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Infection and Immunity, December 2006, p. 7005-7009, Vol. 74, No. 12
0019-9567/06/$08.00+0 doi:10.1128/IAI.01402-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
Surfactant Protein D Increases Fusion of Mycobacterium tuberculosis- Containing Phagosomes with Lysosomes in Human Macrophages
J. Scott Ferguson,1
Jennifer L. Martin,1
Abul K. Azad,3
Travis R. McCarthy,2,3
Peter B. Kang,2,
Dennis R. Voelker,4
Erika C. Crouch,5 and
Larry S. Schlesinger3*
Division of Pulmonary and Critical Care Medicine,1
the Division of Infectious Diseases, Departments of Medicine and Microbiology, Iowa City Veterans Affairs Medical Center and the University of Iowa, Iowa City, Iowa 52242,2
Center for Microbial Interface Biology, Division of Infectious Diseases, Department of Internal Medicine, The Ohio State University, Columbus, Ohio 43210,3
The Lord and Taylor Laboratory for Lung Biochemistry, Department of Medicine, National Jewish Center for Immunology and Respiratory Medicine, Denver, Colorado 80206,4
Department of Pathology and Immunology, Washington University School of Medicine, St. Louis, Missouri 631105
Received 31 August 2006/
Accepted 23 September 2006

ABSTRACT
Lung surfactant protein D (SP-D) binds to
Mycobacterium tuberculosis surface lipoarabinomannan and results in bacterial agglutination,
reduced uptake, and inhibition of growth in human macrophages.
Here we show that SP-D limits the intracellular growth of bacilli
in macrophages by increasing phagosome-lysosome fusion but not
by generating a respiratory burst.

