Protective efficacy of a plasmid DNA encoding Japanese encephalitis virus envelope protein fused to tissue plasminogen activator signal sequences: studies in a murine intracerebral virus challenge model.

We report the construction of chimeric DNA vaccine vectors in which secretory signal sequence derived from tissue plasminogen activator (TPA) was fused to the full length (pCMVTE) or 398 amino terminal amino acids (pCMVTdeltaE) of Japanese encephalitis virus (JEV) envelope (E) protein. Transfection studies indicate that E protein expressed from pCMVTdeltaE-transfected cells but not pCMVTE-transfected cells is secreted into the culture medium. Analysis of the potency of various DNA vaccine constructs in a murine intracerebral (i.c.) JEV challenge model indicates that pCMVTdeltaE confers the highest level (71%) of protection. Immunization with pCMVTdeltaE induces a mixed Th1 and Th2 T helper cell response while immunization with plasmids encoding nonsecretory forms of E protein induces a Th1 T helper response. Only low levels (<1:20) of virus neutralizing antibody titres were observed in DNA vaccinated mice which did not increase further after i.c. JEV challenge. Thus, immunization with a plasmid encoding secretory E protein results in an altered cytokine response and better protection against i.c. JEV challenge than that conferred by immunization with plasmids encoding nonsecretory forms of E protein. We also demonstrate that unlike peripheral JEV challenge, i.c. JEV challenge does not result in an increase in anamnestic antibody response suggesting that other components of immune system such as cytotoxic T cells and T helper cells contribute to protection against i.c. JEV challenge of DNA vaccinated mice.