The origin and turnover of alveolar macrophages in experimental pneumonia

Respiratory Tract and Mediastinum

Normal cytology, abnormal and atypical cells, non-cellular components, and infectious cell changes are largely described together with benign, malignant, and neuroendocrine lesions regarding exfoliative and aspiration cytology of the lung. A separate section broadly addresses diagnostic findings and differential diagnoses in bronchoalveolar washings.

The section ‘Fine needle aspiration biopsy of mediastinal disorders’ covers in particular biopsy techniques, accuracy of liquid-based cytology, and the complex lesions of the thymus gland. Cytodiagnostic algorithms of the major benign and malignant pulmonary and mediastinal lesions and their respective differential diagnoses are additionally presented in synoptic setups.

Bacterial Infections

In Osier’s time, bacterial pneumonia was a dreaded event, so important that he borrowed John Bunyan’s characterization of tuberculosis and anointed the pneumococcus, as the prime pathogen, “Captain of the men of death.”1 One hundred years later much has changed, but much remains the same. Pneumonia is now the sixth most common cause of death and the most common lethal infection in the United States. Hospital-acquired pneumonia is now the second most common nosocomial infection.2 It was documented as a complication in 0.6% of patients in a national surveillance study,3 and has been reported in as many as 20% of patients in critical care units.4 Furthermore, it is the leading cause of death among nosocomial infections.5 Leu and colleagues6 were able to associate one third of the mortality in patients with nosocomial pneumonia to the infection itself. The increase in hospital stay, which averaged 7 days, was statistically significant. It has been estimated that nosocomial pneumonia produces costs in excess of $500 million each year in the United States, largely related to the increased length of hospital stay.

Augmented Proliferation of Human Alveolar Macrophages After Allogeneic Bone Marrow Transplantation

After allogeneic bone marrow transplantation (allo-BMT), recipient alveolar macrophages (AM) are gradually replaced by AM of the donor origin. An influx of mononuclear phagocytes of donor origin to the lung is responsible for the repopulation, but the detailed kinetics remain unclear. We therefore studied 24 BMT recipients who underwent bronchoalveolar lavage (BAL) from 24 to 83 days after BMT. AM cell number, size, morphology, proliferating ability, and genotype of AM were measured. Before day 50, the number and size of AM in BAL fluid were similar to those of normal nonsmokers. However, after day 50, the mean number of AM increased threefold and the mean cell size decreased due to the increase of small AM. These small cells are presumably of donor origin based on DNA fingerprinting analysis and based on fluorescence in situ hybridization for the Y chromosome in a sex-mismatched case. Immunohistochemistry and cell cycle analysis demonstrated that the increase in AM number coincided with a remarkable increase of AM expressing proliferating cell nuclear antigen, suggesting that small AM are proliferating. This is the first report representing that augmented proliferation of donor AM in situ may contribute to the reconstitution of AM population after BMT.

Augmented Proliferation of Human Alveolar Macrophages After Allogeneic Bone Marrow Transplantation

After allogeneic bone marrow transplantation (allo-BMT), recipient alveolar macrophages (AM) are gradually replaced by AM of the donor origin. An influx of mononuclear phagocytes of donor origin to the lung is responsible for the repopulation, but the detailed kinetics remain unclear. We therefore studied 24 BMT recipients who underwent bronchoalveolar lavage (BAL) from 24 to 83 days after BMT. AM cell number, size, morphology, proliferating ability, and genotype of AM were measured. Before day 50, the number and size of AM in BAL fluid were similar to those of normal nonsmokers. However, after day 50, the mean number of AM increased threefold and the mean cell size decreased due to the increase of small AM. These small cells are presumably of donor origin based on DNA fingerprinting analysis and based on fluorescence in situ hybridization for the Y chromosome in a sex-mismatched case. Immunohistochemistry and cell cycle analysis demonstrated that the increase in AM number coincided with a remarkable increase of AM expressing proliferating cell nuclear antigen, suggesting that small AM are proliferating. This is the first report representing that augmented proliferation of donor AM in situ may contribute to the reconstitution of AM population after BMT.

