Circulating levels of endogenous complement inhibitors correlate inversely with complement consumption in systemic lupus erythematosus.

Systemic lupus erythematosus (SLE) is marked by excessive complement activation, contributing to tissue damage. Complement activation can be detected in many organs including the skin, kidney, and brain. The involvement of the central nervous system is particularly relevant to understanding neuropsychiatric SLE (NPSLE), one of the poorest understood manifestations of SLE for which no biomarkers are available. We studied the levels of complement inhibitors in SLE in relation to disease activity and as possible biomarkers to identify NPSLE. Serum levels of complement inhibitors C1-inhibitor (C1-INH), C4b-binding protein (C4BP), Factor I, and Factor H were measured in 345 SLE patients (including 102 with NPSLE) and 108 healthy controls. Compared with controls, SLE patients had higher C1-INH and C4BP but lower Factor I and H levels. All inhibitors positively correlated with total C3 and C4 levels. While correlating with the SLE Disease Activity Index (SLEDAI), no distinction in inhibitor levels was found between SLE and NPSLE patients. Over time, C1-INH and Factor H levels normalized, but no significant changes were observed for C4BP and Factor I. In SLE the levels of circulating complement inhibitors are inversely correlated to complement consumption but do not serve as biomarkers for NPSLE.

DNA Nanostructure-Templated Antibody Complexes Provide Insights into the Geometric Requirements of Human Complement Cascade Activation.

The classical complement pathway is activated by antigen-bound IgG antibodies. Monomeric IgG must oligomerize to activate complement via the hexameric C1q complex, and hexamerizing mutants of IgG appear as promising therapeutic candidates. However, structural data have shown that it is not necessary to bind all six C1q arms to initiate complement, revealing a symmetry mismatch between C1 and the hexameric IgG complex that has not been adequately explained. Here, we use DNA nanotechnology to produce specific nanostructures to template antigens and thereby spatially control IgG valency. These DNA-nanotemplated IgG complexes can activate complement on cell-mimetic lipid membranes, which enabled us to determine the effect of IgG valency on complement activation without the requirement to mutate antibodies. We investigated this using biophysical assays together with 3D cryo-electron tomography. Our data revealed the importance of interantigen distance on antibody-mediated complement activation, and that the cleavage of complement component C4 by the C1 complex is proportional to the number of ideally spaced antigens. Increased IgG valency also translated to better terminal pathway activation and membrane attack complex formation. Together, these data provide insights into how nanopatterning antigen-antibody complexes influence the activation of the C1 complex and suggest routes to modulate complement activation by antibody engineering. Furthermore, to our knowledge, this is the first time DNA nanotechnology has been used to study the activation of the complement system.

Targeted complement inhibition using bispecific antibodies that bind local antigens and endogenous complement regulators.

Complement activation protects against infection but also contributes to pathological mechanisms in a range of clinical conditions such as autoimmune diseases and transplant rejection. Complement-inhibitory drugs, either approved or in development, usually act systemically, thereby increasing the risk for infections. We therefore envisioned a novel class of bispecific antibodies (bsAbs) which are capable of site-directed complement inhibition by bringing endogenous complement regulators in the vicinity of defined cell surface antigens. Here, we analyzed a comprehensive set of obligate bsAbs designed to crosslink a specific target with either complement regulator factor H (FH) or C4b-binding protein (C4BP). The bsAbs were assessed for their capacity to inhibit complement activation and cell lysis in an antigen-targeted manner. We observed that the bsAbs inhibited classical, lectin, and alternative pathway complement activation in which sufficient endogenous serum FH and C4BP could be recruited to achieve local inhibition. Importantly, the bsAbs effectively protected antigen-positive liposomes, erythrocytes, and human leukocytes from complement-mediated lysis. In conclusion, localized complement inhibition by bsAbs capable of recruiting endogenous human complement regulators (such as FH or C4BP) to cell surfaces potentially provides a novel therapeutic approach for the targeted treatment of complement-mediated diseases.