Histological examination of RKl-8 (Panel A-C) and RPn-8 (Panel D-F). (A) Meningoencephalitis in RKl-8 brain, characterized by perivascular infiltrates ("perivascular cuffs") of mononuclear leukocytes (arrows) within the cerebral parenchyma, typical of viral encephalitis. Rarefaction of the white matter, consistent with demyelination, is also present (yellow arrow). (B, C) SV40 meningoencephalitis in RKl-8. SV40-positive cells were localized in the same regions by immunohistochemistry (IHC) (B) and in situ hybridization (ISH) (C). IHC for large T antigen shows SV40 positive cells (arrows) adjacent to a vessel (V) surrounded by inflammatory cells. SV40 DNA is localized within cells by ISH (blue NBT/BCIP chromogen) in a serial section of panel B. (D) Rhesus lymphocryptovirus infection. IHC for EBNA 2 on serial section of tongue shown in additional file 5A, demonstrating widespread localization of EBNA 2 protein expression in nuclei of mucosal epithelial cells (brown chromogen). (E) Mycobacterial infection in RPn-8. Acid fast stain of mesenteric lymph node reveals large numbers of mycobacteria-filled macrophages (magenta color; arrows). (F) Pneumocystis pneumonia in RPn-8. Section of lung stained by Gomori methenamine silver (GMS) technique to localize fungal organisms. Pneumocystis organisms (arrows) within the foamy exudate appear as crescent-shaped or folded spheres.