Eosinophil and foam cell accumulation in lungs of Sprague-Dawley rats fed purified, biotin-deficient diets

One hundred twenty male Sprague-Dawley rats (3 weeks old) were given biotin-deficient diets containing ovalbumin as the protein source. Ten control rats of the same origin were fed a commercially available purified diet that used casein as a protein source. Eosinophils and histiocytes were observed at a higher frequency in lungs of rats fed the purified diets containing ovalbumin than in the controls. Foam cells were confined to subpleural and peribronchial regions, reacting positively to anti-lysozyme antibody. The incidence of pulmonary histiocytosis was 76/120 rats (63.3%) in the groups fed the ovalbumin-containing diets as compared with 1/10 (10.0%) in the controls. The accumulation of eosinophils in lung was highest (6/24 rats, 25%) at 3 months. This lesion was not seen in the controls. Eosinophils were first observed in the perivascular and peribronchiolar regions. In advanced lesions, macrophages and mast cells also appeared in the lesions, which at this stage resembled so-called idiopathic chronic eosinophilic pneumonia of human beings. Neither foam cells nor eosinophils were present in any of the other organs. Because there was no difference in the composition of the diets with the exception of the protein source, these lung lesions may be due to biotin deficiency resulting from the use of ovalbumin as the protein source.

Production of tumor necrosis factor-alpha by alveolar macrophages of lung cancer patients

The abilities of human alveolar macrophages (AM) obtained from healthy donors and patients with lung cancer to produce tumor necrosis factor (TNF) were compared with those of their blood monocytes after activation with lipopolysaccharide (LPS). TNF activity was assayed by measuring cytotoxicity against actinomycin D-treated L929 cells and TNF was determined quantitatively by sandwich enzyme-linked immunosorbent assay (ELISA) with polyclonal and monoclonal antibodies against TNF-alpha. Unstimulated AM from healthy donors released variable amounts of TNF spontaneously, whereas blood monocytes did not. When treated with LPS for 24 h, AM and monocytes produced TNF dose-dependently, but TNF production by AM was significantly more than that by blood monocytes. This TNF activity was inhibited completely by monoclonal anti-TNF-alpha antibody. Macrophages generated by in vitro maturation of monocytes induced by granulocyte-macrophage colony-stimulating factor (GM-CSF) produced more TNF than freshly isolated monocytes. No difference was found in the abilities of AM from healthy donors and patients with lung cancer to produce TNF after activation stimuli. These observations suggest that human AM may be important in in vivo antitumor defense of the lung through TNF-alpha production.

Cathepsin B and D activity in alveolar macrophages from rats with pulmonary granulomatous inflammation or acute lung injury

Cathepsin B and D activity was determined using specific synthetic substrates in alveolar macrophages (AMs) obtained from Sprague-Dawley rats with experimentally induced pulmonary granulomatous inflammation. Increased cathepsin B activity was found 4 days after intravenous injection of complete Freund's adjuvant (CFA), but not after injection of live bacillus Calmette-Guérin organisms (BCG), indicating that the enzyme response was unrelated to the subsequent development of granulomatous inflammation. Findings of comparable increases in enzyme activity following injection of mineral oil (MO) indicate that the response to CFA was due to the oil component. Significantly, oleic acid (OA), a natural fatty acid, did not stimulate enzyme activity although the agent, like MO, caused acute lung injury as assessed by 125I albumin uptake. At 7 and 28 days following injection of CFA, cathepsin B levels in AMs were the same as those in animals given normal saline (NS), although bronchoalveolar lavage (BAL) samples still contained increased numbers of AMs, and cells obtained at 28 days phagocytosed more polystyrene microspheres. Cathepsin D activity did not increase 4 days after injection of CFA or BCG + CFA; at 28 days following injection of BCG + CFA activity was significantly decreased as compared to animals given NS. The data reveal a differential response of two lysosomal enzymes during the early phases of granulomatous inflammation.

