Immunological consequences of mineral dust inhalation. Final report on CEC contract 7248-33.050-08

The alveolar macrophage plays an important role in the phagocytosis of dust particles such as coalmine dust. In addition, through various functions and secretions, the alveolar macrophage also influences other leukocytes to control immune and inflammatory responses and also controls tissue growth and turnover. Subsequently, there has been a suggestion that the immune system may be modulated in the pathological response to mineral dust, via the alveolar macrophage. We set out to study whether we could detect evidence of immunomodulation following exposure to dust collected from the air of British coalmines. In the first sequence of experiments, splenocytes obtained from rats inhaling coalmine dust, quartz as a positive control or titanium dioxide as a negative control, were assessed for non-specific responsivity to the common T cell mitogens conconavalin A and phytohaemagglutinin. Rats inhaled dust collected from the air of coalmines mining (a) anthracite; (b) high rank coking coal; (c) low rank butiminous coal, rutile titanium dioxide or Euro-standard quartz; airborne mass concentrations of 10 and 50mg/m3 were used. None of the above treatments produced any significant change in the responses of splenic T cells to mitogen. A change in responsivity to mitogen would have been anticipated if there had been other ongoing T cell immune responses. Occasional differences between treated rats and control rats were attributable to chance experimental variation. In the second sequence of experiments, localised changes in the immunomodulatory functions of alveolar macrophages were assessed in rats which had been instilled with Img of low rank bituminous coalmine dust, anthracite coalmine dust, TiO 2 or quartz. Normal bronchoalveolar leukocytes were predominantly macrophages and were markedly suppressive to T lymphocytes. Instillation of any of the coalmine dusts, TiO2 or quartz, caused a transient inflammation but this was greatest for thelow rank coalmine dust and quartz, the latter causing a sustained severe inflammation. Whole populations of bronchoalveolar leukocytes from any of these dust-inflamed lungs showed the same degree of suppression of T lymphocytes as control bronchoalveolar leukocytes.Separation of neutrophils from one day coalmine dust or seven day quartz bronchoalveolar leukocytes showed these cells to be significantly less suppressive than control bronchoalveolar leukocytes, although they were not stimulatory to lymphocyte proliferation; the separated macrophages showed the same degree of suppression as control alveolar macrophages. Bronchoalveolar leukocytes from rats exposed to both types of coalmine dust for seven days failed to show any differences from controls in their ability to suppress T lymphocyte responses.When splenocytes from control rats were treated with dust in vitro and the effects on splenocyte mitogenesis determined, there were complex dose effects with anthracite coalmine dust and titanium dioxide causing suppression, low rank coalmine dust having no effect and quartz causing marked stimulation. This effect did not appear to be simply related to the production of Interleukin-1 (IL-1) by quartz since quartz did not stimulate the greatest amount of IL-1 production by splenic macrophages, with only coalmine dust L causing more release of IL-1 than titanium dioxide. IL-1 production by alveolar macrophages in vitro was stimulated by a low dose of quartz but at higher doses there was no significant stimulation compared to controls, presumably due to toxic effects. For all of the other dusts there was a gradual increase in IL-1 production with increase in dose in vitro but only low rank coalmine dust was significantly stimulatory compared to titanium dioxide. When bronchoalveolar leukocytes were obtained from coalmine dust-instilled rats and cultured to assess Interleukin-1 production, there was no significant production compared to titanium dioxide treated controls. When quartz bronchoalveolar leukocytes were separated into quartz macrophages and quartz neutrophils and the cells cultured, the quartz neutrophils produced more Interleukin-1 than the whole population, but this did not reach statistical significance.In the third sequence of experiments, titanium dioxide, quartz or coalmine dusts were instilled into the peritoneal cavity of mice and the splenocyte mitogenic response was measured. The effect was a suppression of response with quartz and a stimulation with low rank coalmine dust.In conclusion, the results suggest that there is no detectable systemic immunological effects of coalmine dust inhalation using splenic responses to T cell mitogens as an end point. They show also that, despite the inflammation in the alveolar region and changes in the bronchoalveolar population, the suppressive effects of the complete bronchoalveolar leukocyte population is conserved. The results of Interleukin-1 production in vitro and on separated populations revealed that complex suppressive effects are very likely to be involved in controlling the release of immunomodulatory molecules by bronchoalveolar leukocytes. The selectivevalue of tightly controlled suppression in the alveolar region is evident in that immune responses in the alveolar region would not be beneficial in view of the number of potentially stimulatory immunological and inflammatory mediators that are released. However, in the milieu of the lymph node where immune responses should occur, and to which dust-activated leukocytes migrate, the spatial separation of neutrophils and macrophages could diminish mutual suppressive effects, possibly even leading to immune stimulation. “”