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R (CD89) Binding of IgA1
* Immunotherapy Laboratory, Department of Immunology, University Medical Center,
Medarex Europe, and
Genmab, Utrecht, The Netherlands
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Abstract |
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 Top Abstract IntroductionMaterials and MethodsResultsDiscussionReferences |
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RI, exists in serum as monomeric
IgA and at mucosal sites as secretory IgA (SIgA). SIgA consists of
dimeric IgA linked by joining chain and secretory components. Human
polymorphonuclear leukocytes (PMN) and mouse PMN transgenic for human
FcRI exhibited spreading and elicited respiratory burst activity
upon interaction with either serum or SIgA. However, PMN devoid of the
2 integrin Mac-1 (Mac-1-/-) were unable to
bind SIgA, despite expression of FcRI. Consistent with this, serum
IgA stimulated Mac-1-/- PMN oxygen radical production, in
contrast to SIgA. Binding studies showed the secretory component, by
itself, to interact with Mac-1-expressing PMN, but not with
Mac-1-/- PMN. These data demonstrate an essential role
for Mac-1 in establishing SIgA-FcRI
interactions. |
Introduction |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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FcRI (CD89) is the IgA receptor, which is uniquely expressed on
polymorphonuclear leukocytes (PMN), monocytes, eosinophils, and select
macrophages (4, 5). FcRI binds serum and SIgA with low
affinity (Ka,
106 M-1)
(6). FcRI expression and function can be up-regulated
by fMLP, GM-CSF, IL-8, IL-1, TNF-, and LPS, and down-regulated by
TGF-. The FcRI ligand-binding chain contains a short cytoplasmic
tail and its expression and function are dependent on FcR 
-chains,
which contain specialized immunoreceptor tyrosine-based activation
motifs (7).
Serum IgA can initiate numerous immune effector functions via FcRI,
which include phagocytosis, cytotoxicity, respiratory burst activity,
release of inflammatory mediators, and Ag presentation (4, 5, 8). Moreover, serum IgA-FcRI interactions on Kupffer cells
may provide a "second-line" of defense in mucosal immunity
(9).
SIgA is considered crucial for the "first-line" mucosal defense
(10, 11, 12), however, its precise in vivo role remains
unclear. SIgA has been shown to interact with microorganisms and to
block virulence factors, which may prevent microbial invasion.
Furthermore, SIgA can trigger phagocyte respiratory burst activity,
degranulation, and TNF- production (13, 14, 15, 16, 17). Another
role suggested for SIgA is neutralization of viruses within infected
epithelia (18, 19). Significant correlations have been
reported between specific titers of SIgA and resistance to infection
(20).
PMN and macrophages are the major phagocytic populations at mucosal
sites. Upon microbial invasion these phagocytes are recruited, increase
receptor levels, and become activated (or "primed"). Fc and
complement receptors are essential for the recognition and elimination
of foreign targets. Mac-1 (CR3, CD11b/CD18, or
m2) is an important
PMN complement receptor that binds multiple ligands, including C3bi,
ICAM-1, fibrinogen, and -glucan. Mac-1 is important for leukocyte
adhesion, migration, phagocytosis, cytotoxicity, and chemotaxis
(21). Furthermore, Mac-1 has been proposed to act as
"signaling partner" for other leukocyte receptors, including IgG
receptors (22). Although Mac-1 has been implicated in FcR
function, this is the first study demonstrating a crucial role for
Mac-1 in FcR ligand binding.
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Materials and Methods |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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dIgA was purified from human serum by HPLC on HR200 Superdex
size-exclusion columns (Pharmacia, San Diego, CA). Anti-CD11b mAb M1/70
(rat IgG2b) and 44a (mouse IgG1) were purified from supernatants of
American Type Culture Collection hybridomas (Manassas, VA). Anti-CD11b
mAb Bear-1 (mIgG1) was obtained from Caltag Laboratories (Burlingame,
CA). mAb 14.1 (human IgG1, anti-FcRI) was produced by immunizing
mice transgenic (Tg) for human Ig (23) with recombinant
soluble FcRI (24) in Ribi adjuvant (Sigma-Aldrich, St.
