Experimental study of interactions between pneumoconiosis and mycobacterial infections

Immunity to the Dual Threat of Silica Exposure and Mycobacterium tuberculosis

Exposure to silica and the consequent development of silicosis are well-known health problems in countries with mining and other dust producing industries. Apart from its direct fibrotic effect on lung tissue, chronic and immunomodulatory character of silica causes susceptibility to tuberculosis (TB) leading to a significantly higher TB incidence in silica-exposed populations. The presence of silica particles in the lung and silicosis may facilitate initiation of tuberculous infection and progression to active TB, and exacerbate the course and outcome of TB, including prognosis and survival. However, the exact mechanisms of the involvement of silica in the pathological processes during mycobacterial infection are not yet fully understood. In this review, we focus on the host's immunological response to both silica and Mycobacterium tuberculosis, on agents of innate and adaptive immunity, and particularly on silica-induced immunological modifications in co-exposure that influence disease pathogenesis. We review what is known about the impact of silica and Mycobacterium tuberculosis or their co-exposure on the host's immune system, especially an impact that goes beyond an exclusive focus on macrophages as the first line of the defense. In both silicosis and TB, acquired immunity plays a major role in the restriction and/or elimination of pathogenic agents. Further research is needed to determine the effects of silica in adaptive immunity and in the pathogenesis of TB.

Animal models of pulmonary infection in the compromised host: potential usefulness for studying health effects of inhaled particles

Pulmonary infection leading to pneumonia is a significant cause of morbidity and mortality worldwide. Airborne particles have been associated with pneumonia through epidemiological research, but the mechanisms by which particles affect the incidence of pneumonia are not well established. The purpose of this review is to examine the potential of animal models to improve our understanding of the mechanisms by which inhaled particles might affect the incidence and resolution of pulmonary infection. The pathogenesis of pneumonia in most animal models differs from that in humans because humans frequently have underlying diseases that predispose them to infection with relatively low doses of pathogens. Normal, healthy animals lack the underlying pathology often found in humans and clear bacteria and viruses rapidly from their lungs. To overcome this, animals are administered large inocula of pathogens, are treated with agents that cause mucosal lesions, or are treated with immunosuppressive drugs. Alternatively, pathogenic bacteria are protected from phagocytosis by encasing them in agar. No one animal model will replicate a human disease in its entirety, and the choice of model depends upon how well the animal infection mimics the particular human response being examined. The advantages and disadvantages of animal models in current use for bacterial and viral infections important in the etiology of human pneumonia are reviewed in detail. Considerable data indicate that prior exposure to particles compromises the ability of experimental animals to resolve a subsequent infection. In addition, information is available on the effects of particle exposure on various portions of respiratory defense including phagocytic function, ciliary movement, inflammation, and antibody response in the absence of infection. In contrast, little research to date has examined the consequences of particle exposure on the host defense mechanisms of animals already infected or on their ability to resolve their infection.

Tuberculosis Mortality and Silica Exposure: A Case-Control Study Based on a National Mortality Database for the Years 1983-1992

A case-control study (8,740 cases; 83,338 controls) was done to evaluate the association between potential occupational exposure to silica and risk of tuberculosis (TB) mortality, using the National Occupational Mortality Surveillance database for 1983-1992. Potential silica exposure was based on the decedent's usual industry and occupation. Assignment of potential exposure to silica was based on the entire range of industries and occupations. Odds ratios (ORs) for mortality from respiratory TB associated with potential high and intermediate exposures to silica were 1.30 (95% CI 1.14-1.48) and 1.07 (95% CI 0.77-1.47), respectively, adjusting for silicosis, other pneumoconioses, age, gender, race, socioeconomic status, and potential exposure to active TB. The elevated OR was seen in all subgroups: male, female, white, and black. The average age at death among respiratory TB cases with potential silica exposure was significantly younger by four years than that among cases without potential silica exposure. The findings suggest that potential occupational exposure to silica alone, in the absence of silicosis, is associated with respiratory TB mortality, with a dose-response relationship.

