Lymphocytes, lymphokines, and silicosis

Silicosis is characterized by mononuclear cell aggregation with mineral particles and fibrosis. Lymphocytes are abundant in these lesions. We exposed inbred strains of mice to a respirable aerosol of cristobalite silica (70 mg/m3, 5 h/d, 12 d) or shamair. Silicosis evolved over months after exposure. The silica-exposed mice showed the accumulation of lymphocytes in alveolar spaces (seen in bronchoalveolar lavage), in lung parenchymal lesions and nodules, and in enlarged bronchial-associated lymphoid tissues and thoracic lymph nodes. The lung lymphocytes were predominantly CD4+ T cells, but numerous CD8+ T cells, natural killer cells, and CD4- gammadelta-TCR+ T cells were present as well. Interferon-gamma (IFN-gamma) production was upregulated, suggesting a THelper-1-like response in silicosis. In silicotic lung tissue, mRNA transcripts for the macrophage-derived cytokines IL-12 and -18 were increased. IFN-gamma gene-deleted mice (C57Bl/6-Ifngtm1 Ts) exposed to silica developed less extensive silicosis and less lung collagen accumulation than wild-type mice. We hypothesize that there is a reiterative amplification cycle in which macrophages with silica may produce cytokines, such as IL-12 and -18, that attract and activate lymphocytes. These activated lymphocytes may then produce additional mediators that in turn attract and activate an expanded secondary population of macrophages. IFN-gamma would be a likely cause of macrophage activation in this cycle. More work is needed to understand the biological events that lead from the inhaled dust to the scarred lung, and to clarify the role of lymphocytes in this process.