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PHAGOCYTES
SHP-1 regulates Fc␥receptor–mediated phagocytosis and the activation of RAC
Anita M. Kant, Pradip De, Xiaodong Peng, Taolin Yi, David J. Rawlings, Jong Suk Kim, and Donald L. Durden
Fc␥receptor–mediated phagocytosis is a
complex process involving the activation of
protein tyrosine kinases, events that are
potentially down-regulated by protein ty-
rosine phosphatases. We used the J774A.1
macrophage cell line to examine the roles
played by the protein tyrosine phosphatase
SHP-1 in the negative regulation of Fc␥
receptor–mediated phagocytosis. Stimula-
tion with sensitized sheep red blood cells
(sRBCs) induced tyrosine phosphorylation
of CBL and association of CBL with CRKL.
These events were completely or partially
abrogated by PP1 or the heterologous ex-
pressionofdominant-negativeSYK,respec-
tively. Heterologous expression of wild-type
butnotcatalyticallyinactiveSHP-1alsocom-
pletely abrogated the phagocytosis of IgG-
sensitized sRBCs. Most notably, overex-
pressed SHP-1 associates with CBL and
this association led to CBL dephosphoryla-
tion, loss of the CBL-CRKL interaction, and
the suppression of Rac activation. These
data represent the first direct evidence that
SHP-1 is involved in the regulation of Fc␥
receptor–mediated phagocytosis and sug-
gest that activating signals mediated by
SRCfamily kinases SYK,CBL, phosphatidyl
inositol-3 (PI-3) kinase, and Rac are directly
opposed by inhibitory signals through
SHP-1. (Blood. 2002;100:1852-1859)
©2002 by TheAmerican Society of Hematology
Introduction
Fc␥receptor–mediated phagocytosis in macrophages is an impor-
tant primary mode of defense in the immune system. Fc␥-receptor
engagement leads to the activation of nonreceptor tyrosine kinases
HCK, LYN, FGR,1,2 and SYK3,4; phosphorylation of adapter
proteins; and activation of effector molecules including Rac, Rho,
and Rab.5,6 Previous results from our laboratory established that
SYK is activated following Fc␥R stimulation and associates with
the Fc␥RI␥-receptor subunit.3Hence, the role played by tyrosine
kinases in this phenomenon has been examined extensively.7-9 In
contrast, there is very limited information on the role of protein
phosphatases in the regulation of the Fc␥R pathway. Herein, we
present the first direct evidence that the protein tyrosine phospha-
tase SHP-1 negatively regulates Fc␥receptor–mediated phagocyto-
sis and controls the phosphorylation state of CBLand the activation
of the small guanosine triphosphatase (GTPase), Rac.
Mouse macrophages express 3 types of Fc gamma receptors—
Fc␥RI, Fc␥RII, and Fc␥RIII10-13—which can bind to the Fc portion of
IgG coating the surface of the foreign invaders. Of these receptors,
Fc␥RI and Fc␥RIIIA are known to transmit the signals through a
tyrosine phosphorylation activation motif (ITAM) contained within the
␥subunit, while Fc␥RIIB has a tyrosine phosphorylation inhibitory
motif (ITIM) in its cytoplasmic domain. Multiple isoforms of Fc␥RIIB
are expressed in J774 cells emanating from alternative splice variants,
suggesting additional complexity of this receptor in macrophage signal-
ing.12 Receptors containing ITIM can act as negative regulators of the
signals initiated by receptors containing ITAM.14 Fc␥RIIB is involved in
inhibition of cell activation by Fc␥R, Fc␥RIIA, andT-cell receptor upon
coaggregation.15 In B cells coligation of B-cell antigen receptor (BCR)
and Fc␥RIIB leads to inhibitory signaling.16 Similarly, ITIM is found in
other receptor systems, such as the gp49 family of proteins on mast cells
and natural killer (NK) cells, killer cell inhibitory receptor (KIR),17 and
inhibitory receptor SHPS-1, abundantly expressed in neurons.18 Upon
activation of these receptors the tyrosine residue in the ITIM gets
phosphorylated by LYN, a Src family kinase,19 to provide a site of
attachment for phosphatases such as SHP-1, SHIP,20 and SHP-2,19
which can start the negative regulatory pathway. Recently, Clynes et al
used the Fc␥RII knockout mouse to demonstrate a role for Fc␥RIIB in
the regulation of Fc␥receptor–mediated phagocytosis.21 These observa-
tions strongly suggest a potential role for phosphatases in Fc␥R signal
transduction, which focused our attention on involvement of SHP-1
(also known as SH-PTP1, HCP, and PTP1C) in phagocytic signal
transduction.
