The long-term goal of the principal investigator is to devise therapeutic strategies for the manipulation of pulmonary alveolar repair after lung injury. Repair of damaged alveolar epithelium,is accomplished through the proliferation and differentiation of type II pneumocytes, which replace lost type I or type II epithelial cells. The efficiency of these processes is now believed to be a critical determinant in the development of pulmonary fibrosis. In recent collaborative work, the applicant found evidence that lung fibroblasts, isolated from patients or experimental animals with fibrotic lung injury, produce a factor(s) capable of inducing apoptosis and necrosis of alveolar epithelial cells of either human or rat origin. The nature of this phenomenon in vitro suggests a mechanism for the generation of the "cuboidal epithelium" of the fibrotic lung: it is hypothesized that fibroblast-derived factors kill type I pneumocytes at a rate faster than that of type II cell division. This project will test that theory through the application of experiment techniques and working hypotheses developed in the applicant's earlier PHS-funded work. In Specific Aim 1, type II cell kinetics during the development of the cuboidal epithelium will be carefully characterized to determine the plausibility of such a mechanism. This will be accomplished through the application of the principal investigator's unique flow cytometric approaches to an established rat model of diffuse interstitial pulmonary fibrosis, from which the lung fibroblasts mentioned above were derived. In Specific Aim 2, the precise actions of the fibroblast-derived factor(s) on type II cell division, differentiation and death will be quantitated through lung epithelial cell culture systems established in the applicants laboratory. In Specific Aim 3 these measurements will be related to in situ studies of human and rat lung tissue. The fibroblasts synthesizing the active factor(s) will be characterized in Specific Aim 4, and the factor itself will be identified in Specific Aim 5. This project will clarify our understanding of a poorly understood, but common phase of lung injury, the "cuboidal epithelium" of later stage lung injuries which lead to pulmonary fibrosis. The proposed experiments will elucidate a previously unrecognized mechanism of lung pathophysiology, and will result in the identification of paracrine pathways and molecules which contribute to the pathogenesis of fibrotic lung diseases. This knowledge will serve as the foundation for future work to design therapeutic interruptions of this mechanism for the improved management of lung disease.