**8.2. IL-20**

IL-10, IL-19, IL-20, and IL-24/mda-7 which are located on chromosome 1q31-32 [167]. IL-19 and IL-20 were predominately expressed in monocytes, as well as non-immune cells under inflam‐ matory conditions [168], whereas IL-22 and IL-26 was only produced by T-cells, especially Th1

Both IL-20 and IL-24 bind to the IL-20R complex which is made up of the cytokine receptor family 2-8/IL-20Rα (IL-20R1) [170], although it was previously shown that IL-19/IL-19 recep‐ tor binding was similar to IL-20/IL-24 receptor binding [170]. IL-19 was also shown to inter‐ act with a DIRS1-like element which is composed of tyrosine recombinase-encoding

In all cases, the binding of IL-19, IL-20 or IL-24 to these receptors caused activation of STAT3 and activation of a minimal promoter region containing those sequences identified as STATbinding sites. Importantly, absent either of the R1 proteins in the two types of receptor com‐ plexes, IL-20R1/IL-20R2 and IL-22R1/IL-22R2 reduced the affinity of IL-19 or IL-24 for these receptors. Furthermore, IL-20R2, and not IL-20R1, was identified as the high affinity recep‐

The functional significance of the IL-10-related cytokines, IL-19, IL-20, IL-21, IL-22 and IL-24 in terms of the pathophysiology of RA and other autoimmune diseases is systematically be‐ ing elucidated. In most cases, the role played by these cytokines has been inferred from

Sakurai et al. [174] showed that IL-19 was produced by cells of human RA synovial tissue. The majority of IL-19 positive cells were vimentin- and CD68-positive, indicating that fibro‐ blasts and macrophages were the main sources of IL-19 in RA synovium. From a functional perspective, synovial tissue lining and sublining layers were both identified with anti-

IL-19 activated synoviocyte STAT3 and, downstream, STAT3 activation caused up-regula‐ tion of IL-6 and IL-19 gene expression whilst decreasing synoviocyte apoptosis induced by serum-starvation [174], a change which may predict the role of IL-19 in the development of synovial hyperplasia [30, 96]. However, the role of IL-19 in RA relative to its activation of signal transduction was further complicated by the findings of Alanärä et al. [175] who showed that IL-1β, an activator of the MAPK pathway [176], also increased the level of IL-19 in peripheral blood mononuclear cells *in vitro*. Combined with other data this result showed that in RA joints IL-19 expression was the highest of all of the IL-10 family cytokines. Fur‐ thermore, these results suggested that IL-19 played a significant role in synovial tissue in‐ flammation, with the caveat that further consideration of IL-19 as a target for intervention in in RA must focus on the relative level of JAK/STAT activation of JAK/STAT versus activa‐

IL-19 was highly expressed in synovial tissue and, in particular, expressed in fibroblasts iso‐ lated from rats with collagen-induced arthritis (CIA) [177]. Of note, treating these rats with a anti-IL-19 antibody, 1BB1, reduced arthritis severity which was accompanied by the lower

measurements in sera of RA patients before and/or after medical therapy.

cells and NK cells, whilst IL-24 synthesis was restricted to monocytes and T-cells [169].

transposons/IL-20Rβ (IL-20R2) [170-172].

tor chain for these cytokines [173].

IL-20R1 and anti-IL-20R2 antibodies.

tion of the other signaling pathways.

**8.1. IL-19**

388 Drug Discovery

IL-20 interacts with IL-20R1/IL-20R2 to activate the JAK/STAT pathway [166] and IL-20 has been implicated in the pathogenesis of autoimmune diseases [178]. However, IL-20R2 sig‐ naling was shown to blunt mouse CD4 and CD8 T-cell responses to antigen *in vitro* and *in vivo* [179]. Thus, it remains to be determined the extent to which IL-20 promotes or sup‐ presses immune-mediated inflammation.

In the CIA model in the rat, treatment with an anti-IL-20 antibody 7E, either alone, or in combination with the TNF blocker, etanercept was compared to etanercept alone for their capacity to 1) ameliorate cartilage damage; 2) stabilize bone mineral density; and 3) alter cy‐ tokine production [180]. In addition, the effect of antibody 7E on expression of various genes implicated in the progression of CIA was evaluated on rat synovial fibroblasts *in vitro.* Treat‐ ment with 7E or etanercept or the combination of 7E and etanercept significantly reduced the severity of arthritis as measured by rat hind paw thickness and swelling. These treat‐ ments also prevented cartilage degradation and bone loss whilst reducing the level of syno‐ vial tissue IL-20, IL-1β, IL-6, RANKL and MMPs. Of note, IL-20 induced the expression of TNF-α in synovial fibroblasts isolated from rats with CIA. Moreover, IL-20 induced RANKL production in synovial fibroblasts, osteoblasts and Th17 cells. In another study, antibody 7E was shown to inhibit mouse osteoclast differentiation induced by macrophage-CSF and RANKL [181]. These results [181] coupled with results from the CIA model [180] indicated that IL-20 was likely to have promoted the increased bone loss in CIA by promoting osteo‐ clast differentiation and the activity of osteoclast-mediated bone resorption.

Correlative human studies of IL-20-mediated responses in RA are just emerging. However, the results have differed somewhat from those seen in the CIA model. Thus, Kragstrup et al. [182] showed that plasma IL-20 levels were increased in RA compared to OA patients with the elevated level of IL-20 primarily localized to mononuclear cells and neutrophils. Stimu‐ lating mononuclear cells isolated from RA synovium with recombinant IL-20 resulted in the increased secretion of the chemoattractant CCL2/MCP-1. However, at variance with find‐ ings in the CIA model, recombinant IL-20 did not alter the expression of TNF-α or IL-6 by mononuclear cells *in vitro*.
