Synergistic proinflammatory responses induced by polymicrobial colonization of epithelial surfaces

The epithelial surfaces of the upper respiratory tract are continuously exposed to a wide variety of commensal microorganisms. In addition to acting as a physical barrier, epithelial cells respond to specific microbial products with the generation of signals, such as cytokines, that trigger inflammation. Because they are common components of the nasopharyngeal flora that share the potential to cause disease, we investigated the effects of Haemophilus influenzae and Streptococcus pneumoniae, alone and in combination, on human respiratory epithelial cells in culture and in a murine model of nasopharyngeal colonization. Exposure of A549 or Detroit 562 epithelial cells to both S. pneumoniae and H. influenzae led to a synergistic increase in production of IL-8, the major neutrophil chemokine in the airway, through an NF-kappaB-dependent mechanism. Likewise, nasal cocolonization of mice caused a synergistic rise in local production of macrophage inflammatory protein 2 in nasal lavage fluid and subsequent recruitment of neutrophils. This synergistic effect depended on production of the pore-forming cytolytic toxin, pneumolysin, by S. pneumoniae and activation of host p38 mitogen-activated protein kinase. Although both H. influenzae and S. pneumoniae have ligands for Toll-like receptors (TLRs) TLR2 and TLR4, synergistic activation was TLR2- and TLR4-independent. Thus, epithelial surfaces are capable of amplifying proinflammatory responses during concurrent stimulation by multiple microbial species. These synergistic responses, demonstrated both in vitro and in vivo, may contribute to inflammation of heavily colonized mucosal barriers.