Complement pathways as potential targets
The available data suggest that the LP and the CP are the two major
complement pathways activated in response to SARS-CoV-2. MBL, collectin
10 and -11 as well as the ficolins 1-3 function as soluble sensors of
the LP (Garred et al., 2016). They interact with the serine protases
MASP1 and MASP2 that act in concert to cleave C4 and C2 and form the C3
convertase C4b2b which proteolytically cleave C3 into C3a and C3b
(Figure 1). MBL comprises multiple carbohydrate recognition domains that
can bind to the high-mannose structure of the SARS-CoV S protein (Ip et
al., 2005). In particular, the N-linked glycosylation site, N330, on the
S protein seems to be critical for MBL interaction. Further, MBL
directly inhibited SARS-CoV-mediated infection in vitro (Zhou et al.,
2010). In addition to the S protein, a recent report uncovered the
interaction of LP MASP-2 with a highly conserved motif in the N protein
of SARS-CoV, SARS-CoV-2 and MERS-CoV. Intriguingly, this interaction not
only potentiated MASP-2-driven activation of the LP but aggravated
LPS-induced pneumonia in a MASP-2-dependent way (Gao et al., 2020).
Given that the N-protein is one of the most abundant structural proteins
in the serum of patients infected by SARS-CoV (Che et al., 2004; Chen et
al., 2005; Guan, Chen, Foo, Tan, Goh & Wee, 2004) these data indicate
that the N protein-mediated interaction with MASP-2 could serve as an
important amplifier of LP activation in highly pathogenic coronavirus
infection. Thus, the available data point toward the LP as an important
target in these virus infections.
C1q is the sensor molecule of the CP that recognizes multiple conserved
molecular patterns including IgM or IgG hexamer molecules that have
bound to their cognate antigen. Similar to MASP-1 and MASP-2, the serine
proteases C1r and C1s form a complex with IgG/IgM-bound C1q to cleave C4
and C2 and generate the C4b2b convertase eventually cleaving C3 into C3a
and C3b (Figure 1). Within the first week after symptoms, COVID-19
infection results in the production of S protein-specific IgG/IgM
antibodies as a target structure for C1q (Wolfel et al., 2020). A recent
study from Wuhan shows that during the first 5 days after clinical
onset, already 30-40% of the infected individuals have generated IgM or
IgG antibodies directed against the S or N proteins of SARS-CoV-2, with
a slightly higher frequency of antibodies against the S protein (Liu et
al., 2020). After 5 weeks, all of the 214 tested patients showed IgG
seroconversion. These data suggest that the seroconversion is somewhat
quicker than what has been observed with SARS-CoV (Peiris et al., 2003).
Taken together, CP activation by IgM and IgG antibodies directed against
the S and N proteins of SARS-CoV and SARS-CoV-2 serves as second
mechanism of complement activation in addition to the initial virus
sensing by the MBL and LP activation (Figure 1).
The C3 convertase C4b2b, which is assembled in response to LP and CP
activation generates the AT C3a from C3, which can degranulate basophils
and mast cells leading to histamine release through activation of its
cognate C3aR (el-Lati, Dahinden & Church, 1994; Kretzschmar et al.,
1993). C3aR expression is triggered in neutrophils upon LPS exposure and
contributes to NETosis (Guglietta et al., 2016). Further, C3a induces
aggregation and serotonin release from platelets, regulates secretion of
IL-6 and TNF-α from B-cells and monocytes and leads to the production of
IL-8 by an epithelial cell line (Fischer & Hugli, 1997; Fischer, Jagels
& Hugli, 1999; Fukuoka & Hugli, 1988). Intracellularly, C3a plays an
important role in activating the NLRP3-inflammasome in human monocytes
(Asgari et al., 2013). Taken together, C3a can mediate a strong
pro-inflammatory environment. The C3 convertase also serves as the
nucleus of the C5 convertase when C3b molecules form a complex with
C4b2b resulting in C4b2b3b, the C5 convertase of the LP and the CP which
cleaves C5 into the AT C5a and C5b (Figure 1) (Ekdahl et al., 2019).
Similar to C3a and in concert with C3a, C5a can drive a strong
pro-inflammatory environment through its strong chemotactic properties
on neutrophils, monocytes, eosinophils, basophils, mast cell and
dendritic cells (DC) and it strong potency to activate such cells to
release ROS, lysosomal enzymes as well as pro-inflammatory cytokines
such as IL-1β, TNF-α, IL-6 and chemokines of the CC and CXC families
(Figure 2). C5a also drives the activation and differentiation of T
cells through DC maturation downstream of C5aR1 (Antoniou et al., 2020;
Weaver et al., 2010). Also, C5a controls histamine-induced increase in
vasopermeability (Kordowski et al., 2019) and drives the production of
metabolites of the arachidonic acid lipoxgenase and cyclooxygenase
pathways resulting in increased production of leukotriene B4 (LTB4) or
prostaglandine E2, both of which increase vasopermeability (Karasu,
Nilsson, Kohl, Lambris & Huber-Lang, 2019; Klos, Tenner, Johswich,
Ager, Reis & Kohl, 2009).
Importantly, such increased vasopermeability of the alveolar-capillary
interface, massive neutrophil recruitment associated with tremendous
production of proinflammatory cytokines and chemokines including IL-1β,
IL-6, IFN-γ, IL-8, CXCL10 and CCL2 leading to the development of
ALI/ARDS has been observed in infections with highly pathogenic
respiratory viruses including H5N1 influenza, SARS-CoV and MERS-CoV
(Jiang et al., 2019; Jiang et al., 2018). In experimental models as well
as in patients infected with influenza (Ohta, Torii, Imai, Kimura, Okada
& Ito, 2011), SARS-CoV (Gralinski et al., 2018), MERS-CoV (Jiang et
al., 2018) or SARS-CoV-2 (Gao et al., 2020; Magro et al., 2020)
increased blood levels and/or lung deposits of complement activation
products have been described.
Taken together, the picture emerges that highly pathogenic coronaviruses
activate complement by the LP and CP. This activation drives the
generation of huge amounts of the highly proinflammatory cleavage
products C3a and C5a, when complement activation is not sufficiently
controlled by complement regulator proteins (Figure 3). Further, TMA
results in C3 and C5 cleavage of by non-canonical pathways through
serine proteases located in the intracellular space of in the
vasculature that exert promiscuous enzyme activity (Figure 2) (Ekdahl et
al., 2016). Below, we will discuss strategies to prevent the initiation
of LP and CP, to attenuate convertase-mediated amplification and to
inhibit the effector functions of C5a/C5aR1 axis activation.