Role of Toll-like receptor 4 in gram-positive and gram-negative pneumonia in mice

Abstract

To determine the role of Toll-like receptor 4 (TLR4) in the immune response to pneumonia, C3H/HeJ mice (which display a mutant nonfunctional TLR4) and C3H/HeN wild-type mice were intranasally infected with either Streptococcus pneumoniae (a common gram-positive respiratory pathogen) or Klebsiella pneumoniae (a common gram-negative respiratory pathogen). In cases of pneumococcal pneumonia, TLR4 mutant mice showed a reduced survival only after infection with low-level bacterial doses, which was associated with a higher bacterial burden in their lungs 48 h postinfection. In Klebsiella pneumonia, TLR4 mutant mice demonstrated a shortened survival after infection with either a low- or a high-level bacterial dose together with an enhanced bacterial outgrowth in their lungs. These data suggest that TLR4 contributes to a protective immune response in both pneumococcal and Klebsiella pneumonia and that its role is more important in respiratory tract infection caused by the latter (gram-negative) pathogen.

FIG. 1.

Survival of TLR4 mutant and WT mice after intranasal inoculation with 4 × 10³ CFU (A), 6 × 10³ CFU (B), or 6 × 10⁴ CFU (C) of S. pneumoniae organisms. A total of 12 mice per group were studied. Survival of TLR4 mutant mice inoculated with 6.5 × 10³ CFU was significantly decreased compared to that seen with WT mice (P < 0.05).

FIG. 2.

Survival of TLR4 mutant and WT mice after intranasal inoculation with 50 (A) or 600 (B) CFU of K. pneumoniae organisms. A total of 8 to 16 mice per group were studied. Survival in TLR4 mutant mice was significantly decreased compared to that seen with WT mice in both experiments (P < 0.05).

FIG. 3.

Bacterial outgrowth in lungs in TLR4 mutant and WT mice at 24 and 48 h after intranasal inoculation with 10⁴ CFU of S. pneumoniae organisms (A) and at 6 and 24 h after intranasal inoculation with 200 CFU of K. pneumoniae organisms (B). Data represent means ± SEM of the results for eight mice. *, P < 0.05 versus results for WT mice.

FIG. 4.

Representative lung histology of WT (A and C) and TLR4 mutant (B and D) mice 24 h after S. pneumoniae (A and B) and K. pneumoniae (C and D) inoculation. Data are representative of the results for five mice per group after hematoxylin and eosin staining. Magnification, ×10.