SHP-1 is a nontransmembrane protein involved in multiple
signaling systems. It contains 2 tandem Src homology 2 (SH2)
domains. The N-terminal SH2 domain serves both a regulatory and
a recruiting function, while the C-terminal SH2 domain functions
predominantly as a recruiting domain.22 SHP-1 plays an important
role in regulating the macrophage proliferative pathway. Macro-
phages from motheaten viable (Mev) mice have a frameshift
mutation in SHP-123 and display hyperproliferation in response to
macrophage colony stimulating factor 1 (CSF-1). Studies from
these mice have indicated that CSF-1 receptor and SHP-1 are
phosphorylated upon growth factor stimulation and associate with
each other. It has been suggested that Grb2, an adapter molecule
From the Herman B. Wells Center for Pediatrics Research, Department of
Pediatrics, Biochemistry and Molecular Biology, Indiana University School of
Medicine, Indianapolis; the Childrens Hospital LosAngeles Research Institute,
University of Southern California School of Medicine, Los Angeles; the
Department of Pediatrics, Division of Immunology/Rheumatology, University of
Washington School of Medicine, Seattle; and the Department of Cancer
Biology, Cleveland Clinic Foundation Research Institute, OH.
SubmittedApril 5, 2001; accepted April 23, 2002.
Supported by a grant from the American Cancer Society (RPG-98-244-
01-LBC) to D.L.D. A.M.K. is a recipient of the Childrens Hospital Los Angeles
Career Development Fellowship. D.J.R is the recipient of a McDonnell Scholar
Award, a Leukemia and Lymphoma Society Scholar Award, and the Joan J.
Drake Grant for Excellence in Cancer Research. This work was supported in
part by National Institutes of Health grants HD37091and CA81140 and by the
American Cancer Society.
A.M.K. and P.D. contributed equally to this work.
Reprints: Donald L. Durden, Herman B. Wells Center for Pediatric Research,
Indiana University School of Medicine, 1044 W Walnut St, Rm 468,
Indianapolis, IN 46202; e-mail: [email protected].
The publication costs of this article were defrayed in part by page charge
payment. Therefore, and solely to indicate this fact, this article is hereby
marked ‘‘advertisement’’in accordance with 18 U.S.C. section 1734.
© 2002 by TheAmerican Society of Hematology
1852 BLOOD, 1 SEPTEMBER 2002 䡠VOLUME 100, NUMBER 5
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with an SH2 domain, binds to SHP-1, possibly recruiting phospho-
tyrosine-containing proteins for dephosphorylation by SHP-1.24
SHP-1–mediated dephosphorylation is also involved in delivery of
the Fas apoptosis signal in lymphoid cells.25 SHP-1 is positively
involved in epidermal growth factor (EGF) and interferon-␥–
induced signal transducer and activator of transcription (STAT)
activation in HeLa cells.26 In macrophages, SHP-1 also selectively
regulates the tyrosine phosphorylation of Stat1 and Jak1 while
leaving Tyk2 and Stat2 unaffected.27 In contrast, Keilhack et al28
have described the binding of SHP-1 to the EGF receptor, leading
to dephosphorylation of the receptor and attenuation of receptor
signaling. In the motheaten mouse, Fc␥RIIB signaling is deficient,
suggesting a role of SHP-1 in control of ITIM function.29 In
contrast, Ono et al 30,31 have shown that inhibitory signaling by
Fc␥RIIB does not require SHP-1 but involves the 5⬘inositol phos-
phatase SHIP. Data from Sharlow et al32 have indicated that
SHP-1 acts as a negative regulator of erythropoietin-induced
differentiation of normal erythroid progenitor cells, preventing
their premature commitment to terminal differentiation. Hence
it has been suggested that tyrosine phosphatases exert both
positive and negative regulation of specific signals and integrate
these signals temporally and spatially within the cell.
