Interaction of Lipopolysaccharides and Lipid A with Complement

A number of lipopolysaccharides derived from Salmonella and Escherichia coli S and R mutant strains were tested for toxicity and anticomplementary activity in the absence of added antiserum. Although all preparations were toxic, only a few exhibited high anticomplementary activity, while others proved to be of low or negligible activity. It was found that isolated lipid A from both active and inactive lipopolysaccharides was strongly anticomplementary as well as toxic, when made water‐soluble with the aid of suitable carriers. Treatment of R form lipopoly‐saccharide with Mg 2+ or Ca 2+ led to complete precipitation of the lipopolysaccharide with consequent loss of toxicity and anticomplementary activity. This treatment had practically no effect on the anticomplementary activity and toxicity of S form lipopolysaccharides. When lipopolysaccharide, after reaction with complement, was reisolated and purified, the resulting preparation was found to be non‐toxic and of negligible anticomplementary activity. No detectable alterations in either the sugar or the fatty acid composition could be detected. The only significant change was the loss of solubility in water. Treatment of the reisolated lipopolysaccharide with EDTA completely restored solubility in water, toxicity, and anti‐complementary activity.