Louis, MO). Splenic lymphocyte suspensions were fused to P3 x
63-Ag8.653 mouse myeloma cells (American Type Culture Collection), and
hybridomas were selected by ELISA and FACS on FcRI transfectants
(7). mAb 14.1 recognizes FcRI within extracellular
domain 2, outside the IgA-binding site. mAb My43 (IgM anti-FcRI)
blocks the FcRI ligand-binding domain (25).
Mice
CD11b knockout mice (Mac-1-/-) in the
C57BL/6 x 129SV background were kindly provided by Dr. T. N.
Mayadas (Harvard Medical School, Boston, MA) (26, 27, 28). To
study Mac-1 involvement in human FcRI function, mice were crossed
with human FcRI (CD89) Tg FVB/N mice (27), yielding
four different genotypes: nontransgenic (Ntg)
Mac-1+/-, Ntg Mac-1-/-,
Tg Mac-1+/-, and Tg
Mac-1-/-. Mac-1-deficiency does not affect PMN
FcR expression levels (27, 28).
PMN isolation
Human PMN were isolated from heparinized venous blood of healthy volunteers by Ficoll-Histopaque (Sigma-Aldrich) density gradient centrifugation. PMN purity determined by cytospin preparations exceeded 95%, and cell viability was >98%.
Before murine PMN isolation, mice were injected s.c. with 15 µg
polyethylene-glycol G-CSF (kindly provided by Dr. J. Andresen; Amgen,
Thousand Oaks, CA) to increase PMN numbers (29, 30). Blood
was collected from the retro-orbital plexus 3 days later. Erythrocytes
were removed by hypotonic lysis, followed by washing remaining
leukocytes with RPMI 1640 medium (Life Technologies, Grand Island, NY)
with 10% FCS. FACS analyses, performed on a FACScan (BD Biosciences,
San Jose, CA), revealed leukocytes to consist of 60% PMN, 35%
lymphocytes, 3% monocytes, and 1% eosinophils. Cell viability
determined by trypan blue exclusion was always >95%.
PMN binding and spreading on IgA
To exclude interference with complement, all Ab preparations
were heat-inactivated before use (30 min, 56°C). Glass slides
(Menzel, Braunschweig, Germany) were coated with 0.5% (w/v) BSA
(Boehringer Mannheim, Mannheim, Germany), 100 µg/ml human
serum IgA (ICN Pharmaceuticals, Costa Mesa, CA), dIgA (kindly provided
by Dr. C. van Kooten, Leiden University Medical Center, Leiden,
The Netherlands), SIgA (Sigma-Aldrich), or IgG (CLB, Amsterdam, The
Netherlands) for 3 h at 37°C, and rinsed with PBS. Isolated
human or mouse PMN (2 x 105 cells) were
incubated in RPMI 1640 medium (with 10% FCS) on coated slides for 10
or 30 min at 37°C. Cells were fixed in 3.7% paraformaldehyde, and
stained for actin with phalloidin-FITC (1:200; Sigma-Aldrich), or
stained for FcRI and Mac-1 (see Colocalization studies)
for 30 min at 20°C. Samples were mounted in Mowiol (with 2.5%
1,4-diazobicyclo-[2.2.2]-octane), and PMN binding and
spreading was analyzed by confocal laser scanning microscopy using a
Leitz DMIRB fluorescence microscope (Leica, Voorburg, The Netherlands)
interfaced with a Leica TCS4D confocal laser microscope (Heidelberg,
Germany). Cell morphology was imaged just above (0.2 µm) the coated
surfaces. PMN diameters were measured orthogonally, and then averaged.
At least 30 PMN from three different experiments were analyzed. In
Mac-1 blocking experiments, PMN were preincubated with 10 µg/ml
anti-CD11b mAb for 30 min at 4°C before SIgA binding. In
additional experiments, PMN were preincubated with 0.1 M
N-acetyl-D-glucosamine (NADG;
Sigma-Aldrich) for 10 min at 20°C, and plated on SIgA-coated slides
in the presence of NADG. Cytochalasin D (Sigma-Aldrich) was used at 10
µg/ml to study the role of actin microfilament polymerization in
FcRI-IgA binding.
