Crystal structures of human SP-D complexed with synthetic oligosaccharides suggest a role for phosphorylated inner core LPS saccharides in host-pathogen interactions

The innate immune protein human surfactant protein D (SP-D) recognises pathogens in the lungs via binding to carbohydrate surface structures. SP-D has been shown to target gram-negative bacterial lipopolysaccharide via calcium-dependent binding, preferentially to the inner core heptose (HepI). To further investigate this recognition, we have determined the high-resolution crystal structures of a trimeric recombinant fragment of human SP-D complexed with synthetic di- and trisaccharides, HepI-Kdo, HepIII-HepII-HepI, and HepII-HepI phosphorylated at either HepI or HepII, inner core motifs common to the lipopolysaccharide of many gram-negative bacteria. In contrast to acid-hydrolysed lipopolysaccharide used in several previous studies, these synthetic saccharides allow presentation of both the innermost Kdo in its natural pyranose form and heptose phosphorylation. The structures confirm the flexibility of SP-D to adopt an alternative binding mode when the preferred epitope is not available, reveal a preference for recognition of the reducing terminal heptose (HepI) via the glyceryl group, indicate that a single Kdo attached to HepI does not have a significant role in ligand recognition, and provide evidence that recognition of phosphorylated inner core diheptosyl ligands varies dependent on whether HepI or HepII is phosphorylated. The disaccharide with HepII O4 phosphorylation binds via the preferred HepI glyceryl-hydroxyls, while HepI O4 phosphorylation reveals HepII binding via the pyranose ring O3 and O4 hydroxyls. The ability of the HepII O4 phosphate to prevent preferred HepI recognition suggests a role for heptose phosphorylation in shielding the bacterial LPS inner core from immune recognition.