In this study, we explored the effect of a C3 complement inhibitor, CRIg/FH, on lupus nephropathy using the widely used MRL/lpr lupus mice. Our results showed that CRIg/FH was able to protect in MRL/lpr mice from nephropathy as presenting lower level of proteinuria, improved renal function and serum immunological markers and minor nephritis.
The complement system actively participates in the pathogenesis of lupus nephropathy. The lupus nephropathy is initiated by the nephrotic deposition of abnormal circulating immune complexes (CIC) formed by autoantibodies against free nucleosomes [
18]. After CIC inducing complement cascade reaction through classical pathway, the proliferation and activation of glomerular mesangial cells was triggered to form glomerular diseases [
19]. The initial activation and further exaggerated complement activation by alternative pathway together contributed to the pathogenesis of overt nephritis in lupus [
20]. It was found that monoclonal antibody against C5 effectively improved lupus nephropathy in both lupus mice [
21] and patients with lupus nephropathy presenting low levels of serum complement [
22]. The C5 blockers which acts effectively against the formation of MAC therefore is currently available and widely used in practice to treat autoimmune diseases [
23]. Nevertheless, the complement inhibiting effect of C5 blockers is often extensive and systematic, resulting in elevated risk of immunosuppression and opportunistic infections [
24]. Although recently study showed that C3 blocker effectively improved proteinuria and preserved renal function in lupus mice [
12], despite its targeting earlier phases in the complement activation, potentency of C3 blocker to broadly suppress complement activation still raised similar concerns [
12]. The CRIg/FH is a fusion protein combining the extracellular domain of CRIg (C3b/iC3b/C3c binding domain) and the alternative pathway inhibitory domain of factor H (SCR1–5) [
14]. The combination of the two domains was shown to not only bind to the C3 digestion product C3b/iC3b/C3c and subsequent reduce activation of classic pathway, but also inhibit the activation of alternative pathway [
15,
16]. The effect of CRIg/FH against complement overactivation induced nephritis was first recognized in rat model of mesangioproliferative glomerulonephropathy (MPGN) [
14]. The ability of CRIg/FH to inhibit classical pathway and remove C3b/iC3b deposition on cell surface and protecting glomerular mesangial cells raised the possibility of its treatment effect in other complement activation related nephritis [
14]. Additional evidence showed that the binding between CRIg/FH and C3b subunit on the surface of macrophages was able to place FH domains to participant and inhibit alternative pathway [
16,
25,
26]. In this study, we found that the administration of complement inhibitor CRIg/FH can reduce the kidney injury in MRL/lpr mice in a dose related manner. Through serum complement profiling, we showed that CRIg/FH prevents the consumption of C3 and C4, suggesting that CRIg/FH act through the inhibition of complement cascade.
The effect of CRIg/FH on lupus nerphritis was confirm by renal pathology and immunofluorescence of the lupus mice. CRIg/FH was able to maintain normal glomerulus morphology and inhibiting levels of local C3 and C4 however not CIC. Interestingly, our results showed that the A-ds-DNA titer decreased in lupus mice treated with CRIg/FH. Although the complement inhibitor does not have directly effect on adaptive immune system, there are preliminary data showing that complement inhibition contributed to the relief of disease activity of lupus in both human subjects and MRL/lpr mice model [
27,
28]. Despite its detailed mechanism is currently unexploited, it was suggested that the clinical value of the drug in treating SLE can act beyond the complement system.