YF 17D Research Articles

Bivalent cholera and typhoid vaccine.

The live attenuated vaccine strains Vibrio cholerae CVD 103-HgR and Salmonella typhi Ty21a can be combined into an oral bivalent vaccine without compromising the immunogenicity of the individual vaccine strains. Seroconversion rates of 87 to 94% for Inaba vibriocidal antibodies and 72 to 91% for anti-S. typhi lipopolysaccharide antibodies (IgG or IgA) were reported in healthy European volunteers receiving a bivalent CVD 103-HgR/Ty21a vaccine-based schedule (bivalent vaccine on day 1 and monovalent Ty21a vaccine on days 3 and 5). The immunogenicity of bivalent CVD 103-HgR/Ty21a vaccine is not adversely affected by concomitant administration of mefloquine, yellow fever vaccine or oral polio vaccine. Chloroquine may reduce the immunogenicity of the CVD 103-HgR component and proguanil may reduce the immunogenicity of the Ty21a component. Bivalent CVD 103-HgR/Ty21a vaccine does not adversely affect the immunogenicity of the yellow fever YF 17D vaccine. The type, incidence and severity of adverse events seen in individuals receiving bivalent CVD 103-HgR/Ty21a vaccine-based schedules are similar to those that occur with the monovalent vaccines.

Trans-Complementation of yellow fever virus NS1 reveals a role in early RNA replication.

Mutational analysis of the nonstructural protein 1 (NS1) of yellow fever virus (YF) has implicated it in viral RNA replication. To further explore this observation, we sought a method for uncoupling NS1 function from NS1 expression and processing as part of the large YF polyprotein. Here we describe a strategy for providing NS1 in trans, utilizing a noncytopathic Sindbis virus vector. Replication of a defective YF genome containing a large in-frame deletion of NS1 was dependent on functional expression of NS1. Recovered mutant virus was shown to contain the deletion and was neutralized by YF-specific antiserum. Complemented mutant virus increased in titer with kinetics similar to those of parental YF 17D but peaked at lower titers. trans-complementation has allowed us to derive high-titer, helper-free stocks of YF defective in NS1 with which to further characterize the role of this gene product in RNA replication. The first cycles of RNA replication were analyzed by using a sensitive strand-specific RNase protection assay. We document these events for mutant and wild-type viruses in the presence or absence of complementation. These data strongly suggest a role for NS1 prior to or at initial minus-strand synthesis.

Conditions for haemolysis by flaviviruses and characterization of the haemolysin.

The 17D vaccine strain of yellow fever virus (YF 17D) was used to establish the optimal conditions for lysis of chick erythrocytes. Tissue culture-grown, polyethylene glycol-concentrated virus showed peak activity at pH 5.4 in citrate buffer when incubated at 37 degrees C. A further two- to fourfold increase in titre was obtained by pretreatment of the chick erythrocytes with 250 micrograms/ml trypsin. These conditions were also shown to be optimal for Japanese encephalitis (JE), West Nile (WN) and dengue-2 (den2) viruses. The ratio of haemagglutination (HA) titre to haemolysis (HL) titre approximated to unity, suggesting that the two functions are associated with the same molecule although as separable entities since selective inactivation of the HL activity of the virus was accomplished using 60 micrograms/ml trypsin. HL could be demonstrated at neutral pH if the chick erythrocytes were first subjected to treatment with acidic pH buffer. The effect on the virus envelope is thus not the sole contribution of a low pH environment to optimal HL. Hyperimmune rabbit antiserum prepared against purified YF 17D virions inhibited HA and HL if added before agglutination had occurred by the virus but when added after agglutination had taken place it showed specific anti-HL activity. Monoclonal antibodies that inhibited HA (HAI) by YF 17D did not inhibit HL (HLI) activity when applied after agglutination had taken place. Moreover, monoclonal antibodies specific for the 54K glycoprotein of YF virus but without HAI activity also had no effect on HL when added either before or after agglutination. As yet, we have been unable to identify a monoclonal antibody displaying specific anti-HL activity but all those directed against the 54K envelope glycoprotein possessing HAI activity showed HA to be a prerequisite for HL.