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Blood, Vol. 93 No. 2 (January 15), 1999:
pp. 667-673
By
From The Institute of Medical Science, The University of Tokyo,
Tokyo, Japan; the Tokyo Metropolitan Komagome Hospital, Tokyo, Japan;
the 4th Department of Internal Medicine, Nippon Medical School, Tokyo,
Japan; and the 2nd Department of Pathology, Niigata University, Medical
School, Niigata, Japan.
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.
ALVEOLAR MACROPHAGES (AM)
represent a major component of the alveolar spaces and play important
roles in host defense. Human lungs are estimated to contain 2.3 × 1010 AM, with 50 to 100 AM per alveolus. This
number is constant in the steady state, although animal studies
demonstrate that large numbers of AM are continuously lost from the
lung, mainly through the airways.1,2 Some reports
demonstrate that peripheral blood monocytes (PBM) are precursors of AM
and that a continuous influx of PBM to the lung is responsible for
maintaining AM numbers.3,4 However, studies using various
techniques have failed to show any significant emigration of PBM into
the alveolar spaces unless the latter are stimulated.5,6
Other reports show that AM are proliferating in the
lung.7-10 Previously, we demonstrated directly that both
murine and human AM proliferate in vitro in the presence of
granulocyte-macrophage colony-stimulating factor (GM-CSF), which is
produced in the lung parenchyma.11,12
In the murine model, AM population kinetics have been investigated
using radiation chimeras, in vivo 3H-thymidine labeling, or
89Sr injection. Although the half life of AM is as short as
11 days,9 they maintain a stable population for long
periods with little or no influx of blood monocytes to the lung. Using
fractionated radiation-induced chimeras that preserve AM precursors in
the lungs, Tarling et al8 demonstrated that approximately
60% of AM are of recipient origin even 11 months after bone marrow
transplantation (BMT). Population renewal is due to in situ
proliferation of AM of recipient origin. Thus, murine AM are supplied
mainly by self-replication, but a small population of precursor cells
is derived from the circulation.
On the other hand, our knowlege of human AM is limited amd mostly based
on in vitro or ex vivo experiments. AM numbers are maintained for a
long period in patients with monocytopenia after chemotherapy,13 suggesting that local proliferation
contributes to the homeostasis of AM. However, large gaps in our
knowledge exist because studying the kinetics of human AM in vivo is
difficult.
In this regard, BMT is a valuable system to study human AM in vivo.
Repopulation of AM with cells of donor origin provides a way of
investigating AM dynamics, even though recipient lungs may not be
normal due to the conditioning therapies or graft-versus-host reactions. Inflammation may promote the entry and repopulation of donor
cells in the lung. Thomas et al14 studied Y chromosomes in
AM obtained from allogeneic BMT (allo-BMT) recipients of sex-mismatched cases and reported that AM are replaced within 90 days after BMT by
cells of donor origin. These observations suggest that influx of PBM to
the lung is the crucial mechanism for the reconstituting AM population
after BMT. After this report, no study has detailed the kinetics of AM
replacement.
In this study, we investigated the kinetics of AM replacement after
allo-BMT. We found that the number of AM in the bronchoalveolar lavage
(BAL) fluid was similar to the control level before day 50 but
increased strikingly after day 50. This increase coincided with the
increase of AM expressing the proliferating cell nuclear antigen,
suggesting the augmented proliferation of AM in situ after day 50. Taken together with previous reports,14 the proliferation of AM in situ may contribute to the reconstitution of the AM population after allo-BMT.
BMT Recipients and Control Subjects
BAL and Preparation of Cells
Morphology and Size of AM
Detection of AM Proliferation DNA cell cycle analysis. Purified AM (1 × 106) were fixed with 70% ethanol at 4°C for 15 minutes, washed twice with cold PBS, and resuspended in 100 µg/mL bovine pancreatic ribonuclease/PBS solution. After 10 minutes of incubation at room temperature, propidium iodide solution was added to a final concentration of 10 µg/mL. After cells were passed through a cell strainer to remove aggregations, cell cycle analysis was performed using a FACScan cytometer (Becton Dickinson). PBMC from healthy donors and murine leukemia cell line P-815 were used as controls for resting and proliferating cells, respectively. The number of cells in G1, S, and G2/M compartments were obtained using the sum of rectangles model. At least 10,000 cells were scored per sample. Detection of proliferating cell nuclear antigen.
