Monoclonal anti-lipid A IgM antibodies HA-1A and E-5 recognize distinct epitopes on lipopolysaccharide and lipid A

Specific binding of two monoclonal IgM antibodies previously investigated as therapeutic agents for treating gram-negative septic shock, HA-1A and E5, was assessed with respect to lipid A and lipopolysaccharide (LPS). Both antibodies bound to lipid A; however, binding of HA-1A was significantly greater than that of E5 to LPS derived from rough strains of bacteria. Reciprocal competitive inhibition experiments supported the concept that HA-1A and E5 bind to distinct epitopes on lipid A. Further, competitive inhibition studies using a monoclonal anti-idiotype antibody with specificity for the variable region of HA-1A suggested that HA-1A and E5 do not share a common idiotype. Finally, studies using double-stranded DNA as antigen indicated that E5 but not HA-1A will bind to DNA. Collectively, these data indicate that HA-1A and E5 are different lipid A-specific antibodies that bind to distinct epitopes on lipid A.