Cancer risk and mortality patterns among silicotic men in Sweden and Denmark

Data from nationwide registry-based cohorts of patients hospitalized for silicosis in Sweden from 1965 to 1983 and Denmark from 1977 to 1989 were linked to national cancer registries in both countries and to mortality data in Sweden to evaluate the risk of cancer and other disorders among hospitalized silicotic patients. The overall cancer standardized incidence ratio (SIR) was 1.5 (95% confidence interval [CI], 1.3 to 1.7) in Sweden and 1.7 (95% CI, 1.2 to 2.3) in Denmark, primarily because of elevations in primary lung cancer in both Sweden (SIR, 3.1; CI, 2.1 to 4.2) and Denmark (SIR, 2.9; CI, 1.5 to 5.2). For Sweden, the all-causes standardized mortality ratio (SMR) was 2.0 (1.9 to 2.2). The SMR for all malignancies was 1.5 (1.2 to 1.7), primarily because of excesses of lung cancer (SMR, 2.9; CI, 2.1 to 3.9). The significant increase in mortality for all infectious and parasitic conditions (SMR, 11.2) was primarily due to tuberculosis (SMR, 21.8). Significant excesses in mortality from silicosis (SMR, 523), bronchitis (SMR, 2.6) and emphysema (SMR, 6.7) contributed to the elevation in nonmalignant respiratory deaths (SMR, 8.8), whereas excess mortality from musculoskeletal disorders (SMR, 5.9) was due to six deaths from autoimmune diseases. Despite limitations of the available data, our findings are consistent with previous reports indicating that silicotic patients are at elevated risk of lung cancer, nonmalignant respiratory diseases, tuberculosis, and certain autoimmune disorders.

Superoxide anion generation by alveolar inflammatory cells in simple pneumoconiosis and in progressive massive fibrosis of nonsmoking coal workers

We have examined superoxide anion (O2-) release by alveolar inflammatory cells recovered by bronchoalveolar lavage from the lower respiratory tract of 10 healthy nonsmokers and 25 nonsmoking pneumoconiotic patients, 11 with radiographic changes of simple pneumoconiosis (SP) and 14 with changes of progressive massive fibrosis (PMF). Significant increased number of cells was recovered from the lower respiratory tract from both patients with SP or with PMF. Alveolitis was made up predominantly of alveolar macrophages (AM) and an increased percentage of neutrophils in patients with PMF (3.3 +/- 0.7%). O2- release was evaluated using a superoxide dismutase (SOD)-inhibitable lucigenin-dependent chemiluminescence method. Spontaneous O2- generation by alveolar inflammatory cells from pneumoconiotic patients with SP was three to four times greater than that from 10 age-matched, healthy control subjects. O2- release by alveolar inflammatory cells from patients with PMF was dramatically increased when compared with that in patients with SP and with that in control subjects and was observed before and after stimulation by phorbol myristate acetate (PMA) (p less than 0.001). The increased O2- release was not due to a lack of enzyme antioxidant system within AM since intracellular superoxide dismutase was not lower in AM from patients than in AM from control subjects (p less than 0.05). Alteration of DLCO correlated with PMA-induced superoxide release by alveolar inflammatory cells in patients with PMF (p less than 0.05). Our data demonstrate that alveolar inflammatory cells from pneumoconiotic patients with PMF are in the activated state and release more oxygen-reactive species that do those from patients with SP.(ABSTRACT TRUNCATED AT 250 WORDS)

Early pulmonary response of the rat lung to inhalation of high concentration of diesel particles

Light microscopy was used to study the pathologic alterations in the rat lung after timed diesel exposures to 6000 micrograms m-3 of diesel particulates ranging from 6 h to 9 weeks. As a general reaction, an increase in alveolar macrophages containing diesel particles, an increase in type II pneumocytes, and an accumulation of inflammatory cells within the alveoli and septal walls was observed. Histochemical differentiation between type II cells and alveolar macrophages was clearly delineated by the use of the PAS (periodic acid Schiff) stain in which macrophages stained positive and type II cells negative for this reaction. After the 9-week exposure, the accumulation of macrophages and accompanying type II cell reaction resulted in the formation of alveolar complexes that seemed to be preferentially localized near the terminal bronchioles. The similarity of the lesion to that of 'macrophage alveolitis' produced in animals repeatedly exposed to coal dust is emphasized, although the ultimate clinical significance of the complex formation cannot be assessed within the time limits of the experimental observation.