Pulmonary alveolar macrophages from patients with active sarcoidosis express type IV collagenolytic proteinase. An enzymatic mechanism for influx of mononuclear phagocytes at sites of disease activity

Alveolar macrophages (AMs) recovered from the bronchoalveolar lavage (BAL) of 44 patients with sarcoidosis were evaluated for their ability to release type IV collagenolytic metalloproteinase (IV-Case). This enzyme, which is produced by peripheral blood monocytes (PBMs) but not by tissue macrophages, degrades type IV collagen, the major structural component of vessel wall basement membranes, and helps to promote the migration of PBMs from the blood compartment to peripheral tissues. Our results demonstrated that AMs from patients with active sarcoidosis released significantly increased levels of IV-Case with respect to patients with inactive disease and control subjects. After in vitro culture, sarcoid AMs secreted IV-Case during the first 24 h of collection; after that time, AMs progressively lost their ability to release IV-Case. Exposition of both sarcoid and normal AMs to recombinant IL 2 or gamma IFN did not influence their property to release IV-Case. The immunoblot analysis of IV-Case demonstrated complete identity between IV-Case released by AMs and the degradative enzyme obtained from PBMs. The increased property to release IV-Case was significantly related to the increase of the absolute number of AMs and, in particular, of AMs bearing two determinants that are usually expressed by most PBMs (CD11b and CD14). Selective depletion of CD11b+/CD14+ AMs from the entire macrophagic population was associated with the recovery of the IV-Case activity to normal values. A positive correlation was also found between the increase in the absolute number of lung T cells and the enhanced CD4/CD8 pulmonary ratio. A 6-mo follow-up study indicated a significant association between the positivity for the 67Gallium scan and the increased property of AMs to release IV-Case. Our data are consistent with the hypothesis that a IV-Case mediated influx of peripheral monocytes takes place in the lung of sarcoid patients. Furthermore, the correlation found between the IV-Case release and disease activity suggests that this assay could represent a useful tool in sarcoidosis disease staging.

Influence of acute pulmonary interstitial inflammation on kinetics of phagocytosis by alveolar macrophages

The pulmonary alveolar macrophage (PAM) is central to lung cellular defenses and is a potential participant in lung injury, but little is known about the influence of the nature and anatomic pattern of acute lung injury on PAM function. To assess the relationship between ongoing pulmonary inflammation and PAM function, we evaluated PAM phagocytic kinetics in a model system of experimental interstitial adjuvant pneumonitis (EIAP) in calves. PAMs were obtained from lung one and seven days postinduction (dpi) of EIAP. Lesions were typical of EIAP, characterized by acute multifocal to coalescing exudative interstitial pneumonitis at 1 dpi, which progressed to granulomatous interstitial pneumonitis by 7 dpi. The total recoverable lung cells and percentage of neutrophils (PMNs) were elevated (P less than 0.01) from animals with EIAP at both 1 and 7 dpi, and there was a four-fold increase (P less than 0.01) in the PAM yield by 7 dpi. Linear regression equations revealed that a larger proportion of control PAMs were phagocytic than were PAMs from animals with EIAP. The mean initial phagocytic rates of PAM following acute lung injury were significantly elevated (P less than 0.05) over controls; this difference was concentration dependent and required a phagocytic bead stimulus concentration in excess of 12.5 x 10(6) beads/ml. PAMs from animals with EIAP had a greater maximum rate of phagocytosis (Vmax) and Km than control PAMs. PAMs from animals with EIAP had a slightly higher proportion of cells which phagocytosed multiple beads. Levels of beta-glucuronidase were elevated (P less than 0.02) in PAM from animals with EIAP at 7 dpi. The results document enhanced PAM phagocytic function in EIAP and differ from our previous experiments in which depressed PAM phagocytic indices were obtained in a model of virus-induced acute bronchiolitis and alveolitis. The functional activities of the PAMs thus appear to be modified by injury-specific events in the lung microenvironment which may, in part, reflect the nature and anatomic pattern of developing pulmonary inflammatory reactions.

