Coal workers' pneumoconiosis: a historical perspective on its pathogenesis.

The earliest observations on coal workers' pneumoconiosis identified fundamental factors and posed particular problems in its genesis. Among the former, intensity of exposure and particle size were recognized, while argument commenced on the roles of stone dust, thus anticipating the quartz question, and of complicating pulmonary states, which introduced the idea of infection. Major studies of the disease were precipitated by its greatly increased prevalence, which became evident among South Wales coal workers from the 1930s. The principal directions of enquiry remained the same as in Scotland a century before, namely the components of coal mine dust responsible for fibrosis and the additional factor required for the development of massive fibrosis. The combined human and experimental evidence now makes possible conclusions in which confidence may be placed.

Minerals, fibrosis, and the lung.

Determinants of pulmonary fibrosis induced by inhaled mineral dusts include quantity retained, particle size, and surface area, together with their physical form and the reactive surface groups presented to alveolar cells. The outstanding problem is to ascertain how these factors exert their deleterious effects. Both compact and fibrous minerals inflict membrane damage, for which chemical mechanisms still leave uncertainty. A major weakness of cytotoxicity studies, even when lipid peroxidation and reactive oxygen species are considered, lies in tacitly assuming that membrane damage suffices to account for fibrogenesis, whereas the parallel occurrence of such manifestations does not necessarily imply causation. The two-phase procedure established that particles, both compact and fibrous, induce release of a macrophage factor that provokes fibroblasts into collagen synthesis. The amino acid composition of the macrophage fibrogenic factor was characterized and its intracellular action explained. Fibrous particles introduce complexities respecting type, durability, and dimensions. Asbestotic fibrosis is believed to depend on long fibers, but scrutiny of the evidence from experimental and human sources reveals that a role for short fibers needs to be entertained. Using the two-phase system, short fibers proved fibrogenic. Other mechanisms, agonistic and antagonistic, may participate. Growth factors may affect the fibroblast population and collagen production, with cytokines such as interleukin-1 and tumor necrosis factor exerting control. Immune involvement is best regarded as an epiphenomenon. Downregulation of fibrogenesis may follow collagenase release from macrophages and fibroblasts, while augmented type II cell secretion of lipid can interfere with the macrophage-particle reaction.

Relationship of lipid secretion and particle size to diffuse interstitial change in pneumoconiosis: a pathogenetic perspective.

Simple pneumoconiosis due to compact particles, notably in coal workers, sometimes departs from its customary focal formations and a more diffuse distribution of dust-impregnated fibrosis is superimposed. To account for this change, which may be reflected in the acquisition of radiologically irregular opacities in addition to rounded ones, attention is directed first toward type II alveolar epithelium. These cells come early into contact with inhaled particles and the lipid secretion provoked prevents their characteristic aggregation, so they remain in a more dispersed state and the consequent fibrotic reaction then tends to become diffuse. Second, submicron particles appear to act not from within the alveolus like the more usual larger ones, but after direct passage through type I epithelium into the interstitium, where lacking focal accumulation they are able to produce diffuse changes. Complexities, however, remain, among which are coalescence of focal lesions as their severity increases and interplay of agents producing interstitial fibrosis in the general population.

Prevalence and pathogenesis of pneumoconiosis in coal workers.

Dust dose and composition do not appear to account wholly for changes in the prevalence of coal workers' pneumoconiosis in Europe. In certain coal pits high progression evidently occurred with relatively low dust exposure or vice versa, whereas progression in relation to dust levels might be variable. Exceptionally high quartz concentrations occur in coal mine dust when pneumoconiosis may progress with unusual rapidity. Under such circumstances lesions resembling silicotic nodules may be found, but with the customarily lower levels of quartz the pathological features assume the form characteristic of coal workers. Morphological changes in relation to dust content of human and animal lungs, as well as cellular behavior, have not accounted completely for the epidemiological findings. Considering all the pathological evidence helps explain the pathogenesis of pneumoconiosis and vagaries of progression. The origin of progressive massive fibrosis cannot be explained simply in terms of dust burden or immunological features, and the role of an infective factor cannot be dismissed. Moreover, lipid secretion by alveolar epithelium introduces a new element that could affect the development of simple and complicated pneumoconiosis. In vitro, cytotoxicity appeared to be too variable for predictive purposes, though direct assay of fibrogenicity using the macrophage fibrogenic factor suggested that dust dose was more important than dust composition. Assessing individual susceptibility presents serious obstacles.

Determinants of pulmonary fibrosis and lipidosis in the silica model.

