Borreliacidal activity of early Lyme disease sera against complement-resistant Borrelia afzelii FEM1 wild-type and an OspC-lacking FEM1 variant

Sera obtained from 14 Lyme borreliosis patients at early stages (stages I a nd II) of the disease were examined for their borreliacidal properties agai nst Borrelia afzelii isolate FEM1 by use of a growth inhibition assay. Five of 14 immune sera exhibited borreliacidal activity against isolate FEM1. H eat-inactivated immune sera failed to kill the spirochaetes. Immunoblotting experiments with outer-membrane preparations showed that OspC and 11 addit ional proteins of 14.0, 16.0, 17.7, 19.3, 21.7, 27.5, 32.7, 40.7, 48.9, 51. 3 and 53.6 kDa were recognised by borreliacidal immune sera. To analyse the borreliacidal properties of anti-OspC antibodies, two sera (EM4 and EM5), which beside antibodies against a 51.3-kDa protein contained exclusively an ti-OspC antibodies, were further investigated by comparative analysis with a FEM1 wild-type and a FEM1 variant lacking OspC in a growth inhibition ass ay. Only FEM1 wild-type and not variant FEM1OspC(-) was killed by immune se ra EM4 and EM5, Complement-dependent killing of FEM1 wild-type was mediated by formation of the terminal complement complex that was found to be attac hed directly to the outer membrane as confirmed by immune-electron microsco py, No complement deposition was observed on the surface of variant FEM1Osp C(-) after incubation with immune sera EM4 and EM5, thereby suggesting that only anti-OspC antibodies in these two immune sera were responsible for bo rreliacidal activity. These results provide direct evidence that anti-OspC antibodies, once developed during the immune response, are of critical impo rtance for efficient killing of borreliae in the early phase of infection.