Alveolar destruction in experimental Klebsiella pneumonia

Sequential histological changes of the lungs were studied in experimental Klebsiella pneumonia, using untreated control mice, cyclophosphamide-treated mice, and carrageenan-treated mice. Cyclophosphamide was used to deplete polymorphonuclear leukocytes and monocytes, and carrageenan was used to deplete mononuclear phagocytes selectively. At 72 hours, varying degree of alveolar necrosis could be seen in untreated control mice. However, the lung lesions of cyclophosphamide- or carrageenan-treated mice were significantly different from those of the control mice. The lung lesions of cyclophosphamide-treated mice indicated that destruction of the alveolar septa was not induced by K. pneumoniae itself but by inflammatory cells, because the alveolar walls were preserved very well in spite of considerable bacterial multiplication in alveolar lumina until infiltration of inflammatory cells occurred. The lung lesions of carrageenan-treated mice showed that alveolar spaces were packed with polymorphonuclear leukocytes, but the alveolar walls were preserved very well as far as the authors could tell after examining the lung lesions by silver impregnation staining. These results suggest that macrophages rather than polymorphonuclear leukocytes and organisms play an important role in alveolar injury in experimental Klebsiella pneumonia.

The effect of heating on the functional activity of iota carrageenan

Iota carrageenan is toxic to cells of the reticuloendothelial system. If carrageenan is dissolved at 40 degrees C and injected intraperitoneally, it will induce a significant reduction of alveolar macrophages. Heating the carrageenan preparation to 121 degrees C, however, results in a marked diminution of this in vivo cytotoxicity. This observation offers an explanation for the variability of effects previously attributed to carrageenan, and suggests that preparation methods should be standardized and carefully monitored.

A biochemical study of the carrageenan-induced granuloma in the rat lung

Intralobular injection of carrageenan in the rat lung induced a chronic granulomatous response, characterized by a prolonged accumulation of macrophages within the affected lobe. This was accompanied by moderate but significant increases in lysosomal beta-acetyl glucosaminidase, cathepsin B1, and neutral protease activity. Beta-acetyl glucosaminidase and cathepsin B1 activities peaked on day 16 post carrageenan injection and cathepsin B1 activity peaked again on day 112. These enzyme peaks correlated with previous morphological findings that numerous PMNs and carrageenan-containing macrophages were present in the alveoli on day 16 and on day 112. Lymphocytes and plasma cells were present in the alveoli on day 112 in addition to the numerous carrageenan-filled large macrophages. The caseinolytic enzyme activity was significantly elevated over controls throughout the duration of the experiment but no distinct peaks of activity were observed. Based on determinations of total collagen, insoluble collagen, salt-soluble collagen, acid-soluble collagen, protein, total proline, and specific activity of proline and hydroxyproline, there was biochemical evidence of increased collagen synthesis or collagen accumulation in the inflamed lobes on days 4 to 32 as compared to the control lobes. No evidence was found to indicate an increase of collagen or collagen synthesis after day 32 in the experimental lobes. These results differ from our earlier histological and ultrastructural findings which reported that no increased collagen deposition was observed on any day in this system, even 500 days post carrageenan injection. Lung to body weight ratios of experimental animals were significantly elevated over controls on all days studied.

Complete Freund's adjuvant-induced pneumonia in swine: a model of interstitial lung disease

Intravenously administered complete Freund's adjuvant, emulsified in saline, induced a lesion in the porcine lung characterized by a disseminated histiocytic interstitial pneumonia, type II pneumocyte proliferation and granulomatous arteriolitis. In addition, there was a persistent increase in numbers of alveolar macrophages. The lesion was well developed in 7 days. The sequential development and early resolution of the light microscopic lesions was studied for a period of 21 days after inoculation. The administration of Freund's adjuvant induced a spectrum of lesions typical of many forms of interstitial lung diseases of man and domestic animals. The pig is a suitable host for the investigation of the pulmonary inflammatory response and the repair mechanisms involved in chronic lung disease produced by infectious agents or environmental antigens.

An ultrastructural study of the macrophages of the carrageenan-induced granuloma in the rat lung