The conditions which might favour development of the fibrotic or the lipid component of the pulmonary reaction to inhaled quartz were examined in rats. Smaller particle size and freedom from surface contamination by amorphous silica or iron oxide, status of the animals whether specific pathogen-free or conventional, and the resistance of cell membranes to damage appeared to bear on fibrogenesis. Increased membrane stability by treatment with polyvinylpyridine-N-oxide abolished not only the fibrosis but also the response of type II cells and hence lipidosis. The rate and intensity of quartz deposition may also affect the response, a low concentration inhaled over a long period favouring nodulation. No other manipulations, environmental or pharmacological, succeeded in inhibiting lipidosis to the benefit of fibrosis. Guinea pigs, however, behaved differently, their reaction being characterized by massive alveolar accumulation of dust-bearing macrophages and type II cell hyperplasia but not by lipidosis. The species variation is unexplained but macrophage predominance may represent a phase that later transforms to lipidosis. The experimental findings may have implications for forms of pneumoconiosis other than silicosis.

Population and ultrastructural changes in murine alveolar cells following 239PuO2 inhalation.

Early changes affecting the principal cellular components of pulmonary alveoli after inhaling 239plutonium dioxide were followed by quantitative and qualitative electron microscopy in mice and rats. Different accumulated doses of a irradiation were achieved. The numbers of alveolar macrophages and interstitial mononuclear cells rose in mice especially after a higher dose of radiation, whilst in rats interstitial fibroblasts were increased. The evidence from mice suggested pronounced secretory activity of type II epithelial cells with subsequent uptake of phospholipid by alveolar macrophages, which developed large cytoplasmic inclusions, but rats were much less severely affected. Pneumonitis was not a feature and with the dosage of radiation employed endothelium escaped structural damage. Sensitivity between species differed, both according to cell type and to intensity of exposure, so demanding caution in the application of experimental findings to man.

Silica, pneumoconiosis, and carcinoma of the lung.

The current argument about the carcinogenicity of inhaled silica is not clarified by reliance on morbidity and mortality experience divorced from or incompletely related to data on environmental exposure. Human evidence provides the ultimate basis for assessing such risks, and numerous studies of the effects of inhaling dusts rich or poor in silica content on the prevalence of pulmonary carcinoma have been performed on large series of cases from major mining areas of the world. When due allowance is made for substances inhaled concomitantly with exposure to silica and for personal pollution by cigarette smoking, the weight of evidence is against a carcinogenic role for uncombined silicon dioxide. Moreover, pneumoconiosis due to compact mineral particles does not appear to determine the onset of lung cancer. Cellular behaviour suggests reasons for the different responses to compact and fibrous particles acting alone.

Pulmonary toxicology of silica, coal and asbestos.

Mineral particles are customarily inhaled as mixtures, though one component may predominate and determine the response. Although the lesions often possess a characteristic structure, according to the main type of particle deposited, morphology affords little indication of pathogenesis. Being a major element in the evolution of dust lesions, macrophage behavior has been examined extensively in vitro after treatment with mineral particles, attention being directed to membrane and biochemical changes; however, no clear lead to the origin of the lesions has emerged. Pulmonary fibrosis, as one of the ultimate consequences of dust accumulation, required a direct in vitro approach in which the products of the macrophage-particle interaction were utilized to provoke collagen formation by fibroblasts in a two-phase system. By this means, silica and asbestos stimulated connective tissue formation and application of the technique to coal dusts appears promising. Coal workers may develop a peculiar type of emphysema in relation to lesions whose fibrous content is comparatively small. Type II alveolar epithelium is also stimulated by inhaled particles and lipid accumulation follows. Alveolar lipidosis interferes with the fibrotic response by preventing contact between macrophage and particles. This phenomenon may account in part for anomalies, apparent in coal workers, between epidemiological findings and dust composition. Carcinogenesis is a well-recognized feature of asbestos exposure, but, as with fibrosis, risk prediction on the basis of in vitro tests of cytotoxicity is premature and may not be valid.

Fibrogenesis by mineral fibres: an in-vitro study of the roles of the macrophage and fibre length.

Evidence on the mechanism by which inhaled mineral fibres lead to pulmonary fibrosis has not been forthcoming. As with silica, a biphasic cell culture system was required to distinguish phagocytosis from collagen formation. Synthesis of total protein and collagen by rat fibroblasts was estimated by incorporation of labelled proline after treatment with the medium from rat peritoneal macrophages that had been cultured in the presence of different types of mineral fibre. The influence of fibre length was also examined. All the main varieties of asbestos reacted with macrophages to produce or release a fibrogenic factor. However, chrysotile and the longer amosite fibres evoked the response only after prolonging the period of incubation with macrophages, presumably by permitting more complete phagocytosis of curled or longer fibres. Short amosite fibres proved to be more active than longer ones and under certain conditions were as potent as quartz. Fibrous glass also possessed stimulatory properties and again a sample having a short length gave a stronger response than a long one. Collagen formation by asbestos thus appears to be mediated by a macrophage factor, so operating in a manner similar to that previously demonstrated for quartz. The conventional view that short fibres are comparatively insignificant in asbestos fibrogenesis cannot, on the present evidence, be sustained. Furthermore, it should not be assumed than man-made mineral fibres of respirable diameter are innocuous or that short ones can be ignored.

