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Alveolar macrophages and their products are thought to be important mediators of the inflammatory lesions and consequent interstitial fibrosis caused by inhalation of inorganic particles. Identification of a homolog of platelet-derived growth factor (PDGF) produced by rat alveolar macrophages that were stimulated with carbonyl iron particles and asbestos fibers motivated our studies on the biologic activity of this potent cytokine. Macrophage-derived PDGF (MD-PDGF) competes for specific membrane receptors on rat lung fibroblasts, initiating DNA synthesis and cell replication. The present report demonstrates that purified human PDGF and the MD-PDGF are chemotactic for early passage rat lung fibroblasts, but not for lung macrophages. Rat lung fibroblasts exhibit a typical bell-shaped, dose-related curve and respond optimally between 2 and 4 ng/ml PDGF. We found that alveolar macrophage-conditioned medium (AMCM), fractionated by gel filtration in 1 M acetic acid, induced a clear chemotactic response in the same fractions (20 to 22 ml) where PDGF was identified by enzyme immunoassay. In contrast, AMCM fractionated by gel filtration in phosphate-buffered saline did not induce any chemotactic activity unless the fractions were treated further with 1 M acetic acid. In this case, chemotactic activity was observed in those fractions with molecular weights of 150 and > 200 kD. All chemotactic activity observed with fractionated AMCM was blocked > 90% by an anti-PDGF antibody. These observations demonstrate that MD-PDGF is chemotactic for rat lung fibroblasts if it first is released from its binding protein, α-macroglobulin (α-M), which is secreted into the medium along with PDGF. Thus, additional experiments were carried out to test whether or not bovine α2-macroglobulin (α2-M), a plasma homolog of the macrophage-derived α-M, modulated the chemotactic activity of human PDGF. Native (“slow”) bovine α2-M caused a concentration-dependent inhibition of PDGF-induced Chemotaxis. This correlates with our finding of no chemotactic activity in the high-molecular-weight regions of fractionated conditioned medium, unless the macrophage-derived binding protein is removed by acidification. These findings demonstrate the potent chemotactic activity of PDGF for primary rat lung fibroblasts and emphasize the importance of binding proteins in modulating biologic activity. It will be essential to establish whether or not the growth factor and its binding protein are active in vivo.