The monocyte-macrophage system in granulomatous inflammation

Distribution and fibrotic response following inhalation exposure to multi-walled carbon nanotubes

BACKGROUND

Prior studies have demonstrated a rapid and progressive acute phase response to bolus aspiration of multi-walled carbon nanotubes (MWCNTs). In this study we sought to test the hypothesis that inhalation exposure to MWCNT produces a fibrotic response and that the response is chronically persistent. To address the hypothesis that inhaled MWCNTs cause persistent morphologic changes, male C57BL/6 J mice were exposed in a whole-body inhalation system to a MWCNT aerosol and the fibrotic response in the alveolar region examined at up to 336 days after termination of exposure.

METHODS

Inhalation exposure was to a 5 mg/m3 MWCNT aerosol for 5 hours/day for 12 days (4 times/week for 3 weeks). At the end of inhalation exposures, lungs were either lavaged for analysis of bronchoalveolar lavage (BAL) or preserved by vascular perfusion of fixative while inflated with air at 1, 14, 84, 168 and 336 days post inhalation exposure. Separate, clean-air control groups were also studied. Light microscopy, enhanced darkfield microscopy and field emission electron microscopy (FESEM) of tissue sections were used to analyze the distribution of lung burden following inhalation exposure. Morphometric measurements of Sirius Red staining for fibrillar collagen were used to assess the connective tissue response. Serial section analysis of enhanced darkfield microscope images was used to examine the redistribution of MWCNT fibers within the lungs during the post-exposure period.

RESULTS

At day 1 post-exposure 84 ± 3 and 16 ± 2 percent of the lung burden (Mean ± S.E., N = 5) were in the alveolar and airway regions, respectively. Initial distribution within the alveolar region was 56 ± 5, 7 ± 4 and 20 ± 3 percent of lung burden in alveolar macrophages, alveolar airspaces and alveolar tissue, respectively. Clearance reduced the alveolar macrophage burden of MWCNTs by 35 percent between 1 and 168 days post-exposure, while the content of MWCNTs in the alveolar tissue increased by 63 percent. Large MWCNT structures containing greater than 4 fibers were 53.6 percent of the initial lung burden and accounted for the majority of the decline with clearance, while lung burden of singlet MWCNT was essentially unchanged. The mean linear intercept of alveolar airspace, a measure of the expansion of the lungs, was not significantly different between groups. Pulmonary inflammation and damage, measured as the number of polymorphnuclear leukocytes (PMNs) or lactate dehydrogenase activity (LDH) and albumin in BAL, increased rapidly (1 day post) after inhalation of MWCNTs and declined slowly with time post-exposure. The fibrillar collagen in the alveolar region of MWCNT-exposed mice demonstrated a progressive increase in thickness over time (0.17 ± 0.02, 0.22 ± 0.02, 0.26 ± 0.03, 0.25 ± 0.02 and 0.29 ± 0.01 microns for 1, 14, 84, 168 and 336 days post-exposure) and was significantly different from clean-air controls (0.16 ± 0.02) at 84 and (0.15 ± 0.02) at 336 days post-exposure.

CONCLUSIONS

Despite the relatively low fraction of the lung burden being delivered to the alveolar tissue, the average thickness of connective tissue in the alveolar region increased by 70% in the 336 days after inhalation exposure. These results demonstrate that inhaled MWCNTs deposit and are retained within the alveolar tissue where they produce a progressive and persistent fibrotic response up to 336 days post-exposure.

Pulmonary fibrotic response to aspiration of multi-walled carbon nanotubes

BACKGROUND

Multi-walled carbon nanotubes (MWCNTs) are new manufactured nanomaterials with a wide spectrum of commercial applications. To address the hypothesis that MWCNTs cause persistent pulmonary pathology, C57BL/6J mice were exposed by pharyngeal aspiration to 10, 20, 40 or 80 μg of MWCNTs (mean dimensions of 3.9 μm × 49 nm) or vehicle. Lungs were preserved at 1, 7, 28 and 56 days post- exposure to determine the potential regions and target cells for impact by MWCNT lung burden. Morphometric measurement of Sirius Red staining was used to assess the connective tissue response.

RESULTS

At 56 days post-exposure, 68.7 ± 3.9, 7.5 ± 1.9 and 22.0 ± 5.1 percent (mean ± SE, N = 8) of the MWCNT lung burden were in alveolar macrophages, alveolar tissue and granulomatous lesions, respectively. The subpleural tissues contained 1.6% of the MWCNT lung burden. No MWCNTs were found in the airways at 7, 28 or 56 days after aspiration The connective tissue in the alveolar interstitium demonstrated a progressive increase in thickness over time in the 80 μg exposure group (0.12 ± 0.01, 0.12 ± 0.01, 0.16 ± 0.01 and 0.19 ± 0.01 μm for 1, 7, 28 and 56 days post-exposure (mean ± SE, N = 8)). Dose-response determined at 56 days post-exposure for the average thickness of connective tissue in alveolar septa was 0.11 ± 0.01, 0.14 ± .02, 0.14 ± 0.01, 0.16 ± 0.01 and 0.19 ± 0.01 μm (mean ± SE, N = 8) for vehicle, 10, 20, 40 and 80 μg dose groups, respectively.

