Monoclonal antibodies reactive with K1-encapsulated Escherichia coli lipopolysaccharide are opsonic and protect mice against lethal challenge.

Seven murine monoclonal antibodies (MAbs) directed against O-side-chain determinants of the K1-encapsulated Bortolussi strain of Escherichia coli (O18:K1:H7) were evaluated for their in vitro and in vivo activities. All the MAbs reacted well in Western blots against E. coli O18 lipopolysaccharide antigens. Two MAbs of the immunoglobulin G (IgG) class promoted in vitro opsonophagocytosis and protected mice lethally challenged with bacteria. Two IgM MAbs showed partial protection, although they had no in vitro opsonic activity, and the remaining three IgM MAbs showed no apparent functional activities. Monoclonal IgG antibodies against bacterial lipopolysaccharide can be opsonic and protective in spite of the presence of the K1 capsule on the bacterium.