To investigate the role played by SHP-1 in phagocytic signal
transduction, we overexpressed wild-type SHP-1 in J774A.1 cells,
using a recombinant vaccinia virus expression system. SHP-1
overexpression led to a complete abrogation of phagocytosis of
sensitized sheep red blood cells (sRBCs) by J774A.1 cells. This is
the first evidence that SHP-1, a tyrosine phosphatase, regulates Fc␥
receptor–mediated phagocytosis. Most notably, SHP-1 associated
with the CBL adapter protein, and this association led to loss of
CBL phosphorylation and the suppression of Rac. These data
support a role for SHP-1 in the control of phagocytosis and suggest
that CBL dephosphorylation mediates, at least in part, negative
control of Fc␥R-dependent phagocytosis.
Materials and methods
Antibodies
Anti-CBL (SC170) and anti-CRKL antibodies (SC-319) were obtained from
Santa Cruz Biotechnology, (Santa Cruz, CA). Plasmids encoding enhanced green
fluorescent protein (EGFP)–tagged SYK kinase and Fc␥RIIA were prepared by
standard subcloning methods in pEGFP and pcDNA3.1, respectively. DNA
constructs encoding SHP-1, SHP-2, catalytically dead SHP-1 (C453S), or
catalytically dead SHP-2 (C486S) were subcloned into the pcDNA3.1 vector.
Anti-SYK antibody was provided by Dr Tamara Hurley (Salk Institute, San
Diego, CA), and anti–SHP-1 antibody was generated in our laboratory. For
detection of Fc␥RIIA expression in COS7 cells, we used an allophycocyanin
(APC) conjugate anti-CD32 monoclonal antibody (FL18.26).
Cell lines and vaccinia virus expression system
J774A.1, a macrophagelike cell line, was obtained from ATCC (Manassas,
VA; catalog no. 67-TIB).The cells were grown in Dulbecco Modified Eagle
Medium (DMEM) containing 10% fetal calf serum (FCS). Recombinant
vaccinia virus vectors were provided by Dr Bernard Moss (Bethesda, MD).
The dominant-negative SYK vaccinia construct (encoding the N terminus
of SYK, residues 1-255) was provided by Dr A. Scharenberg, as previously
described.33 Recombinant vaccinia viruses containing SHP-1 and dominant-
negative SYK were prepared as described. Briefly, recombinant vaccinia
virus was propagated in 149B cells grown in RPMI containing 10% FCS.A
confluent culture of cells was infected with recombinant vaccinia virus at a
concentration of 0.5 plaque-forming units (pfu) per cell for 48 hours. The
cells were scraped in the same medium, pelleted down, and resuspended in
5 mL of 10 mM Tris-HCl pH 9. The cells were disintegrated by freezing in
liquid nitrogen and thawing at 37°C 3 times, after which the volume was
made up to 20 mL with 10 mM Tris-HCl pH 9 and the cells were subjected
to 40 strokes in a homogenizer. Nuclei and cell debris were separated from
the cell lysate by centrifugation at 1000 rpm for 5 minutes. The cell lysate
containing the recombinant vaccinia virus was then subjected to sonication
for 1 minute. The cell lysate was loaded on a cushion of 36% sucrose
solution and centrifuged at 13 000 rpm for 80 minutes at 4°Cinan
ultracentrifuge (Beckman, PaloAlto, CA) using an SW.28 rotor.Viral pellet
obtained at the bottom was suspended in 1 mLof 10 mM Tris-HCl pH 9 and
loaded on a sucrose gradient composed of 6.6 mL each of 40%, 36%, 32%,
28%, and 24% sucrose solutions made in 10 mM Tris-HCl pH 9 to be
centrifuged at 12 500 rpm for 50 minutes at 4°C in an ultracentrifuge
(Beckman) using an SW.28 rotor. A bluish white ring containing purified
virus was collected, diluted with 10 mM Tris-HCl pH 9, and recentrifuged
at 13 000 rpm for 60 minutes at 4°C in an ultracentrifuge (Beckman) using
an SW.28 rotor to pellet the virus down. Purified recombinant vaccinia virus
thus obtained was suspended in 10 mM Tris-HCl pH 9 and titered as
follows. An aliquot was used for making serial dilutions of the viral
suspension. These were used to infect a confluent lawn of 149B cells grown
in a 6-well plate for 2 hours at 37°C in 1 mL RPMI containing 10% FCS.
The medium was then replaced with 3 mL of fresh RPMI containing 10%
FCS. After 24 hours the medium was discarded and the plaques were
visualized by staining with crystal violet to determine the titer.
