Functional heterogeneity in the antibodies produced to Borrelia burgdorferi

Antibodies to outer surface molecules of Borrelia burgdorferi (Osp) that have a bactericidal action in the absence of complement have been described. These antibodies are primarily monoclonal to antigenic determinants in OspA and OspB. One of these, CB2, is an IgG1 monoclonal antibody that recognizes an epitope in the carboxy terminus of OspB. The specificity of CB2 is critically dependent on the presence of a lysine (Lys) residue in position 253, not only for binding but also for killing the spirochete. This antibody has been used successfully to select escape variants or mutants that are missing the Lys residue either by a mutation or by deletion as a result of premature stop codons. Other antibodies to OspA, OspB, and p39 have also been characterized with similar properties. Another important feature of CB2 is that its bactericidal action is not dependent on agglutination, since Fab fragments of the whole immunoglobulin molecule can also kill in the absence of complement synergy. The killing action of CB2 is not inhibited by protease inhibitors, and is dependent on the presence of calcium. Upon contact with Borrelia burgdorferi, CB2 causes lysis of the outer membrane and the formation of a spheroplast. The bactericidal mechanism of this antibody is not known. The sequence of the heavy and light chain variable regions of CB2 have striking homology to murine antibodies of the autoimmune repertoire, and some of these antibodies have catalytic properties. In general, catalytic antibodies have enzymatic rates of acceleration that are significantly less than those of proteolytic enzymes. If CB2 were a catalytic antibody, its substrate specificity may be expected to be broader. CB2 does not cleave recombinant OspB, nor does it cleave other protein substrates. Its killing activity is not dependent on proteolysis. Because the bactericidal action of CB2 involves the destruction of the outer membrane, it is possible that a phospholipase could be associated with the mechanism. The mobility of spirochetal lipids is altered after incubation with CB2, and the bactericidal activity is reduced in the presence of phospholipase inhibitors. These studies suggest that the bactericidal mechanism of CB2, and other similar antibodies, is novel.