Evasion of host recognition by phase variation in Haemophilus influenzae

Haemophilus influenzae colonizes the human respiratory tract, and is a major source of disease. Antibody and complement contribute to the limitation of H. influenzae colonization. This work explores bacterial factors that aid in the evasion of antibody recognition and subsequent complement-mediated lysis. Antibody recognition of H. influenzae is affected by the phase variation of lipopolysaccharide (LPS) structures. Phase variation refers to the stochastic, high frequency on/off switching in gene expression. One phase variable gene, lic1A, controls the attachment of the small molecule phosphorylcholine (ChoP) to the LPS. We found that ChoP-expressing bacteria had reduced antibody binding and increased survival in the presence of complement. Also, ChoP attachment to the LPS increased the stability of the outer membrane, reducing accessibility to host molecules including antibodies. Next, we conducted a screen of genes required for complement resistance in H. influenzae. We found that several LPS phase variable biosynthesis genes are critical for survival in the presence of human antibody and complement. In addition to these, we identified vacJ and the associated yrb genes, which encode an ABC transporter that increases membrane stability and reduces antibody binding. These findings present two different examples of how alterations in outer membrane stability can affect antibody binding to bacterial surface structures. Finally, we examined the dynamics of the contributions of multiple phase variable LPS biosynthesis genes to bacterial survival. We found that exposure to antibody and complement drives selection for phase variants expressing ChoP, di-galactoside (galα1-4gal), and an alternative glucose structure. Each of these LPS structures, in addition to the phase variable molecule sialic acid, had an independent effect on bacterial survival, and these effects were additive in combination. Bacteria with the maximum number of LPS modifications had the greatest survival, and this correlated with reduced recognition of conserved inner core LPS structures. In summary, LPS phase variation in H. influenzae contributes to bacterial evasion of antibody binding and complement-mediated killing. The expression of several LPS phase variable modifications shields conserved surface structures on H. influenzae from host recognition to contribute to the successful colonization of this extracellular pathogen