Preparation of recombinant vaccinia virus containing
catalytically inactive mutant
A construct (C2mSHP-1S453KT3) containing catalytically inactive SHP-1
mutant was kindly provided by Dr Taolin Yi. The insert was amplified by
polymerase chain reaction (PCR), using the following 5⬘and 3⬘primers:
CTCGTCGACAGGATGGTGAGGTGGTTTCAC and AGTCCCGGGAGAT-
CACTTCCTCTTGAGAGAA, respectively. The amplified product was sub-
cloned into PCR2.1 vector with a TA cloning kit (Invitrogen, San Diego, CA),
isolated by using SmaI and SalI restriction digest, and ligated to the recombinant
vaccinia vector pSC65. The construct C2pSC65 was used to make a recombinant
vaccinia virus using packaging cell line CV1 and the wild-type vaccinia virus.
Recombinant virus was purified from the wild-type virus by single-plaque
purification. It was amplified, purified, and titered as described above.
Phagocytic assays
J774A.1 cells were plated at a density of 2 ⫻105cells per well in a 12-well
plate (Costar, Corning, NY) overnight. Cells were infected with recombi-
nant vaccinia virus pSC65 or pSC65-SHP-1 at a density of 2 pfu/cell for 4
hours at 37°Cin5%CO
2. After 4 hours the medium was changed and the
cells were subjected to sRBCs coated with IgG at subagglutinating
concentration. The target-to-effector ratio was kept at 100:1. The cells were
scraped after 2 hours; cytospins were prepared, fixed, and stained with
Wright Giemsa stain (Dade,AG, Switzerland); and the slides were observed
under a microscope for rosette formation. The rest of the cells were
subjected to water shock to lyse the uningested sRBCs. The cells were
suspended in DMEM containing 20% FCS. The cells were spun down on
glass slides and fixed and stained with Wright Giemsa stain. A minimum of
150 cells were counted for each slide and the phagocytic index was
calculated as follows: phagocytic index (PI) ⫽number of sRBCs internal-
ized by 100 J774 cells counted in 10 random fields of slide. In the case of
the inhibitors the cells were subjected to treatment with the inhibitor at
different concentrations along with an appropriate dimethyl sulfoxide
(DMSO) control for 1 hour in DMEM with 10% FCS before the
phagocytosis assay was carried out as described previously.
␤-Galactosidase and rosette formation assays
As a control for effects of recombinant vaccinia virus, J774A.1 cells were
tested for equal viral load by quantitation of ␤-galactosidase activity
measured with X-gal. The plasmid pSC65, used for cloning of dominant-
negative SYK, contains the gene ␤-galactosidase, which cleaves X-gal to
give a color product that can be quantitated colorimetrically. In every
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experiment, we quantitated recombinant viral load for control (cells
infected with empty vector recombinant vaccinia virus) compared with
experimental sample (cells infected with vaccinia virus containing dominant-
negative SYK) and they were equivalent (data not shown). The capacity of
J774A.1 cells to form rosettes via Fc␥receptor was not altered by vaccinia
virus. Cells infected with empty vector recombinant vaccinia virus or cells
infected with vaccinia virus containing dominant-negative SYK showed
100% rosette formation within 1 minute of addition of sensitized sRBCs,
indicating that the surface expression of Fc␥receptors and the extracellular
function of the Fc␥Rs as it relates to binding of sRBC targets was not
affected by recombinant vaccinia virus alone (data not shown). Rosette
formation and phagocytosis did not occur in absence of sensitizing antibody
against sRBCs, which establishes the Fc␥R specificity of this response.
Cells equivalent to 1⫻105were suspended in 400 ␮Lof DMEM containing
10% FCS to which 50 ␮L of 1% X-gal (Sigma, St Louis, MO) was added.
After incubation at 37°C the color of the medium turns blue. The
supernatant was diluted 1:10 and the optical density was measured at 595
nm in a spectrophotometer (Molecular Devices, Menlo Park, CA).
Overexpression of SHP-1 and dominant-negative SYK in
J774A.1 cells
Cells (2 ⫻105) infected with different viruses were lysed with 50 ␮L of sample
buffer. The lysates were resolved on acrylamide gels and probed for appropriate
protein expression with corresponding antibodies as described above.