Respiratory burst measurements
The luminol-ECL method was used for analysis of real-time
respiratory burst activity. Polystyrene tubes were coated with PBS, 100
µg/ml serum IgA, or SIgA for 3 h at 37°C, and blocked with
HEPES+ buffer (containing 20 mM HEPES, pH 7.4,
132 mM NaCl, 6 mM KCl, 1 mM MgSO4, 1.2 mM
NaH2PO4, 1 mM
CaCl2, 5 mM glucose, and 0.5% (w/v) BSA) for
1 h at 37°C. Isolated mouse PMN (4 x
105 cells) in HEPES+ buffer
were incubated in these tubes, and placed in a 953 LB Biolumat
(Berthold, Wildbad, Germany). Luminol (150 µM) was injected in all
tubes and light emission was recorded continuously for 30 min at
37°C. As a positive control, PMN were stimulated with 100 ng/ml PMA
(Sigma-Aldrich), and PMN incubated with luminol only served as negative
control. In blocking experiments, PMN were incubated with FcRI mAb
My43 supernatant during the assay (25).
SC binding to cells
Purified recombinant human SC (kindly provided by Dr. B. Corthesy, State University Hospital, Lausanne, Switzerland) was incubated at 50 µg/ml with isolated human or mouse PMN (2 x 105 cells) for 60 min at 4°C. PMN were washed with FACS buffer (0.5% (w/v) BSA, 0.05% azide in PBS), incubated with mouse IgG1 anti-human SC (Sigma-Aldrich) (1:50) for 45 min at 4°C, washed, and incubated with FITC-labeled goat anti-mouse IgG F(ab')2 (Jackson ImmunoResearch Laboratories, West Grove, PA) (1:100) for 30 min at 4°C. Controls included PMN incubated with both mouse anti-human SC, and FITC-labeled goat anti-mouse IgG F(ab')2. FITC fluorescence of PMN was determined by flow cytometry.
Colocalization studies
Human or mouse PMN were incubated on BSA or IgA-coated glass
slides for 15 min at 37°C, washed with PBS, and incubated with 20
µg/ml of mAb 14.1 and mAb M1/70 in PBS containing 0.2% (w/v) BSA and
5% rabbit serum (Jackson ImmunoResearch Laboratories) for 30 min at
4°C. PMN were then fixed in 3.7% paraformaldehyde (30 min, 20°C),
quenched with 20 mM NH4Cl in PBS (5 min, 20°C),
washed, and incubated with F(ab')2 of
FITC-labeled rabbit anti-human IgG (DAKO, Glostrup, Denmark) (1:50)
and F(ab')2 of TRITC-labeled donkey anti-rat
IgG (Jackson ImmunoResearch Laboratories) (1:125) in PBS with 0.2% BSA
for 45 min at 20°C. Slides were washed, mounted, and analyzed by
confocal microscopy (see above). As a control, colocalization between
FcRI and the 2 integrin LFA-1 (CD11a/CD18)
was analyzed using mAb M17/4 (rat IgG2a anti-mouse CD11a), which
was a kind gift from Dr. G. Kraal (Vrije Universiteit, Amsterdam, The
Netherlands).
Statistical analyses
Data are expressed as means ± SD. Statistical significance was determined by two-tailed unpaired Student’s t tests. Values of p < 0.05 were considered significant.
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Results |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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RI-IgA binding
To assess Mac-1 involvement in FcRI-ligand binding, we studied
PMN from human FcRI-Tg mice crossed with CD11b knock-out mice
(Mac-1-/-). Because FcRI represents a low
affinity IgAR, we analyzed PMN binding to IgA-coated surfaces. Tg
Mac-1-expressing PMN efficiently bound and spread onto serum IgA (IgA),
dIgA, and SIgA-coated slides within 10 min (Fig. 1
, upper panels). PMN
spreading capacity on IgA-coated surfaces varied among different IgA
forms, and was quantified by measuring PMN diameters (18 ± 4.2
µm on serum IgA, 15 ± 3.3 µm on dIgA, and 14.6 ± 3.7
µm on SIgA). Tg Mac-1-/- PMN bound properly
to serum IgA and dIgA, but did not interact with SIgA (Fig. 1, lower panels). Furthermore, Mac-1-/-
PMN, exhibited impaired spreading on slides coated with serum IgA
(12.1 ± 2.5 µm) or dIgA (11.9 ± 4.2 µm). PMN of Ntg
Mac-1+/- littermates, expressing Mac-1, but not
FcRI, served as controls and did not bind human IgA. These data were
supported by studies with human PMN. Binding and spreading capacity of
human PMN on serum IgA, dIgA, and SIgA was similar to that of Tg
Mac-1+/- PMN. However, F(ab')2 of
-Mac-1 mAb inhibited spreading of human PMN on SIgA, but not on
serum IgA slides (data not shown). In addition, all phagocytes bound
IgG- or BSA-coated surfaces (Fig. 1), irrespective of FcRI/Mac-1
expression. Binding of both human and Tg mouse PMN to SIgA was
abolished in the presence of cytochalasin D. These results indicate
Mac-1 to be required for phagocyte-FcRI binding of
SIgA.
