# An Overview of Japanese Encephalitis in Australia: Trends, Impact and Interventions
## 2 Host Invasion by JEV, Pathogenesis and Clinical Course
One of the most significant inflammosomes responsible for the neuroinflammatory effects seen in JE is the NLRP3 inflammasome [26]. This inflammasome consists of a sensor (NLRP3; NOD-, LRR- and pyrin domain-containing protein 3), an effector (caspase 1) and an adaptor (known as PYCARD). The NLRP3 gene codes for an intracellular protein sensor that detects an extensive range of microbial antigens, endogenous damage-associated molecular patterns and environmental toxins, causing cellular stress [27]. The exact molecular mechanisms of how the NLRP3 protein senses cellular stress and terminates NLRPP3 activation and inflammasomeformation remain to be determined [27]. However, it is well established that when the NLRP3 protein becomes activated, it dissociates from the endoplasmic reticulum in the cytoplasm and interacts with the mitochondria and the mitochondrial membrane, thus escalating mitochondrial stress, which forms a positive feedback cycle generating toxic reactive oxygen species and inducing mitophagy. This process ultimately aims to eliminate any mitochondria that are damaged or dysfunctional [27]. Animal studies have shown that JEV infection of mouse brain tissues, microglia cells and astrocytes led to increased production of inflammatory cytokines via the activation of various inflammatory signalling pathways (e.g., ERK/MAPKp38/AP-1/NF-\(\kappa\)B) that were positively correlated with the expression of the NLRP3 gene and the inflammasome [28]. These molecular observations align with findings from NLRP3 gene studies conducted on JE patients, confirming the importance of NLRP3 inflammasome-dependent processes in the neuroinflammation and death of brain tissues as patients progress in JE [25]. The CASPASE 1 gene encodes an evolutionarily conserved inflammatory mediated enzyme that plays a pivotal role in regulating the innate immune system primarily through acting as a protease that cleaves and activates pro-inflammatory cytokines, such as interleukin 1 \(\beta\) (IL-1\(\beta\)) and interleukin-18 (IL-18) [29]. The caspase 1 protein also regulates inflammatory-induced cell death through proteolytic activation of Gasdermin D, as well as metabolic regulations, such as homeostasis and lipid metabolism [30; 31]. In the context of JE, caspase 1 regulates JEV-induced inflammatory responses through regulating the extent of pro-inflammatory cytokine production, a process that involves other JE-related inflammosomes. In a mouse model of JE, JEV infection induced the activation of caspase 1, resulting in the production of pro-inflammatory factors proIL-1\(\beta\) and proIL-18 [26]. It has been well established that the role of caspase 1 in innate immunity is interlinked with the NLRP3 inflammasome, which regulates neuroinflammation in JE [26]. Indeed, caspase 1 governs the production of the above pro-inflammatory cytokines in an NLRP3-dependent manner, as the depletion of NLRP3 not only reduced caspase-1 activity but also decreased the subsequent production of these cytokines [26]. Collectively, caspase 1 plays an integral role in the signalling pathways associated with the inflammatory response to JEV infection. These findings are also supported in patients with JE, in which the expression of both CASPASE 1 and NLRP3 genes appears to positively correlate with the progression of disease severity, with a marked elevation identified in severe cases but not among patients with mild symptoms [25]. Although preliminary, findings from this study provide new insights into our understanding of the role that caspase 1 plays in regulating JE progression and ultimately therapeutic development. The PYCARD gene encodes for an adaptor protein consisting of the PYRIN-PAAD-DAPIN domain (PYD) and the caspase-recruitment domain (CARD) at the N and C terminals, respectively [32]. These domains are members of the six-helix bundle death domain superfamily, which, via the activation of Caspase, mediates the assembly of signalling complexes involved in inflammatory and apoptotic pathways [32].