Complement-mediated killing of Borrelia burgdorferi by nonimmune sera fromsika deer

Various species of cervid deer are the preferred hosts for adult, black-leg ged ticks (Ixodes scapularis and Ixodes pacificus) in the United States. Al though frequently exposed to the agent of Lyme disease (Borrelia burgdorfer i), these animals, for the most part, are incompetent as transmission reser voirs. We examined the borreliacidal activity of normal and B. burgdorferi- immune sera from sika deer (Cervus nippon) maintained in a laboratory setti ng and compared it to that of similar sera from reservoir-competent mice an d rabbits. All normal deer sera (NDS) tested killed >90% of B. burgdorferi cells. In contrast, normal mouse and rabbit sera killed less than or equal to 22% of the Borrelia. Anti-B. burgdorferi antibodies could not be detecte d in any normal sera by indirect fluorescent antibody assay (IFA). Sera col lected from deer 6 wk after exposure to B. burgdorferi by tick feeding exhi bited IFA titers of 1:256, whereas sera from mice and rabbits similarly exp osed had titers of >1:1,024. Heat treatment (56 C, 30 min) of NDS reduced b orreliacidal activity, with <20% of the B. burgdorferi cells killed, sugges ting complement-mediated killing. The chelators EGTA and EDTA were used to block the classical or both the classical and alternative complement pathwa ys, respectively. Addition of 10 mM EGTA to NDS had a negligible effect on borreliacidal activity, with >90% of the cells killed. Addition of 10 mM ED TA reduced the killing to similar to 30%, whereas the addition of Mg2+ (10 mM) restored borreliacidal activity to NDS. The addition of zymosan A, an a ctivator of the alternative pathway, increased the survival of B. burgdorfe ri cells to similar to 80% in NDS. These data suggest that the alternative complement activation pathway plays a major role in the borreliacidal activ ity of NDS. Additionally, 10 mM EGTA had almost no effect on the killing ac tivity of B. burgdorferi-exposed deer sera, suggesting that the classical p athway is not involved in Borrelia killing, even in sera from B. burgdorfer i-exposed deer.