Abstract
The complement system is an efficient anti-microbial effector mechanism. On the other hand abnormal complement activation is involved in the pathogenesis of multiple inflammatory and hemolytic diseases. As general inhibition of the complement system may jeopardize patient health due to increased susceptibility to infections, the development of pathway-specific complement therapeutics has been a long-lasting goal over the last decades. In particular, pathogen mimicry has been considered as a promising approach for the design of selective anti-complement drugs. The C-terminal domain of staphylococcal superantigen-like protein 7 (SSL7), a protein secreted by Staphylococcus aureus, was recently found to be a specific inhibitor of the terminal pathway of the complement system, providing selective inhibition of cell lysis mediated by the membrane attack complex (MAC). We describe here the selection by phage display of a humanized single-domain antibody (sdAb) mimicking the C-terminal domain of SSL7. The antibody, called sdAb_E4, binds complement C5 with an affinity in the low micromolar range. Furthermore, sdAb_E4 induces selective inhibition of MAC-mediated lysis, allowing inhibition of red blood cell hemolysis and inhibition of complement deposition on apopto-necrotic cells, while maintaining efficient bactericidal activity of the complement terminal pathway. Finally, we present preliminary results indicating that sdAb_E4 may also be efficient in inhibiting hemolysis of erythrocytes from patients with paroxysmal nocturnal hemoglobinuria. Our data provide a proof of concept for the design of a selective MAC inhibitor capable of retaining complement bacteriolytic activity and this study opens up promising perspectives for the development of an sdAb_E4-derived therapeutics with application in the treatment of complement-mediated hemolytic disorders.
Highlights
Upon detection of pathogenic or damaged cells in our body, the complement cascade of the innate immunity is activated
We hypothesized that in order to obtain an antibody with the same properties as staphylococcal superantigen-like protein 7 (SSL7)-CT, i.e., selective inhibition of the membrane attack complex (MAC), we should search for an antibody recognizing the same epitope as SSL7 on C5
As our structure of the CVF:C5:SSL7 complex revealed that SSL7 interacts with cobra venom factor (CVF), we predicted that with a conventional IgG antibody mimicking SSL7, there would be a significant risk of interfering with C5 binding to the C3b or C4b moiety in the C5 convertases mimicked by CVF in our structure [40]
Summary
Upon detection of pathogenic or damaged cells in our body, the complement cascade of the innate immunity is activated It results in a complex chain of proteolytic reactions that leads to cleavage of the complement factor C5 and initiation of the terminal pathway (TP) of the complement system [1, 2]. This cleavage generates the small C5a anaphylatoxin, which triggers the inflammatory response, and the large C5b fragment, which recruits complement proteins C6-C9 to form the membrane attack complex (MAC) responsible for the lysis of the targeted cells [3,4,5] Whereas this process is crucial for host defense upon pathogenic invasion, it has to be tightly regulated to prevent aberrant complement activation in non-infected areas and the ensuing risk of damaging healthy tissues. Dysregulation of the complement system, often linked to genetic deficiencies in these regulatory factors, can lead to various pathological conditions, including inflammatory and hemolytic disorders that result from an excessive C5a production and/or the targeting of host red blood cells by the MAC [1, 4, 8, 9]
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