Bacterial pathogens use complex strategies to survive and replicate, causing disease as they do so. But how these strategies, which are usually mediated by molecules called virulence factors, are regulated during infection is poorly understood. In a paper published in Cell, Arpaia et al.1 elegantly demonstrate that the Typhimurium serovar of the bacterium Salmonella enterica activates the host immune system and then uses this innate response as a signal to induce its own virulence genes.
Salmonella species are Gram-negative intestinal pathogens that cause disease ranging from gastroenteritis to typhoid fever, which can be fatal2. They are transmitted mainly through contaminated food or water, and colonize their host's intestine. In typhoid fever, the pathogens can cross the intestinal barrier and spread to the spleen and liver by entering and replicating in phagocytic immune cells such as macrophages2.
The innate immune system is crucial for controlling infectious agents. It recognizes general pathogen-associated molecular patterns (PAMPs), and its function is to both kill pathogens and alert the adaptive immune system3. Integral to innate immunity is a family of proteins called Toll-like receptors (TLRs), which recognize various PAMPs and initiate signalling cascades that act to protect the host3.
Mammals have many TLRs, each of which recognizes certain PAMPs. For example, TLR4, found on the cell surface, recognizes lipopolysaccharide, a component of the bacterial cell wall3. TLR2, which is also located in the cell membrane, recognizes bacterial surface lipoproteins. And TLR9, located in the membrane of cellular organelles called endosomes, recognizes non-methylated CpG sequences in bacterial DNA.
When a TLR binds its specific PAMP, host-cell adaptor molecules — such as MyD88 and TRIF — are recruited, and downstream signalling is initiated3. TLR signalling helps to resolve infection not just by activating specific bactericidal mechanisms, but also indirectly by inducing the production of pro-inflammatory proteins such as cytokines. The function of these receptors has generally been investigated in cell lines or in mice lacking MyD88 or TRIF. But this can be problematic, because MyD88 and TRIF also act in non-TLR-associated cellular pathways1, 3.
Arpaia et al.1 show that Salmonella activates its virulence genes by using as a signal TLR-induced acidification of the phagosome — the intracellular compartment that encloses the engulfed pathogen. The molecules encoded by these genes allow the bacterium to replicate in the normally bactericidal phagosome.
The authors find that mice lacking TLRs 2, 4 and 9 (TLR2×4×9 mice) were less susceptible to Salmonellainfection than those lacking only TLRs 2 and 4 or TLRs 4 and 9. Moreover, bone-marrow-derived macrophages (BMMs) from TLR2×4×9 mice or MyD88×TRIF mice were more resistant to Salmonella infection than the equivalent cells from TLR2×4 mice. In fact, Arpaia et al. report that TLR2×4×9 BMMs can support the replication of several intracellular pathogens, but not Salmonella. These observations indicate the importance of TLR signalling in Salmonella virulence.
How does Salmonella exploit TLR signalling? During an infection, these pathogens normally transform the phagosome into a special vacuole, termed the Salmonella-containing vacuole (SCV), in which they replicate2. In TLR2×4×9 BMMs, however, SCV formation is impaired, and the bacteria are detected in the cytoplasm and associate with lysosomes — intracellular organelles that have bactericidal activity (Fig. 1).
Salmonella forms, and survives within, the SCV by activating a set of genes that occur in a locus within their genome termed SPI-2. These genes encode a type-III secretion system that functions as a syringe to translocate SPI-2 effector proteins from the bacterial cytoplasm into the host-cell cytoplasm2, 4. The effectors manipulate the host cell to allow bacterial replication2. Arpaia et al. studied levels of SPI-2 gene expression and effector translocation and found that — unlike the case in normal or TLR2×4 BMMs — SPI-2 was not induced in TLR2×4×9 cells. The authors could restore bacterial replication in TLR2×4×9 BMMs using a Salmonella mutant that constitutively expresses SPI-2 genes5.
This paper1 clearly shows that, rather than protecting against Salmonella infection, functional TLR signalling contributes to the pathogen's full virulence. Arpaia et al.1 shed light on this apparent paradox by demonstrating that the vacuole of TLR2×4×9 and MyD88×TRIF BMMs does not acidify as rapidly or to the same extent as the SCV of normal and TLR2×4 BMMs. It is widely accepted6 that Salmonella uses pH as a signal for SPI-2-gene induction and effector translocation. So it seems that the bacteria take advantage of the essential, and additive, effects of TLRs on vacuole acidification to modulate the expression of virulence genes.
The study also raises several questions. What are the exact pathway(s) linking TLRs and vacuolar acidification? Some SPI-2 effectors also manipulate aspects of the host immune system — for instance, the activity of the NF-κB transcription factor7. Such specific effectors might also modulate TLR signalling to induce the expression of virulence genes. Alternatively, more-complex regulatory steps may be involved.
And how do TLRs interact with intracellular Salmonella? Does Salmonella release specific ligands such as nucleic acids into the phagosome to activate TLRs? There is a precedent for this in other species8. So it is possible that Salmonella also uses such ligands to induce a robust immune response.
More broadly, do Salmonella and other bacteria exploit other host immune responses? Aparia and colleagues' work underlines the importance of collaboration between workers in immunology and microbiology. Together, these research areas are set to shed light on the intricate interactions between pathogens and their hosts