Influenza A Virus Neuraminidase Enhances Meningococcal Adhesion to Epithelial Cells through Interaction with Sialic Acid-Containing Meningococcal Capsules

The underlying mechanisms of the epidemiological association between influenza virus infections and Neisseria meningitidis invasive infections are not fully understood. Here we report that adhesion of N. meningitidis to human Hec-1-B epithelial cells is enhanced by influenza A virus (IAV) infection. A potential role of the viral neuraminidase (NA) in facilitating meningococcal adhesion to influenza virus-infected epithelial cells was examined. Expression of a recombinant IAV NA in Hec-1-B human epithelial cells increased the adhesion of strains of N. meningitidis belonging to the sialic acid-containing capsular serogroups B, C, and W135 but not to the mannosamine phosphate-containing capsular serogroup A. Adhesion enhancement was not observed with an inactive NA mutant or in the presence of an NA inhibitor (zanamivir). Furthermore, purified IAV NA was shown to cleave sialic acid-containing capsular polysaccharides of N. meningitidis. On the whole, our findings suggest that a direct interaction between the NA of IAV and the capsule of N. meningitidis enhances bacterial adhesion to cultured epithelial cells, most likely through cleavage of capsular sialic acid-containing polysaccharides. A better understanding of the association between IAV and invasive meningococcal infections should help to set up improved control strategies against these seasonal dual viral-bacterial infections.Neisseria meningitidis is commonly found in the human naso-oropharynx, among other commensal bacterial species. Asymptomatic carriers represent about 10% of the population (41). In Europe and North America, cases of invasive infection leading to meningococcal disease (MD), mainly septicemia and meningitis, occur sporadically. The annual incidence of MD varies between 0.3 and 4.35 per 100,000 inhabitants (36). Both bacterial virulence factors (32) and host susceptibility factors (34, 38) contribute to the development of invasive infections, but the exact mechanisms involved remain largely unknown. The serogroups of N. meningitidis, which are defined by the nature of the capsular polysaccharide, are distributed differently among carried and disease-associated isolates (42). Serogroups B and C (whose capsules are composed of polymers of sialic acids) and serogroups Y and W135 (whose capsules are composed of repeated units of sialic acid with d-glucose and d-galactose, respectively [4]) are prominent in MD in Europe and North America. Serogroup A, whose capsule is composed of α1,6-linked N-acetylmannosamine-1-phosphate (18), is most frequently involved in MD epidemics in Africa.A number of clinical observations of MD occurring in patients with influenza have been reported (6, 13, 43). Epidemiological studies clearly showed a spatiotemporal association between influenza and N. meningitidis invasive infections (2, 14, 25). Recent data from the French National Reference Center for Meningococci and from the Sentinelles Network confirmed the overlap between the winter peaks of incidence of bacteriologically confirmed MD cases and influenza-like illnesses during the period from 2000 to 2008 (Fig. ​(Fig.1),1), in agreement with data published previously (14).Open in a separate windowFIG. 1.Invasive meningococcal infections and influenza-like illnesses recorded in France by the Reference Center for Meningococci and the Sentinelles Network from January 2000 to May 2008. Reporting of invasive meningococcal infections is mandatory. All invasive meningococcal isolates in France are sent to the National Reference Center for Meningococci for full characterization and typing. The general practitioners of the Sentinelles Network report on influenza-like illnesses on a weekly basis by sending patient deidentified data via the Internet to a GIS database (11). The monthly incidence of MD (right axis) and the weekly incidence of influenza-like illnesses (left axis) during the period of January 2000 to May 2008 are represented on the same graph.The mechanisms by which influenza virus infection may favor bacterial superinfection with N. meningitidis, as well as with staphylococci, pneumococci, streptococci, and Haemophilus influenzae, have been investigated using cellular and animal models (15, 19, 22, 33). Virus-induced immune dysregulation, such as impairment of phagocytic functions or alterations in the production of cytokines, can be involved (12). In mice convalescing from influenza A virus (IAV) infection, the production of interleukin-10 in the lungs enhanced susceptibility to meningococcal (3) and to pneumococcal (37) superinfection. Influenza virus infection can also enhance bacterial adhesion by disrupting the respiratory epithelium or by increasing the accessibility or expression of membrane receptors (26). Conflicting results about the effects of influenza virus infection on meningococcal adherence to epithelial cells have been reported. One study reported that meningococci bind IAV-infected epithelial cells more efficiently than uninfected cells (10), whereas another concluded that coinfection with influenza B virus does not affect the association of meningococci with cultured human nasopharyngeal mucosa (29). Adhesion to epithelial cells in the nasopharynx is the early step enabling N. meningitidis to colonize the upper respiratory tract before it possibly goes through the epithelial barrier into the blood to induce bacteremia and reaches the meningeal spaces to induce meningitis. The adhesion process is associated with a downregulation of the capsule (9). We hypothesized that sialic acid-containing capsules could be a substrate for the neuraminidase (NA) of IAV and that this direct virus-bacterium interaction could play a major role in enhancing meningococcal adhesion to the respiratory epithelium. This hypothesis was tested on isolates belonging to various serogroups of N. meningitidis, using an in vitro model of adhesion to epithelial cells transiently expressing IAV-derived recombinant NAs.