Content uploaded by Wilfried Bautsch
Author content
All content in this area was uploaded by Wilfried Bautsch on Apr 24, 2015
Content may be subject to copyright.
Content uploaded by Wilfried Bautsch
Author content
All content in this area was uploaded by Wilfried Bautsch on Apr 24, 2015
Content may be subject to copyright.
Modulation of C3a Activity: Internalization of the Human C3a
Receptor and its Inhibition by C5a
1
Britta Settmacher, Daniel Bock, Henry Saad, So¨ren Ga¨rtner, Claudia Rheinheimer, Jo¨rgKo¨hl,
Wilfried Bautsch, and Andreas Klos
2
The C3a receptor (C3aR) is expressed on most human peripheral blood leukocytes with the exception of resting lymphocytes,
implying a much higher pathophysiological relevance of the anaphylatoxin C3a as a proinflammatory mediator than previously
thought. The response to this complement split product must be tightly regulated in situations with sustained complement acti-
vation to avoid deleterious effects caused by overactivated inflammatory cells. Receptor internalization, an important control
mechanism described for G protein-coupled receptors, was investigated. Using rabbit polyclonal anti-serum directed against the
C3aR second extracellular loop, a flow cytometry-based receptor internalization assay was developed. Within minutes of C3a
addition to human granulocytes, C3aR almost completely disappeared from the cell surface. C3aR internalization could also be
induced by PMA, an activator of protein kinase C. Similarly, monocytes, the human mast cell line HMC-1, and differentiated
monocyte/macrophage-like U937-cells exhibited rapid agonist-dependent receptor internalization. Neither C5a nor FMLP stim-
ulated any cross-internalization of the C3aR. On the contrary, costimulation of granulocytes with C5a, but not FMLP, drastically
decreased C3aR internalization. This effect could be blocked by a C5aR-neutralizing mAb. HEK293-cells transfected with the
C3aR, with or without G
a
16, a pertussis toxin-resistant G protein
a
subunit required for C3aR signal transduction in these cells,
did not exhibit agonist-dependent C3aR internalization. Additionally, preincubation with pertussis toxin had no effect on C3a-
induced internalization on PMNs. C3aR internalization is a rapid negative control mechanism and is influenced by the C5aR
pathway. The Journal of Immunology, 1999, 162: 7409–7416.
T
he anaphylatoxins C3a,
3
C5a, and C5a-desArg are gener-
ated during complement activation. Through binding to
the C3aR and C5aR on neutrophils, monocytes, basophils,
mast cells, and eosinophils, they function as potent proinflamma-
tory mediators (1–9). Agonist binding stimulates a pertussis toxin-
sensitive (PTX) increase in free cytosolic [Ca
21
]
i
(10–12) and
initiates a repertoire of host defense actions, from secretory gran-
ule release from neutrophils (12, 13) to chemotaxis in mast cells
(6; for a review, see Ref. 14). Recent studies on both receptors
suggests a much broader tissue distribution than previously sur-
mised. C3aR is expressed during inflammation in the brain and on
activated B lymphocytes (15–18); in addition, C5aR is expressed
on hepatocytes, lung, smooth muscle, and endothelial cells (19–
24). The complement system is strictly regulated by a variety of
positive and negative feedback mechanisms to avoid self-destruc-
tion of the organism. Similarly, the signaling mediated by the ana-
phylatoxins must also be tightly regulated. Serum carboxypepti-
dase N, acting on the anaphylatoxins’ C-terminal arginine residue,
rapidly inactivates newly generated C3a and greatly reduces the
biologic activity of C5a (25). An additional receptor control mech-
anism is homologous desensitization. Details of the negative feed-
back mechanisms of C5aR have been described. The cytosolic
C-terminus of the C5aR is phosphorylated within minutes of C5a
addition (26–28), and the receptor is rapidly internalized (29, 30).
Major and minor phosphorylation sites at the C5aR C-terminus,
which seem to be important for internalization and receptor recy-
cling, have been identified (29, 31, 32). Less is known about the
regulation of the C3aR, but, as with C5aR, homologous desensi-
tization has been noted at least in the guinea pig system (33).
Several groups have used the
b
2
-adrenergic receptor as a model
system to study G protein-coupled receptor internalization; C-ter-
minal Ser and Thr residues are phosphorylated by G protein-cou-
pled receptor kinases (for a review, see Refs. 34 and 35). Kinase
activation is enhanced by their
bg
subunit-dependent trans-loca-
tion from the cytosol to the membrane (36, 37) and their phos-
phorylation by protein kinase C (38, 39). A member of the
b
-ar-
restin family binds to the phosphorylated receptor, thereby
uncoupling it from its G protein, but improving its attachment to
the clathrin-coated vesicle-mediated endocytic pathway (40–42).
The receptor is then rapidly internalized, and the cells are desen-
sitized. Receptor class desensitization (43) has also been described
after stimulation of transfected RBL-2H3 cells by C5a or FMLP,
which results in the cross-phosphorylation and desensitization of
IL-8R A (44), whereas nonchemotactic receptors are not affected.
These pathways are dependent on protein kinases C or A (45–47).
Simultaneously, negative feedback mechanisms distal from the G
protein exist, such as the phosphorylation of phospholipase C
b
3
(48). The internalized receptors are either degraded or dephospho-
rylated and recycled to the cell surface (29).
The aim of this study was the detailed characterization of human
C3aR internalization as one negative feedback mechanism on
granulocytes, as the largest leukocyte population naturally bearing
the C3aR. A subset of experiments was performed on the human
Institute of Medical Microbiology, Hannover Medical School, Hannover, Germany
Received for publication July 8, 1998. Accepted for publication March 24, 1999.
The costs of publication of this article were defrayed in part by the payment of page
charges. This article must therefore be hereby marked advertisement in accordance
with 18 U.S.C. Section 1734 solely to indicate this fact.
1
This work was supported by a grant from the Deutsche Forschungsgemeinschaft
(DFG Kl 603/4-2).
2
Address correspondence and reprint requests to Dr. Andreas Klos, Institut fu¨r
Medizinische Mikrobiologie der MHH, Carl-Neubergstrasse 1, D 30623 Hannover,
Germany. E-mail address: klos@mikrobio.mh-hannover.de
3
Abbreviations used in this paper: C3a and C3aR, the complement component ana-
phylatoxic peptide C3a and its receptor; C5a and C5aR, the complement component
anaphylatoxic peptide C5a and its receptor (CD88); PTX, pertussis toxin; [Ca
21
]
i
,
concentration of free cytosolic Ca
21
.
Copyright © 1999 by The American Association of Immunologists 0022-1767/99/$02.00
mast cell line HMC-1, human monocytes, and differentiated mono-
cyte-like U937 cells to demonstrate that internalization is not a reg-
ulatory mechanism limited to granulocytes. Transiently transfected
HEK293 cells were additionally investigated. In particular, the influ-
ence of C5a and FMLP on C3aR internalization was analyzed.
Materials and Methods
Reagents
Human C3a was purchased from Advanced Research Technologies (San
Diego, CA). The C3a analogue synthetic peptide P117 (LRRQAWRAS-
ALGLAR; aa 63–77 of human C3a) and the control peptide P252 (YTTD-
DYGHYDD) (49) were prepared by solid phase synthesis. FITC-labeled
goat anti-rabbit IgG was obtained from Dianova (Hamburg, Germany). All
other materials, including FMLP and recombinant C5a, were obtained from
Sigma (Deisenhofen, Germany). The polyclonal anti-C3aR serum, specific for
the second extracellular loop, was a gift from R. Ames (SmithKline Beecham,
Philadelphia, PA); its generation and its use in characterization of the C3aR
expression pattern of leukocytes have recently been described (49).
