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Dust elimination from pulmonary alveoli
ENVIRONMENTAL
RESEARCH
23, 224-227 (1980)
SHORT COMMUNICATION Dust Elimination from Pulmonary
Alveoli
P. F. HOLT Department
of Chemistry,
The
University,
Reading
RG6 2AD,
England
Received December 20, 1979 Some alveolar macrophages that engulf asbestos particles move through alveolar walls and spaces to the larger bronchioles. They pass through the,bronchiolar wall to reach the lumen where they are elevated by cilia and pass out of the lung.
Removal of dust from the human lung has been studied by observing the rate of elimination of inhaled 51Cr-labeled polystyrene spheres (Booker et al., 1967). The spheres are eliminated rapidly during the first 10 hr but subsequent elimination is slow, corresponding to a biological half-life of 150-300 days. The biological halflife of plutonium has been found to be about 500 days. Slow elimination of industrial dusts from the human lung has often been observed; for example, diatoms (Wende, 1962) and asbestos (Luton et al., 1947) have been found in the sputum of work people several years after they ceased to handle diatomaceous earth or asbestos. The initial stage of rapid dust elimination relates to the removal of particles from larger air vessels which, as far as the terminal bronchioles, are lined with ciliated epithelium. Dust particles that settle on this surface are moved toward the trachea. There are no cilia in the alveolar region. Dust particles that are deposited beyond the terminal bronchioles are eventually ingested mainly by macrophages, large phagocytic cells that may be part of an alveolar wall or may be free. The stage of slow elimination of dust relates to the removal of particles from alveoli in macrophages. The macrophages eventually enter the larger bronchioles and are escalated by cilia, but how they migrate from the alveoli to the bronchioles has not been explained; details of alveolar clearance are lacking (Morrow, 1972). It is difficult to see why a macrophage should move along an airway, through the respiratory and terminal bronchioles. Bowden (1971), quoting Hatch and Gross, pointed out that the distance from the air sac to cilia in a bronchiole is about 0.5-3.0 mm and movements of this magnitude cannot be explained on the basis of amoeboid motion; there is no known tropism that would account for such purposeful movement toward the bronchiole. Evidence has been sought to explain the route by which alveolar macrophages reach the bronchiolar lumen. For this purpose, a study was made of the lungs of guinea pigs that had inhaled chrysotile asbestos. METHODS
Specific pathogen-free guinea pigs were exposed to dust prepared from Rhodesian chrysotile in the apparatus previously described (Holt et al., 1964) for 24 hr. 224 0013-9351/80/050224-04$02.00/O Copyright All rights
@ 1980 by Academic Press, Inc. of reproduction in any form reserved.
SHORT
COMMUNICATION
225
A dust concentration of about 1000 fibers > 1 pm/ml was maintained. Fibers up to 70 pm in length were in the atmosphere. Histological sections from wax blocks were processed and stained with eosin and Perls’ reagent; macrophages that have ingested the dust are strongly Perls positive and may be readily identified in the absence of nuclei stained with hematoxylin. Sections were scanned over their entire area using a 25 objective and examining every macrophage. The few macrophages that appeared to give information about the transport of dust were photographed. RESULTS
AND DISCUSSION
The reaction of the lung to dust resembles a reaction to infection in that leukocytes and erythrocytes leave capillaries by diapedesis. Dust and extravascular erythrocytes are engulfed by macrophages. Iron, from the breakdown of hemoglobin, is stored in phagosomes some of which fuse so that inclusions in the cytoplasm of these macrophages stain intensely blue with Perls’ reagent (Fig. 1). The Perls reaction thus acts as a marker for macrophages and it can be assumed
FIG. 1. Macrophage with Perls-positive inclusions passing through an alveolar wall. Guinea pig inhaled chrysotile asbestos for 24 hr and was killed 20 weeks later. Perls + eosin. x700.
226
SHORT
COMMUNICATION
that Perls-positive macrophages found in bronchiolar walls and lumen have moved from alveolar regions. In all sections there were macrophages that carried the debris of asbestos fragments. Some macrophages had fused to give giant cells. Particularly in the sections from animals killed about 40 weeks after inhaling dust, a few macrophages were observed that had been in a phase of active migration. Some had extended processes, and some were passing through an alveolar wall (Fig. 1). Morgan et al. (1975), from autoradiographs of the lungs of rats that had inhaled irradiated dusts (5gFe and 51Cr), demonstrated that while the dust was at first fairly evenly distributed, it later accumulated in certain limited areas, notably near the periphery of the lung. These macrophages could not have moved along airways toward the pleura since, once they reached terminal bronchioles, they could not move back toward alveoli against the action of the cilia. Apparently they moved to these sites by penetration of alveolar walls. Our sections revealed some accumulations of macrophages in the neighborhood
FIG. 2. tiacrc lphages in the wall and one macrophages in the lumen of a large bron Guine; b pi ed chrysotile asbestos for 24 hr and was killed 41 weeks later. Perls to emphasize the macrophages. Green fi1t el r was used in photography
iiole (al eosin.
