Diseases featuring tissue ischemia are wide-spread and among the main cause of death
in the Western world.
1
To date, the exact mechanisms of revascularization of tissues after arterial obstruction,
usually as a result of atherosclerosis,
2
remain incompletely understood. This is underscored by the fact that pharmacological
approaches to enhance vessel growth in patients with chronic vascular occlusions are
not successfully established for clinical use, yet.
3
Several reports implicate complement activation as a contributor to ischemic tissue
injury via interaction with reactive oxygen species,
4
for example, the lectin
5
or the classical pathway.
6
7
Thus, the complement system may be a possible therapeutic target in this disease setting
(reviewed in Markiewski et al
8
).
The complement system is a very well-preserved and phylogenetically old part of the
immune system serving several functions from immune protection to tissue homeostasis
and dysfunction.
9
Previously, the complement receptor C5aR1 was demonstrated to be pro-angiogenic,
10
but has also been attributed an inhibitory role for neovascularization.
11
Interestingly, platelet activation can colocalize with areas of increased complement
activity and several functional links have been described connecting complement and
platelets.
12
13
14
Platelets play a decisive role in cardiovascular diseases featuring thrombosis, where
they are the decisive cellular part of any thrombus and contribute to several mechanisms
of thrombus formation.
15
Beyond that, platelets were recently implicated in tissue remodeling processes such
as apoptosis,
16
17
immune patrolling,
18
or adaptive immunity.
19
Furthermore, platelets contribute to the immediate response after vascular injury
by promoting vascular inflammation,
15
immunomodulation,
20
21
22
and atherosclerosis.
23
24
Recently, we could show that complement receptors are expressed on platelets, and
that the anaphylatoxin C3a receptor modulates primary hemostasis.
25
26
Moreover, platelets express C5a receptor 1 (C5aR1).
27
Absence of this anaphylatoxin receptor, however, had no effect on in vivo thrombus
formation.
25
Thus, it might be of alternative fictional relevance for platelet effects in tissue
remodeling.
The group of J. Italiano could visualize that platelets store pro- and antiangiogenic
factors in distinct granules and can release them upon stimulation.
28
Recently, we demonstrated that C5aR1-induced CXCL4 release modulates revascularization.
14
Here, we present further data on the importance of C5aR1 on platelets for the modulation
of tissue revascularization.
First, we analyzed C5aR1 expression on murine platelets (
Fig. 1A
). Then, we assessed receptor expression levels on platelets in a cohort of peripheral
artery disease patients, which were not symptomatic. Previously, this condition has
been linked to improved formation of collateral vessels, and presumably thereby lack
of pain symptoms.
29
Interestingly, we observed an increased expression of C5aR1 on platelets of patients
with an asymptomatic disease (
Fig. 1B
). This led us to investigate the platelet C5aR1 in the hindlimb ischemia model. In
this mouse model of ischemic disease, the femoral artery is ligated to induce tissue
ischemia (see
Supplementary Materials
for further details). One week after induction of tissue ischemia, the distal gastrocnemius
muscle was explanted and processed for immunofluorescence microscopy. Co-staining
of CD42b (red, “a platelet marker”) and C5aR1 (green) revealed expression of C5aR1
on DAPI (blue)-negative platelets in the ischemic tissue (
Fig. 1C
). To decipher the role of platelets in a controlled proangiogenic environment, we
used a further mouse model, the Matrigel plug assay, whereby a gel-like substance
is injected into the skin of mice forming an extracellular-space-like matrix. If the
matrix is supplemented with vascular growth factors such as bFGF, vessels grow into
the plugs, which can then be quantified. We injected Matrigel supplemented with bFGF
and freshly isolated platelets with (wild type [WT]) or without C5aR1 (C5aR1
−/−
) into C57/Bl6 mice. Addition of platelets to the Matrigel resulted in reduced bFGF-mediated
angiogenesis (
Fig. 1D
). Interestingly, presence of C5aR1 on WT platelets was associated with a reduced
extent of vessel growth into the plugs and supplementation with C5aR1
−/−
platelets resulted in significantly more revascularization (
Fig. 1E
).