Cell lines and cell culture conditions
The culture conditions of U937 cells (American Type Culture Collection,
Manassas, VA) and C3aR induction by IFN-
g
(1000 U/ml for 3 days) have
been described previously (50). HEK293 cells (human embryonic kidney;
American Type Culture Collection) were grown in DMEM/nutrient mix
F-12 (Life Technologies, Eggenstein, Germany) supplemented with 10%
heat-inactivated FCS, penicillin (50 U/ml)/streptomycin (50
m
g/ml), 2 mM
L-glutamine, and 1 mM sodium pyruvate at 37°C in a humidified atmo-
sphere with 5% CO
2
. HMC-1 cells (provided by J. H. Butterfield) were
cultured in RPMI 1640 medium (Life Technologies) supplemented as in-
dicated above.
Transient transfection of HEK293 cells
Using lipofectamine reagent from Life Technologies (Eggenstein, Ger-
many) according to the manufacturer’s instructions, HEK293 cells (;5 3
10
6
cells in a 75-cm
2
cell culture flask) were transiently cotransfected with
4
m
g of C3aR in pCDNA3/neo vector (Invitrogen, De Schelp, The Neth-
erlands) and 2
m
g of pCDM8 (Invitrogen) encoding G
a
16. As a negative
control, cells were transfected with plasmid encoding C5aR or pCDM8
without any insert. The transfected HEK293 cells were harvested on day 3
for additional experiments, at which time they expressed approximately
50,000–150,000 receptors/cell, with a K
d
in the range of 1–5 nM (12, 49).
C3aR internalization assay based on flow cytometry
PBLs were prepared from EDTA blood of healthy donors; erythrocytes
were lysed by NH
4
Cl. HEK293 cells were harvested using cell dissociation
solution (Sigma). The remaining PBLs and the HEK293 cells as well as
harvested HMC-1 cells, which grow in suspension, were washed twice and
resuspended in PBS (kinetics and dose-response curves) or with RPMI
without phenol red (all other experiments) at 4°C. For all experiments, cells
were resuspended at a density of 1 3 10
7
/ml. Cells and stimuli were pre-
incubated separately at 37°C for 5 min. The internalization was started in
a total volume of 100
m
l by adding a cell sample (8 3 10
5
cells in 80
m
l)
to the agonist (20
m
l). The sample was halved when the indicated incuba-
tion time was reached. One part (50
m
l) was immediately added to 100
m
l
of ice-cold polyclonal rabbit anti-C3aR serum (1/4000), the other to 100
m
l
of ice-cold preimmune serum (1/4000; as a negative control for C3aR-
independent binding of rabbit IgG). Both were incubated in parallel for 30
min in a microtiter plate. In addition, the samples were incubated with
buffer providing the negative control for nonspecific binding of the sec-
ondary Ab. To stop any further C3aR internalization, this and all following
steps until FACS analysis were performed at 4°C. PBS was used as buffer.
After two washes, the cell pellet was resuspended and incubated for 30 min
in buffer containing FITC-labeled goat anti-rabbit IgG (1/200). For analysis
of C3aR internalization on monocytes, PBLs were washed twice, then re-
suspended in PBS and stained with a R-PE-conjugated CD14 mAb (TUK4,
Dako, Glostrup, Denmark) according to the manufacturer’s procedure. Af-
ter two additional washes, the PBLs were resuspended in 150
m
l of ice-cold
buffer containing 1% formaldehyde.
Finally, the cells were assessed in the flow-cytometer FACScan using
CellQuest software (Becton Dickinson, Heidelberg, Germany). Gating for
granulocytes was based on a combination of forward scatter and side scat-
ter. Staining with anti-CD16 (Dianova), Kimura staining (51), and Diff-
Quick staining (Baxter Dade, Dudingen, Switzerland) confirmed that
.95% of this population were neutrophils, and the majority of the remain-
ing cells were eosinophils (49, 52, 53). Monocytes were gated additionally
based on their CD14 staining. The list-mode files were analyzed subse-
quently with WinMDI software (version 2.5 for Window95; http://facs.
scripps.edu).
The difference in the mean fluorescence intensity obtained with specific
polyclonal rabbit anti-C3aR serum and that obtained with the preimmune
serum was calculated for each value and used to determine the level of
C3aR expression. The C3aR-specific difference in fluorescence intensity
obtained on nonstimulated cells, equivalent to 100% of the C3aR detect-
able on the cell surface, was defined as 0% C3aR internalization. To obtain
this standard for dose-response curves, buffer containing 0.25% BSA with-
out any stimulus was used, incubating the cells at 37°C for the same time
as all other samples. In contrast, to obtain this standard of nonstimulated
cells for kinetics, one cell aliquot was kept after harvesting at 4°C; ice-cold
buffer (0.25% BSA) was added instead of stimulus, and immediately af-
terward cell samples were removed and incubated with the specific and
nonspecific sera, respectively.
Before this flow cytometric assay was used in a quantitative manner, the
following control experiments were performed (data not shown). Dilution
experiments proved that the concentrations of rabbit antiserum and of the
FITC-labeled goat anti-rabbit mAb used in our assay were not limiting for
the C3aR-dependent fluorescence signal. The C3aR-dependent fluores-
cence signal was not decreased significantly in competition experiments at
4°C, when the C3aR antiserum was added only after 100 nM C3a had been
previously applied for 30 min. This is in good agreement with data ob-
tained with recombinant phage Abs directed against the same part of the
C3aR (54). Only at the relatively high concentration of $1
m
M C3a was
a slight decrease in the C3a-dependent fluorescence of about 10–15% no-
ticed. This small effect was nonspecific and negligible, because equal con-
centrations of other irrelevant proteins produced the same decrease. The
accuracy of our flow cytometer was checked using fluorescence standards.
C3aR internalization assay based on
125
I-C3a binding and the
acid wash technique
A modification of the acid/buffer wash technique described by Haigler et
al. (55) was used as a second independent method to monitor C3aR inter-
nalization. In this type of assay the
125
I-labeled ligand will dissociate from
its receptor on the intact cells during the acid wash only as long as the
ligand-receptor complex is not internalized. C3a was radioiodinated with
Iodogen (Pierce, Oud-Beijerland, The Netherlands), resulting in a specific
activity of approximately 450 Ci/mmol as previously described (50). To
induce the expression of the C3aR, U937 cells were incubated for 3 days
with 1000 U/ml IFN-
g
(Bioferon, gift from Rentschler, Laupheim, Ger-
many) as previously described (50). The cells were harvested, washed
twice with PBS in 50-ml Falcon tubes (Greiner, Frickenhausen, Germany)
at 500 3 g at room temperature, resuspended in HAG-CM buffer (20 mM
HEPES, 125 mM NaCl, 5 mM KCl, 0.5 mM glucose, 0.25% BSA, 1 mM
CaCl
2
, and 1 mM MgCl
2
, pH 7.4) to a density of 2 3 10
7
/ml, and main-
tained at room temperature until further analysis. HAG-CM buffer was
used in all subsequent steps. For each single value, 90
m
l of U937 cells
were preincubated in a microfuge tube for 15 min at 37°C. Then, to start
C3aR internalization, 30
m
l of prewarmed
125
I-C3a (;60,000 cpm) plus
either 30
m
l of buffer (for total binding) or 30
m
lof2
m
M unlabeled C3a
(for nonspecific binding) was added. After the indicated incubation periods
at 37°C, internalization was stopped by the addition of 50
m
l of ice-cold
buffer or the acid solution (12.4 M acetic acid and 0.5 M NaCl) and by the
immediate transfer of the tube into an ice bath. After 5–10 min at 4°C, the
sample was split into three 60-
m
l aliquots, and cell-bound
125
I-C3a was
separated from free tracer by centrifugation through a 10% (w/v) sucrose
cushion (12,000 3 g, 6 min, 4°C) as previously described (12). The samples
were counted on a gamma counter (Canberra Packard, Dreieich, Germany).
