Therapeutic Targeting of T Cells in Systemic Sclerosis
Therapeutic Targeting of T Cells in Systemic Sclerosis
Clearly, the presence of these T cells at the site of fibrosis in patients with SSc implicates a role for the T-cell repertoire in the pathogenesis of the disease. The distorted T-cell population is reflected in the polarized cytokine milieu present in SSc patients. The increase in Th2 and Th17 cells, plus the apparently nonresponsive Treg population, results in an increase in Th2 and Th17 cytokines including IL-4, -6, -13, -17 and TNF-α in the serum of SSc patients, which may induce fibrosis, directly or indirectly.
IL-4 is a pleiotropic cytokine involved in the differentiation of T cells that can induce the expression of MHC class II molecules and IgE secretion on B cells. IL-4 signals via its receptor, which consists of the Il-4Rα chain, that binds IL-4 with high affinity, and the γ common chain, which contains the signaling domain. Binding of IL-4 to IL-4R can result in activation of the Janus kinase (JAK) and the STAT signaling cascades. It has been shown that there is an increase in IL-4-producing T cells in the bronchoalveolar lavage fluid of SSc patients. However, the role of IL-4 in causing fibrosis is controversial, since opposing studies suggest either an antifibrotic or a profibrotic role for IL-4. Izbicki et al. used an IL-4 knockout and IL-4 transgenic bleomycin mouse model to demonstrate the effect that the presence or absence of IL-4 has in causing fibrosis. In bleomycin-treated IL-4 mice, an increase in lung fibrosis was observed, whereas in mice overexpressing IL-4 lung fibrosis was decreased, suggesting an antifibrotic function of IL-4. Huaux et al. demonstrated the pleiotropic characteristics of IL-4 using an IL-4 bleomycin-treated mouse model and examining the extent of pulmonary fibrosis. In this model, an increase in lymphocytes was observed in the bronchoalveolar lavage fluid but pulmonary fibrosis was decreased. However, Huaux et al. also showed that stimulation of pulmonary fibroblasts with IL-4 did not induce proliferation of these cells. In contrast to these observations, it has been shown that stimulation of healthy dermal fibroblasts with IL-4 results in an increase in collagen production by these cells due to an activation of the ERK pathway. In addition, Postlethwaite et al. demonstrated that stimulation of dermal fibroblasts with recombinant IL-4 induces collagen production, which is inhibited by the addition of IL-4 antibody. Furthermore, use of the tight skin model of SSc implicates a role for IL-4 in causing dermal fibrosis. In IL-4 tight skin models, a decrease in skin fibrosis is observed. Further to this, it was found that the stimulation of tight skin dermal fibroblasts with IL-4 induced collagen production, which was attenuated with the addition of anti-IL-4 antibody.
IL-6 is another pleiotropic cytokine that is increased in the serum of SSc patients. IL-6 signaling requires the expression of two receptors: IL-6R and gp130. IL-6R is only expressed on hepatocytes, neutrophils, monocytes and leukocytes, whereas gp130 is ubiquitously expressed. gp130 is a critical shared component of IL-6-related cytokine ligands. A soluble form of the receptor (sIL-6R) also exists, owing to the cleaving of IL-6R from activated lymphocytes or hepatocytes by the enzyme ADAM17. IL-6 signaling can occur via classical signaling, in which two IL-6 molecules bind two IL-6R chains, this in turn recruits two gp130 molecules; or via trans-signaling, in which two IL-6 molecules bind to two sIL-6R molecules. This IL-6–sIL-6R complex then binds to gp130, which contains the phosphorylation domains, resulting in signaling via the JAK–STAT and RAS–MAPK pathways. Thus, trans-signaling enables IL-6 to signal to cells that do not usually express IL-6R. As previously stated, IL-6 is increased in the serum of SSc patients and correlates with modified skin thickness score and internal organ fibrosis in diffuse SSc. The same study demonstrated that IL-6 induction of collagen production was dependent on the JAK2–STAT3 signaling cascades and that inhibiting these pathways attenuates collagen production. It has also been shown that stimulated T cells from SSc patients, incubated with TNF-α, produced higher levels of IL-6 and sIL-6R. These T cells also induced an increase in collagen production by healthy dermal fibroblasts, which can be inhibited with the addition of anti-IL-6 antibody, thus implicating trans-signaling in causing fibrosis. Duncan et al. demonstrated that addition of recombinant IL-6 to healthy dermal fibroblasts results in an increase in collagen production, which can be inhibited by the addition of an anti-IL-6 antibody. A bleomycin mouse model of fibrosis shows that there is an increase in IL-6 in the serum and IL-6 mRNA in these mice. Treatment of bleomycin mice with MR16-1 (an anti-IL-6R antibody) results in a decrease in dermal thickening, myofibroblast proliferation and αSMA production. In addition, in an IL-6 bleomycin mouse model, a decrease in dermal sclerosis and αSMA was observed (Figure 2).
