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Published online before print April 1, 2004
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RIIa expression with FcRI results in C-reactive protein- and IgG-mediated phagocytosis
* Immunology Unit, Department of Infectious and Tropical Diseases, London School of Hygiene and Tropical Medicine, United Kingdom; and
Department of Physiology, University College, Cork, Ireland
1 Correspondence at current address: School of Biomedical and Molecular Sciences, University of Surrey, Guildford, Surrey GU2 7HX, UK. E-mail: k.bodman-smith{at}surrey.ac.uk
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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R)I and mediate phagocytosis and signaling through the -chain. To date, binding of monomeric CRP to FcRII has been contentious. We demonstrate that erythrocytes opsonized with CRP bind FcRIIa-transfected COS-7 cells. In addition, we demonstrate that FcRI can use FcRIIa R131 and H131 to phagocytose erythrocytes coated with IgG or purified or recombinant CRP in the absence of the -chain. COS-7 cells expressing FcRIIa or FcRI alone did not phagocytose opsonized erythrocytes. Such phagocytosis required the cytoplasmic domain of FcRIIa, as mutation of tyrosine at position 205 and truncation of the cytoplasmic domain from the end of the transmembrane region (position 206), resulting in the loss of the immunoreceptor tyrosine activatory motif, abrogated phagocytosis. FcRIIa R131 was more efficient than FcRIIa H131 at mediating CRP-dependent phagocytosis.
Key Words: pentraxin • Fc receptors • phosphorylcholine • COS-7
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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CRP-mediated phagocytosis of several organisms has been demonstrated via binding to phosphocholine-containing ligands such as lipotechoic acid and C-polysaccharide on Streptococcus pneumoniae [11
] and lipophosphoglycan on the surface of metacyclic Leishmania donovani [12
]. Until recently, the receptors to which CRP engages to increase opsonization directly were not clearly defined; now, however, it is evident that the Fc receptors for immunoglobulin G (IgG; FcRs) are the main candidates. Receptors for the constant region of Igs (FcR) play a pivotal role in linking the cellular and humoral arms of the immune response [13
, 14
]. FcRs comprise a multigene family divided into three classes defined by their affinity for IgG, receptor structure, and tissue distribution. FcRI and FcRIII generally comprise a unique ligand-binding 
-chain associated with a common signaling chain, the -chain, which has immunoreceptor tyrosine activation motifs (ITAMs) present in the cytoplasmic domain. The human receptor FcRIIa, however, bears an ITAM within the cytoplasmic domain, capable of initiating efficient signal transduction, independent of the -chain. It has been shown recently that in U937 cells, differentiated to a more macrophage-like phenotype with dibutyryl cyclic adenosine monophosphate (dbcAMP), FcRI no longer signals through the -chain but rather recruits FcRIIa to initiate tyrosine phosphorylation [15
]. In addition, FcRIIa has been reported to colocalize with the glycosylphosphatidylinositol-anchored FcRIIIb upon cross-linking in human neutrophils [16
].
CRP binding to the high-affinity receptor for IgG, FcRI, was suggested when the binding of CRP-opsonized erythrocytes to monocytes could be partially inhibited by monomeric IgG [17
] and was further demonstrated when transfection of COS cells with FcRI increased CRP binding [18
]. More recently, we have demonstrated CRP binding to the extracellular portion of FcRI when captured on a surface plasmon resonance chip with an affinity approximately tenfold higher than that of IgG for the receptor. CRP was able to induce phospholipase D signaling in monocytes, consistent with responses through FcRI, and furthermore, CRP-opsonized phosphatidylcholine (PC)-coupled erythrocytes (CRP-PCE) were phagocytosed by COS-7 cells transfected with the FcRI + -chain [19
].