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To study the significance of PMN interaction with immobilized IgA,
we investigated respiratory burst activity of mouse (Ntg
Mac-1+/-, Tg Mac-1+/-, Tg
Mac-1-/-) and human PMN. Both serum IgA and
SIgA induced comparable oxygen radical production in Tg
Mac-1+/- PMN (Fig. 2, left graph). However, Tg
Mac-1-/- PMN were stimulated by serum IgA, but
not by SIgA (Fig. 2, middle graph). Importantly,
Mac-1-expressing and Mac-1-deficient PMN exhibited similar respiratory
burst activity in response to PMA. Ntg Mac-1-expressing PMN did not
initiate respiratory activity in response to serum IgA or SIgA (Fig. 2, right graph). Human PMN exhibited comparable oxygen radical
production upon interaction with serum IgA or SIgA, albeit that serum
IgA-induced respiratory burst activity was significantly faster (data
not shown). Blocking studies with My43, an IgM recognizing the FcRI
ligand-binding domain (25), decreased oxygen radical
production close to background levels (data not shown), showing
IgA-induced respiratory burst to depend on interaction with FcRI.
These data demonstrate requirement of both FcRI and Mac-1 for
SIgA-induced respiratory burst activity.
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RI
Because our data pointed to an essential role for Mac-1
in FcRI-IgA interactions, we next studied the distribution of Mac-1
and FcRI in PMN membranes. Isolated Tg
Mac-1+/- PMN were incubated on
surfaces coated with BSA, serum IgA, or SIgA, whereupon FcRI and
Mac-1 were stained with FITC (green), and TRITC (red), respectively. Tg
Mac-1+/- PMN mediated spreading on both serum
IgA (Fig. 3A, upper
panel) and SIgA (Fig. 3A, middle panel).
FcRI was expressed on the plasma membrane of PMN, but also in
filipodia-like outgrowth of cells. A similar staining was found for
Mac-1, and colocalization was indicated in yellow (Fig. 3; merged
pictures). Tg Mac-1+/- PMN did not spread on
BSA-coated slides, and exhibited overall membrane staining of FcRI
and Mac-1 (Fig. 3A, lower panel). As a control,
FcRI and 2 integrin LFA-1 membrane staining
was examined, which revealed no colocalization (Fig. 3B).
Importantly, experiments performed with human PMN revealed FcRI and
Mac-1 expression to colocalize as well (see below).
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Our results demonstrated only Mac-1-expressing PMN capable of
binding SIgA-coated surfaces. Next, we evaluated the capacity of
Mac-1+/- and Mac-1-/-
PMN to bind aggregated IgA by flow cytometry. Ntg
Mac-1+/- PMN were ineffective in binding serum
IgA-complexes, but interacted well with SIgA complexes. On the
contrary, Ntg Mac-1-deficient PMN were unable to bind SIgA complexes
(data not shown). These observations pointed to a role for the SC in
PMN-SIgA binding. Therefore, we examined the capacity of recombinant SC
to interact with isolated human and mouse PMN. As shown in Fig. 4A, Mac-1-expressing PMN
readily bound SC, irrespective of the presence of FcRI. However,
binding of SC to Mac-1-deficient Tg and Ntg PMN, was abrogated. SC
interacted with human PMN as well (Fig. 4B). These data show
SC, either recombinant or present in its normal configuration (i.e.,
complexed with dIgA), to be capable of interacting with Mac-1.
