Human Toll-Like Receptors Mediate Cellular Activation by Mycobacterium tuberculosis

Recent studies have implicated a family of mammalian Toll-like receptors (TLR) in the activation of macrophages by Gram-negative and Gram-positive bacterial products. We have previously shown that different TLR proteins mediate cellular activation by the distinct CD14 ligands Gram-negative bacterial LPS and mycobacterial glycolipid lipoarabinomannan (LAM). Here we show that viable Mycobacterium tuberculosis bacilli activated both Chinese hamster ovary cells and murine macrophages that overexpressed either TLR2 or TLR4. This contrasted with Gram-positive bacteria and Mycobacterium avium, which activated cells via TLR2 but not TLR4. Both virulent and attenuated strains of M. tuberculosis could activate the cells in a TLR-dependent manner. Neither membrane-bound nor soluble CD14 was required for bacilli to activate cells in a TLR-dependent manner. We also assessed whether LAM was the mycobacterial cell wall component responsible for TLR-dependent cellular activation by M. tuberculosis. We found that TLR2, but not TLR4, could confer responsiveness to LAM isolated from rapidly growing mycobacteria. In contrast, LAM isolated from M. tuberculosis or Mycobacterium bovis bacillus Calmette-Guérin failed to induce TLR-dependent activation. Lastly, both soluble and cell wall-associated mycobacterial factors were capable of mediating activation via distinct TLR proteins. A soluble heat-stable and protease-resistant factor was found to mediate TLR2-dependent activation, whereas a heat-sensitive cell-associated mycobacterial factor mediated TLR4-dependent activation. Together, our data demonstrate that Toll-like receptors can mediate cellular activation by M. tuberculosis via CD14-independent ligands that are distinct from the mycobacterial cell wall glycolipid LAM.

Activation of Human Monocytic Cells by Borrelia burgdorferi and Treponema pallidum Is Facilitated by CD14 and Correlates with Surface Exposure of Spirochetal Lipoproteins

Here we examined the involvement of CD14 in monocyte activation by motile Borrelia burgdorferi and Treponema pallidum. B. burgdorferi induced secretion of IL-8 by vitamin D3-matured THP-1 cells, which was inhibited by a CD14-specific mAb known to block cellular activation by LPS and the prototypic spirochetal lipoprotein, outer surface protein A. Enhanced responsiveness to B. burgdorferi also was observed when THP-1 cells were transfected with CD14. Because borreliae within the mammalian host and in vitro-cultivated organisms express different lipoproteins, experiments also were performed with “host-adapted” spirochetes grown within dialysis membrane chambers implanted into the peritoneal cavities of rabbits. Stimulation of THP-1 cells by host-adapted organisms was CD14 dependent and, interestingly, was actually greater than that observed with in vitro-cultivated organisms grown at either 34°C or following temperature shift from 23°C to 37°C. Consistent with previous findings that transfection of Chinese hamster ovary cells with CD14 confers responsiveness to LPS but not to outer surface protein A, B. burgdorferi failed to stimulate CD14-transfected Chinese hamster ovary cells. T. pallidum also activated THP-1 cells in a CD14-dependent manner, although its stimulatory capacity was markedly less than that of B. burgdorferi. Moreover, cell activation by motile T. pallidum was considerably less than that induced by treponemal sonicates. Taken together, these findings support the notion that lipoproteins are the principle component of intact spirochetes responsible for monocyte activation, and they indicate that surface exposure of lipoproteins is an important determinant of a spirochetal pathogen’s proinflammatory capacity.

Cutting Edge: Recognition of Gram-Positive Bacterial Cell Wall Components by the Innate Immune System Occurs Via Toll-Like Receptor 2

Invasive infection with Gram-positive and Gram-negative bacteria often results in septic shock and death. The basis for the earliest steps in innate immune response to Gram-positive bacterial infection is poorly understood. The LPS component of the Gram-negative bacterial cell wall appears to activate cells via CD14 and Toll-like receptor (TLR) 2 and TLR4. We hypothesized that Gram-positive bacteria might also be recognized by TLRs. Heterologous expression of human TLR2, but not TLR4, in fibroblasts conferred responsiveness to Staphylococcus aureus and Streptococcus pneumoniae as evidenced by inducible translocation of NF-κB. CD14 coexpression synergistically enhanced TLR2-mediated activation. To determine which components of Gram-positive cell walls activate Toll proteins, we tested a soluble preparation of peptidoglycan prepared from S. aureus. Soluble peptidoglycan substituted for whole organisms. These data suggest that the similarity of clinical response to invasive infection by Gram-positive and Gram-negative bacteria is due to bacterial recognition via similar TLRs.