T-LYMPHOCYTE ACTIVATION IN THE ENLARGED THORACIC LYMPH-NODES OF RATS WITH SILICOSIS

Silicosis is primarily a mononuclear cell inflammatory and fibrotic di sease of the pulmonary parenchyma. It is known that lung-associated ly mph nodes are also affected. To study the involvement of lymphocytes i n silicosis, we examined lymph nodes of rats 12 months after an 8-day silica aerosol exposure. We found that 2 thoracic lymph nodes close to the thymus were enormously enlarged in silicotic rats and contained a 49-fold higher cell number than control lymph nodes. The higher cell number was caused by parallel increases in T- and B-lymphocytes, natur al killer (NK) cells, and macrophages without change in the relative p roportions when compared with control thoracic lymph nodes. By examini ng interleukin-2 (IL-2) receptor and intercellular adhesion molecule-1 expression, we detected a significantly higher percentage of activate d CD8(+) T cells and, to a lower degree, of CD4(+) T cells in thoracic lymph nodes of silicotic animals. In contrast, no differences in the activation state were found in T cells obtained from cervical or mesen teric lymph nodes of silicotic and control rats. The occurrence of act ivated T cells in thoracic lymph nodes of silicotic rats was documente d further by selectively enhanced interferon-gamma (IFN-gamma) mRNA ex pression in the absence of IL-2 and IL-4 mRNA changes. These data show that T-lymphocytes of thoracic lymph nodes have become activated with an enhanced IFN-gamma gene transcription which may be an important ca use of macrophage activation during silicosis.