Cross-binding activity and protective capacity of monoclonal antibodies to lipid A.

Six hybridoma clones (1M, 4M, 9M, 11M, 18M and 31G), secreting monoclonal antibodies (mAbs) against lipid A were obtained after fusion between cells of mouse myeloma line X63-Ag8.653 and spleen cells from BALB/c mice immunized with acid treated Salmonella minnesota bacteria coated with additional free lipid A. The specificity and cross-binding activity of the mAbs were characterized in ELISA by using synthetic lipid A analogs as well as different lipid A and lipopolysaccharides (LPS) extracted from R- and S-form bacteria. It was found that the antibodies recognize epitopes in which phosphate groups, especially those at the C4' position of the glucosamine backbone of lipid A, were present. These epitopes were accessible also for the antibodies in purified intact LPS. By using a set of core glycolipids with increasing completion of the core region of the molecule and S-LPSs it was shown that the mAbs cross-reacted with a variety of R- and S-form LPS. The binding activity decreased with increasing length of the polysaccharide chain. The mAb did not prevent ultimate lethality of mice challenged with Klebsiella pneumoniae B and Salmonella typhimurium C5. However a delay of mortality rate of mice pretreated with antibodies 18M and 31G and infected with K. pneumoniae was seen.