This study demonstrates that IL-7 strongly increases the severity of arthritis and joint destruction in mice. Administration of IL-7 augmented radiology-assessed joint destruction, consistent with IL-7-increased intensity of cell infiltrates, bone erosions, cartilage damage, and osteoclast activity. This was associated with expansion of total splenic cell number, CD4+ T cells, CD8+ T cells, CD19+ B cells, and germinal center B cells. IL-7 expanded circulating central and effector memory T cells as well as Th1, Th2 and Th17 cell numbers. Furthermore, collagen type II-driven activation of draining lymph node cells was associated with increased cytokines indicative of Th1 and Th17, but not Th2 activation. Finally, activation of T and B cells was associated with increases in proinflammatory cytokines and tissue-destructive mediators.
In RA patients, IL-7Rbright
T cells are highly proliferative and largely lack Foxp3 expression, compared with poorly proliferating IL-7Rlow/-
) T cells, indicating that IL-7R+
T cells in particular may promote immune activation [11
]. In addition, blockade of IL-7R-mediated immune activation by soluble human IL-7 receptor inhibited IFN-γ production (indicative of Th1 cell activity) in mononuclear cells from RA patients [11
]. Furthermore, IL-7 induces IL-17 production by mononuclear cells from RA patients. Th1 and Th17 activity in experimental arthritis promote joint destruction [21
]. Consistent with these findings, we demonstrated that increased joint destruction induced by IL-7 is associated with increased antigen-driven Th1 and Th17 cell activity of draining lymph node cells.
At a systemic level, IL-7 also increased Th2 cell numbers. In contrast, antigen activation of draining lymph node cells that contain reactive T cells derived from a site of inflammation only resulted in limited amounts of IL-4, which was reduced to undetectable levels on IL-7 treatment. These data suggest that, although as a consequence of the homeostatic potential of IL-7, CD4+
T cells producing IL-4 may be increased systemically, this may not result in significant intraarticular IL-4 production, contributing to the predominance of Th1 and Th17 activity. The local predominance of Th1 and Th17 over Th2 activity in joints of RA patients is in line with these data [23
]. Supporting these findings, IL-7R blockade was associated with reduced IFN-γ and IL-17 levels and with reduced arthritis severity and immunopathology [19
IL-7 has significant effects on T-cell numbers in many experimental studies and in humans, and has been shown to increase B-cell numbers profoundly in mice in contrast with humans [25
]. The stimulatory effects of IL-7 on CIA in this study, associated with expansion of central and effector memory CD4+
T cells, and increased B cells and germinal center B cells, are in line with these results. Although we did not analyze B-cell activation markers in our study, the significant increase in numbers of germinal center B cells, which are potent antibody-producing cells, suggests that IL-7 facilitates B-cell activation either direct or indirect via T cells or other cell types and, through this, plays a significant role in the IL-7-induced increased immunopathology. In line with this, we demonstrated that IL-7 induces T-cell- and monocyte-dependent B-cell activation in a human in vitro
Decreased MPO might reflect reduced circulating neutrophil numbers as a consequence of granulocyte migration to the site of inflammation, but this remains to be demonstrated. In line with this, we demonstrated that local levels of MPO were abundantly present and strongly reduced with IL-7R blockade, indicating that IL-7R-induced T-cell activation and increased arthritis are associated with increased granulocyte activity in the inflamed joints [19
In support of the T-cell activation seen in this study, serum concentrations of CD40L, indicative of T-cell activation, were increased by IL-7. In addition to inducing T-cell-related cytokines, IL-7 can induce other proinflammatory cytokines, largely produced by human monocytes, including TNF-α, IL-1β, IP-10, MIG, MIP-1γ, TARC, and IL-8 [8
]. In line with these proinflammatory capacities, we demonstrated that IL-7 administration in CIA mice significantly upregulates chemokines that are capable of facilitating migration of lymphocytes (MIP1γ, MIP-3β, and lymphotactin), monocytes/macrophages (MCP-5), and granulocytes (MDC). The immune-stimulatory effect of IL-7 was also reflected by a significant increase in von Willibrand factor and mediators indicative of endothelial cell adhesion and angiogenesis: VCAM-1, VEGF, and TPO. The latter two have been described as very important in initiation of synovial hyperplasia in RA [28
]. Together, this strongly suggests that IL-7 facilitates arthritis by increasing chemotaxis of leukocytes, together with enhancement of vascular permeability and neovascularization, eventually leading to increased synovitis and tissue destruction.
IL-7-induced activation of human effector CD4+
T cells upregulates expression and production of receptor activator of nuclear factor (NF)-κB ligand (RANKL), a key factor in osteoclastogenesis regulation [30
]. Supporting the described capacity of IL-7 to increase joint destruction, the present study showed that IL-7 increased leukemia inhibitory factor (LIF) and FGF-basic. LIF is an IL-6-like cytokine, involved in regulation of bone remodeling and bone cell function by stimulation of RANKL [32
]. FGF-basic induces RANKL expression and osteoclast maturation [34
]. These data coincide with T cell-dependent osteoclast activity/formation, leading to osteopenia and increased bone resorption in IL-7-overexpressing transgenic mice [35
Increased loss of joint space, indicating cartilage destruction, was observed on treatment with IL-7. This may result from a local IL-7-induced increase in proinflammatory mediators that induce cartilage destruction, such as TNF-α and oncostatin M. IL-7R blockade has been demonstrated to reduce local expression of such cytokines, thereby preventing cartilage destruction [19
]. Unfortunately, in the present study, mediators such as TNF-α and oncostatin M could not be detected in the serum of arthritic mice. However, IL-7 administration in CIA did increase concentrations of FGF-basic, which has been suggested to be involved in cartilage degradation, mostly through production of MMPs. In addition, cartilage damage may result from direct effects of IL-7 on articular chondrocytes. Chondrocytes were found to express IL-7R and respond to IL-7 stimulation with production of MMP-13 and with proteoglycan release from cartilage explants [36
]. Furthermore, increased IL-7 expression has been detected in chondrocytes from RA patients [37
]. Although in the present study, a direct effect of IL-7 on cartilage could not be demonstrated, this study demonstrates IL-7-induced cartilage degradation in addition to bone destruction.