Biologic Properties of Nontoxic Derivatives of a Lipopolysaccharide from Escherichia Coli K235 Free

Lipopolysaccharide (LPS)² from Escherichia coli K235 was treated with o-phthalic anhydride to obtain a high degree of esterification of available hydroxyl groups, leaving a free carboxyl for each hydroxyl esterified (SPLPS). Although there was no demonstrable loss of fatty acids, this conversion of LPS to a polyanionic molecule altered dramatically the spectrum of biologic properties, most of which are normally attributed to the lipid A (LA) moiety. Mitogenicity for mouse B cells was decreased several hundred-fold; reaction with antibodies to LPS was abolished; pyrogenicity and toxicity were decreased by factors of 10⁵ and 10⁴; the ability to induce the Shwartzman reaction in rabbits was decreased 500-fold, and the ability to stimulate production of interferon in mice was decreased by more than 2 × 10³. However, despite the loss of these properties, SPLPS retained the ability to act as an immunologic adjuvant. The nature of the anionic group is important, e.g., sodium succinyl-LPS (SuLPS) is 10-fold more pyrogenic and toxic than sodium phthalyl-LPS (SPLPS). Data on another LPS derivative, from which ester-linked fatty acid residues were removed before phthalylation, suggest that the ester-linked fatty acid groups in the lipid A moiety of SPLPS may not be necessary for its immunologic adjuvant effect.