Fig. 5. Selected examples of the American Society of Transplantation Infectious Disease Community of Practice polyomavirus histologic classes. Class A: (A) <10% interstitial inflammation (ti0) and <10% tubulointerstitial scarring (interstitial fibrosis/tubular atrophy [IFTA] 0; PAS, ×100). (B) Minor interstitial inflammation with few infected tubular cells arrow; Jones methenamine silver, ×100). Class B2: (C) moderate interstitial inflammation (ti2) with moderate (26%–50%) parenchymal tubular cell cytopathic changes (arrows) in the medulla (Jones methenamine silver, ×100). (D) Moderate inflammation (arrows) and mild tubular atrophy with interstitial fibrosis (IFTA 1) in the cortex (arrowheads; Jones methenamine silver, ×100). Class B3: (E) interstitial inflammation involves 80% of the cortex (ti3; Jones methenamine silver, ×100). (F) Moderate (>25%–50%) interstitial fibrosis and tubular atrophy (IFTA 2; arrowheads) with tubular cell viral cytopathic changes in >50% of the parenchyma (Jones methenamine silver, ×200). (G) Higher magnification showing marked tubular cell cytopathic features (arrows; H&E, ×400).
