bottles or in suspension culture on Cytodex beads (Pharmacia) were solubilized in a non-denaturing buffer containing 0.l M-Tris- HC1 ph 7.5, 0.15 M-NaCI, 1~ Triton X-100, 1~o NP40, and 1 mm-phenylmethysulphonyl SGM

2 854 Short communication (a) 1 2 (b) U 68-- U m --E 45- ~ ~ ~ ~.~ --NS1 Fig. 1. (a) Silver stain of purified DEN 2 NS1. Proteins were electrophoresed on an IlK SDSpolyacrylamide gel. Lane 1, mol. wt. markers; lane 2, 10 gg DEN 2 NS1. (b) Immunoprecipitation of [35S]methionine-labelled DEN 2-specific proteins. Labelled DEN 2-infected Vero cell lysates were precipitated with sera from ovalbumin-immunized mice (lane 1), sera from NSl-immunized mice (lane 2), anti-den 2 NS1 monoclonal antibody 9A9 (lane 3) or mouse anti-den 2 hyperimmune ascites (lane 4). fluoride. Monoclonal antibodies directed against DEN 2 NS1 were partially purified by ammonium sulphate precipitation from mouse ascitic fluid and coupled to cyanogen bromideactivated Sepharose 4B beads for use as a ligand. Two hybridomas (9A9, 9G5) secreting IgG1 antibody against DEN 2 NSI were produced by fusing non-secretor X63-Ag8-653 myeloma cells with spleen cells from BALB/c mice immunized with DEN 2 (New Guinea-C)-infected suckling mouse brain (SMB). An additional anti-den 2 NS 1 IgG 1 monoclonal antibody, D7-3E9-4, and an anti-den 2 envelope protein monoclonal antibody, 4G2, prepared against the New Guinea- C strain of DEN 2 (Gentry et al, 1982), were provided by Dr M. K. Gentry (WRAIR, Washington, D.C., U.S.A.). NS1 was eluted with 0-1 u-glycine buffer ph 2-5 and samples were immediately buffered to ph 7.0 with Tris-HCl followed by exhaustive dialysis against phosphate-buffered saline (PBS) ph 7.4. Fractions were analysed for NS1 by SDS-PAGE followed by silver staining (Bio-Rad). The protein content of pooled samples was estimated by a Coomassie Brilliant Blue technique (Bio-Rad) using bovine serum albumin as a standard. DEN 2 NS1 migrated as a single band of approximately mol. wt. after SDS-PAGE (Fig. 1 a). To evaluate the protective capacity of NS1 immunization, 3-week-old Swiss CD1 mice (Charles River Breeding Laboratories, Wilmington, Mass., U.S.A.) were injected intraperitoneally (i.p.) with 20 gg purified NS1 emulsified in complete Freund's adjuvant followed by a second i.p. injection of 20 gg NS1 in PBS 2 weeks later. Control mice were injected with 20 p.g ovalbumin in the same schedule. An additional control group of 3-week-old mice was immunized with a single i.p. dose of 1 x 106 p.f.u. DEN 2 (New Guinea-C) grown in C6/36 Aedes albopictus cells. DEN 2 (New Guinea-C) passaged 35 times in SMB was provided as a

3 Short communication Table 1. Protection of mice against intracerebral challenge with DEN 2 by vaccination with DEN 2 NS1 Pre-challenge serum titre (reciprocal) A Antigen* Complementfixation Neutralization No. paralysed/ total No. dead/total DEN 2 (New Guinea-C) < /10 1/10 NSl/CFA OA/CFA 256 < 8 <20t < 20 6/33:~ 18/34 4/33~ 14/34 * Mice were given a single i.p. dose of 106 p.f.u. DEN 2 (New Guinea-C) or primed with 20 p.g NS1 or ovalbumin (OA) in complete Freund's adjuvant (CFA) and boosted with 20 gg of each protein in solution. t There was no plaque reduction at 1/20 serum dilution. :~ P < 0-02, Chi square. NSl-immunized against OA-immunized. 855 Table 2. Binding of anti-den antibodies to DEN 2 NS1 Immune ascites* At Polyclonal DEN 1 (Hawaii) DEN 2 (New Guinea-C) DEN 3 (PR6) DEN 4 (H241) Monoclonal anti-den 2 NS1 Monoclonal 9A anti-den E 4G * 1/100 final dilution of mouse immune ascites. t Bound alkaline phosphatase-labelled rabbit anti-mouse IgG. homogenate of virus-infected SMB by Dr W. E. Brandt (Fort Detrick, Frederick, Md., U.S.A.) and was passaged once in C6/36 A. albopictus cells for immunization of mice or used directly to challenge mice at 6 weeks of age. Mice were inoculated intracerebrally with 0.04 ml of a 1/100 dilution of virus-infected SMB, which was the dose determined to result in the highest morbidity and mortality in 6-week-old mice. Mice were bled from the orbit the day before challenge and the sera of each group were pooled. The results of challenge after immunization with infectious DEN 2, purified NS1 or negative ovalbumin control are shown in Table 1. Mice immunized with infectious DEN 2 developed neutralizing antibody (90~o plaque reduction in Vero cells at 1/640) and were protected. The proportion of mice that developed hindleg paralysis and died was significantly (P < 0.02) reduced by NS1 immunization. Sera taken pre-challenge from these mice were monospecific for NS1 as determined by radioimmunoprecipitation and SDS- PAGE (Fig. I b) and gave no plaque reduction but fixed complement in high titre (1/256) against SMB-derived DEN 2 antigen (Casey, 1965); there was little or no complement-fixing activity against other serotypes of DEN (DEN 1:1/8; DEN 3, 4: < 1/8). High titre neutralizing antibody (1/640) was present in the serum of surviving mice 5 weeks after challenge, indicating that virus replication had occurred. An antigenic determinant common to all four DEN serotypes has been shown by crossabsorption studies to be present on the DEN soluble complement-fixing antigen, e.g. NS1 (Russell et al., 1970). To test for the presence of DEN antigenic sites on NS 1, the relative binding to NS1 of DEN serotype-specific antibodies was measured by a conventional ELISA. The results are summarized in Table 2. Hyperimmune ascitic fluids prepared against all four DEN serotypes bound to NS1. A monoclonal anti-nsl antibody also bound whereas a monoclonal antibody against the DEN 2 envelope glycoprotein (E) did not. Concordant with these results, rabbits hyperimmunized with DEN 2 NS1 developed complement-fixing antibody in high titre against representative strains of each DEN serotype

