Cyclic enterobacterial common antigens from Escherichia coli O157 as microbe-associated molecular patterns

In a previous study, we described 2 forms of cyclic enterobacterial common antigen (ECACYC), a tetramer and a pentamer, from Escherichia coli O157. ECACYC is present in several representatives of the Enterobacteriaceae. To date, functional studies on ECACYC are sparse. Cyclic oligosaccharides in other bacteria, like the cyclic β-glucans in Rhizobiaceae, represent microbe-associated molecular patterns involved in host-bacteria interaction. This observation determined the aim of the present study: to test whether the tetrameric and pentameric ECACYC from E. coli O157 can be recognised by host humoral and cellular mechanisms. ELISA tests designed to compare the 2 ECACYC with the O157 lipopolysaccharide showed that both ECACYC were not recognised by polyclonal anti-O157 serum but were good ligands for mannan-binding lectin. The lectin had a higher affinity for the tetramer than the pentamer. ECACYC deposited more C3b than did the lipopolysaccharide. To examine the interactions with human circulating neutrophils, the antigens were loaded onto fluorescent latex beads and applied in a phagocytosis experiment. Spheres coated with the 2 ECACYC occasionally adhered to phagocyte surfaces but, unlike O157-loaded spheres, failed to induce free-radical release. The results show that the 2 ECACYC represent microbe-associated molecular patterns recognised by host humoral non-self-recognition mechanisms.