The degree of receptor internalization was calculated from the ratio of specific
125
I-labeled ligand bound obtained after the acid wash compared with that
obtained after a buffer wash. All acid and buffer washes were performed in
quadruplicate; nonspecific binding (acid wash) was performed in triplicate.
Due to technical limitations, a maximum of four time points can be determined
per assay. For a complete kinetic study, as depicted in Fig. 4, three experiments
with overlapping time points were performed.
Results
C3aR flow cytometric internalization assay
A quantitative flow cytometric assay was used to estimate C3aR
internalization. C3aR-dependent fluorescence signals of granulo-
cytes that had been stimulated were compared with those of
controls. Human granulocytes (in the presence of other leukocyte
7410 C3aR INTERNALIZATION AND ITS INHIBITION BY C5a
populations and gated according to FSC and SSC) were incubated
with increasing concentrations of C3a for 3 min at 37°C and an-
alyzed for C3aR expression. C3a (100 nM) lead to an almost com-
plete disappearance of the C3aR-specific fluorescence, comparable
to fluorescence values obtained with preimmune serum and equiv-
alent to maximal C3aR internalization (Fig. 1, upper panel). The
half-maximal response was reached at about 13 6 4 nM C3a (n 5
3). Three independent C3aR internalization kinetic studies, using
cells from different donors, are depicted in Fig. 2 (left panel).
Following a 10-min incubation with 13 nM C3a, virtually all the
C3aRs are internalized.
As additional controls for the specificity of our flow cytometric
assay, a subset of experiments on PMNs was repeated using the
mAb 8H1 (IgG1) raised against the identical part of the receptor as
the polyclonal rabbit antiserum (W. Bautsch et al., submitted). Es-
sentially the same internalization pattern was noted as that ob-
tained above with the polyclonal antiserum. However, the differ-
ence in fluorescence intensity between that obtained with this
C3aR-specific mAb and that of a nonrelevant control IgG1, which
served as relative measure for the number of C3aR on the cell
surface in this experiment, was smaller than the difference ob-
tained with specific polyclonal rabbit anti-C3aR serum and control
serum. Therefore, a higher relative error of the calculated C3aR
internalization resulted using the mAb, restricting its use to less
quantitative control experiments (data not shown). Synthetic pep-
tides were used as stimuli, and internalization after 10-min incu-
bation was determined. Stimulation with 100 nM C3a analogue
peptide P117 caused on granulocytes 81 6 9% C3aR internaliza-
tion and 100 6 6% internalization at a concentration of 1
m
M
compared with negligible internalization of 6.6 6 7.3 and 3.0 6
3.3% with the irrelevant peptide P252 (n 5 3). Both peptides have
been used by us before as controls for C3a specificity (49, 56).
The human mast cell line HMC-1 and human monocytes were
also partially analyzed. On HMC-1 cells, the maximal C3aR in-
ternalization (;60–70%) was reached within 5 min (filled dia-
monds in the right panel of Fig. 2). These cells seemed to be more
sensitive than PMN to the manipulations necessary in this assay. In
the buffer control (open diamonds), there was a slow decrease in
detectable C3aR, corresponding to a slight spontaneous C3aR in-
ternalization. Therefore, the experiment was repeated under mod-
ified conditions, making it unlikely that the lack of complete C3aR
internalization was due to the fact that the HMC-1 cells had been
overstressed; C3aR internalization was triggered by the addition of
100 nM C3a (final concentration) directly into the cell culture me-
dium of HMC-1 cells that had been cultured undisturbed for 48 h.
By 15 min the maximal internalization was still only approxi-
mately 70% (data not shown). The half-maximal response after 3
min of incubation with the stimulus was reached at about 41 6 18
nM C3a (n 5 3; Fig. 1, middle panel). A typical histogram of the
fluorescence obtained on HMC-1 cells is depicted in Fig. 3. Stim-
ulation with 100 nM C3a analogue peptide P117 caused 9.1 6
4.0% C3aR internalization and 51.3 6 4.8% internalization at a
concentration of 1
m
M compared with negligible internalization of
6.6 6 4.7 and 8.0 6 3.4% with the irrelevant peptide P252 (n 5 3).
A similar dose-dependent shift of the C3aR-specific fluores-
cence was observed on monocytes (Fig. 1, lower panel). The his-
togram in Fig. 3 (lower panel) demonstrates that the C3aR-depen-
dent fluorescence signal was much smaller on these cells than on
neutrophils (Fig. 5) or HMC-1 cells, most likely corresponding to
a smaller number of C3aR on native peripheral monocytes. Be-
cause of that smaller signal-to-background ratio, the statistical er-
ror of the calculated percentage of internalized C3aR in a dose-
response curve increased (Fig. 1, lower panel). In monocytes, the
half-maximal response in three independent experiments ranged
from 15–50 nM C3a. Stimulation with 100 nM C3a analogue pep-
tide P117 caused 17.9 6 8% C3aR internalization, and there was
66.7 6 16.5% internalization at a concentration of 1
m
M compared
with negligible internalization of 3.0 6 11.5 and 9.0 6 15.7% with
the irrelevant peptide P252 (n 5 3). Since monocytes attached
rapidly onto the surface of the vials (different plastic surfaces were
compared) during the incubation at 37°C, a process that led to a
drastic loss of flow cytometrically detectable cells and that might
simultaneously lead to C3a-independent cell activation, time
points beyond 3 min were not analyzed on these cells.
FIGURE 1. Dose-response curve of C3aR internalization on PMNs,
HMC-1 cells, and monocytes. The cells were incubated with increasing
concentrations of C3a for 3 min at 37°C. Receptor internalization (mean 6
SE calculated from three single samples) was detected antigenically by
flow cytometry. A representative experiment (n 5 3) is depicted.
FIGURE 2. Kinetics of ligand-dependent C3aR internalization. Human
granulocytes (left panel, with curves from three independent experiments)
and the human mast cell line HMC-1 (one typical experiment of four; right
panel, l) were incubated at 37°C for an increasing period of time with 13
or 100 nM C3a, respectively. C3aR internalization was detected antigeni-
cally by flow cytometry. Values represent the mean 6 SE calculated from
three single results. Control experiments were performed using buffer in-
stead of C3a. For PMNs, the percentage of C3aR internalization increased
only slowly to ,20% after 1800 s (data not shown). HMC-1-cells were
more sensitive to the experimental conditions’ the buffer control increased
to almost 40% (right panel, L).
7411The Journal of Immunology
The acid wash technique using tracer concentrations of
125
I-C3a
confirms C3aR internalization for IFN-
g
induced
myelomonoblastic U937 cells
The values obtained by the flow cytometric assay are usually very
exact as long as relatively high numbers of C3aR are present on the
surface of the analyzed cell (;24,000 molecules/neutrophil) (49)
and as long as a relatively large proportion of receptors disappear
from the cell surface. However, this type of assay is insensitive to
relatively low agonist concentrations. For this reason and to con-
firm our data by an independent method, a second assay based on
the acid/buffer wash technique (55) was developed. Tracer con-
centrations of
125
I-C3a (,0.2 nM, about 1/10th of the K
d
) were
used to induce and determine C3aR internalization. A similar sys-
tem has been used for analysis of C5aR internalization (31). As
depicted in Fig. 4, IFN-
g
-treated, monocyte-related U937 cells
(50) internalized the
125
I-labeled ligand-receptor complexes, but
relatively slowly. Unfortunately, a very limited number of samples
can be processed simultaneously using the acid wash technique,
and, as depicted, it is additionally hampered by a relatively high
SD, mainly resulting from a relatively high nonspecific
125
I-C3a
and a relatively small specific binding. Therefore, the flow cyto-
metric assay was used for all further analyses. Unfortunately,
U937 cells themselves were not suited for flow cytometric analy-
sis; only the related monocytes can be used for comparison.
