Pathogenesis of mesothelial reactions to asbestos fibers. Monocyte recruitment and macrophage activation.

Exposure to asbestos fibers leads to a variety of mesothelial reactions: pleural effusions, fibrotic pleural plaques, and malignant mesotheliomas. An animal model was developed to reproduce these lesions in C57B1/6 mice using weekly intraperitoneal injections of asbestos fibers. After exposure to asbestos fibers, monocytes were recruited into the abdominal cavity and acquired the characteristics of inflammatory or nonspecifically activated macrophages. Nontoxic titanium dioxide or toxic silica particles did not produce activation of the free peritoneal macrophage population. Aggregates of asbestos fibers were found on the diaphragm and other peritoneal surfaces within only 24 h after a single injection. Macrophage recruitment to these sites peaked between 3 and 5 days, while activated macrophages persisted up to 14 days. Recruitment and activation of macrophages by repeated exposures to asbestos fibers may contribute to chronic damage of the mesothelial lining caused by these mineral fibers.