JEV Infection in the Brain and its effects on the Immune Response

Abstract

Using mouse as a model system, the effects of viral infection in the brain on: (A) Peripheral lymphoid organ - spleen (B) Central lymphoid organ - thymus were studied.

Key Findings Mice infected with 400 pfu of JEV through the intra-cerebral route succumbed to infection within 9-12 days.

A 5.6-fold induction of MHC-I, but not MHC-II, was observed on thymocytes during acute JEV infection.

MHC-I levels were unchanged in mice challenged with inactivated virus or in mice protected by prior immunization.

Induction of MHC-I was observed during acute JEV infection in both Balb/c and C3H mouse strains, and with two different JEV strains (P20778 and JaOAR). This suggested that sustained JEV replication in the brain is necessary for MHC-I induction on thymocytes.

Indirect mechanisms were involved in thymocyte induction of MHC-I by JEV replication in the brain.

Induction of MHC-I was also observed in mice challenged intracerebrally with two other neurotropic viruses - WNV and vaccinia.

Thymus Findings Thymic atrophy and a drastic decrease in the number of double-positive (DP) cells were observed in JEV-infected mice.

Depletion of thymocytes increased progressively as disease advanced, with 90% of DPs depleted by the 9th day post-infection.

Similar changes were observed in thymocytes from mice infected intracerebrally with WNV and vaccinia viruses.

In newborn pups infected with JEV, the decrease in DPs was much lower compared to adult mice.

Staining studies with propidium iodide and annexin V indicated a significant increase in thymocyte apoptosis, contributing to thymic atrophy.

Spleen Findings Since the peripheral pool of cells originates from the thymus, phenotypic and functional changes in spleen cell populations during acute JEV infection were analyzed.

Intracerebral challenge with JEV, WNV, and vaccinia, and their active multiplication in the brain, led to impaired cell-mediated responses of splenocytes.

Splenocytes from acutely infected mice showed decreased proliferation to mitogens (ConA, anti-CD3, and LPS).

Expression of IL-2R chain on splenocytes from JEV-challenged mice was decreased. Splenocytes also showed depressed IL-2 production upon activation.

A decrease in the number of APCs was observed, leading to depressed T cell activation and IL-2 production.

MHC-II expression was drastically decreased on both B cells and adherent cells in the spleen. Peripheral macrophages also showed a two-fold decrease in MHC-II expression. Thus, APCs from JEV-infected mice were defective in providing adequate stimulus for T cell activation.

Addition of irradiated normal APCs improved proliferation of acute splenocytes, while addition of infected APCs to normal spleen had no effect.

Restoration of T cell proliferation in infected splenocytes by normal APCs was contact-dependent.

Macrophages from JEV-challenged mice produced less nitric oxide (NO) when activated with IFN- compared to normal cells, indicating depressed APC function.

A drastic decrease in LPS-stimulated expression of co-stimulatory molecules B7.1 and B7.2 was observed on APCs from JEV-challenged mice, leading to reduced activating signals to T cells.

Splenocytes from infected mice failed to mount recall responses to ovalbumin and JEV antigen.

APCs from WNV- and vaccinia-infected mice also failed to induce optimal T cell activation.

Thus, APCs from mice acutely infected with neurotropic viruses (JEV, WNV, vaccinia) are defective in generating co-stimulatory signals required for T cell proliferation.

Splenocytes from JEV-infected mice (9 days post-challenge) produced less IL-4 and IFN- when stimulated with ConA, but produced increased amounts when provided with co-stimulatory signals. This confirmed that splenocytes from acutely infected mice are defective in co-stimulation.