Mechanisms of lipopolysaccharide-initiated rabbit platelet responses. II. Evidence that lipid A is responsible for binding of lipopolysaccharide to the platelet.

The mechanism of bacterial lipopolysaccharide-(LPS) initiated, complement-(C) mediated rabbit platelet lysis has been examined. The results of these studies support our previous observations that activation of the alternative C pathway is required for platelet lysis and that preparations of LPS that activate only the classical pathway (e.g., lipid A) do not cause lysis. The temporal relationship of the interaction of the LPS with the platelet before the addition of plasma suggests a time-dependent association of the LPS with the platelet. On the basis of a number of experiments, including inhibition with polymyxin B, treatment of LPS with alkali, and blocking experiments with polysaccharide-free LPS preparations, it is concluded that the lipid A region of the LPS molecule is responsible for attaching the LPS to the platelet. Finally, a comparison of the activity of lipid A-associated protein-LPS complexes with protein-free LPS demonstrated that an equivalent extent of platelet lysis was achieved with one-one hundredth the concentration of the former as that required for protein-free LPS. The data suggest that LAP facilitates attachment of the LPS to the platelet.