SHORT
COMMUNICATION
227
of larger bronchioles. Macrophages were visible in adjacent alveoli, in the bronchiolar wall, and in the lumen (Fig. 2). The walls of larger bronchioles are distorted and the tissue is compressed by postmortem contraction of the muscular layer so that macrophages are seen as elongated flattened cells in the tissue. Apparently, alvelolar macrophages with dust reach the cilia of larger bronchioles not by following the airways but by a route through alveolar and bronchiolar walls. The long period that elapses before all the dust is removed from the alveoli may be due partly to the time taken by the macrophages to move over the comparatively long distances, and partly to a slower rate of phagocytosis than has previously been assumed. REFERENCES Booker, D. V., Chamberlain, A. C., Rundo, J., and Muir, D. C. F. (1967). Elimination of 5~ particles from the human lung. Nature (London) 215, 30-33. Bowden, D. H. (1971). The alveolar macrophage. Curr. Top. Pathal. 55, l-36. Holt, P. F., Mills, J., and Young, D. K. (1964). The early effects of chrysotile asbestos dust on the rat lung. J. Path& Burt. 87, 15-23. Luton, P., Champeix, J., and Faure, P. (1947). The significance of the presence of the asbestos body in the expectoration of asbestos workers. Arch. Mal. Prof. Med. Trav. 8, 56-58. Morgan, A., Evans, J. C., and Holmes, A. (1975). Deposition and clearance of inhaled fibrous minerals in the rat. Studies using radioactive tracer techniques. In “Inhaled Particles” (W. H. Walton, Ed.), Vol. IV, pp. 259-272. Pergamon, Oxford. Factors in Respiratory Disease” Morrow, P. E. (1972). Airborne contaminants. In “Environmental (D. H. K. Lee, Ed.), pp. 71-85. Academic Press, New York. Wende, E. (1962). Gewerbehygiene und Klinik der Kieselgursilikose zugleich ein Beitrag zur Gewerbehygiene Iungenaggressiver St&be. Zentbl. ArbMed. ArbSchutz. Suppl. No. 6.
RESEARCH
23, 224-227 (1980)
SHORT COMMUNICATION Dust Elimination from Pulmonary
Alveoli
P. F. HOLT Department
of Chemistry,
The
University,
Reading
RG6 2AD,
England
Received December 20, 1979 Some alveolar macrophages that engulf asbestos particles move through alveolar walls and spaces to the larger bronchioles. They pass through the,bronchiolar wall to reach the lumen where they are elevated by cilia and pass out of the lung.
Removal of dust from the human lung has been studied by observing the rate of elimination of inhaled 51Cr-labeled polystyrene spheres (Booker et al., 1967). The spheres are eliminated rapidly during the first 10 hr but subsequent elimination is slow, corresponding to a biological half-life of 150-300 days. The biological halflife of plutonium has been found to be about 500 days. Slow elimination of industrial dusts from the human lung has often been observed; for example, diatoms (Wende, 1962) and asbestos (Luton et al., 1947) have been found in the sputum of work people several years after they ceased to handle diatomaceous earth or asbestos. The initial stage of rapid dust elimination relates to the removal of particles from larger air vessels which, as far as the terminal bronchioles, are lined with ciliated epithelium. Dust particles that settle on this surface are moved toward the trachea. There are no cilia in the alveolar region. Dust particles that are deposited beyond the terminal bronchioles are eventually ingested mainly by macrophages, large phagocytic cells that may be part of an alveolar wall or may be free. The stage of slow elimination of dust relates to the removal of particles from alveoli in macrophages. The macrophages eventually enter the larger bronchioles and are escalated by cilia, but how they migrate from the alveoli to the bronchioles has not been explained; details of alveolar clearance are lacking (Morrow, 1972). It is difficult to see why a macrophage should move along an airway, through the respiratory and terminal bronchioles. Bowden (1971), quoting Hatch and Gross, pointed out that the distance from the air sac to cilia in a bronchiole is about 0.5-3.0 mm and movements of this magnitude cannot be explained on the basis of amoeboid motion; there is no known tropism that would account for such purposeful movement toward the bronchiole. Evidence has been sought to explain the route by which alveolar macrophages reach the bronchiolar lumen. For this purpose, a study was made of the lungs of guinea pigs that had inhaled chrysotile asbestos. METHODS
Specific pathogen-free guinea pigs were exposed to dust prepared from Rhodesian chrysotile in the apparatus previously described (Holt et al., 1964) for 24 hr. 224 0013-9351/80/050224-04$02.00/O Copyright All rights
@ 1980 by Academic Press, Inc. of reproduction in any form reserved.
