A Factor I-Like Activity Associated with Chikungunya Virus Contributes to Its Resistance to the Human Complement System

Abstract

Chikungunya virus (CHIKV) is an emerging pathogen capable of causing explosive outbreaks. Prior studies showed that exacerbation in arthritogenic alphavirus-induced pathogenesis is attributed to its interaction with multiple immune components, including the complement system. Viremia concomitant to CHIKV infection makes exposure of the virus to complement unavoidable, yet very little is known about CHIKV-complement interactions. Here, we show that CHIKV activated serum complement to modest levels in a concentration- and time-dependent manner, but the virus effectively resisted complement-mediated neutralization. Heat-inactivated serum from seropositive donors could actively neutralize CHIKV due to the presence of potent anti-CHIKV antibodies. Deposition of key complement components C3 and C4 did not alter the resistance of CHIKV to complement. Further, we identified a factor I-like activity in CHIKV that limited complement by inactivating C3b into inactive C3b (iC3b), the complement component known to significantly contribute to disease severity in vivo, but this activity had no effect on C4b. Inactivation of C3b by CHIKV was largely dependent on the concentration of the soluble host cofactor factor H and the virus concentration. A factor I function-blocking antibody had only a negligible effect on the factor I-like activity associated with CHIKV, suggesting that this activity is independent of host factor I and could be of viral origin. Thus, our findings suggest a complement modulatory action of CHIKV which not only helps the virus to evade human complement but may also have implications in alphavirus-induced arthritogenic symptoms. IMPORTANCE Chikungunya virus is a vector-borne pathogen of global significance. The morbidity associated with chikungunya virus (CHIKV) infection, neurovirulence and adaptability to Aedes albopictus, necessitates a deeper understanding of the interaction of CHIKV with the host immune system. Here, we demonstrate that CHIKV is resistant to neutralization by one of the potent barriers of the innate immune arm, the complement system. Chikungunya virus showed marked resistance to complement despite activation and deposition of complement proteins. Interestingly the C3 component associated with the virion was found to be inactive C3b (iC3b), a key factor implicated in the pathogenesis and disease severity in the mouse model of Ross River virus infection. CHIKV also had an associated unique factor I-like activity that mediated the inactivation of C3b into iC3b. We have unraveled a smart strategy adopted by CHIKV to limit complement which has serious implications in viral dissemination, pathogenesis, and disease.

Viral Evasion of the Complement System and Its Importance for Vaccines and Therapeutics

“…However, NiV is capable of C3b cleavage into iC3b with factor I cofactors (factor H and soluble CR1) to protect against virus neutralization (168). Chikungunya virus (CHIKV) also exhibits factor I-like activity, likely of viral origin and dependent on host factor H concentrations, to cleave C3b into inactive iC3b and resist complement-mediated neutralization (169). MuV, SV5, and HIV-1 can all recruit host cell CD46 into the viral lipid membranes during the budding process to protect from complement deposition and neutralization (170,171).…”

Virus-Encoded Complement Regulators: Current Status

“…Both these viruses display resistance to complement-mediated neutralization [ 33 , 45 ]. Examination of their surface following incubation with complement showed a marked reduction in C3b deposition, suggesting a lack of complement activation on the viral surface [ 33 , 45 ]. Studies performed to identify the mechanism of C3b inactivation indicated that the viral particles display factor I-like activity.…”

In the Crosshairs: RNA Viruses OR Complement?

“…Other alphaviruses investigated for their interactions with the CS include the Ross River virus (RRV) and Chikungunya virus (CHIKV), wherein the role of complement was rather destructive to the host than protective during virus infection. CHIKV was also found to activate complement, resulting in the deposition of complement components C3b and C4b on the surface of the virus ( 57 ).…”

“…Interestingly, similar observations were made during our investigations with CHIKV, wherein the virus was found to resist complement-mediated neutralization. Complement activation by CHIKV did result in deposition of C3b and C4b on the virus, but to a lesser degree ( 57 ). The inherent resistance to complement observed in the case of both CHIKV and NiV alludes to a virus-associated mechanism for complement resistance discussed in much detail later.…”

“…Like NiV, CHIKV-associated fI-like activity was incapable of inactivating C4b. Although electron microscopic analysis pointed to an association of fI to the CHIKV envelope, immunoblotting and function blocking cofactor activity assays with an antibody targeting fI protease activity showed that CHIKV-associated fI-like activity is not of host origin ( 57 ). Since the source of fI or fI-like activity is not yet determined at least in the case of CHIKV, detailed investigations are required to gain a better understanding on the precise mechanisms involved.…”