The immunobiology of Japanese encephalitis virus

Abstract

Japanese encephalitis virus is the leading cause of viral encephalitis in Asia. In spite of this, the immunological correlates for recovery from primary Japanese encephalitis virus (JEV) infection remain poorly defined. In this work, I have first established a mouse model for Japanese encephalitis in which a low-dose virus inoculum was administered subcutaneously into adult C57BL/6 mice. In this model, ~60% of the mice developed fatal encephalitis, concomitant with virus burden in the central nervous system (CNS). JEV infection induced a peak natural killer (NK) cell response on day 4 post-infection (pi), a peak CD8+ T cell response on day 7 pi, a peak anti-JEV IgM response on day 8 pi, and an increasing anti-JEV IgG responses from day 8 pi onwards. Second, to investigate the relative importance of immune effector cell populations, splenocyte transfer experiments, and JEV-challenge of mice with acquired or induced cellular deficiency were performed. Using mice lacking B cells ({u03BC}MT^-/-^ mice) and immune B cell transfer to wild-type mice, I find a critically important role for humoral immunity in preventing virus spread to the CNS. T cell help played an essential part in the maintenance of an effective antibody response necessary to combat the infection, since mice lacking major histocompatibility complex class II showed truncated IgM and blunted IgG responses, and the infection was uniformly lethal. On the other hand, NK cells were dispensable for protection against lethal Japanese encephalitis, while CD8+T cells, by itself, have no significant effect on the survival of JEV-infected mice. Third, cytolytic effector pathways and interferon gamma (IFN-y) help protect against lethal JE. Granule exocytosis and Fas-Fas ligand pathways of cytotoxicity act within the CNS to reduce disease severity, and show a redundancy in control of JEV infection. Only mice defective in both cell contact-dependent cytotoxic effector mechanisms display increased susceptibility to subcutaneous challenge with a low dose of JEV. Also, I demonstrate that IFN-y is critical in recovery from primary infection with JEV by a mechanism involving suppression of virus growth in the CNS, but not in extraneural tissues, and that T cells are the main source of the cytokine that promotes viral clearance from the brain. Fourth, the chemokine receptor, CCR5, serves as a host antiviral factor important against lethal JE. CCR5 deficiency resulted in increased mortality, and viral burden in the CNS. Absence of CCR5 did not only result in impaired trafficking of leucocytes into the brain, but also resulted in a multifaceted deficiency of cellular immune responses characterized by reduced NK cell activity, reduced CD8+ T cell activity, and low splenic cellularity. This result is consistent with a mechanism by which CCR5 expression enhances lymphocyte activation and thereby promotes host survival after JEV infection. Fifth, a novel immunodeficient mouse strain, Tuara, was established. Tuara mice displayed enhanced susceptibility to flavivirus infections, due to a loss-of-function mutation that was originally derived from an inbred knockout mouse strain, MHCII-A{u03B1}-/-. The gene of interest was segregated from the MHCII defect, and after customized sequence capture and re-sequencing, was localized within ~50 Mb region of Chromosome 1. Among the candidate list, a G>T transversion on mouse genome reference position 52,179,547 bp of STAT! gene was identified as the most likely causal mutation. Finally, towards application of the insights gained on the basic immunobiology of JE, vaccination experiments using 'new generation' JE vaccines were performed. They revealed a dominant role of humoral immunity in vaccine-mediated protection of mice from lethal challenge with JEV, and in cross-protection against related viruses belonging to the JE serocomplex. CD8+ T cells were not required for protection, while CD4+ T cells provided substantial helper function for the protective humoral immune responses induced by vaccination. These immune correlates for homologous and heterologous protection against JE serocomplex viruses were observed in vaccination with either a live attenuated vaccine (ChimeriVax-JE), or a candidate inactivated Vero cell culture-derived JE vaccine (ccJE) formulated with Advax{u2122}, a novel immunologically active polysaccharide adjuvant. Show more