BINDING OF SURFACTANT PROTEIN-A TO THE LIPID-A MOIETY OF BACTERIAL LIPOPOLYSACCHARIDES

Surfactant protein A (SP-A) enhances the phagocytosis of opsonized and non-opsonized bacteria by alveolar macrophages, but it is not known w ith which component of the bacterial surface it associates. We investi gated the interaction of SP-A with lipopolysaccharides (LPS), which ar e important biologically active constituents of the outer membranes of Gram-negative bacteria. Flow cytometry was used to study the binding of fluorescein isothiocyanate-labelled SP-A either to LPS of various c hain lengths coupled to magnetic beads or to Gram-negative bacteria. T he binding of SP-A to LPS-coated beads was saturable, both time- and c oncentration-dependent, and required both Ca2+ and Na+. SP-A bound to the lipid A moiety of LPS and to LPS from either the Re-mutant of Salm onella minnesota or the J5-mutant of Escherichia coli. In contrast, it did not bind to 0111 LPS of E. coli, suggesting that SP-A binds only to rough LPS. The binding of SP-A to LPS was not affected by mannan an d heparin or by deglycosylation of the SP-A, indicating that the carbo hydrate-binding domain and the carbohydrate moiety of SP-A are not inv olved in its interaction with LPS. We also observed saturable and conc entration-dependent binding of SP-A to the live J5 mutant of whole E. coli, but not to its 0111 mutant. In addition, Re LPS aggregated in th e presence of SP-A. Ca2+ and Na+. We conclude that SP-A associates wit h LPS via the lipid A moiety of rough LPS and may be involved in the a nti-bacterial defences of the lung.