## 4 The Intersection of Vitamin D and the Inflammasome  
Inflammasome pathways form an integral role in the innate immune system, and inappropriate inflammosome activity is strongly implicated in an array of human disease. Data from genetic studies suggested a role for the VDR affecting inflammasome activity. [PERSON] et al. showed that a single nucleotide polymorphism (SNP) in the VDR gene is strongly associated with increased inflammasome activity and circulating levels of IL-1\(\beta\) and IL-18 [44].  
This effect appears to be due to a direct interaction between VDR and NLRP3. [PERSON] et al. performed co-immunoprecipitation of VDR and NLRP3 in ex vivo murine CD4+ T cells, showing VDR complexing with NLRP3 in nuclear extracts. These complexes could also be formed in vivo, and appeared to be functionally significant with chromatin precipitation, demonstrating higher NLRP3 and RNA polymerase II activity at the IL-4 locus in VDR knockout cells [19].  
[PERSON] et al. further investigated the interaction between VDR and the NLRP3 inflammasome. Notably, co-transfection of VDR with other components of the inflammasome did not find any interaction with either ASC or caspase-1. The interacting elements were found to be the ligand-binding domain of VDR with the amino-terminal pyrin domain of NLRP3 [20]. They further demonstrated that VDR knockout macrophages have increased IL-1\(\beta\) and IL-18 when activated by either the canonical or non-canonical pathways, without a concomitant increase in inflammasome-independent cytokines such as tumour necrosis factor \(\alpha\). These effects were then attenuated with the restoration of VDR expression [20].  
Deubiquitination of NLRP3 is a critical step in inflammasome activation and the binding of NLRP3 to ASC. BRCC3 is a deubiquitinase that has a crucial role in posttranscriptional activation of NLRP3, the inhibition of which almost completely blocks NLRP3 activation [24]. [PERSON] et al. further demonstrated that the VDR-binding of NLRP3 appears to attenuate BRCC3-mediated deubiquitination of the LRR domain of NLRP3 [20]. This increase in NLRP3 ubiquitination leads to increased protein degradation [18].  
Independent of VDR-NLRP3 binding and inhibition, VDR can also increase levels of the mitochondrial membrane protein UCP2 (uncoupling protein-2). This protein directly prevents the production of ROS, and its activity is inhibited by the inflammasome. [PERSON] et al. showed that UCP2 levels were significantly increased when in vitro 1,25(OH)-VD\({}_{3}\) solution was added to canonically activated macrophage cell cultures [18]. These actions are summarised in Figure 2.  
VDR agonists can induce these effects by their actions on the VDR to reduce inflammasome activity. Studies have shown that 25(OH)-VD\({}_{3}\) and 1,25(OH)\({}_{2}\)-VD\({}_{3}\) are both capable of inhibiting NLRP3 inflammasome activation and downstream IL-1\(\beta\) signalling [20; 45; 46].  
These actions seem to be beneficial in multiple models of disease. In in vitro models of air pollution, 1,25(OH)\({}_{2}\)-VD\({}_{3}\) attenuated the inflammatory response mediated by p38/NF-kB/NLRP3 [47]. In a murine model of liver injury, VDR agonist calcipotriol demonstrated NLRP3-attenuating effects that reduced the severity of liver injury and fibrosis [48]. Similarly, in a murine model of ulcerative colitis, 1,25(OH)\({}_{2}\)-VD\({}_{3}\) treatment reduced colonic inflammation, likely due to its anti-inflammatoryine effects [18]. Meanwhile, a murine model of diabetic retinopathy demonstrated significant downregulation of the NLRP3 inflammasome response in the setting of hyperglyaemia, with a subsequent reduction in apoptosis and preservation of normal retinal structure [49].