West Nile virus nonstructural protein NS1 inhibits complement activation by binding the regulatory protein factor H

Abstract

The complement system, by virtue of its dual effector and priming functions, is a major host defense against pathogens. Flavivirus nonstructural protein (NS)-1 has been speculated to have immune evasion activity, because it is a secreted glycoprotein, binds back to cell surfaces, and accumulates to high levels in the serum of infected patients. Herein, we demonstrate an immunomodulatory function of West Nile virus NS1. Soluble and cell-surface-associated NS1 binds to and recruits the complement regulatory protein factor H, resulting in decreased complement activation in solution and attenuated deposition of C3 fragments and C5b-9 membrane attack complexes on cell surfaces. Accordingly, extracellular NS1 may function to minimize immune system targeting of West Nile virus by decreasing complement recognition of infected cells.flavivirus ͉ immune evasion ͉ innate immunity ͉ pathogenesis

Flavivirus nonstructural protein NS1: Complementary surprises

“…Suspicion that the protein might be a ligand for NS1 led to its identification as bovine complement fH, and further tests revealed the same propensity of NS1 to bind to human fH. After confirming that fH bound to NS1, Chung et al (9) established that the NS1-fH complex accelerated C3b digestion by fI in solution and that NS1 had no intrinsic cofactor activity. They then demonstrated the very same effect on Chinese hamster ovary (CHO) cells that displayed recombinant NS1 in amounts equivalent to those found on WNV-infected cells.…”

“…The finding by Chung et al (9) that fH binding by a viral protein (WNV NS1) interferes with C3b function has not been previously recognized as a mechanism of immune evasion by any viruses. It will be immediately important to find out whether their discovery applies to other flavivirus family members, especially in cell types known to support replication of specific flaviviruses in vivo, such as cells of nervous or liver tissue, and cells of monocyte/macrophage lineage that are likely to be the earliest site of flavivirus replication after insect bite.…”

“…In view of the large amounts of circulating NS1 early in flavivirus infection, it also would be interesting to see whether soluble NS1 deposited on uninfected cells will behave in the same fashion with respect to fH as NS1 displayed on transfected (or naturally infected) cells. If so, NS1 would then appear to perform double duty by enhancing infection of flavivirus-susceptible cells (16) and then, as the results of Chung et al (9) might suggest, by defending them against the action of activated complement. A key question that arises from their findings is how they might relate to NS1 as the target of protective antibodies.…”

“…Remarkably, active immunization with YFV NS1 also protected monkeys against classic yellow fever (8). The work of Chung et al (9) in this issue of PNAS brings us yet another surprise about NS1 that may enhance our knowledge of flavivirus pathogenesis and immunity: WNV NS1 is reported to exhibit an immunomodulatory function by regulating complement activity.…”

Last 20 aa of the West Nile virus NS1′ protein are responsible for its retention in cells and the formation of unique heat-stable dimers

“…Produced as a single polyprotein, it is cleaved by both host and viral proteases to give rise to the individual proteins (Lindenbach & Rice, 2003). NS1 is a multifunctional glycoprotein involved in the formation of the replication complex (Chu & Westaway, 1992;Khromykh et al, 1999Khromykh et al, , 2000Lindenbach & Rice, 1997;Westaway et al, 1997;Youn et al, 2012) and the modulation of the host immune response (Avirutnan et al, 2006(Avirutnan et al, , 2010Chung et al, 2006;Crook et al, 2014;Kurosu et al, 2007;Muller & Young, 2013;Schlesinger, 2006;Wilson et al, 2008). After cleavage from the polyprotein (Falgout & Markoff, 1995;Nowak et al, 1989), NS1 is glycosylated and forms a heat-labile dimer in the endoplasmic reticulum (ER) (Pryor & Wright, 1994;Winkler et al, 1988Winkler et al, , 1989.…”