Although the type I IFN response is the predominant antiviral signature associated with immunity to viruses, the cytokines belonging to the IL-1 family such as IL-1β and IL-18 also play an important role in the antiviral response [44
]. These cytokines have potent proinflammatory functions and act in a number of ways to enhance antiviral immunity. IL-1β and IL-18 exert antiviral effects through distinct mechanisms. While IL-18 is mainly involved in coordinating IFN-γ production from NK cells and T cells at the early and late phases of infection, respectively, IL-1β governs the recruitment of inflammatory cells such as neutrophils to the site of infection and is important in the generation of optimal adaptive immunity.
Cytosolic sensing pathways contribute to the generation of IL-1β and IL-18 during virus infection. IL-18 and IL-1β are synthesized as inactive forms either constitutively or following NF-κB activation, respectively. The processing of these zymogens into biologically active cytokines is facilitated by the cysteinyl aspartate protease caspase-1, which is also present in the cell as an inactive zymogen. Caspases are responsible for crucial aspects of inflammation and cell death and can be broadly divided into two classes based on their substrate specificity: those that are pro-apoptotic and those that are proinflammatory. Caspase-1 is part of this latter group, which also includes caspase-4, caspase-5, caspase-11, and caspase-12 (reviewed in [45
]). The requirement for two distinct stimuli to regulate IL-1β production ensures that IL-1β is not inappropriately released, which could have deleterious consequences for the host. Indeed, excess production of IL-1β is associated with a number of hereditary periodic fever syndromes as well as autoimmune and inflammatory diseases such as gout and rheumatoid arthritis [46
A multiprotein complex known as the inflammasome catalyzes the conversion of procaspase-1 to active caspase-1, which in turn catalyzes the conversion of proIL-1β and proIL-18 into the mature cytokines [44
]. Inflammasome complexes form in the cytosol in response to a variety of both pathogenic as well as environmental and endogenous danger signals (reviewed extensively in [48
]). Two distinct families of proteins can form inflammasomes including members of the NLR and ALR (also called pyrin and HIN domain (PYHIN)) families ().The NLRs are a large family of cytosolic sensors (23 members in humans, 34 members in mice) whose crucial role in the immune system is now well accepted. The NLRs have a tripartite structure, consisting of a C-terminal leucine-rich repeat domain, a central nucleotide-binding oligomerization (NOD or NACHT) domain, and a variable N-terminal protein–protein interaction domain, which can be either a CARD, a Pyrin domain (PYD) or a baculovirus inhibitor of apoptosis repeat domain (BIR) [49
]. The PYD, CARD or BIR domains facilitate downstream signaling through protein–protein interactions. The PYD domain (also known as a DAPIN or PAAD domain) is a death domain (DD) protein fold, which forms homotypic interactions with other PYD-containing proteins to form higher complexes with known roles in inflammation, apoptosis, and the cell cycle [51
]. The best understood role of PYD domains relates to their ability to form ‘inflammasomes’. Inflammasome complexes assemble upon activation by an appropriate stimulus leading to the multimerization of the adaptor molecule ASC. Subsequently, procaspase-1 is recruited to ASC by means of interactions between the CARDs of ASC and that of caspase-1. These events lead to the auto-cleavage of caspase-1. The two resulting subunits p10 and p20 assemble into the active caspase-1 that then cleaves IL-1β and IL-18.
Figure 2 Cytosolic receptors that sense viruses and induce inflammasome activation. NLRP3, AIM2, IFI16 sense viral infections and recruit ASC to form caspase-1-containing inflammasome complexes. The autoproteolytic processing of procaspase-1 results in the generation (more ...)
Like the NLRs, the PYHIN proteins AIM2 and IFI16 have a pyrin domain and associate with ASC via pyrin domain interactions to activate caspase-1 and IL-1β/IL-18 processing (). Six verified PYHIN proteins have been uncovered in mice (p202a, p202b, p203, p204, MNDAL, and AIM2) and four in humans (IFI16, AIM2, MNDA, and IFIX). Sequence analysis of the mouse genome predicts that seven additional members of this family likely exist [52
]. The N-terminus of the PYHIN proteins, with the notable exception of p202a and p202b, contains a pyrin (PYD) domain. In addition to a PYD domain, the PYHIN proteins contain a DNA-binding HIN-200 domain.
The NLRP3 inflammasome
Although NLRP3 is the best-studied inflammasome pathway, the precise mechanism of NLRP3 activation remains elusive. NLRP3 is activated by diverse signals including microbial products (bacterial pore forming toxins and mRNA), endogenous products (uric acid and ATP) as well as crystalline particles (silica, asbestos and alum). All of these agonists trigger the assembly of the NLRP3 inflammasome (reviewed in detail in [53
]). NLRP3 has been implicated in the recognition of both DNA and RNA viruses. Several groups have shown that NLRP3 contributes to antiviral responses in influenza A virus infection to varying extents [55
]. Additionally, NLRP3 is involved in recognizing DNA viruses such as adenovirus and modified vaccinia virus Ankara strain [60
]. Two potential mechanisms for the NLRP3 activation by viruses have been proposed. One of these proposes a model whereby viral RNAs lead to NLRP3-dependent IL-1β processing [55
]. Additionally, an ion channel protein M2 of influenza A virus has been shown to trigger the NLRP3 inflammasome by causing perturbations in intracellular ion homeostasis [57
]. How NLRP3 is activated during virus infection therefore remains unclear.
The AIM2 inflammasome
AIM2 forms a caspase-1 activating inflammasome upon sensing dsDNA from the cytosolic bacterial pathogens Fransicella tularensis
and L. monocytogenes
or the DNA viruses vaccinia and mouse cytomegalovirus (mCMV) [62
]. AIM2 does not appear to sense particular sequences of DNA, rather the length of the DNA is the determining feature. Double stranded DNA of less than ~50 base pairs is a poor agonist for AIM2. AIM2 is essential for caspase-1-dependent maturation of IL-1β and IL-18 in response to murine cytomegalovirus and vaccinia virus. In the case of mCMV infection, the ability of AIM2 to regulate IL-18 processing is particularly important. AIM2-deficient mice fail to make optimal amounts of IL-18 and as a consequence have compromised IFN-γ production by NK cells. Accordingly, mice lacking AIM2 are compromised in their ability to control mCMV early during infection. Surprisingly AIM2 is not involved in sensing other DNA viruses such as HSV1. While the mechanistic basis for this observation is unclear it suggests that certain viruses may have evolved mechanisms to escape AIM2-mediated surveillance.
The IFI16 inflammasome
Recently, IFI16 has been reported to form an ASC-containing inflammasome complex following infection with Kaposi sarcoma-associated herpesvirus (KSHV) in endothelial cells [67
]. In contrast to other inflammasomes, IFI16 seems to trigger the inflammasome assembly in response to KSHV, most likely via recognition of its DNA, in the nucleus. However, the basis of differential recognition of viral and host DNA in the nucleus by IFI16 remains unknown.