| Summary: | Japanese encephalitis virus (JEV), a member of the family flaviviridae, is one of the most important pathogens of the developing countries, causing high mortality and morbidity amongst children. The present study is aimed at the development of a DNA vaccine for Japanese Encephalitis (JE). As a first step towards developing a DNA vaccine for JE, an eukaryotic expression plasmid encoding the envelope (E) glycoprotein of Japanese Encephalitis Virus (pCMXENV) was constructed. This plasmid expresses the E protein intracellularly, when transfected into Vero cells in culture. Several independent immunization and intracerebral (i.e.) JEV challenge experiments were carried out and the results indicate that 51% and 59% of the mice are protected from lethal i.e. JEV challenge, when immunized with pCMXENV via intramuscular (i.m.) and intranasal (i.n.) routes respectively. JEV-specific antibodies were not detected in pCMXENV-immunized mice either before or after challenge. JEV-specific T cells were observed in mice immunized with pCMXENV, which increased significantly after JEV challenge indicating the presence of vaccination-induced memory T cells. Enhanced production of interferon-y (EFN-y) and complete absence of interleukin-4 (IL-4) in splenocytes of pCMXENV-immunized mice on restimulation with JEV antigens in vitro indicated that the protection is likely to be mediated by T helper (Th) lymphocytes of the Thl sub type. These results demonstrated that immunization with a plasmid DNA expressing intracellular form of JEV E protein confers significant protection against i.e. JEV challenge even in the absence of detectable antiviral antibodies. We then examined the potency of JEV DNA vaccines as well as that of the inactivated mouse brain derived BIKEN vaccine in the i.e. challenge model. The results indicate that all the mice immunized with BIKEN JE vaccine were protected against i.e. JEV challenge while 50% protection was observed in case of mice immunized with pJME or pJNSl and 38% protection was observed in pCMXENV-immunized mice. Immunization with both pJME and pJNSl resulted in 66% protection. These results indicate that the BIKEN JE vaccine confers better protection against i.e. JEV challenge than DNA vaccines. The fact that the BIKEN vaccine conferred better protection against i.e. JEV challenge than DNA vaccines indicated that the i.e. JEV challenge model can be exploited further to examine the potency of different DNA vaccine constructs. Towards this goal, we constructed plasmids that encode secretory or nonsecretory forms of JEV E protein and examined their potency in the i.e. JEV challenge model. Our results indicate that i.m. immunization of mice with plasmid encoding secretory form of JEV E protein confers higher level (75%-80%) protection than those encoding nonsecretory forms. Cytokine analysis of splenocytes isolated from DNA immunized mice after stimulation in vitro with JEV revealed that immunization with plasmid encoding secretory form of JEV E protein induces both Thl and Th2 responses while those encoding nonsecretory forms induce only Thl type of response. Thus, synthesis of secretory form of JEV E protein results in an altered immune response leading better protection against i.e. JEV challenge. Based on our studies, we propose that both cellular and humoral immune responses play a key role in protective immunity against i.e. JEV challenge and DNA vaccines that can induce higher levels of neutralizing antibodies will be as efficient as the BIKEN vaccine in conferring protection against i.e. JEV challenge.
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