To date, CRP has also been reported to bind to the human low-affinity IgG receptor FcRIIa and preferentially to the "high responder" allotype (arginine at position 131, R131) compared with the "low responder" allotype (histidine at position 131, H131) [20
]. In addition, human CRP has been described to bind to murine FcRIIb [21
]. The majority of the literature describing these binding patterns has come from studies using fluorescein-activated cell sorter (FACS) analysis of the binding of whole detecting antibodies directed against CRP. This may, however, result in the generation of misleading binding data as a result of the Fc domains of these antibodies nonspecifically interacting with the FcRs. Indeed, other studies using F(ab')2-detecting reagents have disputed some of these findings, stating that binding of monomeric CRP to FcRIIa-transfected COS-7 cells could not be detected [22
]. Another report using biotinylated CRP also failed to observe binding to white blood cells [23
]. Indirect evidence, however, for CRP binding to FcRII still remains; for example, 100 µg/ml CRP added to human FcRIIa R131-expressing neutrophils resulted in Ca++ flux [20
], and 10–200 µg/ml CRP triggered phosphorylation of FcRIIa in granulocytic HL-60 cells [24
]. In addition, CRP-mediated protection from lipopolysaccharide is abrogated in FcRIIb-deficient mice [25
]. An explanation for the above data might be that interaction between CRP and FcRII does exist but is low avidity.
The first aim of this paper, therefore, was to investigate CRP binding to FcRIIa R131 and H131 using a CRP-opsonized particle with low background uptake as opposed to monomeric CRP. Second, as FcRIIa has been reported to be recruited by FcRI to initiate tyrosine phosphorylation in U937 cells treated with dbcAMP, we looked to see whether FcRIIa could be recruited by FcRI following binding to CRP or IgG to initiate phagocytosis in transfected COS-7 cells.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Generation of F(ab')2 fragments of anti-human CRP
Rabbit polyclonal IgG to human CRP (10 mg, Dako, Ely, Cambridgeshire) was incubated with 10,000 U pepsin-agarose (Sigma-Aldrich UK, Poole, Dorset) for 30 min in 0.1 M citrate buffer, pH 3.5. Protein was eluted with citrate buffer, dialyzed against 0.1 M acetate buffer, pH 5.0, and passed through 1 ml protein G-agarose. Unbound protein was dialyzed against phosphate-buffered saline (PBS), and absence of whole antibody or Fc fragment was analyzed by polyacrylamide gel electrophoresis before use.
COS-7 cell culture and transient expression of FcRs
COS-7 cells were maintained in Dulbecco’s modified Eagle’s medium supplemented with 2 mM glutamine, 100 IU/ml penicillin, 100 µg/ml streptomycin, and 0.015 mg/ml gentamicin. Cells were plated at 1–4 x 105/ml in 60 mm petri dishes to reach 50% confluence overnight.
The open reading frames of FcRI (clone p135) [27
] and FcRIIa (R131; clone PC23) [28
] were subcloned as HindIII/NotI fragments into the expression vector pcDNA3.1 (Life Technologies, Paisley, Renfrewshire, UK). Arginine 131 of FcRIIa (R131) was mutated to histidine by site-directed mutagenesis (QuickChange, Stratagene, La Jolla, CA), using the oligos 5'-GAAATTCTCCCATTTCGATCCCACCTTCTCC-3' and 5'-GGAGAAGGTGGGATCGAAATGGGAGAATTTC-3' to create the clone FcRIIa (H131). The -chain was expressed in the pSVK3 vector (Anachem Ltd., Luton, Bedfordshire, UK) [29
].
Transient expression of the cDNAs was performed in COS-7 cells using the diethylaminoethyl–dextran method [27 ]. All experiments were performed 48–72 h post-transfection when surface expression is maximal.
Generation of mutant FcRIIa
Truncated FcRIIa R131 (from position 206) with a mutation of tyrosine at position 205 to phenylalanine was generated by polymerase chain reaction (PCR) from the FcRIIa R131. The PCR primers used were the forward primer T7 (Sigma Genosys, Cambridge, UK) and the reverse primer 5'-AAAGCGGCCGCtcaGAAGATCAAGGCCACTACAGCAGC-3' containing the NotI site (underlined), stop codon (lower case), and mutation (italics). The product was restriction-digested using XbaI and NotI and subcloned into PCI-neo (Promega UK, Southampton, Hampshire), and the sequence of the cloned fragments was confirmed by dideoxy sequencing. The construct was transiently transfected into COS-7 cells as described above.