The intralobular injection of 0.17 ml of 2 per cent. carrageenan into the rat lung induced an inflammatory granulomatous response. This inflammation was characterized in the early stages by immediate polymorphonuclear (PMN) leucocyte infiltration into the alveoli. Within 2 days the PMN's began to disappear and were replaced by carrageenan-containing macrophages. Alveolar macrophages actively phagocytised the injected carrageenan and were the dominant cell type in he alveoli for the length of this study (365 days). These low-turnover alveolar macrophages, which with light microscopy stained pinkish-red metachromatically with toludine blue due to their carrageenan content, underwent changes in vacuole morphology as well as changes in size and shape. Throughout the course of this inflammation, these macrophages had carrageenan-containing vacuoles which could be seen undergoing fusion to form larger vacuoles which at times constituted half the size of the entire macrophage cytoplasm. The carrageenan in the vacuoles was an amorphous flocculent appearing material for the initial 14 days but changes to a more fine filamentous form for the remainder of the study. This was probably due to its partial digestion by lysosomal enzymes. The general composition of the cytoplasm remained fairly constant during the 365 days. Pinocytotic vesicles and free ribosomes were abundant and the many mitochondria were small and rounded. Smooth endoplasmic reticulum lysosomes, and Golgi apparati, although not prominent, were present but the most striking feature of the cytoplasm was the presence of numerous lamellar bodies (phagocytosised surfactant). Some of the macrophages increased greatly in overall size (five to seven times) compared to their initial size. A few isolated macrophages could be seen degenerating but no general necrosis was seen. Except for one isolated case, no epithelioid cells were observed in this carrageenan inflammation. Fibrosis, if present at all, was very localised and was only evident at day 340 post-injection. This fibrosis generally involved one or two carrageenan-containing macrophages encapsulated by a few collagen fibres. No widespread fibrosis was ever observed in this study which confirmed earlier histological and biochemical investigations.

The current view on the origin of pulmonary macrophages

In the present contribution the current view on the origin of macrophages is briefly summarized. Recent studies on the kinetics of pulmonary macrophages in the normal steady state and during an inflammatory reaction induced by heat-killed BCG applied intravenously or by aerosol are reported in more detail, and the effect of glucocorticosteroids on the kinetics of these cells is described. The results support the general conclusion that pulmonary macrophages derive mainly from circulating monocytes, exclude the existence of an interstitial pool of dividing precursors cells, and provide evidence that the limited local production of pulmonary macrophages is the result of division of mononuclear phagocytes recently derived from the circulation.

The bovine pulmonary inflammatory response: adjuvant pneumonitis in calves

Twenty one- to two-month-old Holstein, Angus, or Holstein-Angus crossbred calves were given intravenous complete Freund's adjuvant and their lungs were examined at 24 hours to 30 days post-injection. Two calves given intravenous saline served as normal controls. The evolving pulmonary inflammatory response was characterized initially by multifocal vasculitis and acute multifocal exudative pneumonitis which progressed to a granulomatous interstitial pneumonitis by seven days post-injection. Discrete granulomas characterized the lesions in the lungs and lymph nodes at 30 days post-injection, and granulomas also were seen in liver, kidney, and spleen. Clinically, the calves were tachypneic and pyrexic during the first week post-injection. Four calves developed acute pulmonary edema and died during, or shortly after, administration of the adjuvant. This model of experimental pneumonia in calves is similar to complete Freund's adjuvant-induced experimental pneumonia in other species. It is reproducible and predictable in its course of development and resolution, and provides a useful model for studying basic mechanisms of pulmonary inflammatory injury and repair in the bovine lung.

The origin of pulmonary macrophages

The controversy as to whether pulmonary macrophages derive from monocytes or from precursor cells in the pulmonary interstitium has been solved by quantitative analysis of 3H-thymidine labeling data on the circulating monocytes and the total population of pulmonary macrophages isolated from mice by lavage and enzyme digestion of lung tissue. It was found that in the normal steady state, during acute inflammatory reactions, and under long-term corticosteroid treatment, pulmonary macrophages derive mainly from circulating monocytes which enter the lungs and become pulmonary macrophages within a few hours. Local division of pulmonary macrophages made only a minor contribution to the maintenance of the population; a dividing precursor cell population located in the interstitium of the lung could not be demonstrated. The few mononuclear phagocytes dividing locally in the lung have most probably arrived recently from the circulation and originate in the bone marrow. These dividing cells already have the characteristics of pulmonary macrophages.