In vitro assessment of the fibrogenicity of mineral dusts.

Relying on in vitro production of the macrophage fibrogenic factor, an attempt was made to quantitate the fibrogenicity of mineral particles. Having determined the optimal conditions by means of quartz, two series of experiments were conducted with respirable coal mine dusts; the first employed artificial mixtures of a mine dust, having a low natural quartz content, with various proportions of quartz, and the second native dusts from European mines. The fibrogenic responses in both series suggest that dust concentration is more important than its composition. Quartz and ash contents of native dusts bore no evident relation to fibrogenicity, high quartz and ash levels sometimes being associated with low collagen levels and vice versa. Comparison with other information disclosed disparities with epidemiology and different experimental assessments. Factors affecting the disarray may well include the neglected lipid response in vivo and individual susceptibility, aspects which do not appear beyond experimental assay.

Replenishment of alveolar macrophages in silicosis: implication of recruitment by lipid feed-back.

The deposition of quartz in the pulmonary alveoli creates a major demand for macrophages to replace those destroyed, but local proliferation of monocytes appeared to be minimal and the role of systemic recruitment was therefore explored. Injected silica and lipids stimulated the phagocytic function of the mononuclear phagocytic system (MPS), whilst inhaled silica provoked lipid accumulation in the lung, thus suggesting that lipid might also induce a proliferative response in the marrow. Using marrow cultures, cells of the rat MPS were identified by size and phagocytic capacity for latex microspheres, and then subjected to kinetic analysis in litter-mate pairs by single and double labelling autoradiography, under normal conditions and after administration of lipid extracted from rat lungs consolidated by silica-induced alveolar lipo-proteinosis. Treatment of the results by a new device facilitated distinction of promonocytes from monocytes and thus afforded a more precise means of assessing MPS kinetics. The duration of DNA synthesis and the cell-cycle time of promonocytes were reduced and the rate of entry into DNA synthesis increased as a result of i.v. injection of lung lipid. These findings support the involvement of systemic recruitment of monocytes from the marrow by a positive feed-back mechanism when a powerful irritant persists in the lungs and the results are discussed in the overall context of silicotic fibrogenesis.

The relative sensitivity of pulmonary parenchymal cells to 239plutonium dioxide.

Alpha particles inhaled by mice affect primarily type II epithelial cells, whereas interstitial mononuclears, alveolar macrophages and type I epitehlium are much more resistant and apparently react secondarily. The cellular responses, qualitative and quantitative, exhibit a time-dose relationship.

Cell kinetics of urethane-induced murine pulmonary adenomas: III. Implications of the disparity between the rates of entry into DNA synthesis and into mitosis.

Metaphase arrest by vincristine in urethane-induced murine pulmonary adenomas became linear after an interval of 60 min. The rate of entry into metaphase was 0-191%/h, which was considerably less than the 1%/h for the rate of entry into DNA synthesis obtained previously by double labelling. The duration of prophase plus metaphase was calculated to be 1-7 h. A growth fraction of 9% and a cell-loss factor of 52% were derived. The disparity between rates of entry into DNA synthesis and into metaphase was investigated by microdensitometry on Feulgen-stained squash preparations of tumours of varying ages. Tne DNA profiles showed an increasing frequency of hyperdiploid nuclei with age. Circumstantial evidence for polyploidy was provided by the presence of many binucleate cells in the tumours. By analogy with the liver, these cells may well represent a stage in the development of polyploidy, and the possible relevance of these findings to the neoplastic process is considered.

Effect of inhaled plutonium dioxide on development of urethan-induced pulmonary adenomas.

Mice were exposed to plutonium dioxide (PuO2) aerosols 2 weeks before or after urethan injection. Both exposures reduced the number and size of adenomas. The incidence of arrested metaphases showed no consistently significant differences between plutonium-exposed and mock-exposed animals. The results are discussed in relation to recent electron microscopic evidence of degenerative changes in the type II epithelial cells of the mouse lung following PuO2 inhalation. It is concluded that damage at the cellular level may account for the observed reduction in growth of pulmonary adenomas in mice whose lungs contained plutonium particles.