Biochemical and functional characterization of the binding of Neisseria Adhesin A (NadA) with Siglec-5 and 14

Neisseria adhesin A (NadA) is a proteic recombinant antigen included in Bexsero, a vaccine against serogroup B Neisseria meningitidis (N. meningitidis). NadA belongs to the trimeric autotransporters family and is composed of a long coiled-coil with three protruding wing-like structures that create an unusual N-terminal head domain. Even though NadA has been extensively described as promoter of both adhesion to and invasion into human epithelial cells, its receptors at the interface with human cells have never been found. To identify the targeted human receptors, we used recombinant NadA (rNadA) as probe on a protein microarray including a expanded collection of surface and secreted human proteins. The screening disclosed three putative interactors, Siglec-5, Siglec-14 and FcγRIIA, expressed on cells of myeloid lineage. We first validated these interactions addressing the biochemical features of the binding, confirming Siglec-5 and -14 as high affinity NadA binding proteins, while FcγRIIA was not. Next, we assessed the membrane properties of NadA binding to Siglecs by using Escherichia coli as an heterologous system. Full-lenght Siglecs, expressed on CHO-K1 cells showed an increase of adhesion of N. meningitidis, whereas the isogenic strain knock-out NadA did not. Although recombinant Siglec-5 ectodomain was shown to bind to NadA on both capsulated and unencapsulated N. meningitidis, we could not find any relevant evidence of NadA/Siglecs interaction on primary and immortalized monocytic cell lines. In recent years, soluble forms of Siglecs were detected in human serum. In this study, the soluble Siglecs presence was confirmed in human pooled sera and in culture media of monocytes and macrophages. Interestingly, we found that during infection assays N. meningitids was able to bind monocytes-released Siglecs in NadA dependent manner. The exogenous addition of the soluble species increased the attachment of bacteria on monocytic cells surface but reduced internalization. We also demonstrated a novel interaction between Siglecs and complement component 1 (C1q), whose in vitro interaction was inhibited by recombinant NadA in a concentration dependent manner. Further, we observed a survival advantage for N. meningitidis in presence of bactericidial antibodies anti-NadA by testing sera, as source of complement, depleted of Siglec-5 and -14. In summary, this work revealed two new human interactors for the neisserial adhesin that could be encountered not only on phagocytes surfaces but also in the bloodstream. On the host side, we characterized a new proteic target for Siglecs on N. meningitidis bacterial surface. Moreover, the C1q binding to Siglec-5 and -14 was revealed for the first time, opening questions on Siglecs biology. In conclusion we found that NadA may not contribute exclusively to the crossing of the epithelial nasopharyngeal barrier but also it may help the bacterium to survive into extracellular host milieu.