Functional Recruitment of the Human Complement Inhibitor C4BP to Yersinia pseudotuberculosis Outer Membrane Protein Ail

Abstract

Ail is a 17-kDa chromosomally encoded outer membrane protein that mediates serum resistance (complement resistance) in the pathogenic Yersiniae (Yersinia pestis, Y. enterocolitica, and Y. pseudotuberculosis). In this article, we demonstrate that Y. pseudotuberculosis Ail from strains PB1, 2812/79, and YPIII/pIB1 (serotypes O:1a, O:1b, and O:3, respectively) can bind the inhibitor of the classical and lectin pathways of complement, C4b-binding protein (C4BP). Binding was observed irrespective of serotype tested and independently of YadA, which is the primary C4BP receptor of Y. enterocolitica. Disruption of the ail gene in Y. pseudotuberculosis resulted in loss of C4BP binding. Cofactor assays revealed that bound C4BP is functional, because bound C4BP in the presence of factor I cleaved C4b. In the absence of YadA, Ail conferred serum resistance to strains PB1 and YPIII, whereas serum resistance was observed in strain 2812/79 in the absence of both YadA and Ail, suggesting additional serum resistance factors. Ail from strain YPIII/pIB1 alone can mediate serum resistance and C4BP binding, because its expression in a serum-sensitive laboratory strain of Escherichia coli conferred both of these phenotypes. Using a panel of C4BP mutants, each deficient in a single complement control protein domain, we observed that complement control protein domains 6–8 are important for binding to Ail. Binding of C4BP was unaffected by increasing heparin or salt concentrations, suggesting primarily nonionic interactions. These results indicate that Y. pseudotuberculosis Ail recruits C4BP in a functional manner, facilitating resistance to attack from complement.

The Yersinia pseudotuberculosis Outer Membrane Protein Ail Recruits the Human Complement Regulatory Protein Factor H

“…Resistance to the AP also appears to be less critical for survival compared with all pathways combined, because ∼1 log of killing was observed with strain YPIII/pIB1 ail 2 in AP-only serum (Fig. 1) compared with the complete killing (∼7 logs) observed with the same strain in NHS (15). This suggests that, despite possessing a poorer ability to bind fH compared with C4BP, under AP-only conditions there are fewer C3 convertases and C3b molecules to inactivate.…”

“…Rck, PagC (Salmonella choleraesuis), and Ail were shown to mediate serum resistance (3-5, 23, 24), whereas adhesion and/or invasion properties have been observed in all of the aforementioned proteins, with the exception of Y. pseudotuberculosis Ail (25)(26)(27)(28)(29). Furthermore, functional fH and C4BP binding have now been described in Rck (21,30) and Y. enterocolitica and Y. pseudotuberculosis Ail (10,11,15). Future characterization of PagC from other species (Salmonella typhi, E. coli O157:H7) will be necessary to determine whether these proteins can also mediate serum resistance, and if so, by the mechanisms described above.…”

“…In concert with C4BP recruitment (15), fH recruitment confers Ail with the ability to mediate resistance to all pathways of complement activation, thus increasing the ability of this pathogen to establish a successful infection.…”

“…Despite this, in previous investigations we observed that Y. pseudotuberculosis Ail, similar to its homolog in Y. enterocolitica, can bind C4BP in a functional manner (15). However, dissimilar to Y. enterocolitica, C4BP binding to Y. pseudotuberculosis Ail was unaffected by increasing heparin or NaCl concentrations (11,15).…”

Yersinia enterocolitica YadA Mediates Complement Evasion by Recruitment and Inactivation of C3 Products

“…Our finding might be contributable to the usage of diverse serotypes (26,27,30), individual serum activity, and strain-specific expression levels of factors involved in serum resistance (YadA, Ail LPS), and also to subtle serotype-specific differences in the YadA protein sequence. Recently, Ho et al (31) showed that the outer membrane protein Ail of Yersinia pseudotuberculosis binds to the complement regulator C4BP regardless of serotype. However, serum resistance was also independent of YadA in two strains of Y. pseudotuberculosis, whereas in another strain serum resistance involved factors in addition to YadA and Ail.…”