Heterologous COS7 cell phagocytosis system
COS7 cells were plated at 1 ⫻105cells per well on a 6-well plate overnight. Cells
were transfected with plasmids using lipofectamine reagent for 4 hours followed
by a washing step and further incubation for 24 or 48 hours. The phagocytosis
assay was carried out as described above for J774 cells. Phagocytic index ⫽
number of sRBCs internalized by 100 COS7 cells randomly sampled. During all
transfections total plasmid DNA concentration and composition were equili-
brated using empty vector plasmids. All transfected proteins, EGFP-SYK,
Fc␥RIIA, SHP-1, and C2-SHP-1, were quantitated by Western blot analysis or
flow cytometry. Thus it can be interpreted that the effect on phagocytic response
(PR) is causally linked to SHP-1 transfection. In all experiments performed, all
transfected proteins were quantitated in parallel populations of COS7 cells to
ensure that the effects observed with transfection of SHP-1 vs SHP-2 were
attributable to this variable and not levels of Fc␥RIIA or SYK kinase. The
determination of conversion of guanosine diphosphate (GDP)–Rac to GTP-Rac
was as described.34
Immunoprecipitation
J774A.1 cells were infected with recombinant vaccinia virus at a concentra-
tion of 2 pfu/mLfor 4 hours. The cells were then collected and suspended at
a concentration of 2 ⫻106cells per mL of DMEM and stimulated with
IgG-coated sRBCs at 37°C for 5 minutes. The samples were centrifuged at
500gin a refrigerated centrifuge and the supernatant was aspirated. The cell
pellet was used for immunoprecipitation as described earlier.35
Results
Dominant-negative SYK inhibits phagocytosis
In order to investigate the role played by nonreceptor tyrosine
kinase SYK in IgG-mediated phagocytosis, we expressed the
dominant-negative form of SYK in J774A.1 cells with recombinant
vaccinia virus. The SYK mutant encodes a truncated form of SYK
containing only the tandem SH2 domains with no catalytic domain
and is expected to dock with the ITAM, thereby preventing the
endogenous catalytically active SYK kinase from interacting with
the Fc␥R␥subunit. Expression of dominant-negative SYK in
J774A.1 cells strongly inhibits phagocytosis of IgG-coated sRBCs
(Figure 1A). Normal phagocytosis occurs in J774A.1 cells infected
with empty vector recombinant vaccinia virus. In comparison,
J774A.1 cells infected with vaccinia virus containing dominant-
negative SYK exhibit minimal phagocytosis. Dominant-negative
SYK expression is confirmed by Western blot (Figure 1B, lane 3).
The data we obtained using dominant-negative SYK are consistent
with other data in the literature, including data from SYK knockout
mice,8strongly supporting a role for SYK in propagating signals
required for IgG-mediated phagocytosis in this J774 system.
Src and phosphatidyl inositol-3 (PI-3) kinase are required for
phagocytosis of IgG coated sRBCs by J774A.1 cells
Recent evidence from HCK/LYN/FGR knockout mice suggests that
members of the Src family of nonreceptor protein tyrosine kinases are
upstream of SYK and PI-3 kinase in myeloid ITAM signaling.8To
examine the role of Src family kinases and PI-3 kinase in Fc␥R
phagocytosis in our system, we treated J774 cells with PP1 (Calbio-
chem, La Jolla, CA), an inhibitor of Src family tyrosine kinases at 10, 5,
and 1 ␮M concentration, or wortmannin, an inhibitor of PI-3 kinase at a
concentration of 10, 1, and 0.1 nM, with appropriate DMSO controls for
1 hour in DMEM with 10% FCS and then added sensitized sRBCs at a
target-to-effector ratio equal to 100:1. Figure 2A shows that PP1
abrogates phagocytosis at 10 and 1 ␮M concentration and the effect is
dose dependent. Figure 2B demonstrates that wortmannin blocks
phagocytosis significantly at 10 and 1 nM concentrations. These
observations are consistent with other reports in the literature, from
studies performed in other cell lines, which strongly suggest that Src
family kinase activity and PI-3 kinase are required for phagocytosis of
IgG-coated sRBCs by J774A.1 cells.
Dominant-negative SYK and PP1 inhibit Fc␥-induced CBL
phosphorylation
Previous reports from our laboratory and others have demonstrated
that Fc␥-receptor cross-linking induces the tyrosine phosphoryla-
tion of the complex adapter protein CBL.36,37 These observations
prompted us to determine whether a phagocytic signaling event
would induce the phosphorylation of CBL. To further understand
the role of specific kinases in this phosphorylation event, we used
dominant-negative SYK and a Src family kinase inhibitor, PP1, to
determine a role for these kinases in CBLtyrosine phosphorylation.