TEXT
Pulmonary surfactant proteins A and D (SP-A and SP-D, respectively)
participate in the lung innate and adaptive immune responses
against multiple pathogens through opsonization and agglutination
of microorganisms, blockade of receptor-ligand interactions,
and/or direct effects on host cells, particularly macrophages
(
5,
27). Experiments using mice with homozygous null alleles
for the SP-A or SP-D genes provide direct evidence in support
of these phenomena (
11,
13,
17-
20).
Both SP-A and SP-D have been implicated in the pathogenesis of tuberculosis. Our work has demonstrated that SP-A increases the phagocytosis of Mycobacterium tuberculosis by human macrophages by up-regulating the macrophage mannose receptor (MR) (2, 10) and reduces NADPH oxidase activity in these cells (6). In contrast, SP-D, through its carbohydrate recognition domain (CRD), binds to the terminal mannose caps of the M. tuberculosis surface lipoglycan lipoarabinomannan (ManLAM) and causes agglutination of the bacilli and inhibition of bacterial uptake by macrophages (7). However, the reduced uptake was not dependent on bacterial agglutination (8). The presence of SP-D also inhibited the intracellular growth of M. tuberculosis in macrophages (8). M. tuberculosis evades several host defense mechanisms including by inhibiting fusion of its own phagosome with lysosomes in macrophages (1) and avoiding the potential stimulation of intracellular oxidant production during phagocytosis (26).
We hypothesized that SP-D inhibits the intracellular growth of M. tuberculosis in macrophages by altering intracellular events that restrict mycobacterial growth following phagocytosis. In this study, we examined whether SP-D coating of M. tuberculosis modulates two major macrophage microbicidal mechanisms: generation of a respiratory burst and regulation of phagosome-lysosome (P-L) fusion events.
Recombinant rat SP-D (referred to as SP-D throughout the paper), produced in CHO cells and purified on a mannose-Sepharose matrix (22, 23), was used in the different assays at a concentration of 0.5 (nonagglutinating for M. tuberculosis) and/or 5.0 (agglutinating for M. tuberculosis) µg/ml. Since SP-D had been shown to inhibit the intracellular growth of M. tuberculosis in macrophages, we first determined whether SP-D had a direct microbicidal effect on M. tuberculosis by performing CFU assays following incubation of the bacilli in the absence or presence of SP-D. No significant effect of SP-D on the viability of bacteria at either 0.5 or 5.0 µg/ml was observed (n = 2; data not shown).
SP-D does not induce a respiratory burst during phagocytosis of M. tuberculosis in human macrophages.
We next determined whether phagocytosis of M. tuberculosis by human macrophages in the presence of SP-D would stimulate the production of reactive oxygen intermediates by using the 2',7'-dichlorofluorescein (DCF) assay (6). Monocyte-derived macrophages (MDMs) were prepared from healthy human volunteers as described previously (24). MDM monolayers on tissue culture plates were preincubated with 32 mM DCF (Molecular Probes) for 30 min at 37°C followed by stimulation with phorbol myristate acetate (PMA; 1 µg/ml), opsonized zymosan (OZ) particles (multiplicity of infection, 20:1), SP-D, M. tuberculosis (multiplicity of infection, 10:1), or M. tuberculosis plus SP-D (0.5 and 5.0 µg/ml). The fluorescence emitted from the stimulated cells was monitored every 2 min for 2 h by a Fluostar32 fluorometer (BMG Lab Technologies). This assay measures production of H2O2-dependent reactive oxygen intermediates (12). PMA and OZ stimulation of MDMs resulted in progressively increased fluorescence (Fig. 1), indicating that the MDMs responded to known soluble and particulate stimuli of the oxidant burst. In contrast, neither SP-D alone, M. tuberculosis alone, nor M. tuberculosis in the presence of SP-D stimulated detectable levels of oxidants over medium-only control. These data indicate that SP-D does not stimulate a detectable respiratory burst in macrophages during phagocytosis of M. tuberculosis.
SP-D increases the fusion of M. tuberculosis phagosomes with lysosomes in human macrophages.
Since SP-D binds to the terminal mannosyl units of ManLAM (
7)
and ManLAM is a bacterial determinant that inhibits
M. tuberculosis phagosome maturation (
9,
14), we hypothesized that binding of
SP-D to the surface of
M. tuberculosis would alter the surface
characteristics of the bacterium and result in enhanced
M. tuberculosis phagosome maturation. In order to test this hypothesis, we performed
complementary transmission electron microscopy (TEM) and confocal
microscopy assays to assess P-L fusion in
M. tuberculosis-infected
MDMs. The TEM assay relies on the labeling of lysosomes and
late endosomes with horseradish peroxidase (HRP) and the subsequent
visualization of these compartments in the form of a black precipitate
within or separate from phagosomes following the addition of
diaminobenzidine (
14,
16). In the absence of HRP, there is no
formation of black precipitate (data not shown).
M. tuberculosis strain Erdman (ATCC 35801) was preincubated with SP-D for 1
h at 4°C before infection of the HRP-loaded MDM monolayers
for 2 h at 37°C. Black precipitate within the
M. tuberculosis phagosome was scored as P-L fusion. In the presence of SP-D,
M. tuberculosis phagosomes demonstrated increased P-L fusion
compared to the control phagosomes (Fig.
2). We found that SP-D
caused relatively higher P-L fusion for phagosomes that contain
single bacilli than for phagosomes containing two or more bacilli
(58.2% ± 5.2% with SP-D versus 38.1% ± 2.6% without
SP-D) (Fig.
2C). As the number of bacilli per phagosome increased
(whether single or clumped), so did P-L fusionan effect
that was further enhanced in the presence of SP-D, except in
the case of phagosomes each with three or more bacilli where
the percentage of P-L fusion was very high. These data indicate
that SP-D coating of
M. tuberculosis has a direct effect on
increasing P-L fusion of single bacilli and also may have an
indirect effect by inducing agglutination of the phagocytosed
bacilli to a certain extent.
The confocal microscopy assay for P-L fusion is based on colocalization
of the phagosomes with the lysosomal/late endosomal markers
LAMP-1 and/or CD63 as described previously (
14,
21). A green
fluorescent protein-expressing
M. tuberculosis H37Rv strain
(
M. tuberculosis-GFP; a kind gift from V. Deretic, University
of New Mexico) was preincubated with SP-D before infection of
the MDM monolayers as described above. The yellow appearance
of individual bacilli on confocal images represents a colocalization
of the bacillus (green) with the lysosomal marker (red). The
association of
M. tuberculosis phagosomes with LAMP-1 was significantly
increased from 33.3% ± 1.5% (control) to 53.5% ±
5.5% in the presence of SP-D (Fig.
3). In separate experiments,
we found that SP-D also increased the colocalization of
M. tuberculosis-GFP
with CD63 (Fig.
3). These data correlate well with our TEM data
and together with them provide strong evidence that SP-D enhances
P-L fusion of
M. tuberculosis-containing phagosomes.
ManLAM beads are phagocytosed by the macrophage MR and reside
in phagosomes with limited P-L fusion (
14). To determine whether
the effect on SP-D of increasing the P-L fusion of
M. tuberculosis-containing
phagosomes was related to its binding to ManLAM and disrupting
the ManLAM-MR pathway, we assayed for P-L fusion of ManLAM beads
or control human serum albumin (HSA) beads in the presence or
absence of SP-D. Purified ManLAM from
M. tuberculosis H37Rv
was used to coat green fluorescent polystyrene beads (Polysciences,
Inc.) as described previously (
25), and the ManLAM-coated beads
or HSA control beads were preincubated with SP-D before they
were added to the MDM monolayers as done above with
M. tuberculosis.
The cells were processed (
14) and analyzed by confocal microscopy.
The yellow color obtained by colocalization of the green beads
with the red-stained CD63 marker was scored as P-L fusion. In
the absence of SP-D, P-L fusion for ManLAM bead phagosomes was
34% ± 2%, whereas in the presence of SP-D, there was
a significant increase in P-L fusion (56% ± 2%, mean
± standard error of the mean [SEM];
n = 2;
P < 0.05).
In contrast, there was no significant difference in the level
of P-L fusion with or without SP-D treatment for HSA control
beads (data not shown).
To determine whether the increased P-L fusion seen with M. tuberculosis and SP-D is due primarily to bacterial agglutination which requires the collagen-like region (CLR) of SP-D (8), we performed similar experiments with M. tuberculosis using the trimeric neck CRD (NCRD) fusion protein, which lacks the CLR but contains the neck and CRD of human SP-D (4). We found that the NCRD protein also increased P-L fusion for M. tuberculosis-containing phagosomes (Fig. 4). These data indicate that the increased P-L fusion due to SP-D does not require the CLR of the protein or agglutination of the bacilli. These data, along with our previous work (7, 8), provide further evidence that SP-D influences the interaction between the macrophage and M. tuberculosis primarily via the CRD portion of the protein that binds to the surface of the bacterium via ManLAM.
SP-D is an important member of the lung collectin family. Lung
collectins are involved in the pathogenesis and host defense
of many respiratory infectious diseases including tuberculosis.
SP-D has been shown to reduce entry of
M. tuberculosis into
macrophages, agglutinate the bacteria potentially to aid their
clearance from the lung, and reduce survival of the SP-D-coated
bacilli that are phagocytosed by macrophages. In the current
study, we provide a potential mechanism for the reduced intracellular
survival of
M. tuberculosis in macrophages, i.e., SP-D enhances
fusion of the
M. tuberculosis phagosome with lysosomes.
M. tuberculosis ManLAM has been shown to be a key molecule in mediating phagosome
maturation arrest (
3,
9). We have previously demonstrated that
the CRD of SP-D binds to the terminal mannose caps of ManLAM
(
7). At the same time, the macrophage MR recognizes the terminal
mannose caps of ManLAM as its principal ligand in mediating
phagocytosis of virulent
M. tuberculosis by human macrophages
(
15,
24,
25). Therefore, the reduced phagocytosis of
M. tuberculosis in the presence of SP-D observed in our previous study (
7) is
likely due to a reduction in the interaction between the ManLAM
caps and the MR. Recently, we have demonstrated that the MR
plays a crucial role in inhibiting maturation of phagosomes
containing
M. tuberculosis or ManLAM-coated beads (
14). Blocking
the MR activity by specific inhibitors or masking the ManLAM
mannose caps by anti-ManLAM antibody redirected the particles
toward other phagocytic receptors and caused higher levels of
P-L fusion in macrophages (
14).
Taken together, our current study clearly supports the idea that SP-D binds and masks the terminal mannose caps of ManLAM on the surface of M. tuberculosis and directs phagocytosis of the bacilli through alternative routes other than the ManLAM-MR pathway, thereby promoting phagosome maturation. Thus, our data indicate that SP-D, in contrast to SP-A, serves as an important innate host defense protein for M. tuberculosis in the lungs.

