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 10(5) and 10(4); 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 x 10(3). 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 moeity of SPLPS may not be necessary for its
immunologic adjuvant effect.