Studies on the cellular response in lung tissue to the inhalation of mineral dust. Final report on CEC Contract 7248-33/025

The prevalence of coalworkers’ pneumoconiosis varies between different collieries in Britain and this has been shown to be related to differences in the mineral content of coalmine dusts which are present in the air of the coalmines. As in other forms of parenchymal fibrosis, the pathobiological processes which lead to pneumoconiosis are considered to be mediated, in large part, by pulmonary leukocytes which can be altered in number and activity by the presence of deposited dust. A study was therefore designed to investigate whether dusts collected from British coalmines of different rank, and hence with different mineralogical content, would produce differences in bronchoalveolar leukocyte response in rats exposed to the dusts by inhalation.To this end, airborne dusts were collected from three collieries producing coals of different type:- anthracite, high rank coking coal and low rank bituminous coal. These dusts were generated as respirable clouds of 10 and 50mg/m3 in purpose built chambers and groups of rats were exposed for up to 75 days. Rats were also exposed, at similar airborne mass concentrations, to the control dusts quartz and titanium dioxide (TiO2); kaolinite at 50mg/m3 was also used. Rats exposed for 32 or 75 days and allowed to breathe room air for 2 further months were also studied.Groups of 4 rats were removed from the exposure chambers on days 2, 4, 8, 16, 32, 52 and 75 and were analysed for bronchoalveolar leukocyte response by analysis of bronchoalveolar lavage profile. The following parameters were measured:- total cells, differential leukocyte count, monocytes, binucleate-, multinucleate- and mitotic macrophages, proportion phagocytic macrophages, level of protein, free lactate dehydrogenase and n-acetyl-0-D-glucosaminidase in the first 10ml of lavage fluid, ability of macrophages to spread on glass in one hour, ratio of lung:body weight (lung index), Fc receptor activity, ability of leukocytes to chemotact, release of chemotaxins by bronchoalveolar leukocytes, and production of superoxide anion and hydrogen peroxide. The study revealed that inhalation exposure to TiO2 caused no inflammation at 10mg/m3 but that there was inflammation present towards the end of exposure to 50mg/m3. Quartz caused substantial airborne mass concentration-, and time-dependent inflammation in the bronchoalveolar region. The coalmine dusts all caused marked airborne mass concentration-,and time-dependent inflammation which was greater in magnitude than that caused by TiO2, but less than that caused by quartz. On cessation of exposure to TiO 2 the inflammation present at the later time points, with the highest dose, decreased in intensity; in contrast rats allowed to recover following exposure to quartz showed marked intensification of the inflammatory response in the period when they were inhaling room air. With coalmine dusts there was persistence of inflammation, but no progression, during recovery. There were no consistent differences between the coalmine dusts of different rank, in causing inflammation except for a suggestion, in some parameters, that high rank dust was less active than low rank, or anthracite dust.The ability of bronchoalveolar leukocytes from dust-exposed lung to chemotact was different depending on the dust to which the rats were exposed. TiO 2-exposed leukocytes showed very little difference from control leukocytes in ability to chemotact whereas leukocytes from both quartz-, and coalmine dust-exposed rats had marked inhibition of their ability to migrate towards a standard chemotaxin.The production of chemotaxin by dust-exposed leukocytes was no different from that produced by control leukocytes and treatment in vitro with the various dusts did not cause enhancement of release; treatment with zymosan however caused release of chemotaxin by all populations. Production of superoxide or hydrogen peroxide by the dusted leukocyte populations showed no significant differences from controls.The results from this study confirm the likely role of inflammatory leukocytes, suggested by many previous studies, in the development of pulmonary lesions following exposure of the lung to pathogenic agents. These previous studies have demonstrated that activated leukocytes, present in the lung parenchyma following exposure to different aetiologic agents, have powerful effector functions which could determine pathological change. Little research has been carried out into the role of such leukocytes in coalworkers’ pneumoconiosis but limited studies in coalworkers have revealed that inflammation is present. The present study has confirmed that exposure to British coalmine dust at airborne mass concentrations of 10 and 50mg/m3 caused marked alveolar inflammatory responses. No reliable difference in ability to cause inflammation were detected between collieries producing coal of very different rank. The mechanisms of the inflammatory leukocyte accumulation is not explained by production of chemotaxins by the leukocytes at the time points where chemotaxin production was measured (8, 32 and 75 days) but these points may have missed peaks of chemotaxin production. Previous research at the IOM has suggested that activation of chemotaxin in lung fluid may also play a role in accumulation of leukocytes in response to toxic dusts but only limited evidence for this was found in the present study where dust-activated serum showed some chemotactic activity.Failure to detect increased oxidant production by coalmine- or quartz-exposed bronchoalveolar leukocytes compared to TiO ..-exposed leukocytes does not preclude a role for oxidants in lung injury during exposure to pathogenic dust. The increased numbers of cells recruited to the lung parenchyma with the pathogenic dusts alone could produce a net increase in total oxidant burden to the lung parenchyma even if oxidant production on a per-cell basis was not increased.The findings of decreased ability to chemotact in bronchoalveolar leukocytes from rats exposed to the pathogenic dust may explain, to some extent, the frequent histological finding of dust-laden leukocyte (generally macrophage) accumulations at the proximal alveoli and terminal bronchioles. Leukocytes in toxic dust-exposed lung, insensitive to the normal chemotactic gradients which govern their movement from the deep lung to the mucociliary escalator for clearance of phagocytosed particles, may thus accumulate. Histological sections of dust-exposed lung showed such accumulations and surrounding septa appeared more thickened and cellular than alveolar walls away from the accumulations of leukocytes suggesting that the dust-laden leukocytes in these accumulations could be releasing factors which lead to septal thickening.