OBJECTIVETo investigate the roles of Toll-like receptor 2 (TLR2) and Toll-like receptor 4 (TLR4) in host defense against Streptococcus pneumoniae infection in the middle ear.

METHODSWild-type (WT) C57BL/6J, TLR2-deficient (TLR2(-/-)) and TLR4-deficient (TLR4(-/-)) mice were inoculated with Streptococcus pneumoniae (1 × 10(6)CFU) through the tympanic membrane. All animals were tested the mouse ABR thresholds and tympanometry measurement before, and 1 day, 3 days and 7 days following pneumococcal challenge. Blood bacterial titer were determined by plating 50 µl volumes of 10-fold diluted blood. Histological analysis of middle ear and inner ear were performed by fixation, decalcification, embedded section, and counterstained with hematoxylin/eosin and toluidine blue staining. Semi-quantitative RT-PCR was applied to determine mRNA accumulation of TLR2 and TLR4 related genes.

RESULTSForty of 68 TLR2(-/-) mice and twenty-one of 59 TLR4(-/-) mice showed bacteremia and died within 3 days after the pneumococcal challenge, however, only 9 of 52 WT mice died. The survive mice were shown have more severe hearing loss in the TLR2(-/-) and TLR4(-/-) mice than in the WT mice, indicated by ABR thresholds, at 3 or 7 days postinoculation. The histological pathology was characterized by effusion and tissue damage in the middle ear, and in the TLR2(-/-) and TLR4(-/-) mice, the outcome of infection became more severe at 7 days. At both 3 and 7 days after challenge, the TLR2(-/-) mice had higher blood bacterial titers than WT mice (P < 0.05). Temporal bone histopathologic change indicated that 3 days after the pneumococcal challenge, the TLR2(-/-) and TLR4(-/-) mice showed effusion and tissue damage in the middle ear, and the infection became more severe at 7 days postinoculation. TLR2(-/-) mice showed severe inflammatory cell infiltration in the cochlear, the organ of Corti showed the outer hair cells damage, the tectorial membrane swelling, degeneration of the stria vascularis, and severe loss of spiral ganglion cells; However, the WT mice was not found the cell infiltration and tissue damage in the cochlear, the organ of Corti shown normal of outer hair cells. Mast cells were not found in the middle ear mucosa of TLR2(-/-) mice, but in the TLR4(-/-) and WT mice, more mast cells were found in the middle ear mucosa of effusion ear by 3 and 7 days postchallenge. Moreover, by 3 days postchallenge, the mRNA accumulation levels of NF-κB, tumor necrosis factor alpha (TNFα), interleukin1β, MIP-1α, MUC5AC and MUC5B were significantly lower in the ears of TLR2(-/-) mice than that in WT and TLR4(-/-) mice.

CONCLUSIONSTLR2(-/-) mice may produce relatively low levels of proinflammatory cytokines following pneumococcal challenge, thus hindering the clearance of bacteria from the middle ear and leading to sepsis and high mortality rate. This study indicated that TLR2 and TLR4 are important in the molecular pathogenesis and host response to otitis media.