Complement, which bridges innate and adaptive immune responses as well as humoral and cell-mediated immunity, is antiviral. Kaposi's sarcoma-associated herpesvirus (KSHV) encodes a lytic cycle protein called KSHV complement control protein (KCP) that inhibits activation of the complement cascade. It does so by regulating C3 convertases, accelerating their decay, and acting as a cofactor for factor I degradation of C4b and C3b, two components of the C3 and C5 convertases. These complement regulatory activities require the short consensus repeat (SCR) motifs, of which KCP has four (SCRs 1 to 4). We found that in addition to KCP being expressed on the surfaces of experimentally infected endothelial cells, it is associated with the envelope of purified KSHV virions, potentially protecting them from complement-mediated immunity. Furthermore, recombinant KCP binds heparin, an analogue of the known KSHV cell attachment receptor heparan sulfate, facilitating infection. Treating virus with an anti-KCP monoclonal antibody (MAb), BSFS, inhibited KSHV infection of cells by 35%. Epitope mapping of MAb BSF8 revealed that it binds within SCR domains 1 and 2, also the region of the protein involved in heparin binding. This MAb strongly inhibited classical C3 convertase decay acceleration by KCP and cofactor activity for C4b cleavage but not C3b cleavage. Our data suggest similar topological requirements for cell binding by KSHV, heparin binding, and regulation of C4b-containing C3 convertases but not for factor I-mediated cleavage of C3b. Importantly, they suggest KCP confers at least two functions on the virion: cell binding with concomitant infection and immune evasion.
This work was funded as part of a Career Development Fellowship by the Wellcome Trust (O.B.S.), a project grant from Cancer Research UK (C7934 to D.J.B. and O.B.S.), and the Swedish Research Council (A.B.) and Cancerfonden (A.B.).
We acknowledge David Evans and David Williams (University of Glasgow) for helpful comments. Tert-HUVEC was kindly provided by Derrick Dargan (University of Glasgow, United Kingdom) and HMVEC and BCBL-1c by Jeff Vieira (University of Washington). Richard Ambinder generously provided the JSC-1 cells. Anti-K8.1 antibody was a gift from Bala Chandran (Rosalind Franklin University, Chicago, Ill.).