EXTRACELLULAR ENVELOPED VACCINIA VIRUS IS RESISTANT TO COMPLEMENT BECAUSE OF INCORPORATION OF HOST COMPLEMENT CONTROL PROTEINS INTO ITS ENVELOPE

Vaccinia virus (VV) produces two antigenically and structurally distin ct infectious virions, intracellular mature virus (IMV) and extracellu lar enveloped virus (EEV). Here we have investigated the resistance of EEV and IMV to neutralization by complement in the absence of immune antibodies. When EEV is challenged with complement from the same speci es as the cells used to grow the virus, EEV is resistant to neutraliza tion by complement, whereas IMV is not. EEV resistance was not a resul t of EEV protein B5R, despite its similarity to proteins of the regula tors of complement activation (RCA) family, or to any of the other EEV proteins tested (A34R, A36R, and A56R gene products). EEV was sensiti ve to complement when the virus was grown in one species and challenge d with complement from a different species, suggesting that complement resistance might be mediated by host RCA incorporated into the EEV ou ter envelope. This hypothesis was confirmed by several observations: ( i) immunoblot analysis revealed that cellular membrane proteins CD46, CD55, CD59, CD71, CD81, and major histocompatibility complex class I a ntigen were detected in purified EEV but not IMV; (ii) immunoelectron microscopy revealed cellular RCA an the surface of EEV retained on the cell surface; and (iii) EEV derived from rat cells expressing the hum an RCA CD55 or CD55 and CD59 were more resistant to human complement t han EEV derived from control rat cells that expressed neither CD55 nor CD59. These data justify further analysis of the roles of these (and possible other) cellular proteins in EEV biology.