The Major Outer Membrane Protein P5 Binds Vitronectin to Mediate Serum Resistance in Nontypeable <i>Haemophilus influenzae</i>

Acquisition of complement regulators is a virulence strategy used by nontypeable Haemophilus influenzae (NTHi) to evade complement-mediated killing by the host. The major outer membrane protein of NTHi, P5, binds C4b-binding protein and factor H to promote bacterial serum resistance. We show that P5 also binds vitronectin, which inhibits the formation of the membrane attack complex at the terminal stage of the complement pathway. Heterologous surface expression of P5 variants from NTHi strains 3655, KR271, KR317, and P652 promoted vitronectin binding to the P5-expressing Escherichia coli. In contrast, deletion of P5 from the NTHi strains reduced vitronectin binding. Vitronectin acquisition conferred serum resistance to P5-expressing E. coli, but not to NTHi ΔompP5 mutants. Using site-directed mutagenesis, extracellular loop 2 of the P5 variants was identified as the binding site for vitronectin. In conclusion, our findings highlight P5 as a receptor for vitronectin that promotes NTHi serum resistance.