Binding of Flavivirus Nonstructural Protein NS1 to C4b Binding Protein Modulates Complement Activation

Abstract

The complement system plays a pivotal protective role in the innate immune response to many pathogens including Flaviviruses. Flavivirus NS1 is a secreted non-structural glycoprotein that accumulates in plasma to high levels and is displayed on the surface of infected cells but is absent from viral particles. Previous work has defined an immune evasion role of Flavivirus NS1 in limiting complement activation by forming a complex with C1s and C4 to promote cleavage of C4 to C4b. Here, we demonstrate a second mechanism, also involving C4 and its active fragment C4b, by which NS1 antagonizes complement activation. Dengue, West Nile or yellow fever virus NS1 directly associated with C4b binding protein (C4BP), a complement regulatory plasma protein that attenuates the classical and lectin pathways. Soluble NS1 recruited C4BP to inactivate C4b in solution and on the plasma membrane. Mapping studies revealed that the interaction sites of NS1 on C4BP partially overlap with the C4b binding sites. Together, these studies further define the immune evasion potential of NS1 in reducing the functional capacity of C4 in complement activation and control of Flavivirus infection.

Dengue Virus NS1 Protein Modulates Cellular Energy Metabolism by Increasing Glyceraldehyde-3-Phosphate Dehydrogenase Activity

“…Most studies have focused on the extracellular interactions of the NS1 protein to explain its involvement in DENV pathogenesis (10)(11)(12). Here, we propose that iNS1 increases glycolytic flux and, consequently, energy production by increasing GAPDH glycolytic activity.…”

“…Secreted NS1 (sNS1) also plays a role in both DENV pathogenesis and protection. It binds several complement components and its regulators, contributing directly to viral immune evasion (7)(8)(9)(10)(11)(12).…”

A Novel Interaction between Complement Inhibitor C4b-binding Protein and Plasminogen That Enhances Plasminogen Activation

“…The polyclonal rabbit anti-human C4BP 9008 Abs were a kind gift of Prof. B. Dahlbäck (Lund University, Sweden). This antibody recognizes all forms of C4BP (plasma purified C4BP-PS, rC4BP, and mutants) equally well (25). Human Glu-plasminogen and polyclonal sheep anti-human plasminogen Abs were purchased from Hematologic Technologies.…”

The contribution of rodent models to the pathological assessment of flaviviral infections of the central nervous system

“…Ablation of frameshifting by disrupting base pairing in a pseudoknot predicted to dictate polymerase slippage was demonstrated to reduce murine neuroinvasiveness of Kunjin virus, indicating the involvement of this extension protein as a flavivirus virulence determinant [101]. Interestingly, NS1 has been associated with disregulation of the host complement cascade [7] as well as TLR3-mediated innate immune responses [192]. Whether the contribution of NS1' to facilitating neuroinvasion emanates from a similar antagonism of host innate immunity remains to be determined.…”