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The complement system is an intricate cascade of the innate immune system and plays a key role in microbial defense, inflammation, organ development, and tissue regeneration. There is increasing interest in developing complement regulatory and inhibitory agents to treat complement dysfunction. In this study, we describe the nanobody hC3Nb3, which is specific for the C-terminal C345c domain of human and mouse complement component C3/C3b/C3c and potently inhibits C3 cleavage by the alternative pathway. A high-resolution structure of the hC3Nb3-C345c complex explains how the nanobody blocks proconvertase assembly. Surprisingly, although the nanobody does not affect classical pathway-mediated C3 cleavage, hC3Nb3 inhibits classical pathway-driven hemolysis, suggesting that the C-terminal domain of C3b has an important function in classical pathway C5 convertase activity. The hC3Nb3 nanobody binds C3 with low nanomolar affinity in an SDS-resistant complex, and the nanobody is demonstrated to be a powerful reagent for C3 detection in immunohistochemistry and flow cytometry. Overall, the hC3Nb3 nanobody represents a potent inhibitor of both the alternative pathway and the terminal pathway, with possible applications in complement research, diagnostics, and therapeutics. [ABSTRACT FROM AUTHOR]

The classical and lectin pathways of the complement system are important for the elimination of pathogens and apoptotic cells and stimulation of the adaptive immune system. Upon activation of these pathways, complement component C4 is proteolytically cleaved, and the major product C4b is deposited on the activator, enabling assembly of a C3 convertase and downstream alternative pathway amplification. Although excessive activation of the lectin and classical pathways contributes to multiple autoimmune and inflammatory diseases and overexpression of a C4 isoform has recently been linked to schizophrenia, a C4 inhibitor and structural characterization of the convertase formed by C4b is lacking. In this study, we present the nanobody hC4Nb8 that binds with picomolar affinity to human C4b and potently inhibits in vitro complement C3 deposition through the classical and lectin pathways in human serum and in mouse serum. The crystal structure of the C4b:hC4Nb8 complex and a threedimensional reconstruction of the C4bC2 proconvertase obtained by electron microscopy together rationalize how hC4Nb8 prevents proconvertase assembly through recognition of a neoepitope exposed in C4b and reveals a unique C2 conformation compared with the alternative pathway proconvertase. On human induced pluripotent stem cell-derived neurons, the nanobody prevents C3 deposition through the classical pathway. Furthermore, hC4Nb8 inhibits the classical pathway-mediated immune complex delivery to follicular dendritic cells in vivo. The hC4Nb8 represents a novel ultrahigh-affinity inhibitor of the classical and lectin pathways of the complement cascade under both in vitro and in vivo conditions. [ABSTRACT FROM AUTHOR]

Pre-exposure prophylaxis (PrEP) is an effective but understudied tool for preventing HIV among women who inject drugs (WWID). This article is the first to explore WWID's rationale for PrEP initiation (or refusal) in a real-world setting. Purposive sampling was used to recruit 25 WWID, participating in a PrEP demonstration project operating within a syringe services program, based on whether they initiated or declined PrEP care. Content analysis of qualitative interviews was used to explore decisions to initiate PrEP (or not). We found that WWID view HIV as severe, perceive themselves to be susceptible to HIV, and believe PrEP is beneficial for HIV prevention. For some, however, real and perceived barriers outweighed benefits, leading to decisions not to initiate PrEP. Barriers included HIV stigma, fear of side effects, and needing assurance that PrEP care will be available long-term. Despite viewing PrEP as an important HIV prevention tool, not all WWID who were offered PrEP initiated it. For these women, supports to buffer perceived barriers to initiation and access to post-exposure prophylaxis may be warranted. For women who initiate, it is possible that adherence will wane if perceived risk does not remain high. Research to understand PrEP persistence is needed. [ABSTRACT FROM AUTHOR]