PTX did not modulate C3aR internalization on PMNs
The data for U937 cells using ,0.2 nM C3a indicated that C3aR-
dependent signal transduction is not an absolute prerequisite for
C3aR internalization. However, these data do not exclude that sig-
nal transduction would modify the C3aR internalization, causing
the faster C3aR internalization detected on granulocytes with
higher concentrations of the ligand (13 vs 100 nM; see Figs. 1 and
2). PTX inhibits C3aR signal transduction (4, 9, 10, 12, 57) and
was tested for its effect on C3aR internalization. The efficiency of
this treatment was checked in a fura-2/AM assay. The C3a-depen-
dent increase in [Ca
21
]
i
was completely blocked by the toxin (data
not shown). In parallel, C3aR internalization was determined using
100 nM C3a (Table I). At this concentration, signal transduction
(e.g., determined as the increase in [Ca
21
]
i
) is maximal (49). In all
experiments performed, C3aR internalization (;90%) was not af-
fected by PTX treatment.
The phorbol ester PMA induced complete C3aR internalization
in human PMNs
Phorbol esters such as PMA are known to induce the sequestration
of a variety of receptors, e.g., the C5aR (31). Granulocytes were
incubated for 30 min at 37°C with increasing concentrations of this
protein kinase activator (Fig. 5). At 400 nM PMA, the histograms
of the fluorescences obtained with C3aR-specific antiserum and
preimmune serum were almost identical, indicating complete
C3aR internalization following PMA stimulation.
C5a, but not FMLP, had a dose-dependent, negative effect on
C3a-induced C3aR internalization
Neither C5a nor FMLP, whose receptor is also highly expressed
(58, 59) and also internalized (30) on human neutrophils, caused
any fast cross-internalization of C3aR (Fig. 6, right panel). Even
after 15 min of incubation with 100 nM C5a or FMLP (at 37°C),
FIGURE 3. Typical histograms of the C3a fluorescence obtained on
buffer-treated and C3a-treated HMC-1 cells (upper panel) and monocytes
(lower panel). The cells were stimulated for 3 min at 37°C with 100 nM
C3a vs buffer as a control and were analyzed using C3aR-immune serum
and preimmune serum, as indicated by the four histograms in each panel:
A, buffer as stimulus/C3aR antiserum for detection; B, 100 nM C3a/C3aR
antiserum; C, buffer/preimmune serum; and D, 100 nM C3a/preimmune
serum.
FIGURE 4. Kinetics of C3aR internalization to
125
I-C3a at tracer con-
centrations (,0.2 nM) on differentiated human U937 cells. U937 cells
were differentiated to a more monocyte/macrophage-like phenotype by
IFN-
g
. Using the acid/buffer wash technique, C3aR internalization was
determined; the mean and SE were calculated from three samples. A rep-
resentative experiment (n 5 3) is depicted.
Table I. PTX-sensitive signal transduction is no prerequisite for C3aR
internalization on PMNs
a
Expt.
C3aR Internalization (%)
n2PTX 1PTX
1 96.9 6 7.3 90.8 6 7.4 3
2 90.9 6 1.3 90.9 6 7.1 4
3 90.0 6 3.3 89.8 6 6.4 6
a
Human granulocytes were pretreated for 3 h with 0.5
m
g/ml PTX or buffer. No
significant change in the degree of C3aR internalization, induced by 100 nM C3a
within 3 min, could be observed. In simultaneously performed control experiments,
the C3a-dependent increase in cytosolic Ca
21
of these granulocytes (fura-2/AM-assay
(12)) was completely inhibited by this treatment (data not shown).
7412 C3aR INTERNALIZATION AND ITS INHIBITION BY C5a
the C3aR-dependent fluorescence signal of PMNs did not decrease
significantly (data not shown). To check whether the internaliza-
tion of the C3aR was somehow modified in the presence of a
related independent second stimulus, C3aR internalization was in-
duced by costimulation of C3a with C5a or FMLP, respectively.
To our surprise, the C3aR-dependent C3aR internalization was sig-
nificantly smaller when granulocytes were coincubated for 2 min
at 37°C with C5a. In contrast, costimulation with 100 nM FMLP
had no effect (Fig. 6). The activity of FMLP was confirmed in a
functional fura-2/AM assay on U937 cells performed in parallel
(data not shown). The inhibitory effect of C5a was dose dependent,
reaching its maximum at 31.6–100 nM (Fig. 6, left panel). The
ligand-dependent C3aR internalization was decreased by 40–55%
in 10 independent experiments. There was no difference when 31.6
nM C5a (final concentration) was added 2 min before C3a (data
not shown). When the incubation with the two stimuli was started
in parallel, the negative effect of C5a decreased over time, but was
still significant after 5 min (data not shown). To show that this
unexpected experimental outcome was mediated by C5aR and was
not due to a direct interaction of the two ligands, granulocytes were
preincubated with a neutralizing anti-C5aR mAb. The pretreatment
with 10
m
g/ml of the anti-C5aR mAb S5/1 (7) reversed by two-
thirds, and at 100
m
g/ml almost completely reversed the negative
effect of 31.6 nM C5a on C3a-dependent C3aR internalization
(Table II).
HEK293 cells lack the ability to ligand dependently internalize
C3aR
Human embryonic kidney 293 (HEK293) cells have previously
been used for functional studies with transiently transfected C5aR
(43). These cells show agonist-induced internalization of other
transfected receptors, such as the histamine H
2
receptor (60) or the
human
b
2
-adrenergic receptor (61, 62). However, the C5aR is
poorly internalized in these cells after transient or stable transfec-
tion (31). As depicted in Fig. 7, after 3 min at 37°C we did not
observe any C3a-induced internalization. C3aR internalization was
also not seen 15 min after addition of 100 nM ligand (data not
shown). Similar results were obtained with HEK293 cells from
three different sources. For functional coupling of the C3aR or
C5aR in HEK293 cells (as determined by phosphoinositol hydro-
lysis in a control experiment; Table III) the receptors must be
coexpressed with G
a
16, a human PTX-resistant, G protein
a
sub-
unit (56). However, cotransfection of HEK293 cells with human
G
a
16 did not improve the internalization of C3aR (Fig. 8).
Discussion
Receptor internalization as one putative control mechanisms of the
C3aR protecting cells against overstimulation was investigated in
detail on human granulocytes using the rapid decrease in antigeni-
cally detectable C3aR in a flow cytometric assay. After stimulation
of human granulocytes and the human mast cell line HMC-1 with
100 nM C3a, maximal internalization was reached within 5–10
min, resembling the fast internalization of the C5aR on polymor-
phonuclear leukocytes or monocytes (30, 63). Although different
blood donors were used for kinetic studies, the three curves ob-
tained on granulocytes were almost indistinguishable, indicating
the high reproducibility of this flow cytometric assay and demon-
strating the invariability between cells from different donors. The
degree of C3aR internalization seemed to be lower on HMC-1
cells than on PMNs. Even under optimized conditions, on HMC-1
cells only approximately 70% of the receptors were internalized
FIGURE 5. Phorbol ester induced C3aR internalization on human gran-
ulocytes. PMNs were preincubated for 30 min at 37°C with 0, 50, 100, and
400 nM PMA. The fluorescence caused by the binding of FITC-labeled
goat anti-rabbit serum to the anti-C3aR rabbit serum (solid lines) decreased
with increasing concentrations of PMA, down to the background fluores-
cence caused by the binding of rabbit preimmune serum (dotted line). A
representative experiment (n 5 3) is depicted.
