Evidence that antibodies to polysaccharide alter platelet responses to endotoxin in tolerant rabbits

When rabbits are made resistant (tolerant) to Salmonella typhi endotoxin by administration of five intraperitoneal injections of small amounts of endotoxin on consecutive days, a plasma factor appears which accelerates the in vitro aggregation response of platelets to endotoxin. Furthermore, in contrast with the reaction in normal animals, this aggregation is often quickly reversible. The present study was undertaken to determine some characteristics of this plasma factor. The humoral factor responsible for altering the platelet aggregation-disaggregation response to endotoxin is apparently antibody against polysaccharide antigen rather than an endotoxin degrading substance because (1) appearance of the factor correlated with presence of antibodies to polysaccharide, (2) the factor was stable at 56 degrees C for 1 h, (3) its effect only took a few seconds to occur, and (4) plasma from tolerant animals reacted only with complete endotoxin, obtained from smooth bacteria, but not lipid A. In fact, lipid A did not induce platelet aggregation unless its specific antibody was also present.