ACKNOWLEDGMENTS
This work was supported in part by a grant from the NHLBI, NIH
HL03885, and a grant from the American Lung Association of Iowa
(J. S. Ferguson); NHLBI, NIH HL51990 (L. S. Schlesinger and
D. R. Voelker); HL45286 and HL66235 (D. R. Voelker); NIAID,
NIH 33004 and NIH 59639 (L. S. Schlesinger); and HL-44015 and
HL-29594 (E. C. Crouch).
We gratefully acknowledge Jordi B. Torrelles for purifying M. tuberculosis ManLAM and coating beads; Thomas Kaufman for his technical assistance; the Central Microscopy and Imaging Facilities both at the University of Iowa and at The Ohio State University; Lee-Ann Allen, Carver College of Medicine, University of Iowa, for her assistance with confocal microscopy; and the Biosafety Level 3 Facilities at the VAMC, Iowa City, IA, and The Ohio State University.

FOOTNOTES
* Corresponding author. Mailing address: Division of Infectious Diseases, Department of Internal Medicine, The Ohio State University, N-1147 Doan Hall, 410 W. 10th Ave., Columbus, OH 43210. Phone: (614) 293-0431. Fax: (614) 293-4556. E-mail:
larry.schlesinger{at}osumc.edu.

Published ahead of print on 9 October 2006. 
Editor: J. L. Flynn
Present address: Division of Science and Research Institute, American Dental Association, Chicago, IL 60611. 

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Infection and Immunity, December 2006, p. 7005-7009, Vol. 74, No. 12
0019-9567/06/$08.00+0 doi:10.1128/IAI.01402-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.
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