Live chimeric and inactivated Japanese encephalitis virus vaccines differ in their cross-protective values against Murray Valley encephalitis virus

Abstract

The Japanese encephalitis virus (JEV) serocomplex, which also includes Murray Valley encephalitis virus (MVEV), is a group of antigenically closely related, mosquito-borne flaviviruses that are responsible for severe encephalitic disease in humans. While vaccines against the prominent members of this serocomplex are available or under development, it is unlikely that they will be produced specifically against those viruses which cause less-frequent disease, such as MVEV. Here we have evaluated the cross-protective values of an inactivated JEV vaccine (JE-VAX) and a live chimeric JEV vaccine (ChimeriVax-JE) against MVEV in two mouse models of flaviviral encephalitis. We show that (i) a three-dose vaccination schedule with JE-VAX provides cross-protective immunity, albeit only partial in the more severe challenge model; (ii) a single dose of ChimeriVax-JE gives complete protection in both challenge models; (iii) the cross-protective immunity elicited with ChimeriVax-JE is durable (>or=5 months) and broad (also giving protection against West Nile virus); (iv) humoral and cellular immunities elicited with ChimeriVax-JE contribute to protection against lethal challenge with MVEV; (v) ChimeriVax-JE remains fully attenuated in immunodeficient mice lacking type I and type II interferon responses; and (vi) immunization with JE-VAX, but not ChimeriVax-JE, can prime heterologous infection enhancement in recipients of vaccination on a low-dose schedule, designed to mimic vaccine failure or waning of vaccine-induced immunity. Our results suggest that the live chimeric JEV vaccine will protect against other viruses belonging to the JEV serocomplex, consistent with the observation of cross-protection following live virus infections.

FIG. 1.

Susceptibility of B6 mice immunized with low and high doses of JE-VAX or ChimeriVax-JE to challenge with MVEV. Groups of 6-week-old B6 mice were immunized with three doses of undiluted (∼0.6 μg) or 100-fold-diluted JE-VAX s.c., given at 2-week intervals, or with one dose of 10⁵ or 10¹ PFU of ChimeriVax-JE i.v. At 14 weeks of age, mice were challenged with 10⁸ PFU of MVEV i.v. (A) Mortality. Groups of mice were monitored twice daily for morbidity and mortality for 21 days. (B) Anti-MVEV NS1-specific antibody titers in prechallenge sera and sera of surviving mice at 28 days post-MVEV challenge as an indirect measure of viral load. Horizontal bars show mean end point titers, the dotted line indicates the detection limit, and asterisks indicate significance of difference in antibody titer relative to the unvaccinated group of mice (*, P < 0.03; **, P < 0.003).

FIG. 2.

Comparison of the protective values of inactivated and live JEV vaccines against homologous and heterologous virus challenge in mice defective in IFN responses. Groups of 6-week-old mice were immunized with JE-VAX (three immunizations with different antigen concentrations, as indicated, given at 2-week intervals) or ChimeriVax-JE (one immunization with different virus doses, as indicated) or left untreated and challenged at 14 weeks of age with 10² PFU of JEV, MVEV, or WNV i.v. (A) IFN-α-R^−/− mice immunized with JE-VAX and challenged with JEV; (B) IFN-α-R^−/− mice immunized with JE-VAX and challenged with MVEV; (C) IFN-α-R^−/− mice immunized with ChimeriVax-JE and challenged with MVEV; (D) IFN-α/γ-R^−/− mice (deficient in both type I and type II IFN responses) immunized with ChimeriVax-JE and challenged with MVEV; (E) IFN-α-R^−/− mice immunized with ChimeriVax-JE and challenged with WNV (strain Kunjin). Mice were monitored twice daily for morbidity and mortality for 21 days.

FIG. 3.

Durability of humoral immunity induced with JE-VAX and ChimeriVax-JE. Groups of 6-week-old IFN-α-R^−/− mice were immunized three times at 2-week intervals with high doses (∼0.3 μg) of JE-VAX s.c. or with one dose of 10⁵ PFU of ChimeriVax-JE i.v. (A) At 28 weeks of age, prechallenge serum samples from individual mice in each group (n = 10) were collected and JEV-specific antibody titers determined; mean titers calculated from ELISA end points ± standard deviations are shown. (B) At 29 weeks of age, groups of five mice were challenged with 10³ PFU of JEV i.p. (protection data are given in Table 2), and 3 weeks later, sera from surviving mice were collected. The anti-JEV NS1 protein-specific antibody responses in prechallenge (n = 10) and postchallenge (n = 5) sera were measured, and mean titers calculated from ELISA end points ± standard deviations are shown.

FIG. 4.

Enhancement of MVEV infection in IFN-α-R^−/− mice suboptimally immunized with JE-VAX. (A) Serum; (B) brain. Groups of 6-week-old IFN-α-R^−/− mice were immunized three times at 2-week intervals with undiluted (∼0.6 μg) or 100-fold-diluted JE-VAX, or left untreated, and challenged with 10² PFU of MVEV i.v. at 14 weeks of age. Blood and brains were harvested at the times indicated and processed as described previously (27), and virus content was measured by plaque assay on Vero cells. Dotted lines indicated detection limits; each symbol corresponds to an individual mouse. For the calculation of mean titers, which are represented by horizontal bars, serum and brain samples with values below the detection limit were set at 20 and 200 PFU/ml or g, respectively.