Flow cytometry
Aliquots of 5 x 105 COS-7 cells transfected with various FcRs were incubated on ice for 45 min with directly conjugated mouse (IgG1) anti-human FcRI–phycoerythrin (PE; clone 10.1, Caltag Laboratories, Burlingame, CA) or the isotype-control mouse IgG1–PE (Caltag Laboratories) at 2 µg/ml, diluted in PBS containing 1% bovine serum albumin and 0.02% sodium azide (FACS buffer) for staining of FcRI. The monoclonal antibody 41H.16 (kindly provided by Professor Jan van de Winkel, Utrecht, The Netherlands) and the isotype-control mouse IgG2a (clone UPC-10, Sigma-Aldrich UK) were incubated at 5 µg/ml with transfected cells as above, washed three times with FACS buffer, and incubated with F(ab')2 goat anti-mouse IgG (heavy- and light-chain-specific) FITC (Jackson Immunoresearch Laboratories, West Grove, PA) for detection of surface FcRII. Finally, all the cells were washed three times in FACS buffer and fixed in 2% paraformaldehyde, and fluorescence was measured on a FACSCalibur flow cytometer (Becton Dickinson, Mountain View, CA).
Sheep erythrocyte coupling to PC and nitro-phenol (NP) followed by opsonization with CRP, IgG1, and IgG2
Sheep erythrocytes were coupled to PC as described previously [19
] using a modification of a methods described previously for erythrocyte labeling [30
]. To opsonize, PCE were washed twice in Hank’s balanced salt solution (HBSS), incubated at 108/ml with CRP [2 µg/ml in HBSS containing 0.5 mM CaCl2 (HBSSC)], or a subagglutinating dilution of rabbit (IgG1) anti-sheep erythrocyte stroma antibody (Sigma-Aldrich UK) for 1 h at 4°C and washed three times in HBSSC.
Sheep erythrocytes were also coupled to the hapten NP using the succinimido-ester of NP (NP-caproate-O-succinimide, Sigma Genosys) as described previously [31 ]. Briefly, sheep erythrocytes were washed three times in isotonic borate buffer, pH 8.5, incubated at 5 x 108/ml with NP (10 mg/ml in dimethylformamide) for 1 h at room temperature, and washed three times with HBSS. To opsonize, NP-coupled erythrocytes (NPE) were washed in HBSS, incubated at 108/ml with a subagglutinating dilution of chimaeric human IgG2 anti-NP (Serotec Ltd., Oxford, UK) for 1 h at 4°C, and washed three times in HBSS.
Phagocytosis assays
Unopsonized, CRP-opsonized, or IgG-opsonized erythrocytes were added to transfected COS-7 cells in 24-well plates containing sterile coverslips at a ratio of 100:1 and were incubated for 2 h at 37°C. Unbound erythrocytes were washed away three times with HBSSC, and hypotonic shock buffer (1 mM PBS, pH 2.5) was added to half of the wells for 2 min to lyse uninternalized erythrocytes. Cells were fixed with 2.5% glutaraldehyde in 0.2 M PBS, pH 7.4, and rosetting and internalized erythrocytes were visualized by staining for myeloperoxidase using hydrogen peroxide and O-dianisidine as described previously [32
]. A total of 200 cells per condition was counted for the percentage of cells rosetted (defined as a minimum of three associated eythrocytes), the percentage of cells containing erythrocytes (% phagocytosis), and the phagocytic index (the number of internalized erythrocytes per 100 cells).
Statistical analyses
SPSS package 11.0 for Windows (2001) was used to perform Mann-Whitney U tests.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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RIIa and preferentially, the R131 allotypeRIIa led us to investigate the binding of CRP-opsonized particles to this receptor. We chose to look at erythrocyte interactions, as these particles have a low rate of nonspecific binding and phagocytosis compared with others. COS-7 cells, therefore, were transiently transfected with FcRIIa R131 or H131 allotype plus the tyrosine kinase Syk, incubated with PCE alone or PCE opsonized with CRP or IgG1. Binding of the erythrocytes was detected as the percent cells rosetted by these particles and phagocytosis, as the percent phagocytosis and the phagocytic index.
To ensure that differences in binding and phagocytosis were not a result of differences in surface expression of these receptors, aliquots of transfected cells were stained for human FcRII expression and detected by flow cytometry. There was no significant difference in the level of cell-surface FcRIIa H131 and R131 expression by transfected COS-7 cells as measured by the mean fluorescence intensity (MFI; mean±SEM of three separate experiments: 28.3±6.05 and 29.6±5.50, respectively).
