It has long been appreciated that Mycobacterium tuberculosis invades the host in small infectious droplets in the lower respiratory tract, rather than in the upper respiratory tract. Now, Ramakrishnan and colleagues suggest an explanation for these long-standing observations: pathogenic mycobacteria express membrane lipids that prevent the activation of microbicidal macrophages in the otherwise sterile lower respiratory tract.

Credit: CORBIS

Microbial pathogen-associated molecular patterns (PAMPs) are recognized by Toll-like receptors (TLRs), which results in the recruitment of macrophages that eradicate the invading organisms. Although mycobacteria are rich in PAMPs, TLR signalling has previously been shown to be dispensable at early stages of infection. Here, the authors used zebrafish larvae that are deficient in Myd88, an adaptor molecule in the TLR signalling pathway, to study macrophage recruitment at early stages of infection with Mycobacterium marinum, a close relative of M. tuberculosis. As cell-surface-associated phthiocerol dimycoceroserate (PDIM) lipids are present on pathogenic mycobacteria, but not on non-pathogenic mycobacteria, the authors speculated that PDIM could mask the PAMPs to avoid induction of TLR signalling. Indeed, infection with M. marinum lacking PDIM was attenuated in the zebrafish model owing to Myd88-dependent macrophage recruitment to the site of infection, whereas macrophage recruitment to the site of wild-type M. marinum infection was independent of Myd88.

iNOS+ macrophages were rarely recruited in response to wild-type M. marinum infection

The microbicidal potential of the different macrophages recruited was investigated by comparing the expression of inducible nitric oxide synthase (iNOS; also known as Nos2a), as the gene encoding this synthase can be expressed in humans, mice and zebrafish upon mycobacterial infection and mycobacteria are sensitive to reactive nitrogen species. Interestingly, iNOS+ macrophages were rarely recruited in response to wild-type M. marinum infection in zebrafish larvae, whereas infection with the PDIM-deficient strain resulted in the recruitment of mainly iNOS+ macrophages.

Finally, the authors investigated whether continual TLR signalling in the upper respiratory tract, owing to the presence of commensal microorganisms, overrides mycobacterial PDIM-dependent immune evasion. Indeed, co-infection of animals with M. marinum and either Staphylococcus aureus or Pseudomonas aeruginosa resulted in the attenuation of wild-type mycobacterial growth. P. aeruginosa was rapidly cleared, which suggests that the attenuation of mycobacterial growth was not due to the physical presence of other bacteria but was rather due to the immune response elicited by the commensals.

These results show that PDIM enables pathogenic mycobacteria to evade TLR detection, which in turn explains why TLR-mediated immunity is dispensable in protection against M. tuberculosis infection. As this immune evasion strategy is only effective in the absence of other TLR-stimulating bacteria, this could explain why mycobacteria only establish infections in the relatively sterile lower lung where they can avoid host commensals.