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*
Clinica Pediatrica,
Cattedra di Chimica, and
Sezione di Patologia Generale ed Immunologia, Universita di Brescia, Brescia, Italy;
§
Dipartimento di Patologia e Medicina Sperimentale e Clinica, Universita di Udine, Udine, Italy;
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Servizio di Immunologia Clinica, Spedali Civili, Brescia, Italy; and
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Istituto Ricerche Farmacologiche "Mario Negri," Milan, Italy
| Abstract |
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compared with normal donors. Conversely,
neither MCP-1 binding to monocytes nor induction of the respiratory
burst by MCP-1 and FMLP is significantly different between WAS patients
and normal donors. Within a few minutes of stimulation, monocytes
respond to chemokines with increased expression of adhesion molecules
and with morphologic changes such as cell polarization. Although
up-regulation of CD11b/CD18 expression following stimulation with FMLP
or MCP-1 is preserved in WAS patients, cell polarization is
dramatically decreased. Staining of F-actin by FITC-phalloidin in
monocytes stimulated with chemoattractants shows F-actin to have a
rounded shape in WAS patients, as opposed to the polymorphic
distribution of F-actin in the polarized monocytes from healthy donors.
These results suggest that WAS protein is involved in the monocyte
response to the chemokines MCP-1 and macrophage inflammatory
protein-1
. | Introduction |
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Abnormalities of cytoskeleton organization, with paucity of leukocyte microvilli, have been reported as a hallmark of the disease and may contribute to the lymphoid and platelet disturbances (1, 7, 8); recent reports have indicated that WASp binds to several intracellular proteins containing SH3 domains, such as Cdc42 (a ras superfamily member), p47phox (a cytosolic subunit of NADPH oxidase), and Fyn (a Src family member), that are all involved in the regulation of leukocyte functions and motility (9, 10, 11, 12, 13, 14, 15).
Leukocytes migrate in response to concentration gradients of chemoattractants, such as bacterial products, complement components, and a new class of cytokines recently identified and called chemokines (16, 17). These chemotactic factors induce leukocyte infiltration into inflamed tissues such as inflamed joints of rheumatic patients, granulomatous reactions, or DTH responses (18, 19). Stimulation by chemotactic factors results in changes in leukocyte morphology, cell polarization and adhesion, and induction of respiratory burst and of leukocyte degranulation and thereby activation of leukocyte antibacterial properties (17, 20, 21). In the present report we investigated whether monocyte activation in response to this new class of chemoattractants is affected in WAS patients and how these defects might relate to the biologic role of WASp. A selective defect in chemotaxis, cell polarization, and F-actin distribution was observed in response to all the chemokines tested, whereas other chemokine-induced responses (e.g., activation of the respiratory burst and up-regulation of CD11b/CD18 expression) were not affected.
| Materials and Methods |
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Four patients with molecularly defined WAS were included in this
study. Patients were evaluated and treated at the Department of
Pediatrics, University of Brescia (Table I
). Monocytes constituted 5 to 15% of
all leukocytes in the patients studied. As a control, PBMC were
obtained, after informed consent was granted, from age-matched subjects
that were hospitalized for minor head trauma.
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PBMC were purified by Ficoll separation medium (Lympholyte-H,
Cedarlane Laboratories, Hormby, Canada) gradient centrifugation as
described previously (22). Monocytes constituted 30 to 40% of PBMC as
determined by a direct immunofluorescence assay using the mAb CD14
(Dako, Glostrup, Denmark). When indicated, monocytes were purified by
Percoll separation medium (Pharmacia Biotech, Uppsala, Sweden) as
described previously (23). Cells were cultured in RPMI 1640 (HyQ,
HyClone Europe, Cramlignton, U.K.) containing 100 U/ml penicillin, 100
µg/ml streptomycin, 2 mmol/l L-glutamine, 20 mmol/l
HEPES (Imperial, Andover Hants, U.K.), and 10% heat-inactivated FCS
(Boehringer Mannheim, Mannheim, Germany). Human recombinant MCP-1,
MIP-1
, and RANTES were obtained from Pepro Tech (Rocky Hill, NJ).
All reagents and media, tested by The Endotoxin Kit (Sigma, St. Louis,
MO), contained endotoxin at levels <12 pg/ml.
Cytokine determination
The MCP-1 concentration was assessed by an ELISA kit purchased from R&D Systems Europe (Abingdon, U.K.), following the manufacturers instructions.
