Background: Enteroaggregative Escherichia coli (EAEC) is a cause of acute and persistent diarrhea in adults and children worldwide. The involvement of host factors in EAEC infections is unresolved. Binding of the recognition molecule ficolin-2 from the lectin pathway of complement to certain EAEC strains has been observed, but the importance is not known. Our aim was to uncover the involvement of ficolin-2 in innate complement dependent immune protection towards EAEC.

Materials and methods: Ficolin-2 binding was screened by a consumption based ELISA method. Flow cytometry determined the deposition of C4, C3 and formation of the bactericidal C5b-9 membrane attack complex (MAC) on the bacteria in combination with inhibitors against C1q, ficolin-2, and factor D from the classical, lectin, and alternative pathway, respectively.

Results and conclusion: Initial screening showed that ficolin-2 bound to three EAEC prototypic laboratory strains. Further screening of 56 EAEC clinical isolates showed that one isolate (E2-1920073) was particularly capable of binding high levels of both recombinant and serum ficolin-2. Thus, we chose to use this isolate to study functional complement deposition. Incubation of the E2-1920073 strain with serum led to deposition of C4, C3 and MAC formation. No inhibition of complement deposition was observed when a C1q inhibitor was added, while partial inhibition was observed when ficolin-2 or factor D inhibitors were used. Combining the inhibitors against ficolin-2 and factor D led to complete inhibition of complement deposition. Our results demonstrate that ficolin-2 is involved in the pathophysiology of EAEC and is crucial in innate complement dependent immune protection against pathogenic EAEC bacteria.