A Fine-Tuned Interaction between Trimeric Autotransporter Haemophilus Surface Fibrils and Vitronectin Leads to Serum Resistance and Adherence to Respiratory Epithelial Cells

Abstract

dHaemophilus influenzae type b (Hib) escapes the host immune system by recruitment of the complement regulator vitronectin, which inhibits the formation of the membrane attack complex (MAC) by inhibiting C5b-C7 complex formation and C9 polymerization. We reported previously that Hib acquires vitronectin at the surface by using Haemophilus surface fibrils (Hsf). Here we studied in detail the interaction between Hsf and vitronectin and its role in the inhibition of MAC formation and the invasion of lung epithelial cells. The vitronectin-binding region of Hsf was defined at the N-terminal region comprising Hsf amino acids 429 to 652. Moreover, the Hsf recognition site on vitronectin consisted of the C-terminal amino acids 352 to 374. H. influenzae was killed more rapidly in vitronectin-depleted serum than in normal human serum (NHS), and increased MAC deposition was observed at the surface of an Hsf-deficient H. influenzae mutant. In parallel, Hsf-expressing Escherichia coli selectively acquired vitronectin from serum, resulting in significant inhibition of the MAC. Moreover, when vitronectin was bound to Hsf, increased bacterial adherence and internalization into epithelial cells were observed. Taking our findings together, we have defined a fine-tuned protein-protein interaction between Hsf and vitronectin that may contribute to increased Hib virulence.

Haemophilus influenzae surface fibril (Hsf) is a unique twisted hairpin-like trimeric autotransporter

“…Recombinant DNA plasmids were obtained as described elsewhere (Singh et al, 2014). For surface expression of the Hsf protein, pET16-hsf54-2300 (Hallström et al, 2006) was freshly transformed into E. coli BL21 (DE3) and plated on LB ampicillin plates.…”

“…Recently, we studied the Hsf-Vn interaction in detail and found that BD2 of Hsf selectively interacts with the C-terminal end of Vn and thus inhibits MAC formation (Singh et al, 2014). Vn also mediates an increased adherence of Hib to epithelial cells.…”

“…We proposed that when Vn is bound to BD2 it may function as a bridging molecule between the bacteria and epithelial surface integrins. However, BD1 may directly bind to host epithelium, leading to a stronger Hsf-mediated interaction of Hib with the host cell surface (Singh et al, 2014).…”

Moraxella catarrhalis Binds Plasminogen To Evade Host Innate Immunity

“…catarrhalis ubiquitous surface proteins (Usps) belong to a family of trimeric autotransporters that are found in several Gram-negative species. The proteins in this group are composed of membrane anchor, stalk, neck, and head domains and thus appear as "lollipops" on the bacterial outer membrane (4,15,16). Ubiquitous surface proteins are generally categorized into three distinct subgroups: UspA1 is present in all clinical isolates, whereas UspA2 and hybrid UspA2 (UspA2H) are found in 75% and 25% of strains, respectively.…”

Haemophilus influenzae Type f Hijacks Vitronectin Using Protein H To Resist Host Innate Immunity and Adhere to Pulmonary Epithelial Cells

“…After these initial steps, bacteria occasionally disrupt the epithelium and penetrate into deeper tissue layers after passing through the basement membrane and extracellular matrix (ECM) (7,8). Encapsulated H. influenzae may also breach the blood-brain barrier and cause meningitis (9,10).…”