SHORT
COMMUNICATION
225
A dust concentration of about 1000 fibers > 1 pm/ml was maintained. Fibers up to 70 pm in length were in the atmosphere. Histological sections from wax blocks were processed and stained with eosin and Perls’ reagent; macrophages that have ingested the dust are strongly Perls positive and may be readily identified in the absence of nuclei stained with hematoxylin. Sections were scanned over their entire area using a 25 objective and examining every macrophage. The few macrophages that appeared to give information about the transport of dust were photographed. RESULTS
AND DISCUSSION
The reaction of the lung to dust resembles a reaction to infection in that leukocytes and erythrocytes leave capillaries by diapedesis. Dust and extravascular erythrocytes are engulfed by macrophages. Iron, from the breakdown of hemoglobin, is stored in phagosomes some of which fuse so that inclusions in the cytoplasm of these macrophages stain intensely blue with Perls’ reagent (Fig. 1). The Perls reaction thus acts as a marker for macrophages and it can be assumed
FIG. 1. Macrophage with Perls-positive inclusions passing through an alveolar wall. Guinea pig inhaled chrysotile asbestos for 24 hr and was killed 20 weeks later. Perls + eosin. x700.
226
SHORT
COMMUNICATION
that Perls-positive macrophages found in bronchiolar walls and lumen have moved from alveolar regions. In all sections there were macrophages that carried the debris of asbestos fragments. Some macrophages had fused to give giant cells. Particularly in the sections from animals killed about 40 weeks after inhaling dust, a few macrophages were observed that had been in a phase of active migration. Some had extended processes, and some were passing through an alveolar wall (Fig. 1). Morgan et al. (1975), from autoradiographs of the lungs of rats that had inhaled irradiated dusts (5gFe and 51Cr), demonstrated that while the dust was at first fairly evenly distributed, it later accumulated in certain limited areas, notably near the periphery of the lung. These macrophages could not have moved along airways toward the pleura since, once they reached terminal bronchioles, they could not move back toward alveoli against the action of the cilia. Apparently they moved to these sites by penetration of alveolar walls. Our sections revealed some accumulations of macrophages in the neighborhood
FIG. 2. tiacrc lphages in the wall and one macrophages in the lumen of a large bron Guine; b pi ed chrysotile asbestos for 24 hr and was killed 41 weeks later. Perls to emphasize the macrophages. Green fi1t el r was used in photography
iiole (al eosin.
SHORT
COMMUNICATION
227
of larger bronchioles. Macrophages were visible in adjacent alveoli, in the bronchiolar wall, and in the lumen (Fig. 2). The walls of larger bronchioles are distorted and the tissue is compressed by postmortem contraction of the muscular layer so that macrophages are seen as elongated flattened cells in the tissue. Apparently, alvelolar macrophages with dust reach the cilia of larger bronchioles not by following the airways but by a route through alveolar and bronchiolar walls. The long period that elapses before all the dust is removed from the alveoli may be due partly to the time taken by the macrophages to move over the comparatively long distances, and partly to a slower rate of phagocytosis than has previously been assumed. REFERENCES Booker, D. V., Chamberlain, A. C., Rundo, J., and Muir, D. C. F. (1967). Elimination of 5~ particles from the human lung. Nature (London) 215, 30-33. Bowden, D. H. (1971). The alveolar macrophage. Curr. Top. Pathal. 55, l-36. Holt, P. F., Mills, J., and Young, D. K. (1964). The early effects of chrysotile asbestos dust on the rat lung. J. Path& Burt. 87, 15-23. Luton, P., Champeix, J., and Faure, P. (1947). The significance of the presence of the asbestos body in the expectoration of asbestos workers. Arch. Mal. Prof. Med. Trav. 8, 56-58. Morgan, A., Evans, J. C., and Holmes, A. (1975). Deposition and clearance of inhaled fibrous minerals in the rat. Studies using radioactive tracer techniques. In “Inhaled Particles” (W. H. Walton, Ed.), Vol. IV, pp. 259-272. Pergamon, Oxford. Factors in Respiratory Disease” Morrow, P. E. (1972). Airborne contaminants. In “Environmental (D. H. K. Lee, Ed.), pp. 71-85. Academic Press, New York. Wende, E. (1962). Gewerbehygiene und Klinik der Kieselgursilikose zugleich ein Beitrag zur Gewerbehygiene Iungenaggressiver St&be. Zentbl. ArbMed. ArbSchutz. Suppl. No. 6.