Quantitation of glycosaminoglycans of rabbit lung during delayed-type hypersensitivity reactions and granuloma formation

The specificity and kinetics of hyaluronic acid (HA) accumulation in relation to other glycosaminoglycans (GASs) were determined in rabbit lungs during an allergic granulomatous response to BCG, an allergic nongranulomatous response to tuberculoprotein, and during a foreign-body granulomatous response to carrageenan. Hyaluronic acid was the only GAG detected in the lung lavage fluids. Hyaluronic acid occurred in the airways on day two of the allergic granulomatous response, but its presence in the airway did not correlate with ensuing granuloma formation in the parenchyma. Generalized increases in GAG of the parenchyma also peaked on day two of the DTH responses. Generalized increases in GAG peaked on day five during the foreign-body granulomatous response to carrageenan. A persistently elevated level of HA in the lung tissue correlated with granuloma formation but not with the intensity of the response.

A histological study of the carrageenan-induced granuloma in the rat lung

Intralobular injection of 0.17 ml of 2% carrageenan, through a ventral slit in the trachea of rats, induced localised areas of inflammation with a high survival rate. This inflammation was characterised by immediate polymorphonuclear leucocyte (PMN) infiltration into the interstitial and alveolar spaces followed in 4 days by replacement of the PMNs by carrageenan-containing macrophages. Between days 10 to 70, the macrophages rapidly increased in size and accumulated numerous large vacuoles which stained for the presence of carrageenan. Several macrophages were so large that they each filled an entire alveolar space. From days 70 to 205, the macrophage appearance was unchanged except that the staining of their carrageenan-containing vacuoles was less metachromatic with toluidine blue. Fibrosis was first noted at day 205 and consisted of several small granulomas located near large airways and blood vessels. These granulomas had a central area filled with macrophages and a peripheral zone consisting of fibroblasts, new collagen, scattered macrophages and blood vessels. The morphology of the macrophages remained essentially unchanged from days 205 to 500 but by day 500, the macrophages were found only in numerous pockets within the inflamed lobe. They still stained positive for the presence of carrageenan at day 500. The extreme longevity of these macrophages and the lack of significant fibrosis may be due to the "un-naturalness", indigestibility, and low toxicity of the irritant, carrageenan. In addition, their size and numerous vacuoles may have inhibited their movement and subsequent removal from the lung. The paucity of significant fibrosis may be due to the lack or inhibition of a "fibroblast stimulating factor" released by the macrophages or possibly the collagen was degraded as soon as it was synthesised. This carrageenan-induced inflammation is a very suitable for the study of alveolar macrophages but appears to be inappropriate for the study of pulmonary fibrosis.

Proliferation activity and bacteriostatic potential of human blood monocytes, macrophages in pleural effusions, ascites, and of alveolar macrophages

Human blood monocytes, macrophages from pleural effusions, ascites, and alveolar macrophages obtained by bronchopulmonary lavage were investigated. The proliferative activity of these cells was determined by the 3H-thymidine labeling index in vitro (3H-TDR L.I.). The bacteriostatic capacity was measured by the potential of the cells to block DNA-synthesis of proliferating Escherichia coli after phagocytosis. In most cases 3H-TDR L.I. of blood monocytes, macrophages from pleural effusions and ascites was less than 1%. However, macrophages of some patients with neoplastic diseases exhibited 3H-TDR L.I. between 4.0--9.6%. This probably reflected the action of factors, possibly lymphokines, which stimulated macrophage proliferation. In contrast, alveolar macrophages seemed to have almost totally lost their proliferative potential. The bacteriostatic capacity of blood monocytes proved to be significantly lower than that of macrophages. This demonstrates the functional immaturity of blood monocytes. In all type of macrophages investigated the bacteriostatic power was very high. No differences could be detected either between macrophages of different sources or between macrophages of benign, inflammatory, or malignant diseases.

Reaction of macrophages to cigarette smoke. II. Immigration of macrophages to the lungs

The present study identifies the source of the elevated pulmonary macrophage population in young adult male mice which results when animals are exposed to cigarette smoke. Light microscopic and autoradiographic analysis of pulmonary tissue from smoke-exposed animals revealed that pulmonary macrophages (free, attached, and septal or interstitial) divide only rarely. Further, it was noted that, during the marked progressive increase in the labeled macrophage population in the lungs, the number of silver grains over the nuclei of labeled macrophages did not become significantly diluted. Thus, the markedly elevated macrophage population which results when animals are exposed to cigarette smoke appears, for the most part, to be due to immigration of cells from bone marrow rather than in situ division of resident macrophages.