Migration assays
Migration of monocytes (1.5 x 106 cells/ml in RPMI 1640 and 1% BSA) was evaluated by a microchamber technique as described previously (24). For monocytes, 5-µm pore size polycarbonate filters (Neuro Probe, Cabin John, MD), were employed. Under the assay conditions employed, only monocyte in PBMC preparations migrated across the filter. At the end of the incubation (90 min), filters were removed, fixed, and stained by Diff-Quick (Harleco, Gibbstown, NJ), and three oil immersion fields were counted in a blind evaluation after coding samples. In each assay, FMLP (Sigma) at a concentration of 10 nM was used as a standard chemoattractant for monocytes.
MCP-1 binding
Biotin-conjugated MCP-1 (R&D Systems, Minneapolis, MN) was used, following manufacturers instructions. Briefly, cells (106/100 µl) were washed twice with PBS, resuspended in 50 µl of RPMI 1% BSA, and incubated at 16 to 20°C with 10 µl of biotin-conjugated-MCP-1 (R&D, Minneapolis, MN). After 15 min, 10 µl of avidin-FITC (R&D Systems) was added to each sample, and the incubation was continued at 4°C for 30 min. Cells were then washed with cold medium (RPMI/1% BSA) twice, resuspended in PBS plus 1% paraformaldehyde, and analyzed by a flow cytometer FACScan (Becton Dickinson, San Jose, CA). Binding activity was expressed as the mean channel fluorescence of MCP-1-avidin binding cells (>90% of monocytes were positive for MCP-1 binding).
FACS analysis
Whole blood treated with FMLP was washed and incubated with saturating concentrations of CD11b (Dako) or control mouse-IgG (Dako) for 30 min at 4°C. Cells were washed twice with PBS, resuspended in 100 µl of PBS, and incubated at 4°C for 30 min with 4 µl of FITC-conjugated rabbit anti-mouse IgG (Dako). RBCs were then lysed by incubating the blood with 4 ml of FACS lysing buffer (Becton Dickinson) for 5 min at room temperature. After three washes with PBS, cells were resuspended in PBS plus 1% paraformaldehyde and analyzed with a FACScan (Becton Dickinson). At least 5000 events were acquired, and on the basis of forward and side scatter, the window for monocyte-gated cell was set.
Monocyte polarization assay
The polarization assay was performed with purified monocytes in suspension stimulated with chemoattractants as described previously (25). Briefly, purified monocytes were stimulated, in duplicate, with chemoattractants or with medium alone for variable lengths of time (210 min) at room temperature in polypropylene tubes. Stimulation was stopped by adding an equal volume of 10% formaldehyde to the medium. After coding samples, at least 200 cells were counted and classified on the basis of their morphology (spherical or head-tail shape) by an independent investigator. Data were expressed as a percentage.
Immunofluorescence staining of F-actin
The monocyte polarization assay in response to FMLP, MCP-1, and
MIP-1
was performed as described above. At the end of the incubation
period, monocytes were centrifuged over a slide at 700 rpm for 10 min
by cytospin. Monocytes adherent to the slide were washed twice with
PBS, incubated with 0.165 µM FITC-phalloidin or with a control IgG
conjugated to FITC for 30 min at 4°C, and washed. Slides were stained
with FITC-phalloidin and then photographed by a fluorescence
microscope. Slides stained with FITC-IgG did not display any detectable
fluorescence.
Superoxide anion production
Superoxide production by monocytes was measured as superoxide dismutase (SOD; Sigma)-inhibitable cytochrome c reduction by a modified Pick and Mizel method (26). Briefly, monocytes, contained in PBMC preparations were resuspended in medium (Hanks containing 5 mM glucose, 0.5 mM calcium chloride, 4 mM sodium azide, and 80 µM cytochrome c; Sigma) and preincubated in 96-well microtiter plates for 5 min at 37°C. Stimulation of cells was performed in triplicate with or without addition of SOD (250 µg/ml) at 37°C for 60 min. At 5-min intervals, the OD at 550 nm of each reaction mixture was determined in an ELISA reader. Superoxide production was calculated from the difference in the ODs at 550 nm between the wells with and without SOD and was converted to nanomoles of superoxide anion per 2 x 105 monocytes.
Statistical analysis
Comparisons between normal donors and WAS patients were performed where indicated using nonparametric analysis for unpaired data.
| Results |
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. With both chemokines, the chemotactic responses of monocytes
obtained from WAS patients were strongly reduced compared with those of
monocytes from normal donors (Figs. 2
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, or FMLP with FITC-conjugated phalloidin, a
fluorescent probe specific for F-actin (Fig. 8
showed a polarized distribution of F-actin with
pseudopod formation. In contrast, monocytes obtained from WAS patients
maintained a rounded shape upon stimulation.