Figure 1. Dominant-negative SYK inhibits phagocytosis. (A) Phagocytosis of
IgG-sensitized sRBCs by noninfected J774A.1 cells (control), cells infected with
recombinant vaccinia virus containing empty vector (pSC65-vector), or cells infected
withdominant-negative SYK (pSC65-D/N SYK). The cells were infected with vaccinia
viruses or empty vector for 4 hours at 37°C with 5% CO2, after which they were
subjected to IgG-sensitized sRBCs in fresh medium at a target-to-effector ratio equal
to 100:1 for 2 hours at 37°C. Nonengulfed sRBCs were lysed by water shock and the
cells were fixed and stained with Wright Giemsa staining before the phagocytic index
was counted. Columns indicate phagocytic index of J774A.1 (mean ⫾SD). (B)
Western blot analysis shows expression of dominant-negative SYK in J774A.1 cells
infected with different recombinant vaccinia viruses: lane 1, no vector; lane 2, empty
vector; lane 3, dominant-negative SYK.
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The expression of dominant-negative SYK kinase representing
the N-terminal SH2 domains completely abrogates the phagocytic
response but has a modest effect on the tyrosine phosphorylation
status of CBL in vivo (Figure 3A). We demonstrated that CBL
tyrosine phosphorylation is induced under conditions of phagocyto-
sis and that PP1 abrogates the tyrosine phosphorylation of CBL
(Figure 3B). This effect is dose dependent (data not shown), as is
the effect of PP1 on Fc␥-receptor phagocytosis (Figure 2A).
Interestingly, dominant-negative SYK inhibited CBL tyrosine
phosphorylation to a lesser extent but completely abrogated the
phagocytic response. Both PP1 and D/N SYK suppressed the basal
tyrosine phosphorylation state of CBL in vivo. These data suggest
that both the Src family kinase catalytic activity and the capacity of
SYK to dock with the ITAM receptor are required for the induction
of the CBL phosphorylation in response to stimulation with
sensitized sRBCs and that these 2 events are required for phagocy-
tosis in vivo. The dominant-negative SYK would not be expected
to alter the upstream activity of SRC kinases, hence SRC-mediated
phosphorylation of CBL is not altered to the same extent. The data
are consistent with the model that SRC and SYK are both involved
in Fc␥R-mediated phagocytosis, likely mediated by the down-
stream activation of PI-3 kinase (Figure 2B), and set the stage for a
biochemical analysis of the negative regulation of this ITAM
receptor by the intracellular phosphatase SHP-1. Hence, we
conclude that the SRC/SYK/CBL signaling axis is a likely target
for protein tyrosine phosphatase (PTPase) action as it relates to the
negative regulation of phagocytosis.
Overexpression of SHP-1 in J774A.1 cells results in the
dephosphorylation of CBL and inhibits phagocytosis
The data presented here as well as in other published work clearly
demonstrate a requirement for tyrosine kinases in phagocyto-
sis1-4,7-9 (Figures 1 and 2). These findings therefore also suggest that
dephosphorylation of specific sites of tyrosine phosphorylation may
negatively regulate this response. To begin to address this question we
overexpressed SHP-1 in J774A.1 cells. Phagocytosis of sensitized
sRBCs was unaltered in cells infected with empty vector recombinant
vaccinia virus or vaccinia expressing a catalytically inactive SHP-1
protein (C2, Figure 4). In contrast, overexpression of wild-type SHP-1 in
J774 cells markedly inhibited phagocytosis of IgG-coated sRBCs
(Figure 4, left lower panel). Figure 4Ashows phagocytosis of sensitized
sRBCs by J774A.1 infected with empty vector recombinant vaccinia
virus, vaccinia virus containing catalytically dead SHP-1 C2 mutant, or
vaccinia virus expressing wild-type SHP-1. Quantitation of the phago-
cytic index in the different groups is shown in Figure 4B and Western
blot analysis for expression of SHP-1 and catalytically dead SHP-1
mutant in these cells is shown in Figure 4C. As a control for effects of
recombinant vaccinia virus, J774A.1 cells were exposed to an equal
multiplicity of infection (MOI) of virus and then quantitated ␤-galactosi-
dase activity was measured with X-gal. There was no effect of SHP-1 or
other protein constructs on the capacity of cells to form rosettes (data not
shown), an assay that assesses the capacity of J774 cells to form
sRBC-J774 cell conjugates. These findings indicate that SHP-1 nega-
tively regulates intracellular events required for the IgG-mediated
phagocytic response in macrophages, and the catalytic activity of SHP-1
is required for this suppression.