Macrophage heterogeneity

Macrophages are a mobile, functionally diverse group of cells which may be recruited and stimulated to a high degree of metabolic activity. Heterogeneity may be detected from one site to another and result from local influences, e.g. lung v. peritoneal cells, or occur within a population and arise dur to different stages of differentiation, maturation or activation or possibly from distinct cell lines. Recruitment and turnover are important determinants of the diversity of cells at any one site. In addition, anti-tumour, anti-microbial and secretory capacities of macrophages are greatly influenced by the degree and nature of stimulation possibly affecting only a subpopulation of the cells. Accessory cell activity is also a function of a minor population of macrophages which have distinct surface antigens. The sources of the heterogeneity and the interrelationship between the macrophages subpopulations remain to be determined.

The effects of asbestos on macrophages

The exact role of the alveolar macrophage in the pathogenesis of asbestosis is not known. Most studies of the effect of asbestos on macrophages have been concerned with the in vitro biochemical or cytotoxic properties of the dust and have made use of peritoneal macrophages. In general, chrysotile had a toxic effect on the macrophages, whereas amphibole varieties did not. Most forms of absetos, however, are actively fibrogenic in man and animals, and there is no clear correlation between in vitro cytotoxicity of various forms of asbestos and their fibrogenicity. Recent experiments in which animals are exposed to asbestos in vivo provide evidence of alteration of macrophage activity, as demonstrated by changes in surface morphology and IgG receptor sites, as well as released of various secretory products. Deposition of complement components found on the surface of alveolar marcophages from animals exposed to asbestos could be a manifestation of a humoral immune response directed against an altered cell. The capacity of macrophages to participate in inflammation, tissue repair, and immunity suggests an immunopathogenic concept for the development of asbestosis.

Ultrastructure of the lung in the rat following exposure to crocidolite asbestos and quartz

Lung tissue from rats that had inhaled U.I.C.C. crocidolite asbestos or quartz particles showed thickening of the interstitial tissue and the presence of collagen fibres. Aggregates of macrophages in the alveolar spaces were a dominant feature of all the sections examined from asbestos exposed rats. According to the ultrastructural studies described here, all the macrophages were mature cells, indicating that the ingested crocidolite asbestos was non-toxic. Lung sections of rats exposed to quartz particles were significantly different. Single cells were found in the alveolar spaces and many macrophages showed a characteristic pattern of vacuole formation. Other cells contained intracellular membranous lamellated bodies, similar to those found in Type II pneumocytes. Cells containing lamellated bodies were also found in the interstitial tissue. These findings suggest that the two mineral dusts have quite different biological effects on the macrophage and that the development of pulmonary fibrosis may, to some extent, be caused by a different mechanism in each instance.

Pulmonary sarcoidosis: A clinico-pathological study

The light and electron microscopic changes in biopsy tissue from the lung of a 30-year-old housewife severeley incapacitated by diffuse pulmonary sarcoidosis with pulmonary hypertension are presented. The lung tissue was distorted by numerous granulomas in the interstitial tissues and within alveoli. Many pulmonary blood vessels including arteries were damaged by the granulomas. The ultrastructural features of the epithelioid cells were found to be distinctive and probably specific. The giant cells which accompanied the epithelioid cells contained two types of inclusion body: one appeared to be related to the Schaumann body but the nature and origin of the second type was not clear. Many of the granulomas were surrounded by avascular fibrous tissue which contained, in addition to mature fibroblasts, myofibroblasts and a primitive form of cell that appeared to be a fibroblast precursor. It was conjectured that the myofibroblasts, through their contractile powers, might increase the distortion of the lung architecture and thereby the patient's disability. The alveolar walls were thickened by a diffuse infiltrate of macrophages and epithelioid cells but there was no excess of collagen and elastic fibres. The evidence suggested that the epithelioid cells developed from macrophages. From the cellular nature of the diffuse infiltration of the alveolar walls and the absence of fibrosis it seemed that the disease was still at an early and active stage, a conclusion strengthened by the fact that treatment with corticosteroids led to marked and sustained clinical improvement.