Fig. 1
(
A
) Washed murine platelets were stained with a C5aR1-specific antibody or isotype control.
Displayed is a histogram representative of four independent platelet samples. (
B
) In patients with peripheral artery disease (PAD), C5aR1 expression was increased
in asymptomatic disease versus symptomatic PAD patients at Fontaine stage IIb. Shown
are representative images of 20 patients. (
C
) Immunofluorescence co-staining of ischemic murine hindlimb gastrocnemius muscle
sections at 630× magnification showing colocalization of the platelet markers CD42b
(red) and C5aR1 (green) 1 week after induction of ischemia. Representative images
confirm that the cells are anuclear (DAPI-negative, blue). Arrows point to platelets.
Scale bars represent 5 µm. (
D
) In vivo Matrigel plug assay experiments were performed using WT mice. Matrigel was
supplemented with bFGF and freshly isolated platelets from WT mice or vehicle control.
After 7 days, Matrigels were explanted, and vessel density way assessed by H&E staining.
We observed decreased vessel growth after addition of isolated platelets. Data are
displayed as % of control.
n
= 6–7 plugs were analyzed. The group with vehicle control represents 100%. *
p
< 0.05. (
E
) Similarly, Matrigel was supplemented with bFGF and freshly isolated platelets from
WT or C5aR1
−/−
mice were additionally injected. After 7 days, Matrigels were explanted, and vessel
density was assessed by H&E staining. We observed increased vessel growth in the absence
of the C5aR1 on platelets. Data are displayed as % of control.
n
= 7 plugs were analyzed. Data are displayed as % of control. The group with WT mice
represents 100%. *
p
< 0.05. (
F
) Mice bearing a platelet-specific knockout of C5aR1 on platelets were generated (PF4-cre
+
C5aR1
fl/fl
). PF4-cre
+
C5aR1
fl/fl
and PF4-cre
−
C5aR1
fl/fl
mice were subjected to hindlimb ischemia and analyzed over 2 weeks. Revascularization
of the hindlimbs after femoral artery ligation was visualized by laser Doppler fluximetry
(LDI). We found increased revascularization in PF4-cre
+
C5aR1
fl/fl
compared with PF4-cre
−
C5aR1
fl/fl
animals, although this reached statistical significance only at some time points.
Data are shown as the mean ± SEM (
n
= 3 animals per group) and are displayed as % of the perfusion in the contralateral
control limb. *
p
< 0.05. (
G
) As prior experiments suggested a platelet-mediated mechanism of C5aR1 control of
revascularization, we stimulated washed human platelets with C5a. The supernatant
was analyzed by ELISA for the level of CXCL4. Preincubation with the C5aR1 inhibitor
PMX205 inhibited C5a-induced CXCL4 secretion. Data are shown as the mean ± SEM.
n
= 4–8. The group with WT mice represents 100%. *
p
< 0.05. (
H
) Platelets from WT mice were stimulated with C5a or control and preincubated with
PMX205 or control. The supernatant was co-incubated with endothelial cells (MHEC-5T).
In some groups, an anti-CXCL4 antibody was added to the endothelial cells. C5a-stimulated
WT platelet supernatant inhibited endothelial tube formation, which was not detectable
when C5aR1 or CXCL4 was inhibited. Data represent mean ± SEM of the total tube length.
Data are displayed as % of control and the group with control stimulated supernatant
treatment represents 100%.
n
= 10–11. *
p
< 0.05. (
I
) Representative images of endothelial tube formation show the inhibitory effect of
C5a-conditioned platelet supernatant compared with control. SEM, standard error of
the mean; WT, wild type.
