Comparison of binding specificity and the function of two human IgM anti-lipid A monoclonal antibodies.

The interactions of two anti-lipid A monoclonal antibodies (mAb)--HA-1A and SdJ5-1.17.15--with their antigenic sites on lipid A, were compared using a dot-blot assay and lipid A structural analogues, as well as lipid A-high-density lipoprotein (HDL) complexes. The reactivities of both mAb were affected by the type of fatty acid side chains and by the phosphate group on the glucosamine residue II; however, the interaction of SdJ5-1.17.15 appeared to be more markedly affected by the fatty acid side chains. A determination of the biological significance of these antigenic differences was made. Human peripheral blood mononuclear cells (hPBMC) challenged with Escherichia coli 055:B5 lipopolysaccharide (LPS) pre-incubated with SdJ5-1.17.15 released significantly less tumor necrosis factor-alpha (TNF-alpha) and interleukin-1 beta (IL-1 beta), compared to hPBMC exposed to vehicle preincubated LPS. HA-1A did not attenuate the in vitro release of either cytokine. The ability of both mAb to neutralize the in vivo toxicity of LPS was also evaluated. Rats administered E. coli 055:B5 pre-incubated with SdJ5-1.17.15 had a significantly reduced 24-hr mortality rate compared to vehicle controls. HA-1A did not attenuate the in vivo mortality rate. Therefore, the reactivity of anti-lipid A mAb with the antigen is preferentially affected by different residues on the lipid A moiety. Thus, the differences in biological activity seen with SdJ5-1.17.15 and HA-1A may be due in part to differences in their recognition sites on lipid A.