Live Chimeric and Inactivated Japanese Encephalitis Virus Vaccines Differ in Their Cross-Protective Values against Murray Valley Encephalitis Virus

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 (≥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.Murray Valley encephalitis virus (MVEV) is a mosquito-borne flavivirus belonging to the Japanese encephalitis virus (JEV) serocomplex which can cause severe, sometimes fatal, disease in humans (reviewed in references 30, 31, 32, and 42). The virus is endemic in northern Australia and Papua New Guinea, where it causes a small number of human cases of encephalitis in most years. In symptomatic patients the case fatality rate is ∼20%, and among those who recover a large number (∼50%) will suffer from neuropsychiatric sequelae. Cases of Murray Valley encephalitis are more common in children or visitors in areas of endemic disease than in adult residents, who have preexisting immunity (7, 42, 46). Sporadically, MVEV spreads to central or southern regions of Australia (e.g., the Murray Valley of southeastern Australia) and causes epidemic viral encephalitis in humans (32). There are no vaccines or antiviral agents available against MVEV, and given the relatively small number of human cases, it is unlikely that a MVEV-specific vaccine for human use will be produced. However, it has been known for many years that at least in animal models, live viral infection with other members of the JEV serocomplex will give cross-protective immunity against heterologous viruses belonging to this group (10, 17, 33, 48, 52). MVEV is genetically and antigenically closely related to JEV (82% amino acid sequence identity in the envelope E] protein), the most important encephalitic flavivirus in terms of human disease incidence and severity (reviewed in reference 4). A number of live and inactivated JEV vaccines have been licensed or are under development (reviewed in references 2, 16, and 34). If effective and long-lasting cross-protective immunity against MVEV was induced by one of the JEV vaccines, a strong case could be made for its prophylactic use in populations at risk of MVEV infection in Australia. A further reason for investigating the suitability of JEV vaccines in the Australian context is the recent emergence of JEV in northern Australia (18, 19, 41). This has raised the prospect that JEV may become established in enzootic cycles on the Australian mainland, necessitating the use of JEV vaccines in regions where MVEV is also endemic. The impact of MVEV infection in JEV vaccine recipients in terms of disease outcome remains unknown.In contrast to its protective value against heterologous flaviviruses, cross-reactive flavivirus immunity has also been associated with infection- and/or disease-enhancing consequences in natural and laboratory settings (1, 9, 20, 39). Antibody-dependent enhancement of infection is thought to account for the more severe forms of dengue sometimes associated with secondary, heterologous dengue virus infections by a mechanism putatively involving the increased uptake of virus bound with nonneutralizing antibody into Fc receptor-bearing cells (14, 15). For the MVEV/JEV pair, it has been reported that transfer of subneutralizing concentrations of JEV-immune serum or sera from mice suboptimally immunized with inactivated JEV vaccine (JE-VAX; Biken, Japan) can prime recipient mice for a more severe disease when challenged with MVEV (3, 50). We have demonstrated this potentially detrimental effect for the first time in the context of the full complement of the vaccine-primed immune response: the administration of an experimental UV-inactivated MVEV vaccine at a suboptimal dose greatly increased the susceptibility of mice (up to 75% mortality) to challenge with a dose of JEV, which was sublethal in unvaccinated animals (29). It is not clear if this phenomenon is an inherent property of inactivated vaccines, which provide relatively poor immunity in terms of quality, magnitude, and duration in comparison to live virus infections. Here we investigate the protective value and risk of disease potentiation of a recombinant, live JEV vaccine candidate (ChimeriVax-JE) and a licensed, inactivated JEV vaccine (JE-VAX) in mouse models of MVEV and West Nile virus (WNV) encephalitis. ChimeriVax-JE is constructed from yellow fever virus 17D vaccine cDNA by replacement of the viral structural prM and E proteins with those of an attenuated JEV strain; it has been shown to protect mice and monkeys from JEV challenge (12, 36) and has undergone phase 2 and phase 3 trials for safety and efficacy in humans (35, 37).