Fimbria-mediated enhanced attachment of nontypeable Haemophilus influenzae to respiratory syncytial virus-infected respiratory epithelial cells

Respiratory syncytial virus (RSV) infection is known to predispose children to otitis media and sinusitis due to bacteria such as nontypeable Haemophilus influenzae (NTHI). In this study, we investigated the role of NTHI surface outer membrane protein P5-homologous fimbriae (P5-fimbriae) in attachment to RSV-exposed A549 epithelial cells. Analysis by fluorescence flow cytometry showed that a live P5-fimbriated NTHI strain (NTHI(F+)) attached to a higher proportion of RSV-exposed A549 cells than to control cells (mean, 68% for RSV versus 29% for control; P = 0.008), while attachment of the P5-fimbriae-deficient isogenic mutant strain (NTHI(F-)) was significantly lower than in control cells and rose only slightly following RSV exposure (mean, 17% for RSV versus 10% for control, P = 0.229). Attachment of NTHI(F+) did not correlate with the amount of RSV antigen expressed by A549 cells. Furthermore, paraformaldehyde-fixed NTH(F+) also demonstrated an enhanced binding to RSV-exposed cells. Observations by transmission electronic microscopy showed that the mean number of bacteria attached per 100 RSV-exposed A549 cells was higher for NTHI(F+) than NTHI(F- ) (99 versus 18; P < 0.001). No intracellular bacteria were identified. UV- irradiated conditioned supernatants collected from RSV-infected A549 cultures (UV-cRSV) also enhanced the attachment of NTHI(F+) to A549, suggesting the presence of a preformed soluble mediator(s) in UV-cRSV that enhances the expression of receptors for P5-fimbriae on A549 cells. In summary, RSV infection significantly enhances NTHI attachment to respiratory epithelial cells. P5-fimbria is the critical appendage of NTHI that participates in this attachment. In clinical settings, blocking of the P5-fimbria-mediated attachment of NTHI(F+) by passive or active immunity may reduce the morbidity due to NTHI during RSV infection.