Toll-like receptor 2 contributes to antibacterial defence against pneumolysin-deficient pneumococci

Abstract

Toll-like receptors (TLRs) are pattern recognition receptors that recognize conserved molecular patterns expressed by pathogens. Pneumolysin, an intracellular toxin found in all Streptococcus pneumoniae clinical isolates, is an important virulence factor of the pneumococcus that is recognized by TLR4. Although TLR2 is considered the most important receptor for Gram-positive bacteria, our laboratory previously could not demonstrate a decisive role for TLR2 in host defence against pneumonia caused by a serotype 3 S. pneumoniae. Here we tested the hypothesis that in the absence of TLR2, S. pneumoniae can still be sensed by the immune system through an interaction between pneumolysin and TLR4. C57BL/6 wild-type (WT) and TLR2 knockout (KO) mice were intranasally infected with either WT S. pneumoniae D39 (serotype 2) or the isogenic pneumolysin-deficient S. pneumoniae strain D39 PLN. TLR2 did not contribute to antibacterial defence against WT S. pneumoniae D39. In contrast, pneumolysin-deficient S. pneumoniae only grew in lungs of TLR2 KO mice. TLR2 KO mice displayed a strongly reduced early inflammatory response in their lungs during pneumonia caused by both pneumolysin-producing and pneumolysin-deficient pneumococci. These data suggest that pneumolysin-induced TLR4 signalling can compensate for TLR2 deficiency during respiratory tract infection with S. pneumoniae.

Fig. 1

TLR2 does not contribute to host defence against WT S. pneumoniae. Survival (A) and bacterial outgrowth (B) of WT mice (closed symbols or bars) and TLR2 KO mice (open symbols or bars) with 5 × 10⁷ cfu S. pneumoniae D39. Mortality was assessed four times daily for 14 days (n = 16 per group). Bacterial loads in WT mice and TLR2 KO mice were determined 24 and 48 h after infection. Data of bacterial loads are mean ± SEM (n = 7–8 per group at each time point).

Fig. 2

TLR2 limits outgrowth of pneumolysin-deficient S. pneumoniae PLN. Survival (A) and bacterial outgrowth (B) of WT mice (closed symbols or bars) and TLR2 KO mice (open symbols or bars) with 5 × 10⁷ cfu S. pneumoniae PLN. Mortality was assessed four times daily for 14 days (n = 13 per group). Bacterial loads in WT mice and TLR2 KO mice were determined 24, 48 and 72 h after infection. Data of bacterial loads are mean ± SEM (n = 7–8 per group at each time point) *P < 0.05 versus WT mice.

Fig. 3

Lung histology in WT and TLR2 KO mice after infection with S. pneumoniae PLN. Representative lung tissue slides from WT mice (A, C and E) and TLR2 KO mice (B, D and F) after infection with 5 × 10⁷ cfu S. pneumoniae PLN. Mice were sacrificed after 24 h (A and B), 48 h (C and D) or 72 h (E and F). HE staining: magnification 4×.

Fig. 4

Reduced lung inflammation in TLR2 KO mice early after infection with S. pneumoniae D39 or S. pneumoniae PLN. Representative lung tissue slides from WT mice (A and C) and TLR2 KO mice (B and D) 6 h after infection with 5 × 10⁷ cfu S. pneumoniae D39 (A and B) or S. pneumoniae PLN (C and D). HE staining: magnification 4×. Insets show Ly-6G staining.