FIGURE 6. C5a, but not FMLP, diminished with increasing concentra-
tions the C3a-induced (31.6 nM) internalization of C3aR on PMNs. As
indicated, human granulocytes were simultaneously incubated for 2 min at
37°C with 31.6 nM C3a or buffer, respectively, and increasing concentra-
tions of C5a (0, 10, 31.6, 100, and 316 nM) or 100 nM FMLP. In the
absence of C3a, neither 100 nM C5a nor FMLP led to any significant C3aR
internalization. In contrast to the inhibitory effect of C5a, coincubation with
FMLP did not alter the C3a-dependent internalization of C3aR. Data are
presented as the mean 6 SE from one representative experiment of three
performed.
Table II. The negative effect of C5a on C3a-dependent C3aR
internalization can be blocked specifically by a mAb inhibiting C5a-
binding to its receptor
a
C3a (nM) C5a (nM)
mAb
(
m
g/ml) IgG2a Control Anti-C5aR mAb
31.6 0 0 88.0 6 4.1 88.0 6 4.1
31.6 100 0 52.3 6 7.2 52.3 6 7.2
31.6 0 10 82.6 6 4.0 86.8 6 1.5
31.6 100 10 51.1 6 8.2* 74.4 6 2.1*
31.6 0 100 90.5 6 3.9 88.5 6 3.0
31.6 100 100 39.0 6 1.9* 82.8 6 2.7*
a
Human granulocytes were preincubated for 30 min at 4°C with 0, 10, or 100
m
g/ml (10
m
g/ml to ;60 nM) of the C5aR-specific mAb S5/1 (7) or a control mAb
of the same subtype (IgG2a), respectively. Then, still in the presence of the mAbs, the
cells were simultaneously stimulated (for 2 min at 37°C) with C3a and/or C5a at the
indicated concentrations. Depicted is one typical out of three independent experiments
(n 5 3 single values, each).
* Significant difference ( p , 0.01) between the effect of mAb S5/1 and control
IgG2a.
7413The Journal of Immunology
compared with almost 100% internalization on granulocytes. The
remaining 30% C3aR detected on the cell surface either were sim-
ply not internalized or may represent a steady state situation re-
sulting from rapid receptor internalization and recycling. Mono-
cytes internalized in a C3a dose-dependent fashion this receptor as
well, demonstrating, just as for the HMC-1 mast cell line, that
internalization of the C3aR is a general control mechanism on a
variety of cells.
C3aR internalization detected on IFN-
g
-induced U937 cells by
the
125
I-C3a acid wash technique 1) supported our data obtained
by flow cytometry, 2) confirmed that human monocyte/macro-
phage-like cells internalize the C3aR, and 3) demonstrated that
internalization of the C3aR takes place at agonist concentrations at
which one can hardly expect any relevant receptor activation and
signal transduction (49). However, it is not only the low C3a con-
centration making it very unlikely that signal transduction is nec-
essary for C3aR internalization. In contrast to cells induced by
dibutyryl cAMP, there is no detectable C3a-dependent increase in
[Ca
21
]
i
in U937 cells induced by IFN-
g
(50).
Internalization on granulocytes was dose dependent in the sense
that it took longer for smaller concentrations of C3a (13 nM) to
reach the same maximum. One possible explanation for the faster
C3aR internalization at higher concentrations could have been that
even if signal transduction is not a prerequisite for C3aR internal-
ization, it could speed up this process. However, pretreatment with
PTX did not alter significantly the fast internalization caused by
100 nM C3a, a concentration at which maximal signal transduction
can be expected.
The phorbol ester PMA, a potent activator of protein kinases, in
particular of protein kinase C (64–66), caused a dose-dependent
internalization of C3aR on granulocytes. Such a rapid phorbol es-
ter-induced internalization has been described for several other
receptors (61, 67, 68), including C5aR (31). Although, C5aR is
phosphorylated after application of PMA (28, 32), the underlying
mechanism among PMA, protein kinases, and receptor internal-
ization seems to be more indirect and complex, since a C5aR mu-
tant in RBL cells lacking any putative protein kinase C phosphor-
ylation motif is still internalized after PMA application,
comparable to the wild-type C5aR (31). The activation of signal
transduction by maximal doses of either C5a or FMLP did not lead
to any cross-internalization of the C3aR, although FMLP, for ex-
ample, can trans-locate protein kinase C to the plasma membrane
of neutrophils (69). Therefore, if there was a physiological corre-
late to the C3a-independent C3aR internalization caused by PMA
in vivo, it should be a more extreme situation, where either de-
sensitization of these signal transduction pathways completely
fails or high or long lasting intracellular activation is caused by the
simultaneous stimulation of granulocytes by a variety of different
mediators.
The inhibitory effect of C5a on C3aR internalization was ob-
served on human granulocytes and not only on a more or less
artificial system, such as C3aR-transfected cells or differentiated
tumor cell lines. The dose-dependent effect, its relative specificity
(FMLP as coactivator had no influence on the amount of deter-
mined C3aR), and its inhibition by an anti-C5aR mAb competing
with C5a for receptor binding suggest that the inhibitory effect
FIGURE 7. Transiently transfected HEK293 cells did not show any ag-
onist-induced C3aR internalization. Human embryonic kidney cells
(HEK293) were transfected with the wild-type C3aR (two upper panels)or
with the plasmid vector without insert (lower panel). C3a (100 nM) did not
cause any shift of the fluorescence determined by C3aR-immune serum
within 3 min of incubation with the stimulus at 37°C. One typical exper-
iment of three performed is shown. Similar results were achieved after 15
min of incubation and with HEK293-cells from two other sources (data not
shown).
Table III. Control experiment demonstrating expression of the G
a
16-
subunit after cotransfection with the C3aR in HEK293 cells by C3a- and
G-protein-dependent phosphoinositide hydrolysis
a
Transfected
Plasmids Buffer (cpm)
100 nM C3a
(cpm)
C3aR 2050 6 180 1930 6 40
G
a
16 2230 6 210 2350 6 235
C3aR 1 G
a
16 1410 6 90 3750 6 220
a
HEK293 cells were cotransfected with plasmids coding for the human C3aR or
G
a
16, as indicated. Accumulation of [
3
H]phosphoinositides upon stimulation with
100 nM C3a or buffer as control, respectively, was determined. Data are presented as
mean 6 SE (n 5 3) from one representative experiment. The experiment was per-
formed as described (56).
FIGURE 8. Cotransfection with G
a
16 of HEK293 cells transiently ex-
pressing the C3aR did not rescue agonist-induced C3aR internalization.
Human embryonic kidney cells (HEK293) were cotransfected with wild-
type C3aR and G
a
16. The cells were stimulated for 10 min at 37°C with
100 nM C3a vs buffer as a control and were analyzed using C3aR-immune
serum and preimmune serum as indicated: A, buffer as stimulus/C3aR an-
tiserum for detection; B, 100 nM C3a/C3aR antiserum; C, buffer/preim-
mune serum; and D, 100 nM C3a/preimmune serum. C3a (100 nM) did not
cause any significant shift of the fluorescence determined by C3aR-im-
mune serum within 10 min of incubation with the stimulus at 37°C. One
typical experiment of three performed is shown. Similar results were
achieved after 30 min of incubation (data not shown).
7414 C3aR INTERNALIZATION AND ITS INHIBITION BY C5a
itself is not an experimental artifact. In vivo one would expect that
C3a and C5a are simultaneously present at sites of complement
activation. Therefore, the experimental costimulation setting
should actually reflect the in vivo situation near the site of com-
plement activation. In general, receptor internalization is considered a
negative feedback mechanism, limiting the amount and duration of
signaling. Consequently, the decreased C3aR internalization in the
presence of C5a could augment the activation of granulocytes and
other C3aR-expressing cells, since more noninternalized receptors
would be present for longer time periods. Conversely, at the periphery
where diverging gradients of the two anaphylatoxin are more likely or
if spontaneous generation of only one anaphylatoxin, for example by
the direct action by proteinases, occurred, C3a-mediated inflammation
would be limited due to fast receptor internalization and cell desen-
sitization. This is difficult to demonstrate experimentally because
there is no way to distinguish between the intracellular signaling of
the two costimulated receptors.