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We next evaluated the region of Mac-1 involved in SIgA-FcRI
binding. Human PMN were plated on immobilized serum IgA or SIgA in the
absence (control) or presence of NADG, which interacts with the Mac-1
lectin-binding domain (31), and were stained for both
FcRI and Mac-1 (Fig. 5). Human PMN
exhibited spreading on both serum IgA and SIgA, like Tg
Mac-1+/- PMN. Incubation of PMN with NADG
resulted in a blockade of PMN binding to SIgA-coated surfaces, whereas
binding to serum IgA was barely affected by NADG (Fig. 5, lower
panel). The partial colocalization of FcRI and Mac-1 observed
in control PMN bound to serum IgA-coated slides was decreased in the
presence of NADG. PMN incubated on BSA-coated surfaces in the presence
of NADG exhibited no change in binding. Furthermore, NADG impaired
binding of Tg Mac-1+/- to SIgA, but not to serum
IgA (data not shown). In contrast, Abs directed against the I-domain of
Mac-1, although partly inhibiting PMN spreading, did not inhibit
binding of human PMN to serum and SIgA (data not shown). These results
implicate the Mac-1 lectin-binding domain to be involved in
FcRI-SIgA interaction.
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Discussion |
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TopAbstractIntroductionMaterials and MethodsResultsDiscussionReferences |
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RI and IgA,
relatively little is known about the biology of FcRI-IgA
interactions. The present study documents a new role for Mac-1, a
2 integrin, in ligand binding of
FcRI. Mac-1 was found to be crucial for interaction of PMN FcRI
with SIgA, and subsequent PMN activation. Our data support Mac-1
binding to SC to underlie this phenomenon, and implicate the Mac-1
lectin-binding domain to be involved.
Modulation of receptor-ligand interactions by accessory molecules is
well-documented for a number of immune receptors, including cytokine
receptors, integrins, and FcR (28, 33, 34, 35, 36). IgG binding to
FcRI (CD64) and FcRIII (CD16a) is dependent on association with
the FcR -chain (36, 37, 38). In this study, Mac-1 is
identified as a novel accessory protein, crucial for FcRI-SIgA
interaction. In SIgA, SC is covalently ligated to the C2 and C3
domains within the dIgA Fc region (39), which may
interfere with the affinity of FcRI for SIgA (40, 41, 42).
The present study demonstrates SC binding to PMN in a Mac-1-dependent
manner. When one appreciates the structure of SC, Mac-1 interaction
with SC seems rational. SC represents a heavily glycosylated protein
belonging to the Ig superfamily (3). Moreover, we observed
Mac-1 lectin-binding domain, bearing carbohydrate-binding specificity
(31), involved in PMN binding of SIgA.
Physical association between leukocyte membrane proteins (such as
FcR-FcR -chain, and FcRIIIb-Mac-1) has been documented before
(22, 37, 38). Despite clear colocalization of FcRI and
Mac-1, a direct FcRI-Mac-1 interaction was not apparent from
immunoprecipitation experiments (our unpublished data).
Importantly, Mac-1 modulates PMN function upon IgA binding, as
indicated by stimulation of a potent respiratory burst. Our data show
requirement of both FcRI and Mac-1 for SIgA-induced respiratory
burst activity, because Mac-1-deficient Tg PMN and Ntg PMN were unable
to elicit oxygen radical production in response to SIgA. A role for
Mac-1 in FcR functions, including respiratory burst has been shown
before (28, 43, 44, 45). CD18 interactions with the actin
cytoskeleton and associated proteins may enable Mac-1 signaling
(46, 47). Furthermore, the present work is consistent with
earlier studies documenting a role for 2
integrins in superoxide production of eosinophils mediated by SIgA, and
not serum IgA (48). SC by itself was described to activate
eosinophil functions, but not PMN functions, which was proposed to
relate to an additional (15 kDa) "SC receptor" on eosinophils
(49). Indeed, we observed Mac-1-deficient eosinophils
capable of interacting with both SC and SIgA (our unpublished
data).
