The molecular workings of a general strategy that bacterial pathogens use to thwart the innate immune response have been published in a recent issue of Nature.

Macrophages use cell-surface receptors known as Toll-like receptors (TLRs) to detect the presence of microbial pathogens by recognizing a variety of signals, known as pathogen-associated molecular patterns (PAMPs). The detection of a PAMP triggers the expression of intracellular effector molecules and signalling pathways. Some successful bacterial pathogens have evolved ways to hijack these pathways to facilitate their own survival and replication.

It was already known that the lethal toxin (LT) of Bacillus anthracis induces apoptosis of activated macrophages by preventing activation of the p38 signalling pathway, an important anti-apoptotic pathway. In this latest paper, Michael Karin's group follow-up on these findings by looking at the molecular mechanisms that are involved in apoptosis induction.

In the presence of a synthetic p38 inhibitor, the induction of apoptosis in wild-type macrophages by heat-inactivated B. anthracis was dependent on the presence of TLR4 and a TLR4 ligand. The importance of TLR4 was confirmed by the fact that bone-marrow-derived macrophages (BMDMs) in which TLR4 is inactivated were resistant to killing by heat-inactivated B. anthracis. Additionally, macrophages transfected with a vector expressing the TLR4 cytoplasmic domain underwent apoptosis in the presence of a p38 inhibitor. TLR4 therefore seems to be the key cell-surface receptor involved in B. anthracis-induced macrophage apoptosis.

Hsu et al. went on to look at the intracellular signalling pathways that are triggered by B. anthracis. They found that PKR, a dsRNA-responsive protein kinase, is the crucial factor in pathogen-mediated apoptosis through TLR4. How does the activation of PKR induce apoptosis? Activated PKR inhibits protein synthesis by phosphorylating the elongation initiation factor eIF2α. Hsu et al. obtained macrophages that express an eIF2α variant in which the key PKR phosphorylation site is mutated. BMDMs expressing this mutant initiation factor were less sensitive to apoptosis by LPS/p38 inhibitor incubation than cells expressing the wild-type factor.

Finally, the effects of infection with live B. anthracis and two other bacterial species, Salmonella enterica serovar Typhimurium and Yersinia pseudotuberculosis, were observed. In PKR−/− macrophages, the levels of apoptosis observed were reduced compared with the levels observed in PKR+/+ macrophages. Although TLR4 recognizes a component of the bacterial cell wall, an additional bacterial virulence factor is also required to induce apoptosis — for B. anthracis, this was already known to be LT; the results from this study show that for Y. pseudotuberculosis, it is YopJ, and for S. typhimurium, it is encoded within the SPI2 pathogenicity island.

This work adds further weight to the idea that inducing apoptosis of macrophages, key components of the innate immune response, is a general strategy used by pathogenic bacteria to avoid detection by the host immune system.