The cross-inhibitory effect on C3aR internalization was not ob-
served by costimulation with FMLP, indicating a specific compo-
nent of the C5aR-C3aR cross-talk. The signal transductions of
C5aR and FMLP receptor are very similar (70, 71). Ca
21
mobi-
lization by FMLP in U937 cells or granulocytes was even higher
and longer lasting than that caused by C5a (data not shown).
Therefore, it seems unlikely that signal transduction by C5a was
the reason for the interaction between the two anaphylatoxin re-
ceptors. Two explanations seem possible. C3aR and C5aR may
share a limiting cell component required for receptor internaliza-
tion, which is not (solely) used by FMLP receptor (58, 59); then,
the cointernalization of .100,000 C5aR (72) would slow down the
internalization of approximately 25,000 C3aR by competition. On
the other hand, there may be a more direct interaction, possibly
mediated by a specific kinase.
C3a-treated HEK293 cells did not internalize C3aR, as recently
described by us for C5aR (31). HEK293 clones from three inde-
pendent sources were used with similar results, suggesting that the
observed deficiency was not a clonal artifact. Transiently trans-
fected HEK293 cells have been successfully used for receptor in-
ternalization studies, suggesting that the observed deficiency is re-
stricted to certain receptors. The lack of internalization cannot be
due to species incompatibilities, because C3aR and HEK293 cells
are both of human origin. The internalization machinery of
HEK293 cells can distinguish between certain receptors, even
within adrenergic receptor subtypes;
b
2
-adrenergic receptors are
selectively internalized to intracellular vesicles, which are distinct
from those containing M
a
2
-4H adrenergic receptors, while M
a
2
-
10H receptors or the rat type 2 angiotensin II receptor remain in
the plasma membrane (73, 74). Therefore, HEK293 cells may
serve as a model system in reconstitution experiments to identify
the missing specific components linking C3aR and C5aR to the
internalization machinery of these cells. Cotransfection of C3aR
and C5aR with G
a
16 is necessary to achieve ligand-dependent
Ca
21
mobilization. However, cotransfection with this PTX-resis-
tant G protein subunit did not improve the poor C3aR internaliza-
tion in HEK293 cells, indicating that signal transduction alone is
not sufficient for reconstitution.
Our study characterized in detail the C3aR internalization,
which was negatively influenced by the C5aR pathway. Future
investigations will show whether C3aR internalization participates
not only in the negative functional control in response to a repet-
itive or prolonged C3a stimulus, but whether it is also a prerequi-
site for dephosphorylation and C3aRrecycling, as has been sug-
gested for C5aR (26).
Acknowledgments
We thank Dr. Robert Ames from SmithKline Beecham Pharmaceuticals for
the critical reading of the manuscript and the C3aR antiserum, Dr. Ulrich
Martin for the assistance with FACS analysis, and Dr. Lubomir Arseniev
and Prof. Dr. Arnold Ganser of the Department of Hematology for their
help and use of their FACS equipment. The HMC-1 cells were kindly
provided by J. H. Butterfield. We thank the head of our department, Prof.
Dr. D. Bitter-Suermann, for his, as always, strong support.
References
1. Chenoweth, D. E., and T. E. Hugli. 1978. Demonstration of specific C5a recep-
tor on intact human polymorphonuclear leukocytes. Proc. Natl. Acad. Sci. USA
75:3943.
2. Van-Epps, D. E., and D. E. Chenoweth. 1984. Analysis of the binding of
fluorescent C5a and C3a to human peripheral blood leukocytes. J. Immunol.
132:2862.
3. Dahinden, C. A., S. C. Bischoff, T. Brunner, M. Krieger, S. Takafuji, and
A. L. de Weck. 1991. Regulation of mediator release by human basophils: im-
portance of the sequence and time of addition in the combined action of different
agonists. Int. Arch. Allergy Appl. Immunol. 94:161.
4. Kretzschmar, T., A. Jeromin, C. Gietz, W. Bautsch, A. Klos, J. Ko¨hl,
G. Rechkemmer, and D. Bitter-Suermann. 1993. Chronic myelogenous leukemia-
derived basophilic granulocytes express a functional active receptor for the ana-
phylatoxin C3a. Eur. J. Immunol. 23:558.
5. Legler, D. F., M. Loetscher, S. A. Jones, C. A. Dahinden, M. Arock, and
B. Moser. 1996. Expression of high- and low-affinity receptors for C3a on the
human mast cell line, HMC-1. Eur. J. Immunol. 26:753.
6. Nilsson, G., M. Johnell, C. H. Hammer, H. L. Tiffany, K. Nilsson, D. D. Metcalfe,
A. Siegbahn, and P. M. Murphy. 1996. C3a and C5a are chemotaxins for human
mast cells and act through distinct receptors via a pertussis toxin-sensitive signal
transduction pathway. J. Immunol. 157:1693.
7. Fureder, W., H. Agis, M. Willheim, H. C. Bankl, U. Maier, K. Kishi,
M. R. Muller, K. Czerwenka, T. Radaszkiewicz, and J. H. Butterfield. 1995.
Differential expression of complement receptors on human basophils and mast
cells: evidence for mast cell heterogeneity and CD88/C5aR expression on skin
mast cells. J. Immunol. 155:3152.
8. Elsner, J., M. Oppermann, W. Czech, G. Dobos, E. Schopf, J. Norgauer, and
A. Kapp. 1994. C3a activates reactive oxygen radical species production and
intracellular calcium transients in human eosinophils. Eur. J. Immunol. 24:518.
9. Hartmann, K., B. M. Henz, S. Kruger-Krasagakes, J. Kohl, R. Burger, S. Guhl,
I. Haase, U. Lippert, and T. Zuberbier. 1997. C3a and C5a stimulate chemotaxis
of human mast cells. Blood 89:2863.
10. Norgauer, J., G. Dobos, E. Kownatzki, C. Dahinden, R. Burger, R. Kupper, and
P. Gierschik. 1993. Complement fragment C3a stimulates Ca
21
influx in neu-
trophils via a pertussis-toxin-sensitive G protein. Eur. J. Biochem. 217:289.
11. Monk, P. N., and L. J. Partridge. 1993. Characterization of a complement-frag-
ment-C5a-stimulated calcium-influx mechanism in U937 monocytic cells. Bio-
chem. J. 295:679.
12. Klos, A., S. Bank, C. Gietz, W. Bautsch, J. Ko¨hl, M. Burg, and T. Kretzschmar.
1992. C3a receptor on dibutyryl-cAMP-differentiated U937 cells and human neu-
trophils: the human C3a receptor characterized by functional responses and
125
I-
C3a binding. Biochemistry 31:11274.
13. Gerard, N. P., and C. Gerard. 1990. Construction and expression of a novel
recombinant anaphylatoxin, C5a-N19, as a probe for the human C5a receptor.
Biochemistry 29:9274.
14. Ember, J. A., M. Jagels, and T. E. Hugli. 1998. Anaphylatoxins and biological
responses. In The Human Complement System in Health and Disease, 1st Ed.
J. Volanakis and M. Frank, eds. Marcel Dekker, New York, p. 241.
15. Ames, R. S., Y. Li, H. M. Sarau, P. Nuthulaganti, J. J. Foley, C. Ellis, Z. Zeng,
K. Su, A. J. Jurewicz, R. P. Hertzberg, et al. 1996. Molecular cloning and char-
acterization of the human anaphylatoxin C3a receptor. J. Biol. Chem. 271:20231.
