TLR4 on hematopoietic cells is crucial for host defense against Klebsiella pneumonia but TLR2 is needed when bacterial numbers are high

Introduction

Klebsiella species are opportunistic pathogens that can give rise to severe infections including pneumonia and sepsis. Typically, Klebsiella infections are nosocomial and mainly caused by Klebsiella pneumoniae, the medically most important species of the genus.

Objective

We set out to validate and extend our previous data using C3H/HeJ mice that demonstrated an important role for TLR4 in K. pneumoniae pneumonia. Moreover, we were interested in the relative roles of cells from hematopoietic origin and parenchymal cells

Methods

Using TLR2 and TLR4 single and TLR2 × 4 double knockout (KO) mice on a C57BL/6 background, the roles of TLR2 and TLR4 were investigated independently and together. We intranasally inoculated C57BL/6 wild-type (WT) and KO mice with K. pneumoniae (4 × 10^-3 CFU per mouse) and studied host defense. Moreover, we performed bone marrow transplantation (BMT) experiments in which we transplanted KO bone marrow into irradiated WT mice and vice versa.

Results

Shortly after infection, both TLR4 and TLR2 × 4 KO mice demonstrated an attenuated proinflammatory response in the lungs. This was associated with higher bacterial counts 24 hours after infection in the lungs, liver and spleens of both TLR4 and TLR2 × 4 KO animals. Interestingly, although no differences in antibacterial host defense of TLR2 KO animals were observed, TLR2 × 4 KO animals were more susceptible to K. pneumoniae infection than the single TLR4 KO mice: after 44 hours of infection, 0/8 WT, 0/8 TLR2 KO mice, 5/8 TLR4 KO mice and 8/8 TLR2 × 4 KO mice had succumbed. Moreover, when infecting all strains with a high dose of K. pneumoniae (10^-4 CFU), no differences in outgrowth were detected between WT, TLR2 and TLR4 KO animals, whereas double KO animals suffered from higher bacterial burdens in the lungs, liver, spleen and blood. BMT of WT bone marrow into irradiated TLR2 × 4 KO mice resulted in a reversed phenotype with similar bacterial growth compared with syngenic transplanted WT mice

Conclusion

These data confirm our previous research that, during low-dose infections, TLR4 is of primary importance in host defense against K. pneumoniae. Nevertheless, when high numbers of bacteria are present, TLR2 acts together with TLR4 to orchestrate the immune response, a protective effect that is primarily mediated by hematopoietic cells.