substrates involved in cytoskeletal maintenance and energy metabolism have been identified, suggesting that, analogous to apoptosis, cleavage of these substrates is what mediates the distinct morphology of pyroptotic cells and eventually leads to their death. Another hypothesis stipulates that pyroptotic cells lyse as a result of the formation of membrane pores, which cause loss of ionic equilibrium, water influx, swelling, and membrane rupture with release of inflammatory intracellular contents. Caspase-1–deficient mice are susceptible to infection by various pathogens. The susceptibility of these mice could be attributed to either lack of a proper innate immune response in the absence of mature IL-1β and IL-18 or to a defect in macrophage cell death. Without being able to experimentally tease apart the two different roles of caspase-1 in inflammation and cell death, it is not possible to make conclusions about the role of pyroptosis in pathogen clearance.

3.2.3. Caspase-independent cell death

Cells possess several mechanisms to execute cell death. Several of these are caspase-independent and have been described for infected cells. For instance, although caspase-1–deficient macrophages are initially resistant to death by many bacteria, they eventually succumb in a caspase-independent fashion. Similarly, in the case of Mycobacterium tuberculosis, infected macrophages undergo apoptosis, but inhibition of caspases does not prevent cell death. A serine protease inhibitor appears to block this caspase-independent death.76 Moreover, at high multiplicity of infection (MOI), M. tuberculosis induces a caspase-independent cell death that is not observed at low MOI.77 In the case of Shigella, the primary death mode is pyroptosis, induced through the Ipaf-caspase-1 inflammasome.78 However, at higher MOI, Shigella induces a caspase-1–independent form of cell death, termed pyronecrosis.79 Disease-associated cryopyrin appears to trigger this death mode as well, which is independent of caspase-1 but presumably requires cathepsin B.79 The IPAF–caspase-1 inflammasome has been recently shown to be essential for the initiation of a proper innate immune response to Pseudomonas aeruginosa.50,56 Virulent P. aeruginosa isolates that evade the immune response express the effector protein exoenzyme U (ExoU). Interestingly, ExoU blocks caspase-1 activity and prevents the production of proinflammatory cytokines. However, despite inhibiting caspase-1, ExoU-expressing P. aeruginosa very efficiently killed macrophages.50 Therefore, it appears that caspase-independent death occurs as a “back-up” strategy or when cells are overwhelmed with a high bacterial

load. Whether it performs a physiologic function similar to that of apoptosis or pyroptosis remains open for debate.

3.2.4. Autophagy and autophagic cell death

Autophagy can be triggered in infected host cells, presumably as a host defense mechanism for eliminating pathogens without disposing of the entire cell.80 In a situation in which normal phagolysosomal maturation is blocked, such as during M. tuberculosis infection, the initiation of autophagy can overcome this inhibition and result in bacterial degradation.81 Listeria monocytogenes, Salmonella enterica, Francisella tularensis, and the parasite Toxoplasma gondii have also been shown to be targeted by autophagy.81,82,83 To demonstrate the importance of autophagy in intracellular pathogen clearance, Nakagawa and colleagues82 have shown the effective elimination of the pathogenic group A Streptococcus (GAS) within nonphagocytic cells via autophagy. Atg5−/− cells allowed GAS survival, replication, and subsequent release to the surroundings, indicating that autophagy is protective for the host. Conversely, autophagosome formation may support the replication of poliovirus, rhinovirus, and Legionella pneumophila in host cells, as these microorganisms have devised ways to subvert the autophagosome machinery to their own benefits.84

The type and outcome of pathogen-induced cell death depend on the nature of the infection itself (Figure 32-11). A wide variety of microorganisms have evolved mechanisms to modulate host cell death and to use a step in cell death to their advantage. Characterization of pathogen-induced cell death not only gives insight into disease pathogenesis, but also helps in the understanding of the basic mechanisms of the different cell death modalities under normal physiologic conditions.

4. CONCLUSIONS

Sequencing of the human genome and that of various pathogens, together with advances in molecular biology and genetic manipulation techniques, have resulted in an outburst of discoveries in the host–pathogen field. However, despite past progress in areas such as vaccination, hygiene, and antimicrobials, the extent and impact of infectious diseases on both developed and developing nations is regaining added prominence in the 21st century, as evidenced, for example, by SARS and West Nile virus outbreaks. Numerous circumstances, including rapid societal and technological changes, have

influenced the emergence and re-emergence of infectious diseases. Many of these factors, including aging populations, a heavier chronic disease burden, therapeutic suppression of host defenses, changing behaviors, and strong antibiotic selection pressure, act by increasing human susceptibility to infection. Further work is therefore required to understand the different measures microbial pathogens employ to infect the host and to discover means to strengthen our response and overcome the infection.

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