4 856 Table 3. Short communication Complement-fixation titres (CF) after immunization of rabbits with DEN 2 New Guinea-C NSI* Suckling mouse brain (SMB) antigen DEN 1 DEN 2 DEN 3 DEN 4 Normal Hawaii NG-C H87 H241 SMB Rabbit no. 1 Pre-immunization CF 8t Post-immunization CF Rabbit no. 2 Pre-immunization CF Post-immunization CF * Rabbits were primed by combined intradermal and subcutaneous injections of about 20 ~tg DEN 2 NS1 in complete Freund's adjuvant (FA) and boosted three times with about 10 ~tg NS1 in incomplete FA at monthly intervals. Serum was collected 1 week after the last boost. Pre-immunization sera were slightly anticomplementary. t Reciprocal. (Table 3). These results suggested that immunization with DEN 2 NS1 might confer crossprotection against challenge with other serotypes of DEN but no such protection was observed among mice immunized with DEN 2 N S 1 following intracerebral challenge with neurovirulent Hawaii strain DEN 1 (provided by Dr R. E. Shope, YARU, New Haven, Conn., U.S.A.). We have shown that immunization with the DEN 2 glycoprotein NS1 confers partial protection in mice against DEN 2 encephalitis in the absence of detectable anti-virion antibody as measured by neutralization and immunoprecipitation assays. These results extend our earlier observation that immunization with YF NS 1 protects mice against YF encephalitis but differ in that protection by YF NS1 immunization was complete. Immunization with DEN 2 NS1 stimulated production of high-titred monospecific complement-fixing antibody. Such antibody might protect by promoting complement-mediated lysis of infected cells bearing surface NS 1. In earlier studies, the capacity of passively transferred anti-yf NS1 antibodies to protect mice against intracerebral YF inoculation correlated with their ability to fix complement and to lyse virus-infected cells (Schlesinger et al., 1985). It was, therefore, not surprising that mice passively immunized with the available non-complement-fixing IgG1 monoclonal antibodies to DEN 2 NSI were not protected (data not shown). Consistent with the results of an earlier cross-absorption study of DEN soluble complementfixing antigen (Russell et al., 1970), an antigenic site(s) common to each of the four DEN serotypes was demonstrated on NS1 by ELISA. Hyperimmunization of rabbits with DEN 2 NS1 stimulated production of antibody with broad DEN serotype reactivity but DEN 2 NS1 immunization failed to provide cross-protection in mice, perhaps because adequate levels of heterotypic complement-fixing antibodies did not appear in the relatively short interval between immunization and challenge. It has been shown that antibodies to DEN virion in subneutralizing concentrations may enhance replication of the virus in macrophages and it has been proposed that antibodies to one DEN serotype may predispose to more severe disease in the event of later infection with a different DEN serotype (Halstead & O'Rourke, 1977). Since immunization with DEN 2 NS1 does not elicit detectable anti-virion antibodies, enhancement of a subsequent dengue infection would be unlikely, The results of the present study suggest that further investigation of DEN NS1 as a potential vaccine component is warranted. It is possible that sequential vaccination with DEN NS1 and active DEN might result in subclinical infection with viral replication, production of neutralizing antibody and subsequent immunity. We thank Susan Chapman and Ronald Robinson for technical assistance, Dr Charles Calisher (Centers for Disease Control) for complement-fixation assays, and Joanne Prives for transcription of the manuscript. This work was funded in part by Grant GA HS 5852 from the Rockefeller Foundation and Grant V22/181/8 from the World Health Organization.

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