Fimbria-mediated enhanced attachment of nontypeable Haemophilus influenzaeto 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 Haemophi lus influenzae (NTHI), In this study, we investigated the role of NTHI surf ace outer membrane protein P5-homologous fimbriae (P5-fimbriae) in attachme nt to RSV-exposed A549 epithelial cells. Analysis by fluorescence flow cyto metry showed that a live P5-fimbriated NTHI strain (NTHIF+) attached to a h igher 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 PS-fim briae-deficient isogenic mutant strain (NTHIF-) was significantly lower tha n in control cells and rose only slightly following RSV exposure (mean, 17% for RSV versus 10% for control, P = 0.229), Attachment of NTHIF+ did not c orrelate with the amount of RSV antigen expressed by A549 cells. Furthermor e, paraformaldehyde-fixed NTHIF+ also demonstrated an enhanced binding to R SV-exposed cells, Observations by transmission electronic microscopy showed that the mean number of bacteria attached per 100 RSV exposed A549 cells w as higher for NTHIF+ than NTHIF- (99 versus 18; P < 0.001). No intracellula r bacteria were identified. UV-irradiated conditioned supernatants collecte d from RSV-infected A549 cultures (UV-cRSV) also enhanced the attachment of NTHIF+ to A549, suggesting the presence of a preformed soluble mediator(s) in UV-cRSV that enhances the expression of receptors for P5-fimbriae on A5 49 cells. In summary, RSV infection significantly enhances NTHI attachment to respiratory epithelial cells. P5-fimbria is the critical appendage of NT HI that participates in this attachment. In clinical settings, blocking of the PS-fimbria-mediated attachment of NTHIF+ by passive or active immunity may reduce the morbidity due to NTHI during RSV infection.