Oxidant production by control and inflammatory bronchoalveolar leukocyte populations treated with mineral dusts in vitro

Neutrophil sequestration in rat lungs

BACKGROUND

The transit of neutrophils through the pulmonary microvasculature is prolonged compared with red blood cells and is increased further during cigarette smoking and in exacerbations of chronic obstructive pulmonary disease. The increased residence time (sequestration) of neutrophils in the pulmonary capillaries in these conditions may be the first step leading to the accumulation of cells within the lung interstitium and in the bronchoalveolar space, so potentiating lung damage. A rat model has been developed to investigate the factors which may influence neutrophil transit through the lung microvasculature.

METHODS

Intratracheal instillation of the heat killed organism Corynebacterium parvum was used to induce an acute neutrophil alveolitis. Neutrophils and red blood cells were isolated from donor rats, labelled with two distinct radioisotopes, and then reinjected into recipient rats to assess their transit through the pulmonary circulation. To ascertain whether peripheral blood neutrophils were minimally altered by the isolation procedure their functional status in vitro was compared with that of inflammatory neutrophils in a number of assays commonly used as descriptors of neutrophil activation. The influence of neutrophil activation on the accumulation of cells in the lungs was assessed by comparing the lung sequestration of control neutrophils, isolated from peripheral blood, with that of inflammatory neutrophils obtained from bronchoalveolar lavage of inflamed rat lungs. Lung sequestration of neutrophils was defined as the fold increase in the ratio of neutrophils labelled with chromium-51 to red blood cells labelled with technetium-99m in lung tissue compared with the same ratio in peripheral blood.

RESULTS

Sequestration of peripheral blood neutrophils occurred in control rat lungs as shown by a 17.5 (2.1) fold increase in the ratio of neutrophils to red blood cells in the pulmonary circulation compared with the ratio of these cells in the peripheral circulation. When inflammatory neutrophils, obtained by bronchoalveolar lavage from C parvum-treated animals, were injected into control rats, the increase was 90.6 (11.0) fold. Induction of an inflammatory response in the lung tissue of the recipient rat also caused an increase in the sequestration of control neutrophils compared with the same cells in control rat lungs which was, however, less marked than when inflammatory neutrophils were used (34.7 (4.7) fold). The mean (SE) pressure developed on filtration of inflammatory neutrophils in vitro through a millipore filter (7.53 (0.2) cm H2O) was greater than that of peripheral blood neutrophils (1.18 (0.2) cm H2O). Increased filtration pressure indicates a decrease in cell deformability and suggests that this may be a contributory factor to the increased sequestration of inflammatory neutrophils in the pulmonary vasculature.

CONCLUSIONS

This study shows that there is sequestration of neutrophils in the pulmonary vasculature in normal rat lungs which increases in acute lung inflammation and when inflammatory neutrophils are injected into control animals. In this model changes in the neutrophil, such as cell deformability, may have a more important role in inducing increased neutrophil sequestration than the inflammatory response in the lungs.

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.

Interferon-gamma and immunoglobulin enhance mineral dust-induced production of reactive oxygen metabolites by human macrophages

The modulating effect of interferon-gamma (IFN-gamma) and polyclonal immunoglobulin (poly-Ig) on mineral dust-induced production of reactive oxygen metabolites (ROM) by human monocyte-derived macrophages was investigated using lucigenin-dependent chemiluminescence. Activation of the cell cultures with IFN-gamma enhanced the subsequent quartz- and asbestos-induced production of ROM, while the addition of poly-Ig only stimulated chrysotile asbestos-induced ROM production. The combination of IFN activation and the addition of poly-Ig to the reactions caused a 20-fold enhancement of the responses to chrysotile. As increased levels of IFN-gamma and immunoglobulins have been observed in patients with mineral dust-induced diseases, we suggest that "immunological activation" caused by, for example, autoimmune diseases or recurrent pulmonary infections might amplify the production of potentially tissue-injuring ROM by mononuclear phagocytes and may possibly determine the individual susceptibility to mineral dust-related disorders.

Bronchoalveolar leukocyte response in experimental silicosis: modulation by a soluble aluminum compound

The biological properties of quartz have been related to its surface reactivity. We have addressed the role of particle surface reactivity in mediating the biological activity of quartz in mixed dusts, by treating the quartz with aluminum lactate. Intratracheal instillation of untreated quartz in rats caused a rapid, sustained alveolitis and bronchoalveolar leukocytes obtained from these animals had enhanced activity in degrading fibronectin, but reduced ability to mount a respiratory burst. Quartz pretreated with aluminum elicited a markedly reduced inflammatory response; the reduced activity of the treated quartz was also reflected in the attenuated change in the key functional parameters, oxidant production and proteolysis of fibronectin. Late intratracheal dosing with aluminum after the quartz-induced alveolitis was well established reduced the inflammatory response and abrogated the effect of quartz on the respiratory burst, but did not alter fibronectin degradation by the leukocytes. Aluminum did not affect the inflammatory response to Corynebacterium parvum and thus the effect was on the quartz particles and not on the inflammatory leukocytes. These findings have implications for the likely pulmonary responses to mixed dusts containing quartz and aluminum silicate clays.