In conclusion, this study provides evidence for a crucial role of Mac-1
in phagocyte SIgA binding. Enhanced IgA responses upon incubation of
PMN with GM-CSF, TNF-, or IL-8 have been reported
(50, 51, 52), although the underlying mechanisms remain to be
addressed. We propose Mac-1 involvement in these phenomena to be
likely, because of the well-recognized ability of these cytokines
and chemokines to activate Mac-1 (53, 54, 55).
Requirement of two molecules interacting with one ligand may provide
immune effector cells with an extra way to regulate their activity.
When "first-line" defense fails, mucosal inflammation leads to PMN
recruitment and priming, accompanied by up-regulation of Mac-1, which
may trigger more potent FcRI-mediated responses. We hypothesize
Mac-1 to increase affinity of FcRI-SIgA interaction via binding
of SC.
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Acknowledgments |
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Footnotes |
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2 Address correspondence and reprint requests to Dr. Jan G. J. van de Winkel, Immunotherapy Laboratory, Department of Immunology, University Medical Center, Room KC02.085.2, Lundlaan 6, 3584 EA, Utrecht, The Netherlands. E-mail address: JanvandeWinkel{at}aol.com
3 Abbreviations used in this paper: SIgA, secretory IgA; dIgA, dimeric IgA; SC, secretory component; pIgR, polymeric IgR; PMN, polymorphonuclear leukocyte; Tg transgenic; Ntg, nontransgenic; NADG, N-acetyl-D-glucosamine.
Received for publication December 7, 2001. Accepted for publication July 26, 2002.
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References |
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R). Crit. Rev. Immunol. 16:423.[Medline]
RI) review. Encyclopedia of Molecular Medicine Wiley, New York.
receptor essential for interaction with IgA. J. Immunol. 162:2146. chain: molecular basis for CD89/FcR chain association. J. Biol. Chem. 270:29781. chain signalling. Blood 97:205.RI-positive liver Kupffer cells: reappraisal of the function of immunoglobulin A in immunity. Nat. Med. 6:680.[Medline]
R) determined by measurement of chemiluminescence induced by serum or secretory IgA1 or IgA2. Immunology 71:328.[Medline]
chain-dependent shedding of soluble CD89. J. Immunol. 163:5806.2 integrin CD11b/CD18 in neutrophil apoptosis: a homeostatic mechanism in inflammation. Immunity 5:653.[Medline]
RI, CD89) function in transgenic mice requires both FcR chain and CR3 (CD11b/CD18). Blood 93:4387.-glucan-binding lectin site of complement receptor type 3 (CD11b/CD18). J. Immunol. 156:1235.[Abstract]
chain: a functional component of the interleukin-7 receptor. Science 262:1877. subunit: enhancement of FcR ligand affinity. J. Exp. Med. 183:2227. receptor and the mast cell receptor for IgE share an identical subunit. Nature 341:752.[Medline]
RI) with the subunit of the IgE receptor. Proc. Natl. Acad. Sci. USA 90:6023.2 and C3 in human IgA1. J. Exp. Med. 183:1579. receptor (FcR) CD89. J. Biol. Chem. 274:23508. receptors. P. L. Ogra, ed. Mucosal Immunology 213. Academic Press, San Diego, CA.
receptor IIIB and the complement receptor type 3. J. Immunol. 153:1769.[Abstract]
M2, CD11b/CD18) and FcRIII cooperate in generation of a neutrophil respiratory burst: requirement for FcRIII and tyrosine phosphorylation. J. Cell Biol. 125:1407.2 (CD11/CD18) integrins can serve as signaling partners for other leukocyte receptors. J. Lab. Clin. Med. 129:492.[Medline]
2-subunit (CD18) and the actin binding protein -actinin. J. Immunol. 151:3795.[Abstract]
of Fc receptor expression and IgA-mediated superoxide generation and killing of Pseudomonas aeruginosa by polymorphonuclear leukocytes. J. Infect. Dis. 170:82.[Medline]
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)-,
serum IgA (IgA, •)-, or SIgA (
)-coated tubes. As positive
control, PMN were stimulated with PMA (x). Oxygen radical
production of PMN was measured in time by chemiluminescence detection.
Experiments were performed four times yielding essentially identical
results. Statistical analyses of areas below the curve verified Tg
Mac-1+/- PMN to bind SIgA significantly better
(p = 0.001) than Tg Mac-1-/-
PMN.