As shown in Figure 1A
, CRP- and IgG1-opsonized PCE rosetted a significantly greater percentage of FcRIIa R131-transfected COS-7 cells when compared with PCE alone (P=0.021 and P=0.021, respectively). CRP-opsonized PCE did not significantly rosette FcRIIa H131-transfected COS-7 cells (Fig. 1A)
, which is in agreement with Stein et al. [20
], and as IgG1-opsonized PCE also showed preferential binding to the FcRIIa R131 allotype, a positive control of IgG2-opsonized NPE was included. These particles significantly rosetted FcRIIa H131-transfected COS-7 cells when compared with NPE alone (P=0.034). No significant uptake was observed as percent phagocytosis (data not shown) or as the phagocytic index (number of erythrocytes per 100 cells) for CRP-, IgG1-, or IgG2-opsonized erythrocytes incubated with FcRIIa allotype-transfected COS-7 cells (Fig. 1B)
.
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RIIa is specific for CRP
Unopsonized and CRP- or IgG1-opsonized PCE were preincubated with or without F(ab')2 fragments of a polyclonal anti-human CRP antibody at 20 or 100 µg/ml and were added to COS-7 cells transfected with FcRIIa R131 or H131 + Syk. Binding of the opsonized particles to the cells was detected as the percent cells rosetted.
Figure 2
demonstrates that at 100 µg/ml, the addition of F(ab')2 anti-CRP antibody significantly inhibited the binding of CRP-opsonized PCE to cells expressing FcRIIa R131 (P=0.05), whereas IgG1-opsonized PCE was not affected. A significant reduction in CRP-opsonized PCE binding to FcRIIa H131 was also observed (P=0.05).
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RI + -chain (Fig. 3
).
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RI and FcRIIa in the absence of the -chainRI-IIa chimaera-transfected COS cells rosette and ingest IgG-opsonized erythrocytes readily [33
], and FcRIIa has been reported to associate with FcRI for signal transduction in the monoblastic U937 cell line upon differentiation to a more macrophage-like stage [15
]. We have demonstrated recently that CRP-opsonized particles bind and initiate phagocytosis through FcRI in the presence of the -chain [19
] and have confirmed binding of these particles to FcRIIa R131. Therefore, it might be hypothesized that IgG or CRP-opsonized particles may also initiate FcRI-mediated phagocytosis through cross-linking of FcRI with FcRIIa. To investigate this, COS-7 cells were cotransfected with FcRI and the -chain or FcRIIa and Syk and were tested for phagocytosis of unopsonized, CRP-opsonized, or IgG1-opsonized PCE.
The level of expression of FcRI, although cotransfected with other FcRs, was analyzed by staining with PE-labeled anti-FcRI antibody or isotype control and analyzing for surface expression by flow cytometry. There were no significant differences among the expression of FcRI upon cotransfection with the -chain, FcRIIa H131, or FcRIIa R131 on the surface of FcRI-transfected COS-7 cells, as expressed by the MFI (mean±SEM: 41.9±6.35, 51.6±17.46, and 48.0±18.0, respectively).
Binding of CRP- and IgG1-opsonized PCE to COS-7 cells transfected with FcRI, FcRI + -chain, FcRI + FcRIIa H131, and FcRI + FcRIIa R131 was demonstrated (as expressed by the percent of cells rosetted, Fig. 4A
): CRP and IgG1 opsonization resulted in significantly higher binding when compared with PCE alone in each case: P = 0.001, P < 0.001, P = 0.004, and P < 0.001, for CRP-PCE rosetting, and P = 0.001, P < 0.001, P = 0.002, and P < 0.001 for IgG1-PCE rosetting versus PCE rosetting of FcRI, FcRI + -chain, FcRI + FcRIIa H131, and FcRI + FcRIIA R131, respectively.
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RI + FcRIIa H131 less than cells transfected with FcRI, FcRI + -chain, and FcRI +FcRIIa R131. This decrease was significantly different for IgG1-opsonized PCE (P=0.008, P=0.020, and P=0.016, respectively), whereas it was not significantly different for CRP-opsonized PCE (P=0.100, P=0.382, and P=0.541, respectively).
