Immunohistological Studies in Viral Encephalitis

Summary

Immunofluorescence (IF) techniques performed on formol-fixed, paraffin-embedded and protease-pretreated sections from autopsy brain for demonstration of rabies and Herpes simplex type 1 (HSV-1) antigens proved to be a simple and effective method to ensure the etiology of some viral encephalitides. In contrast to fresh material, the use of formol-fixed tissues rules out any risk in handling highly infectious material. Another major advantage is the possibility of comparing IF appearance with normal histology of the same section by post-staining with hematoxilin-eosin (H.E.) or other stains.

In 2 animal and one of 2 human rabies cases, neuronal cytoplasmic inclusions were more prominent in IF than in H.E. stains. In 31 cases with a histopathological diagnosis of necrotizing encephalitis, HSV-1 antigen was demonstrated in 17 of 25 acute cases but absent in 6 cases with clinical courses longer than 4 weeks. Specific fluorescence of nuclear inclusion bodies was inconstant and less prominent than that of neuronal cytoplasm and neuronal/glial nuclear membranes. Since HSV-1 positive cells are patchily distributed, limited tissue sampling (e.g. in diagnostic biopsies) may yield false negative results. Paucity of HSV-1-positive cells in the leptomeninges renders CSF immunocytology of limited use for diagnostic purposes.