# The Significance of IL-36 Hyperactivation and IL-36R Targeting in Soriasis
## 2 Structural and Functional Characterization of II-36 Receptor
### IL-36R-Dependent Intracellular Signaling  
IL-36R-dependent intracellular pathways has been widely investigated in human keratinocytes, which constitutively express high levels of IL-36 Rrp2 and IL-1 RAcP1 subunits and vividly respond to IL-36s stimulation (Table 1). Previous studies demonstrated in fact that IL-36\(\alpha\), \(\beta\) or \(\gamma\) agonists strongly activate NF-kB, and STAT3 pathways, thus inducing the expression of _IL-36\(\alpha\)_, _S100a8_, _Defh3_, and _IL-17C_ mRNAs in wild-type keratinocytes, but not in cells lacking of IL-36R. [40,41] (Figure 2). Other than NF-kB signaling, IL-36\(\gamma\) also activates the phosphorylation of P38 kinase, known to participate to the induction of neutrophil chemokines IL-8, CXCL and CCL20 in human keratinocytes upon Toll-like receptor (TLR) activation [18,42]. Recently, the intracellular IrB\(\zeta\) mediator has been also uncovered as having an important role in the IL-36-induced intracellular signaling in epidermal keratinocytes. In particular, [PERSON] et al. demonstrated that IL-36\(\alpha\) and IL-36\(\gamma\) agonists are potent inducers of IxB\(\zeta\) expression, an atypical nuclear IxB protein, which acts not only as a repressor but, more importantly, as an activator of a selective subset of NF-kB-dependent inflammatory genes in human keratinocytes [43]. Endothelial cells also constitutively express IL-36R complex and respond to IL-36\(\gamma\) by activating the phosphorylation of NF-kB and c-Jun kinases, known to have pro-inflammatory functions [21] (Table 1) (Figure 2). Consistently, we recently observed that in these cells, IL-36\(\gamma\) strongly induces the phosphorylation of NF-kB/p65, but also of ERKI/2, which is known to induce proliferative responses in different cell types (unpublished data) (Figure 2). Human dermal fibroblasts also express active IL-36R complex and respond to IL-36 cytokines (as discussed in Section 5) (Table 1), although the corresponding intracellular signaling have not been investigated. Previous studies showed that the phosphorylation of P38 and HSP27 cascades is induced by IL-36\(\gamma\) in human synovial fibroblasts via activation of IL-36R complex [44]. In psoriatic skin, dendritic cells and macrophages efficiently respond to IL-36 stimulation in an IL-36R-dependent manner (Table 1), as discussed in Section 5, although a detailed characterization of the intracellular signaling is lacking. Unlike IL-36 agonists, the binding of the two natural antagonists, IL-36 Ra and IL-38, to IL-36R inhibits the activation of the corresponding intracellular pathways [21,44]. In vitro studies demonstrated that IL-36 Ra binds to IL-36R much stronger than either IL-36\(\alpha\) or IL-36\(\gamma\) and, in the presence of agonists, it competes with them for IL-36R occupancy [45] (Figure 2). Consequently, IL-36 Ra binding to IL-36R impedes the recruitment of IL-1 RacP for the ternary IL-36R: IL-1 RAcP1:IL-36 complex formation, thus inhibiting the intracellular pathways, as clearly demonstrated in keratinocytes, endothelial cells, and synovial fibroblasts [18,21,44]. IL-38, the newest member of the IL-1 family, shares 41% homology with IL-1 Ra, 43% homology with IL-36 Ra and lower homology (14-30%) with IL-1\(\beta\) and other IL-1 subfamilies [46]. Similar to IL-36 Ra, IL-38 binds to IL-1 Rrp2 and inhibits the downstream cascades, even if it is unknown whether it blocks the recruitment of IL-1 RacP to the receptor subunit [46]. We recently observed that, similarly to IL-36 Ra, IL-38 impairs the phosphorylation of P65/NFkB and P38 in human keratinocytes induced by IL-36\(\gamma\)[18]. However, in addition to IL-36R, IL-38 can bind, albeit with low affinity, to IL-1R1, but also to the orphan receptor Three Immunoglobulin Domain-containing IL-1 receptor-related 2 (TIGIRR-2), formerly known as interleukin 1 receptor accessory protein- like 1 (IL-1 RAPL1) [32] (Figure 2). [PERSON] et al.