CONCLUSIONS

The distribution of lung burden was predominately within alveolar macrophages with approximately 8% delivery to the alveolar septa, and a smaller but potentially significant burden to the subpleural tissues. Despite the relatively low fraction of the lung burden being delivered to the alveolar tissue, the average thickness of connective tissue in the alveolar septa was increased over vehicle control by 45% in the 40 μg and 73% in the 80 μg exposure groups. The results demonstrate that MWCNTs have the potential to produce a progressive, fibrotic response in the alveolar tissues of the lungs. However, the increases in connective tissue per μg dose of MWCNTs to the interstitium are significantly less than those previously found for single-walled carbon nanotubes (SWCNTs).

Alteration of deposition pattern and pulmonary response as a result of improved dispersion of aspirated single-walled carbon nanotubes in a mouse model

Nanoparticles have a fundamental dimension of <100 nm. However, on suspension in media, agglomerates of nanoparticles are the more common structure. This is particularly evident in prior intratracheal instillation or aspiration studies of single-walled carbon nanotubes (SWCNT), in which granulomatous lesions encased by epithelioid macrophages were produced by large agglomerates. In this study, we tested the hypothesis of whether exposure to more dispersed SWCNT structures would alter pulmonary distribution and response. A dispersed preparation of single-walled carbon nanotubes (DSWCNT) with a mean diameter of 0.69 microm was given by pharyngeal aspiration to C57BL/6 mice. Electron microscopy demonstrated a highly dispersed, interstitial distribution of DSWCNT deposits by 1 day postexposure. Deposits were generally <1 microm. Macrophage phagocytosis of DSWCNT was rarely observed at any time point. Lung responses were studied by lavage and morphometry at 1 h, 1 day, 7 day, and 1 mo after a single DSWCNT exposure of 10 microg/mouse. Lung sections and lavage cells demonstrated an early, transient neutrophilic and inflammatory phase that rapidly resolved and was similar to that observed with large agglomerates. No granulomatous lesions or epithelioid macrophages were detected. Morphometric measurement of Sirius red staining was used to assess the connective tissue response. The average thickness of connective tissue in alveolar regions was 0.10 +/- 0.02, 0.09 +/- 0.02, 0.10 +/- 0.01, 0.48 +/- 0.04, and 0.88 +/- 0.19 microm for PBS and 1-h, 1-day, 7-day, and 1-mo postexposure groups, respectively. The results demonstrate that dispersed SWCNT are rapidly incorporated into the alveolar interstitium and that they produce an increase in collagen deposition.

Quantitative analysis of apoptotic cell death in granulomatous inflammation induced by intravenous challenge with Cryptococcus neoformans and bacillus Calmette-Guérin vaccine

Apoptotic cell death of macrophage has become recognized as a significant mechanism responsible for the resolution of inflammation. The purpose of this study was to examine how the apoptotic cell death involves the formation and resolution of granulomas in rats intravenously inoculated with Cryptococcus neoformans (Cr. neoformans) and Mycobacterium bovis-derived bacillus Calmette-Guérin (BCG) vaccine. The number and size of granulomas in the livers obtained on days 5, 10, 15, 20 and 25 after inoculation were examined by morphometric image analysis, as well as the occurrence of apoptotic cell death quantitatively analyzed by terminal deoxynucleotidyl transferase (TdT)-mediated dUTP-biotin nick end-labeling (TUNEL) procedure on tissue sections. In both groups the number and size of granulomas were maximized on day 10, then the granulomas were almost resolved until day 25 when the inoculated Cr. neoformans and BCG almost disappeared. From the induction to the resolving stages of granulomatous inflammation, TUNEL-positive cells constantly appeared in granulomas, and the highest frequency of apoptotic cells in granulomas was observed in the earlier stage of granuloma formation. These results indicate that the maintenance and resolution of infectious granulomas are regulated by the balance between the influx of newly recruited macrophages and the apoptotic elimination of granuloma macrophages. The apoptosis of granuloma macrophages actively involves the cellular turnover in both granuloma formation and resolution.

Three-dimensional study of epithelioid cells by a quick-freezing and deep-etching method in muramyl dipeptide-induced granulomas

The three-dimensional ultrastructure of epithelioid cells was studied by the quick-freezing and deep-etching (QF-DE), as well as the freeze-substitution (QF-FS) methods. The granulomas were induced in rats by injecting muramyl dipeptide (MDP) into the hind footpads. At 3 weeks after the injection, the footpads were perfused with a fixative, excised, and quickly frozen to prepare the replica membranes. Some unfixed footpads were also quickly frozen and freeze-substituted. Dense networks of intermediate filaments, connected with the nuclei, mitochondria and other vesicular cell organelles, were observed throughout the cytoplasm of epithelioid cells by the QF-DE method. A few actin filaments were located in filopodia and just beneath the cell membranes. Interdigitation of the cell membranes between adjacent cells was clearly demonstrated by the QF-FS method and clathrin-coated pits were identified at the base of interdigitating filopodia. In addition, the exact moment of fusion between endosomes and lysosomes was ascertained by the same method. These results suggest that the cytoskeletal organization of epithelioid cells resembles that of epithelial cells rather than actively motile macrophages.