INTRODUCTION

A balance of apoptosis is necessary to prevent abnormal cellular accumulations or, conversely, heightened cell turnover. A small shift in either direction may have serious clinical and pathophysi­ological repercussions.

In general, infectious agents (bacteria, viruses, and protozoa) and their toxins subvert normal apoptotic pathways in order to achieve one or more of four specific goals: invasion, immune escape, tissue damage, or cellular transformation.

Invasion: Cryptosporidium parvum causes a self-limited diarrheal disease in immunocompe­tent individuals but life-threatening biliary tract disease in those with immunodeficiency. In vitro, C. parvum directly induces apoptosis of human biliary epithelial cells through a Fas/Fas ligand-mediated pathway in which both are upregulated after contact with C. parvum. The result is caspase-dependent biliary cell apoptosis (see below).¹ A similar mechanism is employed by Pseudomonas aeruginosa, which enhances endogenous Fas/Fas ligand production and, consequently, induces apoptosis of pulmonary epithelia.2,3 Other bacterial pathogens, including Legionella pneumophila,⁴,⁵ Clostridium difficile toxin B,⁶ and group A streptococcus,⁷ also induce epithelial apoptosis as part of their pathogen­esis, and although the phenomena are well described, the molecular mechanisms are undefined. In contrast, intestinal pathogens, including Salmonella species and enteroinva- sive Escherichia coli, induce intestinal epithelial cell apoptosis that is only partially inhibited by administration of anti-tumor necrosis factor (TNF)- antibodies or nitric oxide scavengers,8 suggesting that this form of apoptosis is mediated through a non-Fas-depen- dent pathway.

Immune escape: The host response to extracellular bacteria typically involves activation of the humoral immune response in order to generate antibodies, which interfere with bac­terial attachment to host cells, neutralize bacterial toxins, facilitate phagocytosis, or directly activate the complement system. Intracellular bacteria typically induce cell-mediated immune responses, most commonly a delayed-type hypersensitivity (DTH) response. CD4 T helper cells (TH) are essential to the generation of both the humoral and DTH responses. TH cells are activated by encountering an antigen. Depending on the prevailing cytokine environment, TH cells can develop into discrete subsets. The predominance of interleukin (IL)-4 promotes a TH₂ cytokine profile, whereas a predominance of IL-12 promotes a TH₁ cytokine profile. Most commonly, naive TH cells, mast cells, and natural killer (NK) or NK-T cells produce IL-4 after exposure to antigen. Conversely, IL-12 production occurs after macrophage exposure to foreign antigens, most often, intracellular pathogens or bacterial products. Specifically, CD4 T cells skewed toward the TH₂ response activate B cells, which results in a humoral response. Conversely, the generation of a TH₁ skewed response mostly produces IL-2 and interferon (IFN)-, which supports inflammation and activates cytotoxic T cells and macrophages. Together, these result in enhanced T cell killing and macrophage-mediated lysis of invading pathogens.

It has recently become apparent that different bacterial pathogens possess mechanisms of in­ducing apoptosis to inhibit the normal host immune response. The following bacteria were described to induce apoptosis of antigen-presenting cells: Shigellaflexneri, Bordetella pertussis, Listeria monocytogenes, Salmonella typhimurium, Salmonella enteritidis, E. coli, Actinobacillus actinomycetemcomitans, P. aeruginosa, Yersinia enterocolitica, Sta­phylococcus aureus, L. pneumophila, Helicobacter pylori, Mycobacterium tuberculosis, and Mycobacterium bovis.⁹ In the case of salmonella, the invasive toxin SipB is both necessary and sufficient for macrophage apoptosis. SipB enters target-cell macroph­ages and directly cleaves and activates procaspase-1, which results in caspase-1 activity and consequent cytotoxicity.¹⁰ Similarly, the secreted IpaB toxin of S.

Bacteria may also alter mitochondrial regulation of apoptosis. Mitochondria are involved in the initiation, decision, and degradation phases of apoptosis.¹⁷ Neisseria meningitidis and Neisseria gonorrhoeae are Gram-negative bacteria that encode pore proteins that localize to the outer Gram-negative cell wall. Porin B (PorB) from N. meningitidis (PorBm) or from N. gonorrhoeae (PorBg) can translocate from the bacterial cell wall into the mitochondria of mammalian cells. Although both proteins associate with the voltage-dependent anion channel (VDAC) component of the mitochondrial perme­ability transition pore complex (PTPC), PorBg inhibits δψ_m,¹⁸ whereas PorBm induces loss of δψ_m and, consequently, initiates apoptosis.¹⁹ The vacuolating cytotoxin of H. py­lori (VacA) is essential for its cytotoxic effects on gastric mucosa. The N-terminal frag­ment of VacA, p34, localizes to mitochondria, resulting in loss of δψ_m and release of apoptotic mediators. This is inhibited by Bcl-2, which suggests a mechanism that in­volves the PTPC,²⁰ although the protein to which p34 binds is not yet defined.

Tissue damage: Many infections cause tissue damage by enhanced apoptosis. Such may be the case for the ocular pathology of toxoplasmosis. In mice inoculated with Toxoplasma gondii, the Fas/Fas ligand pathway is activated and destroys ocular tissue.²¹ Similarly, the induction of apoptosis was implicated in the effects of Clostridium difficile toxin B on the intestinal surface, which ultimately results in a profound, watery, life-threatening diarrhea.⁶ Bartonella infections may also represent an example of altered apoptotic regulation by bacteria; however, in this context, bartonella seems to inhibit endothelial cell apoptosis, resulting in endothelial cell accumulation and angioproliferation.²²

Transformation: A number of viral pathogens may promote the oncogenic transformation of target cells teleologically in an effort to enhance their survival and increase production of progeny viruses. Viral transformation is often associated with the expression of gene products that inhibit apoptosis, thereby promoting survival of infected target cells. Exam­ples include activation of the PI3 kinase-Akt survival pathways by Epstein-Barr virus (EBV), papillomavirus, Kaposi’s sarcoma-associated herpesvirus (KSHV), polyomavirus, hepatitis B virus (HBV), and hepatitis C virus (HCV).²³ Many of the viruses that more efficiently induce transformation may have multiple mechanisms of inhibiting apoptosis, as exemplified by KSHV. KSHV not only activates Akt survival pathways but also directly enhances NFκB activity as well as inhibits caspase-8 activation through viral FLICE- inhibitory protein (FLIP) expression.²⁴

This chapter will illustrate one disease of patients with human immunodeficiency virus (HIV) that is characterized by excessive apoptosis (cryptosporidium) and one disease characterized by insufficient apoptosis (EBV).