To detect proliferating cell nuclear antigen (PCNA) in AM, 1 × 105 of AM were fixed with 70% ethanol for 15 minutes at
4°C immediately after cytospin preparation. The slides were washed
twice with PBS, immersed in cold PBS overnight, dried, and stocked at
Fluorescence In Situ Hybridization (FISH) for Detection of Y Chromosome in AM In situ hybridization was performed as described previously.18 Cytospin preparation of AM were wet-fixed with 70% ethanol for 20 minutes at 4°C and dried at room temperature. Slides were pretreated with RNase and proteinase K before in situ hybridization. For hybridization, 10 ng of biotinylated DY Cocktail probe (Oncor, Gaithersburg, MD) was mixed in 20 mL of hybridization buffer (55% formamide, 1× SSC, 10% dextran sulfate), denatured at 75°C for 10 minutes, and applied to the slides previously denatured at 70°C for 2 minutes in 70% formamide, 2× SSC. Hybridization was performed at 37°C overnight. Posthybridization washes were performed at 42°C for 15 minutes, twice in 50% formamide/1× SSC and once at room temperature in 1× SSC. Fluorescence signals were visualized and amplified by fluorescein isothiocyanate (FITC)-conjugated avidin and biotinylated antiavidin system (Vector Laboratories, Burlingame, CA). After rinsing, slides were stained with the combined use of antifading buffers (PBS containing 90% glycerol, 1.25% diazabicyclo-[2,2,2]-octane [Sigma Chemical Co] and 0.5 µg/mL propidium iodide [Sigma Chemical Co], and PBS containing 90% glycerol and 1 mg/mL D-phenylenediamine [Sigma Chemical Co]). The fluorescein yellow-green signals and propidium iodide-stained red chromosomes were excited at 450 to 490 nm (filter B) using a Olympus fluorophoto microscope and were photographed with Fujichrome 100 film (Fujifilm, Tokyo, Japan). The size of AM was measured by direct observation using a photomicrometer as described above.DNA Fingerprinting of Genomic DNA From AM or Monocytes For DNA fingerprinting, we applied the method described by Wong et al.19 Briefly, genomic DNA was extracted from purified AM, monocytes, or whole blood by incubating in sodium dodecyl sulfate (SDS)/protease solution followed by phenol/chloroform extraction and ethanol precipitation. The extracted DNA was digested to completion with HinfI, and 2 µg of DNA was electrophoresed in 0.7% agarose gel until a 2.3-kb marker DNA fragment had migrated 20 cm from the gel origin on control tracks. DNA was transferred by Southern blotting to Hybond N membrane (Amersham, International plc, Amersham, UK), fixed by UV irradiation, prehybridized, and hybridized overnight with a 32P-labeled cocktail of single locus, minisatellite probes MS1 + MS31, MS43a, MS8, and g3 (Cellmark Diagnostics, Oxfordshire, UK), which recognize chromosomes 1p33-p35, 7p22-pter, 12q24.3-qter, and 7q36-qter, respectively. After washing in 0.1× SSC at 65°C, the membrane was autoradiographed from 20 to 46 hours at 80°C in the presence of an intensifier screen.
Statistical Analysis Data were analyzed by Stat view-J 4.5 using Macintosh computer (Apple Japan, Inc, Tokyo, Japan) and expressed as mean ± standard error. The difference between two groups was evaluated using the Student's t-test. A P value less than .05 was considered significant.
Characterization and Time Course of BAL Cells From BMT Recipients Twenty-four BMT recipients underwent 27 BAL, including 3 second BAL and 10 normal nonsmokers underwent BAL as a control. BAL recovery was similar in both groups. There was a strong correlation between the number or percentage of AM and the time after BMT (Fig 1A and B). Before day 50, the percentage and number of AM from 17 recipients were 91.3% ± 1.3% and 6.4 ± 0.9 × 106/mL, respectively, which were similar to those in controls (91.6% ± 1.5% and 8.2 ± 1.6 × 106/mL, Table 2). The mean values after day 50 (96.5% ± 1.3% and 25.0 ± 2.2 × 106/mL, respectively) were significantly higher than those before day 50 or of controls. This tendency was confirmed in 3 recipients who underwent serial bronchoscopy; in each case the percentage and number of AM increased at the later time point (Table 3).
Morphology of AM After Allo-BMT The size of AM also changed according to days after BMT (Fig 2). The mean diameter of AM after day 50 was 20.0 ± 0.7 µm, which was significantly smaller than those before day 50 (23.9 ± 0.6 µm) or controls (24.7 ± 0.7 µm). The mean diameter of AM inversely correlated with the cell number in ELF (r = .62, P < .001, n = 27). In Diff-Quick
staining, most AM before day 50 had large bright cytoplasm and deviated
eosinophilic nuclei. After day 50, the morphology showed two cell
patterns: one with large, bright cytoplasm and one with a smaller
diameter and basophilic cytoplasm. The small AM were regularly round
and had nuclei with dense chromatin. These cells were adherent,
phagocytic, and nonspecific esterase-positive cells (data not shown).
