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Moraxella catarrhalis  is a Gram negative bacterium and a leading causative agent of otitis media (OM) in children. Several recent reports have provided strong evidence for an association between toll like receptors and OM. It has been found that both  Streptococcus pneumoniae  and nontypeable  Haemophilus influenzae  activate host protective immune responses through toll like receptors (TLRs), however, the precise mechanism by which  Moraxella catarrhalis  initiates the host immune response is currently unknown. In this report, using murine macrophages generated from a series of knock-out mice, we have demonstrated that  M. catarrhalis  lipooligosaccharide (LOS) and either heat killed or live bacteria are recognized by one or more TLRs. LOS activates the host immune response through a membrane bound CD14-TLR4 complex, while both heat killed and live  M.cat  require recognition by multiple toll like receptors such as TLR2, TLR4 and TLR9 without the requirement of CD14. We have also shown that  M.cat  stimuli are capable of triggering the host innate immune response by both MyD88- and TRIF- dependent signaling pathways. We further showed that  M.cat  induced activation of mitogen activated protein kinase (MAPK) is essential in order to achieve optimal secretion of pro-inflammatory cytokine TNF-α. We finally showed that TLR4 mutant C3H/HeJ mice produce significantly lower levels of pro-inflammatory cytokines TNF-α and IL-6  in vivo , An increased bacterial loads at 12 and 24 hours ( P<0.001 ) in their lungs upon challenge with live  M.cat  in an aerosol chamber compared to wild-type (WT) control mice. These data suggest that TLRs are crucial for an effective innate immune response induced by  M.cat . The results of these studies contribute to an increased understanding of molecular mechanism and possible novel treatment strategies for diseases caused by  M.cat  by specifically targeting TLRs and their signaling pathways.

Moraxella catarrhalis Activates Murine Macrophages through Multiple Toll Like Receptors and Has Reduced Clearance in Lungs from TLR4 Mutant Mice