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-Type II epithelial cells are essential for lung development and remodeling, as they are precursors for type I cells and can produce vascular mitogens. Although type II cell proliferation takes place after hyperoxia, it is unclear why alveolar remodeling occurs normally in adults whereas it is permanently disrupted in newborns. Using a line of transgenic mice whose type II cells could be identified by their expression of enhanced green fluorescent protein and endogenous expression of surfactant proteins, we investigated the age-dependent effects of hyperoxia on type II cell proliferation and alveolar repair. In adult mice, type II cell proliferation was low during room air and hyperoxia exposure but increased during recovery in room air and then declined to control levels by day 7. Eight weeks later, type II cell number and alveolar compliance were indistinguishable from those in room air controls. In newborn mice, type II cell proliferation markedly increased between birth and postnatal day 7 before declining by postnatal day 14. Exposure to hyperoxia between postnatal days 1 and 4 inhibited type II cell proliferation, which resumed during recovery and was aberrantly elevated on postnatal day 14. Eight weeks later, recovered mice had 70% fewer type II cells and 30% increased lung compliance compared with control animals. Recovered mice also had higher levels of T1␣, a protein expressed by type I cells, with minimal changes detected in genes expressed by vascular cells. These data suggest that perinatal hyperoxia adversely affects alveolar development by disrupting the proper timing of type II cell proliferation and differentiation into type I cells. alveoli; cell proliferation; differentiation; enhanced green fluorescent protein; proliferating cell nuclear antigen THE ALVEOLUS IS COMPOSED OF two epithelial cell types that can be identified by their distinct morphology and expression of unique genes. Type I cells are thin, flat cells that cover pulmonary vascular endothelial cells and comprise 95% of the alveolar surface (56). These cells are important for gas exchange, regulation of alveolar fluid levels, and stretch-induced modulation of surfactant secretion. Type I cells can be identified by their expression of T1␣ (also known as RTI 40 ), aquaporin-5, caveolin-1, or the cyclin-dependent kinase inhibitor p15 (41,42). Type II cells, on the other hand, are large,

CD8+ T cells contribute to the pathophysiology of Pneumocystis pneumonia (PcP) in a murine model of AIDS-related disease. The present studies were undertaken to more precisely define the mechanisms by which these immune cells mediate the inflammatory response that leads to lung injury. Experimental mice were depleted of either CD4+ T cells or both CD4+ and CD8+ T cells and then infected with Pneumocystis. The CD4+-depleted mice had significantly greater pulmonary TNF-α levels than mice depleted of both CD4+ and CD8+ T cells. Elevated TNF-α levels were associated with increased lung concentrations of the chemokines RANTES, monocyte chemoattractant protein 1, macrophage-inflammatory protein 2, and cytokine-induced neutrophil chemoattractant. To determine whether TNFR signaling was involved in the CD8+ T cell-dependent chemokine response, TNFRI- and II-deficient mice were CD4+ depleted and infected with Pneumocystis. TNFR-deficient mice had significantly reduced pulmonary RANTES, monocyte chemoattractant protein 1, macrophage-inflammatory protein 2, and cytokine-induced neutrophil chemoattractant responses, reduced inflammatory cell recruitment to the alveoli, and reduced histological evidence of PcP-related alveolitis as compared with infected wild-type mice. Diminished pulmonary inflammation correlated with improved surfactant activity and improved pulmonary function in the TNFR-deficient mice. These data indicate that TNFR signaling is required for maximal CD8+ T cell-dependent pulmonary inflammation and lung injury during PcP and also demonstrate that CD8+ T cells can use TNFR signaling pathways to respond to an extracellular fungal pathogen.