receptors. However, for this recognition to be possible, PAMPs must reach the cytosol. Various mechanisms of PAMP delivery to the cytosol have been described that result in inflammasome activation. For instance, the facultative intracellular Gram-positive bacteria Listeria monocytogenes escapes the phagosome into the host cytosol, its replicative niche, via the actions of a pore-forming toxin named listeriolysin O (LLO).52 In the cytosol, Listeria is sensed by multiple inflammasomes.53 Bacterial secretion systems have also been shown to deliver PAMPs to the cytosol. Salmonella typhimurium SipB, thought previously to activate caspase-1 through direct binding,54 mediates the activation of the Ipaf inflammasome indirectly by facilitating the delivery of bacterial flagellin into the cytosol through a type III secretion system.55 Flagellin, from Legionella pneumophila49 and pseudomonas aeruginosa, is similarly delivered via bacterial secretion systems and sensed by the Ipaf inflammasome, leading to caspase-1 activation.50,51,56 This mechanism of delivery to the cytosol is not exclusive to flagellin, as bacterial secretion systems have been also proposed to translocate peptidoglycan derivatives from extracellular bacteria, such as Helicobacter pylori57 and EHEC,38 resulting in the activation of Nod signaling. An alternative mechanism through which bacterial products could reach the cytosol is through the pannexin pore.58,59 Pannexin is a hemichannel that gets recruited to the cell membrane in response to activation of the P2X7 receptor by adenosine triphosphate and opening of K+ channels.60 This leads to the gradual formation of a larger pore that has been shown to mediate the intracellular delivery of muramyl dipeptide, leading to caspase-1 activation.59,61

Pore formation in the plasma membrane by various microbial toxins such as nigericin, maitotoxin,53 and aerolysin62 also results in inflammasome activation, although independently of bacterial product translocation into the cytosol. It is proposed that K+ efflux from the cell through these pores or through K+ channels is sensed as a danger signal by the Nlrp3 inflammasome.53 Unlike Ipaf activation, which seems to be specific to flagellin and bacterial type III secretion systems, the Nlrp3 inflammasome is activated by a wide array of triggers, some derived from pathogens, but others derived from the host. The list of Nlrp3 activators is expanding steadily and includes thus far toxins (nigericin, maitotoxin, aerolysin, listeriolysin O, gramicidin, and α-toxin),53,62,63 K+ efflux,64 UVB,65 asbestos and silica,66 viral and bacterial RNA,67 uric acid crystals,68 and microbial and host DNA.69 The mechanism by which the Nlrp3 inflammasome responds to all these divergent activators possibly relies on the generation of a downstream signal that itself feeds down on the activation of the

inflammasome. Perturbation of the intracellular ionic milieu and/or production of intermediate metabolites, such as ROS, have been proposed to be the link to the Nlrp3 inflammasome.64,70

3.3. Cell death

Cell death is a key process that tailors host–pathogen interactions and is the most common outcome during infections. The death of an infected cell is oftentimes concomitant with the death of the infecting agent and can promote efficient pathogen clearance. Destruction of infected tissues may also eliminate a pathogenic niche, thereby hampering microorganism replication and dissemination.

Pathogens have devised strategies to inhibit cell death for a successful infection (Figure 32-10). Conversely, certain pathogens induce death of immune cells as a means to subvert normal host defense mechanisms and of epithelial and endothelial cells for invasion to deeper layers of an organ and the bloodstream. Killing of phagocytes impairs pathogen clearance and is detrimental to the host. By producing cytotoxic pore-forming exotoxins, bacteria such as Bacillus anthracis,71 Actinobacillus actinomycetemcomitans,72 and Pseudomonas aeruginosa73 kill macrophages before they themselves are phagocytosed and destroyed. Similarly, Bordetella pertussis adenylate cyclase hemolysin secretion during the early stages of infection may allow for successful colonization of alveolar tissue by eliminating the local macrophage population.74

3.3.1. Apoptosis and pathogen clearance

The mechanism of pathogen-induced cell death often involves the modulation of the apoptotic response. Apoptosis of infected cells is thought to dampen pathogen spread yet protect the integrity of infected tissues. Several pathogens induce apoptosis by interfering with the NF-κB and/or MAPK cell survival pathways. For example, Salmonella AvrA and Yersinia YopJ proteins inhibit NF-κB activation, whereas Bacillus anthracis’s protease lethal factor (LF), a component of its LT, targets MKK6 to dampen MAPK signaling (Figure 32-10). Other pathogens have devised strategies to inhibit cell death for a successful infection. For obligate intracellular organisms such as Rickettsia rickettsii, a viable host cell is required for bacteria to replicate and thrive. By stimulating NF-κB signaling, Rickettsia prevents host cell death and continues to replicate unabated. Another intracellular pathogen, Chlamydiae spp., also protects infected cells from death during the early invasive stages of the disease, presumably by blocking cytochrome c release from the mitochondria. Genetic studies provide

S. pneumoniae

BH3

BH3

the most stringent evidence of apoptosis induction by a pathogen and allow the evaluation of the effects of cell death on host resistance to infection. An excellent example is that of alveolar macrophage apoptosis by pneumococci. Over-expression of the antiapoptotic protein Mcl-1 in a transgenic mouse model blocks apoptosis and renders mice susceptible to infection, indicating that apoptosis is indeed triggered by pneumococci and that it is protective for the host. On the other hand, apoptosis could be pathogenic. During sepsis, lymphocytes are depleted by apoptosis, which leads to anergy and immunosuppression. In an experimental model of sepsis, inhibition of apoptosis by selective caspase-3 inhibitors or by Bcl-2 over-expression was shown to lower sepsis-related mortality.

3.3.2. Pyroptosis

Cell death triggered by intracellular pathogens such as Salmonella and Shigella was initially thought to occur by apoptosis, a death executed by apoptotic caspases.

Later, however, caspase-1, a prototypical inflammatory caspase, was demonstrated to be the central effector of this cell death. This is evidenced by the resistance of caspase-1–deficient macrophages to the quick death induced by these pathogens. Unlike apoptosis, caspase- 1–dependent cell death is accompanied by the release of proinflammatory mediators from the cell. The term pyroptosis has been proposed to describe this proinflammatory programmed cell death (pyro relating to fire or fever and ptosis denoting falling). Caspase-1 is essential for the processing, maturation, and release of the cytokines IL-1β and IL-18. It has also been recently proposed to act as a regulator of unconventional protein secretion,75 a process that might mediate the release of “danger” proteins to the extracellular milieu. These functions of caspase-1 contribute to the proinflammatory nature of pyroptosis (Figure 32-11). The release of inflammatory factors by pyroptotic cells is, however, not what kills the cell. The mechanisms by which caspase-1 executes cell death have been recently addressed. Multiple direct caspase-1