We then created a platelet-specific C5aR1-deficient mouse strain using the cre-lox
system (for details please refer to the Material and Methods section in the
Supplementary Information
14
). We assessed platelet reactivity by stimulating diluted whole blood with collagen-related
protein and measured the platelet marker β
3
integrin (GPIIIa, CD61), which mediates binding to fibrinogen. There was no significant
difference in between both strains (
Supplementary Fig. S1A
), suggesting that there are no effects on the fibrinogen receptor. However, addition
of C5a induced some activation in platelets on the level of fibrinogen binding and
activated GPIIbIIIa (measured by activation-specific PAC1 antibody), but not on P-selectin
upregulation (
Supplementary Fig. S2
). These effects could be blocked by the C5aR1 antagonist PMX205 (
Supplementary Fig. S3
). In the absence of C5a ligation, WT and C5aR1
−/−
platelets did not display differences in binding to fibrinogen (
Supplementary Fig. S4
). We monitored revascularization over 14 days post induction of hindlimb ischemia
and observed that the C5aR1
flox/flox
PF4cre
−
mice show a lower degree of revascularization than the C5aR1
flox/flox
PF4cre
+
littermates, where C5aR1 is absent in platelets (
Fig. 1F
). Thus, the platelet C5aR1 is an important modulator of revascularization.
As all previous data suggest a platelet-mediated effect of C5aR1 on platelets, we
stimulated platelets from WT mice with C5a in vitro. Using an ELISA-based analysis,
we could measure a release of antiangiogenic CXCL4 (platelet factor 4, PF4) from platelets
upon C5a stimulation, which could not be observed when C5aR1 was blocked by the C5aR1
antagonist PMX205 (
Fig. 1G
). To verify that the observed effect is indeed mediated through CXCL4 secreted from
platelets, we assessed endothelial tube formation in vitro and co-incubated cells
with C5a-conditioned platelet supernatant (
Fig. 1H, I
). Confirming our hypothesis, we could show that C5a-conditioned platelet supernatant
inhibited endothelial tube formation, which could be blocked by administering the
C5aR1 antagonist PMX205 (
Fig. 1H
). Interestingly, a neutralizing antibody against CXCL4 did not have an additive effect
on top of PMX205 in reversing the effect of C5a-conditioned platelet supernatant (
Fig. 1H
). As the collagen pathway of platelet activation has gained recent attention,
30
we excluded involvement of glycoprotein VI in C5a-mediated release of CXCL4 from platelets
(
Supplementary Fig. S5
). Together, we demonstrate that C5a-mediated release of CXCL4 is one mechanism how
platelets modulate vessel growth.
Here, we identified a novel mechanism for inhibition of neovascularization via CXCL4
secretion induced by platelet C5aR1. Targeting the complement system may offer novel
approaches to treat patients with diseases featuring tissue ischemia and inflammation.
These diseases are largely caused by atherosclerosis.
2
Interestingly, the anaphylatoxin receptors C5aR1 and C3aR are expressed in atherosclerotic
plaques.
31
Importantly, in patients with atherosclerosis, expression of C3aR and C5aR shows a
significant correlation with platelet activation markers.
25
27
32
In a mouse model, anti-C5aR1 compounds have already proven effective in enhancing
revascularization.
14
The potential for the treatment of human patients, however, will first have to be
evaluated in future clinical studies.
Recently, the C5a–C5aR1 axis was suggested to be an important marker of inflammation
associated with COVID-19.
33
Interestingly, the platelet C5aR1 has been recognized as an important player in the
complex pathophysiology of SARS-CoV-2 infection.
34
Thus, evaluating the C5aR1–CXCL4 axis in this context may improve our understanding
of disease mechanisms and provide new treatment options for COVID-19 and other diseases
featuring thromboinflammation and complement involvement.
Complement-active drugs are available and already in use.
35
For instance, eculizumab, which inhibits the cleavage of C5 by the C5 convertase into
C5a, has been clinically established for the treatment of aHUS (atypical hemolytic
uremic syndrome) and for PNH (paroxysmal nocturnal hemoglobinuria).
36
Indeed, several novel pharmacological approaches have recently been introduced, i.e.,
Pegcetacoplan
37
or Avacopan,
38
a C5aR1 inhibitor, to the clinic. In light of our findings, implications of treatment
regimen for vascular remodeling and regeneration should be assessed in future studies
and should be taken into account when designing trials on complement therapeutics.
In conclusion, understanding the crosstalk of platelets with the complement system
is important to apprehend the exact role of this interplay for platelet and complement
activation and resulting diseases featuring thromboinflammation.