Discussion
All three complement activation pathways converge at the generation of
C3 convertase that cleaves C3 into anaphylatoxin C3a and opsonin C3b
[18]. The association of C3b with the C3 convertase results in the
formation of a C5 convertase cleaving C5 into anaphylatoxins C5a and
C5b. This cleavage triggers the terminal complement cascade leading to
the assembly of MAC (C5b-9). In our and previous studies, the terminal
pathway components levels, that is C5a and MAC, were elevated in
patients with active AAV compared to healthy controls. This finding
indicates that complement activation occurs in the course of systemic
vasculitis. In our patients, effective remission induction treatment was
associated with a decrease in C5a levels, whereas MAC levels did not
change. Nevertheless, the meta-analysis showed that both C5a and MAC
levels in patients with remission of AAV were lower compared to patients
with active disease. Remission induction therapy also resulted in a
decrease in C3a levels. Activation of complement in AAV occurred
irrespective of ANCA-serotype, severity of disease or previous
immunosuppression, since we detected no difference between C5a and MAC
levels in the groups of patients with MPO-ANCA or PR3-ANCA vasculitis,
active MPA or GPA, severe or non-severe vasculitis, newly diagnosed or
relapsing disease, predominant granulomatous or vasculitic or mixed
disease, immunosuppressive-naïve or previously immunosuppressed
patients. C5a and MAC levels did not correlate with BVAS, proteinuria,
serum creatinine or laboratory markers of inflammation and were
increased only in a proportion of patients with active AAV. Of note, C5a
concentrations exceeded the upper reference level only in a quarter of
patients who showed clinical signs of active AAV. Therefore, analysis of
both complement components alone does not allow for distinction between
active disease and remission.
We also measured factor B and properdin levels that are components of
the alternative complement pathways. Factor B can be cleaved by factor D
into Ba and Bb and is necessary for the formation of the alternative
pathway C3 convertase (C3bBb), whereas properdin increases the half-life
of the convertase activity and promotes constant cleavage of C3 into C3a
and C3b. Median factor B levels were elevated in patients with active
AAV compared to healthy controls and decreased after remission. However,
we detected no difference between median properdin levels in AAV patents
and healthy volunteers. These data were confirmed by our meta-analysis.
Deposition of Bb in glomeruli of patients with AAV correlated with the
proportion of crescents, the extent of interstitial infiltrates,
interstitial fibrosis and tubular atrophy, and inversely with the
proportion of normal glomeruli [19]. Moreover, plasma Bb
concentrations correlated with common pathway components levels (C3a,
C5a, and MAC), clinical and laboratory signs of vasculitis activity
(BVAS, erythrocyte sedimentation rate (ESR)), and the proportion of
total and cellular crescents in kidney biopsies [8]. These findings
indicate that circulating Bb levels may reflect both systemic and renal
disease activity of AAV. In another study that analyzed 187 renal biopsy
samples from patients with AAV, properdin staining was associated with
the proportion of cellular crescents, and the presence of properdin
correlated with the level of proteinuria [5]. In our study, factor B
or properdin levels did not correlate with proteinuria, serum
creatinine, BVAS or laboratory markers of inflammation (ESR, C-reactive
protein). In a small group of patients with kidney biopsies, factor B
levels correlated with C3 deposition in the glomeruli, whereas all other
correlations between complement components and histological parameters
were not significant.
A crucial role of complement activation in AAV pathogenesis makes
targeting complement components an attractive therapeutic strategy.
Currently, two C5a inhibitors are in clinical development for AAV:
avacopan, an oral C5a receptor (C5aR) inhibitor, and IFX-1, a monoclonal
antibody to C5a [20]. Efficacy, safety and steroid-sparing effects
of avacopan in patients with GPA/MPA were shown in two phase II trials,
CLEAR and CLASSIC [20, 21], whereas IFX-1 has entered phase II
development. The positive results of the phase III trial of avacopan,
ADVOCATE, in AAV patients were recently announced, though not published
yet. C5a also interacts with C5L2 receptors that compete with C5aR for
binding of anaphylotoxins and, therefore, may have anti-inflammatory
effects. Knockout of C5L2 in mice resulted in a more severe MPO-ANCA
associated glomerulonephritis [4]. On the contrary, C5L2 was
upregulated in glomeruli in patients with ANCA-associated
glomerulonephritis [7]. These contradictory experimental and
clinical data suggest that C5L2 may have different roles and functions
in various systems, cells, tissues, organs, and/or species [22].
A major limitation of our study is the relatively small sample of
patients, particularly studied sequentially both prior to and after
achievement of remission. However, our findings are in accordance with
previous studies and were reinforced by the meta-analysis of all
available data on activation of the complement system in AAV.
In summary, our findings and the results of the meta-analysis provided
additional evidence for activation of the complement system via the
alternative pathway in AAV. Signs of complement activation were found
only in a proportion of patients with active AAV, whereas achievement of
clinical remission following immunosuppressive therapy did not always
lead to normalization of various complement component levels. Therefore,
we cannot conclude that any plasma complement component alone may be
useful as a biomarker of the disease activity or a guide for treatment
decisions. However, studies of the complement system in AAV and other
autoimmune diseases pave the way to new drugs development and can turn
the dream of steroid-free regimens into a reality. In addition,
investigations of key signaling pathways and molecules (i.e., macrophage
migration inhibitory factor, sphingosine-1-phosphate, high mobility
group box 1) of C5a-mediated neutrophil priming and activation by ANCA
may provide new insights into the complex AAV pathogenesis.