Steroid Hormone Synthesis by Vaccinia Virus Suppresses the Inflammatory Response to Infection

The 3β-hydroxysteroid dehydrogenase (3β-HSD) isoenzymes play a key role in cellular steroid hormone synthesis. Vaccinia virus (VV) also synthesizes steroid hormones with a 3β-HSD enzyme (v3β-HSD) encoded by gene A44L. Here we examined the effects of v3β-HSD in VV disease using wild-type (vA44L), deletion (vΔA44L), and revertant (vA44L-rev) viruses in a murine intranasal model. Loss of A44L was associated with an attenuated phenotype. Early (days 1–3) after infection with vΔA44L or control viruses the only difference observed between groups was the reduced corticosterone level in lungs and plasma of vΔA44L-infected animals. Other parameters examined (body weight, signs of illness, temperature, virus titres, the pulmonary inflammatory infiltrate, and interferon [IFN]-γ levels) were indistinguishable between groups. Subsequently, vΔA44L-infected animals had reduced weight loss and signs of illness, and displayed a vigorous pulmonary inflammatory response. This was characterized by rapid recruitment of CD4⁺ and CD8⁺ lymphocytes, enhanced IFN-γ production and augmented cytotoxic T lymphocyte activity. These data suggest that steroid production by v3β-HSD contributes to virus virulence by inhibiting an effective inflammatory response to infection.