Date Published: August 7, 2014
Publisher: Public Library of Science
Author(s): Adam J. Caulfield, Wyndham W. Lathem, Joseph Heitman.
Fas ligand (FasL, CD95L) is a type-II membrane protein within the tumor necrosis factor (TNF) superfamily of death receptors . FasL shares 25%–30% sequence homology with related family member proteins such as tumor necrosis factor alpha (TNFα) and TNF-related apoptosis-inducing ligand (TRAIL), with the most similarity present in the C-terminal homology ectodomain that extends into the extracellular space for receptor binding . FasL engages and trimerizes the death receptor Fas (CD95) on cell surfaces to initiate the extrinsic apoptosis pathway . The Fas-FasL interaction recruits the Fas-associated death domain adapter protein (FADD) via death domain binding, which interacts with dimerized procaspase-8 to form the death-inducing signaling complex (DISC) . Caspase-8 catalyzes its autoactivation, followed by the proteolytic conversion of downstream effector caspases such as caspase-3 and -7 into their mature forms . Effector caspases direct cell death by apoptosis, which results in nuclear and cytoplasmic condensation followed by cellular fragmentation into membrane-bound apoptotic bodies . Caspase-activated DNase (CAD) cleaves genomic DNA between nucleosomes to generate short fragments prior to cellular condensation and membrane blebbing . Membrane fragments are usually taken up by other cells and degraded in phagosomes via a process known as efferocytosis. Efferocytosis of apoptotic cells contributes to the resolution of inflammation by rapidly clearing cytotoxic cellular debris, and defects in this process can lead to inflammatory diseases such as acute lung injury .
In response to many bacterial pathogens, the host responds by triggering FasL-dependent cell death as an inflammatory innate immune response . Fas-mediated apoptosis of epithelial cells induces the release of proinflammatory cytokines, including TNFα, interleukin 8 (IL-8), macrophage inflammatory protein 2 (MIP-2), monocyte chemotactic protein 1 (MCP-1), and interleukin-1 beta (IL-1β) , . In addition to these cytokines, Fas signaling positively affects CXC chemokine production that leads to enhanced neutrophil infiltration . This apoptotic response is usually a protective mechanism by the host during bacterial infections. Optimal levels of cell death may eliminate replicative niches for intracellular pathogens and enhance further immune cell recruitment through the secretion of cytokines and chemokines, while excessive cell death often leads to an exaggerated immune response, self-tissue damage, and possibly death of the host. Experimentally, the contribution of FasL to inflammatory diseases can be assessed using C57BL/6 FasLgld mice, which contain a single residue mutation (F275L) within FasL that prevents binding to the Fas receptor . Similarly, Fas-FasL signaling may be studied using Faslpr mice, which lack a functional Fas receptor and thus cannot be activated by FasL . In models of pulmonary inflammation, these mice exhibit reduced airway epithelial cell apoptosis, cytokine secretion, neutrophil influx, and tissue damage . Similar results are obtained during knockdown of Fas by small interfering RNA (siRNA) .
We recently reported that Y. pestis prevents Fas-FasL signaling as a distinct pathogenic strategy to reduce apoptosis and enhance disease during mammalian infection . Y. pestis produces the plasminogen activator Pla, a surface-exposed protease that is critical for the progression of the pneumonic (respiratory) form of plague and is required for bacterial outgrowth in the lungs . While cell death via apoptosis predominates during the early, anti-inflammatory stage of pneumonic plague, as the infection progresses, cellular apoptosis is reduced as the synthesis of Pla increases and the bacterial burden rises , .
As with pathogenic Yersinia species, enteric bacterial pathogens have also been shown to manipulate apoptotic signaling to successfully colonize the gut , ; however, direct interactions with the Fas-FasL signaling pathway were only discovered recently. Two independent groups showed that the EPEC T3SS effector NleB disrupts FADD-mediated apoptosis downstream of Fas-FasL engagement within target cells to counteract host defenses and enhance colonization , . After injection into host cells, the N-acetylglucosamine (GlcNAc) transferase activity of NleB post-translationally modifies FADD at a single arginine residue (Figure 1). This residue is conserved among the related proteins TNF receptor type-1 associated death domain protein (TRADD) and receptor-interacting serine/threonine protein kinase 1 (RIPK1), which are also modified by NleB. GlcNAcylation of these proteins prevents death domain oligomerization and thus aborts apoptotic signaling downstream of the TNF family death receptors TNFR1, Fas, and TRAIL. The GlcNAc transferase activity of NleB is specifically required for bacterial gut colonization in a mouse model of EPEC, suggesting that EPEC and related pathogens disrupt Fas-induced apoptosis to overcome the otherwise protective host response conferred by this signaling pathway.
It is becoming clear that the manipulation of cell death is a major strategy by which bacterial pathogens enhance virulence, although the specific mechanisms through which this occurs appear to be different from species to species. Some pathogens actively promote host apoptosis, while others inhibit Fas-FasL signaling. Understanding the in vivo effects of FasL on the virulence of a pathogen is made even more complex since different microbes stimulate varying levels of cell death (and by different pathways) and are likely to produce factors that both induce and abrogate apoptosis, with fine-tuning of cell death pathways for maximal virulence.