To determine whether the CBL adapter protein is a potential
substrate for SHP-1 in vivo, we expressed SHP-1 or the C2 mutant
of SHP-1 in J774 cells and then stimulated the cells with sensitized
sRBCs followed by immunoprecipitation of CBL. Both the wild-
type and mutant SHP-1 coimmunoprecipitated with CBL. Most
notably, only the expression of wild-type SHP-1 resulted in the
dephosphorylation of CBL in vivo (Figure 5). We next evaluated
whether changes in CBL phosphorylation led to alterations in
downstream CBL-dependent signaling events, including the forma-
tion of the CBL-CRKL signaling complex. The CBL-CRKL
interaction has been implicated previously in ITAM and Fc␥
receptor signaling.38 In the presence of SHP-1 (but not C2 SHP-1),
phosphorylation decreased markedly and the interaction of CBL-
CRKL was abrogated (Figure 5). Upon dephosphorylation of CBL,
the CBL-CRKL adapter protein interaction is lost. The expression
of C2 (catalytically dead SHP-1) is noted to associate with CBL to
a lesser extent and not to induce the dephosphorylation of CBL or
to disengage the CBL-CRKL interaction in vivo. From these data
we conclude that CBL is an in vivo substrate for SHP-1 under
conditions of Fc␥-receptor engagement in myeloid cells and that
dephosphorylation of CBL alters the generation of phospho-CBL-
dependent downstream signaling complexes in vivo.
Figure 2. SRC and PI-3 kinases are required for Fc␥receptor–mediated
phagocytosis. J774A.1 cells were treated with (A) PP1 or (B) wortmannin at the
indicatedconcentrations along with an appropriateDMSO control for 1 hourin DMEM
with10% FCS andthen IgG-sensitizedsRBCs were addedat a target-to-effectorratio
equal to 100:1. Columns indicate phagocytic index of J774A.1 cells treated with
DMSO (control), PP1, or wortmannin (mean ⫾SD).
Figure 3. Effect of dominant-negative SYK and PP1 on tyrosine phosphoryla-
tion of CBL in response to stimulation with IgG-sensitized sRBCs. (A-B)
Western blot analysis of CBL immunoprecipitates to assay the phosphorylation of
CBL following treatment of IgG-sensitized sRBCs in J774A.1 cells expressed by
dominant-negative SYK, treated with Src family kinase inhibitor, PP1 (10 ␮M).
J774A.1 lysates prepared from resting cells or cells stimulated with sensitized sRBCs
for 5 minutes were immunoprecipitated with polyclonal anti-CBL antibody and
immunoblottedwithmonoclonal antiphosphotyrosine antibodytodetermine phosphor-
ylation of CBL or immunoblotted with polyclonal anti-CBLantibody to determine total
protein amounts of CBL under the same nitrocellulose membrane.
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Specificity of SHP-1 in control of phagocytosis
To begin to address the specificity of SHP-1 in these signaling events,
we compared the effects of SHP-1 transfection and SHP-2 transfection
on Fc␥RIIA phagocytosis (Figure 6A). A heterologous COS7 cell
system reconstituted with the Fc␥RIIA receptor and SYK kinase was
used to ask the question, Is the effect of SHP-1 on Fc␥receptor ITAM
signaling specific?34 Equivalent levels of Fc␥RIIA and EGFP-SYK
were confirmed in COS7 cell transfectants (Figure 6B) and levels of
SHP-1 and SHP-2 expression were quantitated in cells analyzed for
phagocytosis and by flow cytometry (Figure 6C). The data clearly
suggest that SHP-2 has no significant effect on the Fc␥RIIA-induced
phagocytic response. This result provides evidence that the effect of
SHP-1 on ITAM signaling seen in our study is specific for this
blood-specific phosphatase.
SHP-1 regulates RAC activation in response to
Fc␥RIIA engagement
It is clear that small GTPases of the Rho family play an important
role in the modulation of the actin-cytoskeletal network of proteins
required for Fc␥receptor–mediated phagocytosis. The small GTPase
Rac must be converted from a GDP-bound state to GTP-Rac in
order to activate downstream effectors required for cytoskeletal
reorganization and polymerization of actin. Data from a Rac2
knockout model developed in our laboratory39 provide direct
evidence that Rac2 is required for macrophage-mediated phagocy-
tosis (D.L.D., unpublished observation,August 2001). Prior reports
have also implicated Rac in Fc␥R-mediated phagocytosis.5To
provide further biochemical evidence for SHP-1 in the regulation
of phagocytosis, we determined the effect of SHP-1 on the
Fc␥RIIA-induced conversion of GDP-Rac to GTP-Rac (Figure 7).
