Optic neuritis in different strains of mice by a recombinant HSV-1 expressing murine interleukin-2

Abstract

Purpose: The authors have shown previously that a recombinant HSV-1 that constitutively expresses two copies of murine IL-2 (HSV-IL-2) induces demyelination by activated CD8(+) T cells in the brain and spinal cord of ocularly infected female BALB/c mice. The present study was conducted to determine whether the ocular infection with this recombinant virus induces optic neuritis independent of virus dose, major histocompatibility complex (MHC) background, or sex.

Methods: Female BALB/c, C57BL/6, SJL/6, and 129SVE mice and male BALB/c mice were ocularly infected with different doses of recombinant HSV-IL-2 virus. Demyelination of optic nerves in infected mice was monitored histologically using Luxol fast blue staining and by measurement of visual-evoked cortical potentials (VECPs).

Results: Both focal and diffuse regions of demyelination of the optic nerves were observed in the HSV-IL-2-infected mice as early as day 10 after infection and as late as day 60 after infection (the final experimental time point) in all strains of mice tested. Optic nerve demyelination was not observed in control mice ocularly infected with HSV-IL-4 or wild-type HSV-1. VECP responses were delayed significantly in the HSV-IL-2-infected mice compared with mice infected with control viruses.

Conclusions: The results demonstrate for the first time that a combination of viral infection and constitutive expression of IL-2, but not IFN-gamma or IL-4, can result in demyelination and visual impairment in the optic nerves of ocularly infected mice.

Figure 1

Histologic analyses of optic nerves obtained from female BALB/c mice 14 days after ocular infection with HSV-IL-2 or control viruses. Female BALB/c mice were infected ocularly with HSV-IL-2, HSV-IL-4, HSV-IFN-γ, KOS, or mock-infected, as described in the footnote to Table 1. The optic nerves were obtained 14 days later, fixed, sectioned, and stained with LFB. Representative photomicrographs are shown. In the photomicrograph of the section of the optic nerve from the HSV-IL-2–infected mouse, the single-headed arrow indicates focal demyelination, and the two-headed arrow indicates diffuse demyelination. (A–C) In HSV-IL-2–infected optic nerve areas of focal demyelination were apparent as plaque areas with loss of LFB staining (single-headed arrow). (D, E) In HSV-IL-2–infected optic nerve, areas of diffuse demyelination were apparent as plaque areas with loss of LFB staining (two-headed arrow). In contrast to (A–E), no demyelination was detected in mice infected with HSV-1 strain KOS (F), HSV-IL-4 (G), HSV-IFN-γ (H), or mock-infected (I) as shown by dense LFB blue staining in myelinated regions, and no detectable focal or diffuse positive material.

Figure 2

Histologic analyses of optic nerves obtained from female BALB/c mice 10 days after ocular infection with HSV-IL-2 or control HSV-IL-4. Female BALB/c mice were infected ocularly with HSV-IL-2 or -IL-4, as described in the footnote to Table 1. The optic nerves were obtained 10 days later, fixed, sectioned, and stained with LFB. Representative photomicrographs are shown. (A) In HSV-IL-2–infected optic nerve, areas of focal demyelination were apparent as plaque areas (arrows). (B) No demyelination was detected in mice infected with HSV-IL-4.

Figure 3

Demyelination of the optic nerve in female BALB/c mice infected with different doses of HSV-IL-2. Female BALB/c mice were infected ocularly with 2 × 10³, 2 × 10⁴, 2 × 10⁵, 2 × 10⁶, or 2 × 10⁷ PFU/eye of HSV-IL-2 (five mice/group). The optic nerves were dissected 14 days later, fixed, sectioned, and stained with LFB. The presence of focal and/or diffuse demyelination in multiple sections was determined.

Figure 4

Effect of ocular HSV-IL-2 infection on optic nerve demyelination in different strains of mice. Female C57BL/6, 129SVE, or SJL/6 mice were infected ocularly with HSV-IL-2 (2 × 10⁵ PFU/eye). Control mice were infected similarly with HSV-IL-4. The optic nerves were dissected 14 days later, fixed, sectioned, and stained with LFB. Representative micrographs are shown. In the photomicrograph of the section of the optic nerve from the HSV-IL-2–infected mouse, the single-headed arrow indicates focal demyelination, whereas the two-headed arrow indicates diffuse demyelination. (A–C) C57BL/6 mice infected with HSV-IL-2; (D) C57BL/6 mice infected with HSV-IL-4; (E–G) 129SVE mice infected with HSV-IL-2; (H) 129SVE mice infected with HSV-IL-4; (I–K) SJL/6 mice infected with HSV-IL-2; and (L) SJL/6 mice infected with HSV-IL-4.

Figure 5

Effect of ocular infection with HSV-IL-2 on visual evoked cortical potentials of female BALB/c mice. Mice were infected ocularly with HSV-IL-2 (2 × 10⁵ PFU/eye) or parental dLAT2903 (2 × 10⁵ PFU/eye). After 11 days, the VECPs were recorded. Representative VECP recordings from a mice infected with (top) dLAT2903 or (bottom) HSV-IL-2.