Figure 4
clearly demonstrates that cells transfected with FcRI + FcRIIa or the -chain display significant phagocytic activity for CRP- and IgG1-opsonized PCE as expressed by the percent phagocytosis (Fig. 4B)
and the phagocytic index (Fig. 4C)
compared with PCE alone. Cells transfected with FcRI alone did not phagocytose these particles.
Once again, the levels of CRP- and IgG1-mediated phagocytosis appeared to be decreased in cells expressing FcRI + FcRIIa H131 in comparison with those expressing FcRI + -chain or FcRI + FcRIIa R131 (Fig. 4B)
. This decrease was significant for the percent phagocytosis of IgG1-opsonized PCE by cells transfected with FcRI + FcRIIa H131 in comparison with those transfected with FcRI + -chain or FcRIIa R131 (P=0.020 and P=0.042, respectively) but not for percent phagocytosis of CRP-opsonized PCE (P=0.222 and P=0.100, respectively).
The phagocytic index (Fig. 4C)
showed a similar pattern and was significantly decreased in cells transfected with FcRI + FcRIIa H131 in comparison with those expressing FcRI + -chain or FcRIIa R131 for particles opsonized with CRP (P=0.048 and P=0.011, respectively) and IgG1 (P=0.002 and P=0.016, respectively).
In addition, no significant differences in phagocytosis were observed between recombinant and purified CRP when cells were transfected with FcRI and FcRIIaR131 (data not shown).
Uptake of CRP-opsonized particles via FcRI in the presence of FcRIIa is dependent on the FcRIIa cytoplasmic ITAM
To confirm the involvement of the ITAM of FcRIIa R131 in the phagocytosis of CRP-opsonized sheep erythrocytes, we truncated the cytoplasmic domain of FcRIIa R131 from position 206. The tyrosine at amino acid 235 in FcRIIb1 has been proposed to play a role in signal transduction as well as the ITAM, and it has been shown to be essential in anchoring the molecule within the plasma membrane [34
]. As FcRIIa and FcRIIb1 are homologous in this region, and an analogous tyrosine at position 205 of FcRIIa has been identified, it may play a similar role in signal transduction; therefore, we mutated it to phenylalanine. When the resultant cDNA was cotransfected with FcRI into COS-7 cells, CRP- and IgG1-opsonized PCE rosetted the transfected cells; however, they were no longer internalized (Fig. 4)
.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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RI) is well established; its specificity for FcRII, however, is less clear. In this study, we demonstrate that PC-coupled erythrocytes, when opsonized with CRP, bind to and rosette COS-7 cells transfected with FcRIIa. The importance of this finding is that until now, groups have investigated the binding of monomeric CRP to FcRIIa, which necessitates the detection of binding with some form of anti-CRP antibody or chemical labeling of CRP. These methods can lead to nonspecific binding of the Fc portions of the detecting antibody and alteration of the structure of the CRP molecule, respectively, and hence, possible disruption of binding specificities. As FcRIIa falls into the group of low-affinity receptors for monomeric IgG, it might be expected that its affinity for CRP would also be low. Indeed, studies using F(ab')2 preparations of detecting antibody did not show any interaction between cells expressing FcRIIa and monomeric CRP [22
]. By using erythrocytes coupled to PC and subsequently opsonized with CRP, we ensure a multimeric complex of CRP molecules, presumably able to associate with and cluster FcRIIa receptors, leading to stable and detectable binding. This is likely to be a more physiologically relevant model compared with monomeric CRP interactions with low-affinity FcRs, especially when comparing binding to immune complexes containing IgG and often CRP.
The data presented here show that CRP binding to FcRIIa demonstrated a slight preference for the R131 allotype, confirming and extending the observations made by the Du Clos group [20
]. However, the difference in binding to R131 compared with H131 was not as pronounced as previously reported [20
]. As a control for the levels of expression of the two allotypes when transfected into COS-7 cells, we included a erythrocyte coupled to the hapten NP and opsonized with chimaeric human IgG2 anti-NP. This particle showed a preference for the H131 allotype and significantly rosetted cells expressing this form of FcRIIa when compared with unopsonized NP-coupled erythrocytes. This further demonstrates the specificity of the system.
