**8.6. IL-7**

IL-7 was shown to be a fundamental contributor to thymocyte development as well as a regula‐ tor of T-cell homeostasis in peripheral blood. IL-7 activates both the PI3K/Akt/mTOR and JAK/ STAT pathways suggesting that IL-7 regulates the survival and/or death of T-cells [208].

The IL-7 receptor provides an indicator of the biological activity of IL-7. IL-7R is composed of a γC and Rα polypeptide. JAK3 associates with γC. The binding of JAK3 to γC allows IL-7 dimer formation to occur between γC and Rα so that JAK3 can phosphorylate Rα and/or JAK1 [209]. In most cases, activation of JAK3 causes STAT5 to be phosphorylated.

With respect to relationship between IL-7 and RA, Kim et al. [210] showed that the levels of IL-1β and TNF-α found in the synovial fluid of RA patients could typically increase IL-7 production by stromal cells in culture. In addition, IL-7 was also a strong inducer of RANKL production by T-cells, independent of TNF-α [210]. Interestingly, van Roon et al. [211] showed that TNF-α blockade in RA patients reduced IL-7 production. However, high levels of IL-7 persisted in RA patients who failed to respond to antagonists of TNF-α.

Hartgring et al. [212] found significantly higher amounts of IL-7Rα in the synovial fluid of RA patients as well as in synovial fluid from patients with undifferentiated arthritis. IL-7 level strongly correlated with the number of activated CD3+ T-cells. IL-7Rα was also identi‐ fied on B-cells and macrophages from RA patients, but importantly IL-7Rα-expressing Tcells did not co-express, FoxP3. *Ex vivo* studies performed on monocytes collected from RA patients revealed that recombinant human IL-7Rα inhibited IL-7 induced T-cell proliferation and IFN-γ production suggesting that blockade of IL-7Rα in RA patients reduced the ex‐ pression of the STAT-responsive gene, IFN-γ.

myeloid cell proliferation whereas JAKs controlled the activation of ERK-2 and associated

The results of another study [203] showed that IL-3 played an important role in regulating SOCS3 and PIAS proteins [16, 20, 21] both of which are important in regulating cytokine sig‐ naling as well as the fine-tuning of the survival and/or cell death pathways for immune and non-immune cells in general. IL-3 plays a particularly critical role in regulating these events in mast cells [203], plasmacytoid dendritic cells [204], osteoclast-like cells, [205] and osteo‐ clasts [206, in particular. All of these cell types are involved in some aspect of RA pathology.

To further illustrate this point, Gupta et al. [206] showed that osteoclasts treated with IL-3 were diverted to the dendritic cell lineage which may also be related to the finding that that IL-3 dampened human osteoclast-mediated bone resorption. Most recently, Srivastava et al. [207] showed that IL-3 increased the number of functionally active Treg cells by stimulating the production of IL-2 by non-Treg cells the latter being dependent on the dose of IL-3. Of note, treating mice with CIA with IL-3 significantly reduced the severity of arthritis and also increased the frequency of Treg cells found in the thymus, lymph nodes and spleen. Al‐ though this study [207] did not directly measure the status of activated STAT proteins in the CIA mice treated with IL-3, these additional results showed that treatment of CIA with IL-3 decreased production of IL-6, IL-17A, TNF-α and IL-1 whilst increasing IFN-γ and IL-10

IL-7 was shown to be a fundamental contributor to thymocyte development as well as a regula‐ tor of T-cell homeostasis in peripheral blood. IL-7 activates both the PI3K/Akt/mTOR and JAK/ STAT pathways suggesting that IL-7 regulates the survival and/or death of T-cells [208].

The IL-7 receptor provides an indicator of the biological activity of IL-7. IL-7R is composed of a γC and Rα polypeptide. JAK3 associates with γC. The binding of JAK3 to γC allows IL-7 dimer formation to occur between γC and Rα so that JAK3 can phosphorylate Rα and/or JAK1 [209]. In most cases, activation of JAK3 causes STAT5 to be phosphorylated.

With respect to relationship between IL-7 and RA, Kim et al. [210] showed that the levels of IL-1β and TNF-α found in the synovial fluid of RA patients could typically increase IL-7 production by stromal cells in culture. In addition, IL-7 was also a strong inducer of RANKL production by T-cells, independent of TNF-α [210]. Interestingly, van Roon et al. [211] showed that TNF-α blockade in RA patients reduced IL-7 production. However, high levels

Hartgring et al. [212] found significantly higher amounts of IL-7Rα in the synovial fluid of RA patients as well as in synovial fluid from patients with undifferentiated arthritis. IL-7 level strongly correlated with the number of activated CD3+ T-cells. IL-7Rα was also identi‐ fied on B-cells and macrophages from RA patients, but importantly IL-7Rα-expressing Tcells did not co-express, FoxP3. *Ex vivo* studies performed on monocytes collected from RA patients revealed that recombinant human IL-7Rα inhibited IL-7 induced T-cell proliferation

of IL-7 persisted in RA patients who failed to respond to antagonists of TNF-α.

anti-apoptotic signals [202].

392 Drug Discovery

(Table 1).

**8.6. IL-7**

With respect to the putative role of IL-7 in regulating certain aspects of cartilage responses in arthritis, Yammani et al. [213] reported that IL-7, IL-6 or IL-8 stimulated the production of the Ca2+-binding protein, S100A4, by cultured human articular chondrocytes. Importantly, IL-7 increased the synthesis of S100A4 to a greater extent than either IL-6 or IL-8 with IL-7 stimulated S100A4 resulting from JAK3/STAT3 activation. In that regard, pre-treating chon‐ drocytes with the experimental JAK3, inhibitor, WHI-P154, or with cyclohexamide blocked S100A4 synthesis which also inhibited the production of MMP-13. Because S100A4 has been implicated as significantly contributing to pannus-mediated destruction of cartilage in RA inflammation [214], blockade of IL-7R may be useful for down-regulating the expression of S100A4 and MMP-13 with associated blunting of pannus invasion into cartilage.

The interaction between S100A4 and the tumor suppressor p53 protein was purported to be related to the role of S100A4 as a promoter of cancer metastasis [215]. IL-7 via S100A4 was also shown to induce the expression of MMP-13 as well as MMP-1, MMP-9 and S100A4 was also shown to be involved in the neoangiogenesis and aberrant cell proliferation of rheuma‐ toid synovium [216]. Importantly a selective inhibitor of MMP-13 reduced the level of carti‐ lage destruction in 2 of 3 animal models of RA, including the SCID-mouse co-implantation model and CIA, but not adjuvant arthritis. [217]. Thus, evidence has gradually accumulated to show that up-regulation of S100A4 via activation of STAT3 significantly alters the pro‐ gression of inflammatory arthritis.