Using the heterologous COS7 cell system, we observed that SHP-1
and not the C2 mutant of SHP-1 blocked Fc␥RIIA phagocytosis
(Figure 4). SHP-1 overexpression suppressed the Fc␥RIIA-induced
conversion of GDP-Rac to GTP-Rac (Figure 7; compare lanes 3
and 6). In the absence of Fc␥RIIA or SYK, Rac is not activated
(Figure 7, lanes 7-9). The transfection of SHP-2 had no effect on
phagocytosis (Figure 6A), Rac activation (data not shown), or CBL
phosphorylation state (Figure 8). These data are internally consis-
tent with the observed effect of SHP-1 on CBL phosphorylation
and the CBL-CRKL interaction and provide confirmatory evidence
that SHP-1 regulates this signaling axis required for ITAM signaling.
Discussion
Because of overlapping binding affinities, Fc␥receptors must
function in concert during the process of phagocytosis. During Fc␥
receptor–mediated phagocytosis by macrophages, all the 3 types of
Fc␥receptors are cocrosslinked by the Fc portion of IgG coating
the surface of the foreign invaders. Of these receptors, Fc␥RI and
Fc␥RIII are involved in activation of nonreceptor tyrosine kinases
such as HCK, LYN, and SYK. In contrast, Fc␥RIIB has an ITIM in
its cytoplasmic domain, which is known to play an inhibitory role
during signal transduction by virtue of its association with protein
and lipid phosphatases such as SHP-1 and SHIP. These phospha-
tases are recruited directly to the signalsomes generated by
Figure 4. Overexpression of SHP-1 in J774A.1 cells inhibits phagocytosis. (A)
Composite photomicrographs of Wright Giemsa–stained J774 cells undergoing
phagocytosis of sRBCs. Original magnification, ⫻100. We show noninfected J774
A.1 cells (control); cells infected with recombinant vaccinia virus containing empty
vector (pSC65-vector); J774 cells infected with recombinant vaccinia virus encoding
wild-type SHP-1 (pSC65-SHP-1); cells containing catalytically dead SHP-1 mutant
(pSC65-C2). The cells were infected with vaccinia viruses or empty vectorfor 4 hours
at 37°C with 5% CO2, after which they were subjected to IgG-sensitized sRBCs in
fresh medium at a target-to-effector ratio equal to 100:1 for 2 hours at 37°C. (B)
Quantitation of phagocytosis of IgG-sensitized sRBCs by J774A.1 cells. Columns
indicate phagocytic index of J774A.1 cells (mean ⫾SD). (C) Western blot analysis
shows expression of SHP-1 protein in J774A.1 cells infected with recombinant
vaccinia virus: lane 1, no vector; lane 2, empty vector; lane 3, catalytically dead
C2-SHP-1 mutant; lane 4, wild-type SHP-1 .
Figure 5. CBL is a substrate for SHP-1. (A-B) Western blot analysis of CBL
immunoprecipitates to assay the tyrosine phosphorylation of CBL and to determine
protein-protein interactions between CBL and SHP-1 or CRKL following treatment of
IgG-sensitized sRBCs in J774A.1 cells expressed by wild-type SHP-1 and catalytically
dead mutant SHP-1. J774A.1 lysates prepared from resting cells or cells stimulated with
sensitized sRBCs for 5 and 10 minutes were immunoprecipitated with polyclonal anti-CBL
antibody and immunoblotted with antiphosphotyrosine antibody (PY-CBL blot), anti-CBL
antibody (CBL blot), anti–SHP-1 antibody (SHP-1 blot), and anti-CRKL antibody (CRKL
blot). (C) Western blot analysis shows expression of SHP-1 proteins in J774A.1 cells
infected with recombinant vaccinia virus: lane 1, no vector; lane 2, empty vector; lane 3,
catalytically dead SHP-1 mutant; lane 4, wild-type SHP-1 phosphatase.
1856 KANT et al BLOOD, 1 SEPTEMBER 2002 䡠VOLUME 100, NUMBER 5
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