It is interesting that CRP binding to FcRIIa did not result in significant levels of phagocytosis by the transfected COS-7 cells. This agrees with reports from other groups, where similarly transfected COS cells did not phagocytose IgG-opsonized erythrocytes efficiently [33
], or only a relatively low percentage of cells expressing FcRIIa underwent complete phagocytosis [35
]. This may be a result of the relatively low affinity of CRP and IgG for FcRIIa, which might be sufficient for stable rosetting or surface binding but not lead to enough clustering for phagocytosis to be initiated. Upon cotransfection of FcRI and FcRIIa R131 in the absence of the -chain, significant binding of IgG1- and CRP-opsonized PCE in comparison with PCE alone was observed. This binding was comparable with the binding of similar particles to FcRI cotransfected with the -chain. Analysis of these cells for phagocytosis demonstrated for the first time that binding of surface-expressed FcRI and FcRIIa leads to the activation of phagocytosis in the absence of the -chain. Percent phagocytosis and the phagocytic index for IgG1- and CRP-opsonized PCE were significantly different when compared with PCE alone and when compared with cells transfected with FcRI alone. These findings complement reports that FcRI on U937 cells [15
] and FcRIIIb on neutrophils [16
] can associate with FcRIIa and use its ITAM to trigger signaling.
To confirm that the combination of FcRIIa and FcRI signals through the FcRIIa intracellular domain to initiate the observed phagocytosis, we mutated the cytoplasmic tail at position 205 and truncated the molecule from position 206. All phagocytic activity was abolished for CRP-PCE and IgG1-PCE. These results suggest that the association of FcRI to FcRIIa R131 by CRP- and IgG1-opsonized PCE initiates signaling mechanisms through the FcRIIa ITAM, resulting in the promotion of phagocytosis. Upon cotransfection and therefore association with the high-affinity FcRI, CRP- and IgG1-opsonized particles may form more stable complexes, leading to recruitment of associated molecules and activation of the signaling cascade. It is possible that this relates to the high affinity of CRP for FcRI [19
].
In these studies, we also cotransfected FcRI with FcRIIa H131 allotype to assess whether the apparently reduced binding of CRP and IgG1 to this receptor affected FcRI-mediated phagocytosis. A decrease in rosetting by CRP- and IgG1-opsonized PCE to cells transfected with FcRI + FcRIIa H131 when compared with cells transfected with FcRI + -chain and those with FcRI + FcRIIa R131 was observed. A similar decrease was observed for percent phagocytosis and phagocytic index when compared with cells transfected with FcRI + -chain and FcRI + FcRIIa R131. These decreases were significant in all cases of IgG1-PCE binding and uptake, although only significant for CRP-PCE upon analysis of the phagocytic index. This observation is not wholly surprising, as the decreased affinity for FcRIIa H131 may lead to less stable complexes on the surface of these cells.
Finally, to overcome the possibility that contaminating IgG in the purified CRP preparation could be responsible for the observed effects of CRP opsonization of PCE, we compared the ability of recombinant CRP with purified CRP to enhance rosetting and phagocytosis in a selection of transfectants. Recombinant CRP showed virtually identical levels of rosetting, percent phagocytosis, and phagocytic index in cells transfected with FcRI + -chain, + FcRIIa R131, and + FcRIIa R131 mutant when compared with PCE opsonized with purified CRP. The possibility that CRP binding altered erythrocyte membrane structure and was the cause of any effects demonstrated was excluded by showing that a F(ab')2 preparation of anti-CRP specifically inhibited responses to CRP-opsonized cells.
The results in this study suggest that human FcRIIb and FcRIIc might also interact with CRP, as they have very similar extracellular domains to FcRIIa, and this should be investigated further. The more widespread distribution of FcRIIa compared with FcRI or -chain suggests CRP could have effects on many cell types.
Our data suggest, therefore, that CRP- or IgG1-opsonized particles can bind to FcRIIa but that the interaction does not result in sufficient association of these receptors to trigger phagocytosis. However, in the presence of FcRI, the binding of CRP- or IgG1-opsonized particles is changed in such a way that sufficient redistribution of FcRIIa receptors can now occur, and this triggers phagocytosis.
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ACKNOWLEDGEMENTS |
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RIIa R131 and H131 and Martin Taylor for advice and discussion. Received July 2, 2003; revised January 27, 2004; accepted February 4, 2004.
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