16. Gasque, P., S. K. Singhrao, J. W. Neal, P. Wang, S. Sayah, M. Fontaine, and
B. P. Morgan. 1998. The receptor for complement anaphylatoxin C3a is ex-
pressed by myeloid cells and nonmyeloid cells in inflamed human central nervous
system: analysis in multiple sclerosis and bacterial meningitis. J. Immunol.
160:3543.
17. Heese, K., C. Hock, and U. Otten. 1998. Inflammatory signals induce neurotro-
phin expression in human microglial cells. J. Neurochem. 70:699.
18. Fischer, W. H., and T. E. Hugli. 1997. Regulation of B cell functions by C3a and
C3a(desArg): suppression of TNF-
a
, IL-6, and the polyclonal immune response.
J. Immunol. 159:4279.
19. Buchner, R. R., T. E. Hugli, J. A. Ember, and E. L. Morgan. 1995. Expression of
functional receptors for human C5a anaphylatoxin (CD88) on the human hepa-
tocellular carcinoma cell line HepG2: stimulation of acute-phase protein-specific
mRNA and protein synthesis by human C5a anaphylatoxin. J. Immunol. 155:308.
20. Gasque, P., S. K. Singhrao, J. W. Neal, O. Gotze, and B. P. Morgan. 1997.
Expression of the receptor for complement C5a (CD88) is up-regulated on reac-
tive astrocytes, microglia, and endothelial cells in the inflamed human central
nervous system. Am. J. Pathol. 150:31.
21. Werfel, T., J. Zwirner, M. Oppermann, A. Sieber, G. Begemann, W. Drommer,
A. Kapp, and O. Gotze. 1996. CD88 antibodies specifically bind to C5aR on
dermal CD117
1
and CD14
1
cells and react with a desmosomal antigen in human
skin. J. Immunol. 157:1729.
7415The Journal of Immunology
22. Haviland, D. L., R. L. McCoy, W. T. Whitehead, H. Akama, E. P. Molmenti,
A. Brown, J. C. Haviland, W. C. Parks, D. H. Perlmutter, and R. A. Wetsel. 1995.
Cellular expression of the C5a anaphylatoxin receptor (C5aR): demonstration of
C5aR on nonmyeloid cells of the liver and lung. J. Immunol. 154:1861.
23. Lacy, M., J. Jones, S. R. Whittemore, D. L. Haviland, R. A. Wetsel, and
S. R. Barnum. 1995. Expression of the receptors for the C5a anaphylatoxin,
interleukin-8 and FMLP by human astrocytes and microglia. J. Neuroimmunol.
61:71.
24. Wetsel, R. A. 1995. Expression of the complement C5a anaphylatoxin receptor
(C5aR) on non-myeloid cells. Immunol. Lett. 44:183.
25. Bokisch, V. A., and H. J. Mu¨ller-Eberhard. 1970. Anaphylatoxin inactivator of
human plasma: its isolation and characterization as a carboxypeptidase. J. Clin.
Invest. 49:2427.
26. Giannini, E., and F. Boulay. 1995. Phosphorylation, dephosphorylation, and re-
cycling of the C5a receptor in differentiated HL60 cells. J. Immunol. 154:4055.
27. Ali, H., R. M. Richardson, E. D. Tomhave, J. R. Didsbury, and R. Snyderman.
1993. Differences in phosphorylation of formylpeptide and C5a chemoattractant
receptors correlate with differences in desensitization. J. Biol. Chem. 268:24247.
28. Tardif, M., L. Mery, L. Brouchon, and F. Boulay. 1993. Agonist-dependent phos-
phorylation of N-formylpeptide and activation peptide from the fifth component
of C (C5a) chemoattractant receptors in differentiated HL60 cells. J. Immunol.
150:3534.
29. Naik, N., E. Giannini, L. Brouchon, and F. Boulay. 1997. Internalization and
recycling of the C5a anaphylatoxin receptor: evidence that the agonist-mediated
internalization is modulated by phosphorylation of the C-terminal domain. J. Cell
Sci. 110:2381.
30. Van-Epps, D. E., S. Simpson, J. G. Bender, and D. E. Chenoweth. 1990. Regu-
lation of C5a and formyl peptide receptor expression on human polymorphonu-
clear leukocytes. J. Immunol. 144:1062.
31. Bock, D., U. Martin, S. Gartner, C. Rheinheimer, U. Raffetseder, L. Arseniev,
M. D. Barker, P. N. Monk, W. Bautsch, J. Kohl, et al. 1997. The C terminus of
the human C5a receptor (CD88) is required for normal ligand-dependent receptor
internalization. Eur. J. Immunol. 27:1522.
32. Giannini, E., L. Brouchon, and F. Boulay. 1995. Identification of the major phos-
phorylation sites in human C5a anaphylatoxin receptor in vivo. J. Biol. Chem.
270:19166.
33. Meuer, S., T. E. Hugli, R. H. Andreatta, U. Hadding, and D. Bitter-Suermann.
1981. Comparative study on biological activities of various anaphylatoxins (C4a,
C3a, C5a). Investigations on their ability to induce platelet secretion. Inflamma-
tion 5:263.
34. Ferguson, S. S., L. S. Barak, J. Zhang, and M. G. Caron. 1996. G-protein-coupled
receptor regulation: role of G-protein-coupled receptor kinases and arrestins.
Can. J. Physiol. Pharmacol. 74:1095.
35. Palczewski, K., and J. L. Benovic. 1991. G-protein-coupled receptor kinases.
Trends Biochem. Sci. 16:387.
36. de Bont, E. S., A. Martens, J. van Raan, G. Samson, W. P. Fetter, A. Okken, and
L. H. de Leij. 1993. Tumor necrosis factor-
a
, interleukin-1
b
, and interleukin-6
plasma levels in neonatal sepsis. Pediatr. Res. 33:380.
37. Pitcher, J. A., J. Inglese, J. B. Higgins, J. L. Arriza, P. J. Casey, C. Kim,
J. L. Benovic, M. M. Kwatra, M. G. Caron, and R. J. Lefkowitz. 1992. Role of
bg
subunits of G proteins in targeting the
b
-adrenergic receptor kinase to mem-
brane-bound receptors. Science 257:1264.
38. Winstel, R., S. Freund, C. Krasel, E. Hoppe, and M. J. Lohse. 1996. Protein
kinase cross-talk: membrane targeting of the
b
-adrenergic receptor kinase by
protein kinase C. Proc. Natl. Acad. Sci. USA 93:2105.
39. Chuang, T. T., H. LeVine, and A. De Blasi. 1995. Phosphorylation and activation
of
b
-adrenergic receptor kinase by protein kinase C. J. Biol. Chem. 270:18660.
40. Ferguson, S. S., W. E. Downey, A. M. Colapietro, L. S. Barak, L. Menard, and
M. G. Caron. 1996. Role of
b
-arrestin in mediating agonist-promoted G protein-
coupled receptor internalization. Science 271:363.
41. Goodman, O. B. J., J. G. Krupnick, F. Santini, V. V. Gurevich, R. B. Penn,
A. W. Gagnon, J. H. Keen, and J. L. Benovic. 1996.
b
-Arrestin acts as a clathrin
adaptor in endocytosis of the
b
2-adrenergic receptor. Nature 383:447.
42. Lohse, M. J., J. L. Benovic, J. Codina, M. G. Caron, and R. J. Lefkowitz. 1990.
b
-Arrestin: a protein that regulates
b
-adrenergic receptor function. Science
248:1547.
43. Didsbury, J. R., R. J. Uhing, E. Tomhave, C. Gerard, N. Gerard, and
R. Snyderman. 1991. Receptor class desensitization of leukocyte chemoattractant
receptors. Proc. Natl. Acad. Sci. USA 88:11564.