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| Discussion |
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. Monocyte
refractiveness to chemoattractants is not related to receptor
desensitization in vivo as hypothesized by Altman et al. (31), as we
failed to detect increased levels of MCP-1 in the plasma of WAS
patients, and their monocytes display a normal desensitization pattern
in response to both FMLP and MCP-1. This observation was further
confirmed by our finding that monocytes from WAS patients display a
binding activity to MCP-1 comparable to that of monocytes derived from
normal donors, as shown by flow cytometric analysis of biotinylated
MCP-1 binding. Furthermore, monocytes from WAS patients, stimulated
with MCP-1 or by FMLP, release superoxide anion and up-regulate cell
surface expression of CD11b/CD18 to the same extent as monocytes
obtained from normal donors, indicating a selective defect in cell
motility rather than a generalized abnormality of chemokine-mediated
responses.
In leukocytes, chemoattractants induce actin polymerization,
integrin up-regulation, and superoxide anion production through G
protein-coupled receptors; these receptors activate a cascade of
intracellular events, including changes in cytosolic free calcium and
release of phosphoinositides (30). In this signaling pathway, proteins
with GTPase activity, such as Rho, Cdc42, and Rac, are involved (14, 27). These proteins belong to the Ras superfamily and determine
membrane ruffling and pseudopodia formation in fibroblasts (14). The
active form of Cdc42 was found to interact with WASp (9, 10, 11). Following
interaction with Cdc42, WASp forms clusters of aggregation of F-actin
(9). We found that monocytes from WAS patients, after stimulation with
MCP-1 or with FMLP, lack the capability of cell polarization and show a
diffuse distribution of F-actin in cells with a rounded shape, in
contrast to the accumulation of F-actin in pseudopods as observed in
normal donors after stimulation with FMLP, MCP-1, or MIP-1
. These
results demonstrate that monocytes from WAS patients have a defective
cell polarization in response to chemoattractants and suggest that WASp
is involved in the regulation of F-actin polymerization in vivo (9). In
addition, our results suggest that the defect of cell polarization
observed in WAS patients may determine the reduction of leukocyte
chemotaxis in response to chemoattractants. A possible explanation for
the monocyte defects observed in WAS patients is based on the
hypothesis that in normal individuals, following stimulation with
chemokines, Cdc42 would interact with WASp and thereby induce F-actin
polymerization. A similar role has been postulated for the GTP binding
protein Rac in the induction of respiratory burst in response to
chemokines in polymorphonuclear cells, but has not been described for
Cdc42 (32). The small GTP binding proteins, Rho and Rac, are essential
intracellular components of the superoxide anion synthase that
translocate to the membrane and activate the enzyme after
polymorphonuclear cells stimulation with FMLP or IL-8 (27, 33). We
speculate that chemokines such as MCP-1 and MIP-1
might activate
Cdc42, and that, in turn, WASp, bound to activated Cdc42, might
constitute the anchor for G-actin polymerization.
The decreased chemotactic response of monocytes in WAS patients
suggests possible defects of leukocyte recruitment in inflamed tissues
in these subjects. Leukocyte migration and homing are induced by local
production of chemokines in inflammatory reactions, as observed in
dermatitis and in DTH; in these cases, chemokine release in the derma
was shown to be required for leukocyte recruitment (34, 35). Our
observation that monocytes from WAS children have deficient chemotactic
responses to MCP-1 and to other chemokines, but produce normal amounts
of MCP-1 when activated by LPS (data not shown) provides a possible
explanation for the impaired DTH that is often observed in WAS
patients, even when the in vitro lymphocyte proliferative response to
mitogens is preserved (1, 3). Furthermore, chemokines such as MIP-1
,
IL-8, and stromal derived factor-1 are involved in the regulation of
chemotaxis and the proliferation of hemopoietic precursors (36, 37).
Whether interactions with marrow stromal cells and a proliferative
advantage of hemopoietic precursors with intact WASp function account
for the nonrandom pattern of X-chromosome inactivation reported in
CD34+ hemopoietic progenitor cells from WAS carrier
females remains to be seen (38).
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| Acknowledgments |
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| Footnotes |
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2 Address correspondence and reprint requests to Dr. Raffaele Badolato, Clinica Pediatrica, Universita di Brescia, c/o Spedali Civili, 25123 Brescia, Italy. E-mail address: ![]()
3 Abbreviations used in this paper: WAS, Wiskott-Aldrich syndrome; DTH, delayed-type hypersensitivity; WASp, Wiskott-Aldrich syndrome protein; MCP-1, monocyte chemoattractant protein-1; MIP-1
, macrophage inflammatory protein-1
; SOD, superoxide dismutase. ![]()
Received for publication August 8, 1997. Accepted for publication March 10, 1998.
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