HISTOPATHOLOGICAL CHANGES IN ENLARGED THORACIC LYMPH-NODES DURING THEDEVELOPMENT OF SILICOSIS IN RATS

Silicosis is primarily a fibrotic lung disease which also affects the draining lymph nodes. In the present study, Me examined the lymph node s of rats from 2 weeks to 52 weeks after an 8-day silica aerosol expos ure. Parallel to the typical silicotic changes in the lungs, profound alterations occurred in both posterior mediastinal lymph nodes. The we ight of the lymph nodes progressively increased from 3.5-fold to 35-fo ld at 52 weeks after silica exposure. The weight increase was accompan ied by an early increase of T cells and preferentially of CD4(+) cells at 2 weeks, which converted into a B cell increase at 6 weeks. Histol ogically, a leukocyte influx without apparent structural changes was n oted at 2 weeks whereas at 6 weeks, germinal centers and T cell region s were disappearing and macrophages accumulated in granuloma-like stru ctures which were randomly scattered throughout the lymphoid tissue. W ithin the granulomas, macrophages were detected that carried ingested silica particles without apparent signs of degeneration or apoptosis. At 52 weeks after silica exposure, macrophage granulomas persisted wit hout induction of fibrosis in both lymph nodes, and T and B cells were now evenly distributed within the tissue. These data extend our previ ous findings on lymphocyte and macrophage activation and indicate that the early and marked desorganization of draining lymph node structure s may contribute to the immune abnormalities in silicosis.