Panel A: Normal lung tissue from an uninfected monkey [the rate of monocyte turnover (RMT) = 1.61%]; Panel B: Lung tissue from an SIV-infected animal (RMT = 22.7%) demonstrates minimal interstitial accumulation of a few mononuclear cells (arrows) and rare alveolar macrophages (<); Panels C and D: Lung tissues from SIV-infected animals with RMT = 40.5% and RMT= 55%, respectively, exhibited mild (Panel C) to moderate (Panel D) interstitial pneumonia characterized by alveolar septa thickening, capillary dilation (C), increased the numbers of mononuclear cells (arrows) and hyperplastic type II pneumocytes (II). Low (Panel C) to moderate (Panel D) numbers of alveolar macrophages (<) and multinucleated giant cells (G) were noted in the alveolar spaces. Asterisk (*) = alveolar septum. Tissues were stained with H&E and viewed at 400× magnification. Lung injury was assessed using a histological scoring system as previously described (4, 16).