Killing of Borrelia burgdorferi by antibody elicited by OspA vaccine is inefficient in the absence of complement

A Lyme disease vaccine, based on the Borrelia burgdorferi lipoprotein OspA, has recently undergone phase In trials in humans. The results of one of th ese trials indicate that vaccine efficacy positively correlates with anti-O spA antibody titer, Spirochete killing within the tick vector midgut, upon which vaccine efficacy appears to depend, may occur chiefly via a mechanism that involves antibody alone, as it has been reported that complement is d egraded by tick saliva decomplementing factors. We compared the in vitro ki lling efficiencies of anti-OspA antibody elicited in rhesus monkeys by the OspA vaccine, in the presence and in the absence of monkey complement. Kill ing in the absence of complement was between 14 and 3,800 times less effici ent than with complement present, depending on the spirochete strain. The r elative inefficiency of the complement-independent killing mechanism by ant i-OspA antibody may explain why OspA vaccine efficacy is critically depende nt on antibody titer.