44. Sabroe, I., T. J. Williams, C. A. Hebert, and P. D. Collins. 1997. Chemoattractant
cross-desensitization of the human neutrophil IL-8 receptor involves receptor
internalization and differential receptor subtype regulation. J. Immunol.
158:1361.
45. Tang, H., H. Shirai, and T. Inagami. 1995. Inhibition of protein kinase C prevents
rapid desensitization of type 1B angiotensin II receptor. Circ. Res. 77:239.
46. Clark, R. B., J. Friedman, R. A. Dixon, and C. D. Strader. 1989. Identification of
a specific site required for rapid heterologous desensitization of the
b
-adrenergic
receptor by cAMP-dependent protein kinase. Mol. Pharmacol. 36:343.
47. Lohse, M. J., R. J. Lefkowitz, M. G. Caron, and J. L. Benovic. 1989. Inhibition
of
b
-adrenergic receptor kinase prevents rapid homologous desensitization of
b
2-adrenergic receptors. Proc. Natl. Acad. Sci. USA 86:3011.
48. Richardson, R. M., H. Ali, B. C. Pridgen, B. Haribabu, and R. Snyderman. 1998.
Multiple signaling pathways of human interleukin-8 receptor A: independent reg-
ulation by phosphorylation. J. Biol. Chem. 273:10690.
49. Martin, U., D. Bock, L. Arseniev, M. A. Tornetta, R. S. Ames, W. Bautsch,
J. Kohl, A. Ganser, and A. Klos. 1997. The human C3a receptor is expressed on
neutrophils and monocytes, but not on B or T lymphocytes. J. Exp. Med. 186:199.
50. Burg, M., U. Martin, D. Bock, C. Rheinheimer, J. Kohl, W. Bautsch, and A. Klos.
1996. Differential regulation of the C3a and C5a receptors (CD88) by IFN-
g
and
PMA in U937 cells and related myeloblastic cell lines. J. Immunol. 157:5574.
51. Kimura, I., Y. Moritani, and Y. Tanizaki. 1973. Basophils in bronchial asthma
with reference to reagin-type allergy. Clin. Allergy 3:195.
52. Hansel, T. T., I. J. De Vries, T. Iff, S. Rihs, M. Wandzilak, S. Betz, K. Blaser, and
C. Walker. 1991. An improved immunomagnetic procedure for the isolation of
highly purified human blood eosinophils. J. Immunol. Methods 145:105.
53. Knapp, W., ed. 1989. Leucocyte Typing, Vol. 4: White Cell Differentiation An-
tigens. Oxford University Press, Oxford.
54. Hawlisch, H., R. Frank, M. Hennecke, M. Baensch, B. Sohns, L. Arseniev,
W. Bautsch, A. Kola, A. Klos, and J. Kohl. 1998. Site-directed C3a receptor
antibodies from phage display libraries. J. Immunol. 160:2947.
55. Haigler, H. T., F. R. Maxfield, M. C. Willingham, and I. Pastan. 1980. Dansylca-
daverine inhibits internalization of
125
I-epidermal growth factor in BALB 3T3
cells. J. Biol. Chem. 255:1239.
56. Crass, T., U. Raffetseder, U. Martin, M. Grove, A. Klos, J. Kohl, and W. Bautsch.
1996. Expression cloning of the human C3a anaphylatoxin receptor (C3aR) from
differentiated U-937 cells. Eur. J. Immunol. 26:1944.
57. Zwirner, J., O. Gotze, A. Moser, A. Sieber, G. Begemann, A. Kapp, J. Elsner, and
T. Werfel. 1997. Blood- and skin-derived monocytes/macrophages respond to
C3a but not to C3a(desArg) with a transient release of calcium via a pertussis
toxin-sensitive signal transduction pathway. Eur. J. Immunol. 27:2317.
58. Allen, C. A., M. F. Broom, and V. S. Chadwick. 1992. Flow cytometry analysis
of the expression of neutrophil FMLP receptors. J. Immunol. Methods 149:159.
59. Yamazaki, M., T. Matsuoka, K. Yasui, A. Komiyama, and T. Akabane. 1988.
Increased production of superoxide anion by neonatal polymorphonuclear leu-
kocytes stimulated with a chemotactic peptide. Am. J. Hematol. 27:169.
60. Smit, M. J., H. Timmerman, A. E. Alewijnse, M. Punin, I. van den Nieuwenhof,
J. Blauw, J. van Minnen, and R. Leurs. 1995. Visualization of agonist-induced in-
ternalization of histamine H2 receptors. Biochem. Biophys. Res. Commun. 214:1138.
61. Fonseca, M. I., D. C. Button, and R. D. Brown. 1995. Agonist regulation of
a
1B-adrenergic receptor subcellular distribution and function. J. Biol. Chem.
270:8902.
62. von Zastrow, M., and B. K. Kobilka. 1994. Antagonist-dependent and -indepen-
dent steps in the mechanism of adrenergic receptor internalization. J. Biol. Chem.
269:18448.
63. Van Epps, D. E., S. J. Simpson, and D. E. Chenoweth. 1992. C5a and formyl
peptide receptor regulation on human monocytes. J. Leukocyte Biol. 51:393.
64. Kikkawa, U., Y. Takai, Y. Tanaka, R. Miyake, and Y. Nishizuka. 1983. Protein
kinase C as a possible receptor protein of tumor-promoting phorbol esters. J. Biol.
Chem. 258:11442.
65. Kraft, A. S., and W. B. Anderson. 1983. Phorbol esters increase the amount of
Ca
21
, phospholipid-dependent protein kinase associated with plasma membrane.
Nature 301:621.
66. Castagna, M., Y. Takai, K. Kaibuchi, K. Sano, U. Kikkawa, and Y. Nishizuka.
1982. Direct activation of calcium-activated, phospholipid-dependent protein ki-
nase by tumor-promoting phorbol esters. J. Biol. Chem. 257:7847.
67. Beeler, J. F., and R. H. Cooper. 1995. Regulation of hepatocyte plasma mem-
brane
a
1-adrenergic receptors by 4
b
-phorbol 12-myristate 13-acetate. Biochem.
J. 305:73.
68. Schonhorn, J. E., T. Akompong, and R. Wessling. 1995. Mechanism of trans-
ferrin receptor down-regulation in K562 cells in response to protein kinase C
activation. J. Biol. Chem. 270:3698.
69. O’Flaherty, J. T., D. P. Jacobson, J. F. Redman, and A. G. Rossi. 1990. Trans-
location of protein kinase C in human polymorphonuclear neutrophils: regulation
by cytosolic Ca2(1)-independent and Ca2(1)-dependent mechanisms. J. Biol.
Chem. 265:9146.
70. Klinker, J. F., I. Schwaner, S. Offermanns, A. Hageluken, and R. Seifert. 1994.
Differential activation of dibutyryl cAMP-differentiated HL-60 human leukemia
cells by chemoattractants. Biochem. Pharmacol. 48:1857.
71. Offermanns, S. 1991. Identification of receptor-activated G proteins with photo-
reactive GTP analog, [
a
-
32
P]GTP azidoanilide. Methods Enzymol. 195:286.
72. Huey, R., and T. E. Hugli. 1985. Characterization of a C5a receptor on human
polymorphonuclear leukocytes (PMN). J. Immunol. 135:2063.
73. Mukoyama, M., M. Horiuchi, M. Nakajima, R. E. Pratt, and V. J. Dzau. 1995.
Characterization of a rat type 2 angiotensin II receptor stably expressed in 293
cells. Mol. Cell. Endocrinol. 112:61.
74. von Zastrow, M., R. Link, D. Daunt, G. Barsh, and B. Kobilka. 1993. Subtype-
specific differences in the intracellular sorting of G protein-coupled receptors. J.
Biol. Chem. 268:763.
7416 C3aR INTERNALIZATION AND ITS INHIBITION BY C5a