These data suggest that the increase of AM after day 50 is due to a
repopulation of small AM in the lung.
Proliferating AM After Allo-BMT To investigate the mechanism of the increase of AM numbers after day 50, we evaluated for AM proliferation using monoclonal anti-PCNA antibody. Normal controls had low PCNA expression (3.0% ± 0.4%). PCNA expression was slightly elevated compared with control in the early period after BMT (7.1% ± 1.3%, P = .03). PCNA expression was strikingly higher in BMT recipients after day 50 than those in recipients before day 50 or the control (18.3% ± 1.5%, P < .001; Fig 3). Moreover, the percentages of proliferating AM correlated with the cell number in the all recipient (r = .59, P < .001). The high PCNA levels seen after day 50 corresponded to increased cells in G2/M phase of the cell cycle. Consistently, DNA cell cycle analysis on 6 recipients showed higher percentages of AM entering G2/M phase after day 50 than those before day 50 or controls (Table 4). Thus, the increase of AM after day 50 seemed to correlate with an augmented, local proliferation of AM in vivo.
Direct Observation of Chimeric State of AM by Y Chromosome Detection in a Sex-Mismatched Case To demonstrate that the small rounded AM present after allo-BMT were derived from donor hematopoietic progenitor cells, we performed FISH to detect the Y chromosome of AM from a sex-mismatched case (recipient, female; donor, male; Fig 4). In this case, recipients underwent BAL on day 48, when AM are expected to be in a chimeric state. As controls, 2 sex-matched cases (male to male and female to female) were also studied. As shown in Fig 4, most AM that were positive for Y chromosome were small AM. Thirty-three percent of AM were positive for Y chromosome in this case, whereas 100% and 0% were detected in the control cases of male to male and female to female, respectively. The histogram of diameter of AM demonstrated that AM of donor origin were mostly small, whereas recipient AM were heterogeneous in size (Fig 5). These data indicate that small AM that increase after allo-BMT are donor origin and therefore entered the lung as circulating mononuclear phagocytes.
Genotypic Evaluation of AM After Allo-BMT The DNA fingerprinting method enables one to detect minor differences in the genotypes between siblings through the use of single locus probes. Using this method, we investigated the replacement of recipient genotypes in AM and blood monocytes by donor genotypes in 3 different recipients (Fig 6). On day 41, the banding pattern showed that the recipient genotype was predominant in AM, although several bands which were characteristic for donor genotype were also observed. On day 55, bands of donor and recipient origin were similar in intensity, indicating that donor AM were increasing in this period. On day 83, the donor bands predominated but the recipient genotype was still clearly seen. In contrast, banding pattern of blood monocytes demonstrated that these cells were exclusively of donor origin as early as day 41. Thus, replacement of monocytes by donor cells is complete by day 41, whereas that of AM proceeds gradually around day 50.
This study was designed to clarify the mechanism of reconstitution of AM populations after allo-BMT. Previous studies have demonstrated that AM are completely replaced by donor AM within 90 days after allo-BMT, indicating that AM are ultimately of bone marrow origin.14 To extend these observations, we investigated the kinetics of reconstitution of AM population by performing BAL from 24 to 83 days post-BMT. Our data demonstrated that the cell number, morphology, and proliferative ability of AM were similar to those of the control AM until day 50 when recipient AM predominated. After day 50, the number of AM in ELF increased to threefold. Morphologically, small immature AM predominated after day 50. Those cells were presumed to be donor origin. Finally, AM after day 50 demonstrated an augmented proliferative ability in vivo. Consequently, these observations combined with previous studies suggest that small AM of donor origin replace recipient AM after allo-BMT.
The authors thank Kirin Brewery Co, Ltd for providing recombinant human GM-CSF and Morinaga Milk Industry Co, Ltd for providing recombinant human M-CSF. We are very grateful to Dr M. Weiden, Dr D. Nam, Dr S. Kanegasaki, and Dr K. Akagawa for valuable discussions and to Teijin Bio Laboratories for their technical help.
Submitted May 13, 1998;
accepted September 21, 1998.
Address reprint requests to Koh Nakata, MD, PhD, Laboratory of Culture Collection, The Institute of Medical Science, The University of Tokyo, 4-6-1 Shirokane-dai, Minato-ku, Tokyo 108, Japan.
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