(Enlarge Image)
Figure 2.
In systemic sclerosis patients, there is a skewed T-cell repertoire, with an increase in Th2 and Th17 cells observed.
Cytokines, including IL-4, -6, -13 and -17, are secreted by T cells and can interact with fibroblasts, resulting in activation of the cell and its differentiation into a myofibroblast. This results in an increase in extracellular matrix components disposition, resulting in the characteristic fibrosis that is observed in systemic sclerosis patients. gp130 is the common cytokine shared subunit for IL-6 and associates with IL-6R to form a functional receptor complex. The box indicates downstream effects of T-cell products in dermal fibroblasts.
R: Receptor.
IL-13 is a 33 amino acid-long cytokine that has similar properties to IL-4 and shares a common receptor subunit. IL-13 binds the IL-13R and signals via JAK kinases to activate STAT6 signaling. IL-13 can stimulate fibroblasts to increase ECM production; however, conflicting studies have suggested that IL-13 stimulated fibrosis may be TGF-β dependent or independent. Fichtner-Feigl et al. demonstrated that IL-13 and TNF-α induced TGF-β production via IL-13Rα, which resulted in fibrosis. However, Kaviratne et al. suggested that liver fibrosis caused by IL-13 was independent of TGF-β. This study used IL-13 mice and showed a decrease in fibrosis of the liver after infection with Schistosoma mansoni. Fuschiotti et al. implicated a role of CD8 T cells in the production of IL-13, as CD8 IL-13-secreting cells were found elevated in the skin of patients with SSc. Culturing these CD8 T cells with fibroblasts resulted in an increase in ECM production, which correlated with IL-13 levels. In addition, inhibition of IL-13 with an IL-13 antibody decreased the collagen mRNA production by these cells. Although CD8 T cells are best known for their ability to lyse virally infected cells, it is now known that they have a much broader role in the immune system.
TNF-α is another pleiotropic cytokine involved in the pathogenesis of SSc. TNF-α has two forms; a membrane-bound (mTNF-α)form and a soluble (sTNF-α)form. TNF-α can signal via two receptors. TNFR1 is a ubiquitously expressed receptor, which both sTNF-α and mTNF-α can bind to, whereas TNFR2 only binds mTNF-α and is only expressed by immune cells. TNF-α is increased in patients with SSc and T cells from SSc patients have increased expression of TNFR1 and 2 on their surface and this correlates with skin thickening. In addition, activation of these T cells prior to stimulation of TNF-α increases IL-6 and IL-13 secretion, resulting in an increase in collagen production by dermal fibroblasts. In addition, as previously stated, DNAX-1 has been shown to be involved in the development of dermal thickness and T-cell infiltration into the skin. DNAX-1 knockout mice have decreased TNF-α expression, which is thought to decrease T-cell infiltration and dermal thickening.
IL-17 is a proinflammatory cytokine produced by Th17 cells that has been found to be increased in the serum, bronchoalveolar lavage fluid, skin lesions and lungs of patients with SSc. IL-17 expression correlates with interstitial lung disease in SSc patients. Kurasawa et al. showed that IL-17 mRNA is increased in SSc patients with lung fibrosis and when IL-17 is incubated with dermal fibroblasts from both healthy controls and SSc patients, proliferation of these cells is observed. However, it was not shown that IL-17 increases collagen production by these cells. More recently it has been shown that the presence of IL-17-expressing T cells and mast cells, in SSc patients, colocalizes with areas of myofibroblast proliferation. However, Truchetet et al. also confirmed that IL-17 does not induce collagen production, rather IL-17 seems to inhibit myofibroblast generation.
T-cell Products that Drive Fibrosis
Clearly, the presence of these T cells at the site of fibrosis in patients with SSc implicates a role for the T-cell repertoire in the pathogenesis of the disease. The distorted T-cell population is reflected in the polarized cytokine milieu present in SSc patients. The increase in Th2 and Th17 cells, plus the apparently nonresponsive Treg population, results in an increase in Th2 and Th17 cytokines including IL-4, -6, -13, -17 and TNF-α in the serum of SSc patients, which may induce fibrosis, directly or indirectly.
IL-4 is a pleiotropic cytokine involved in the differentiation of T cells that can induce the expression of MHC class II molecules and IgE secretion on B cells. IL-4 signals via its receptor, which consists of the Il-4Rα chain, that binds IL-4 with high affinity, and the γ common chain, which contains the signaling domain. Binding of IL-4 to IL-4R can result in activation of the Janus kinase (JAK) and the STAT signaling cascades. It has been shown that there is an increase in IL-4-producing T cells in the bronchoalveolar lavage fluid of SSc patients. However, the role of IL-4 in causing fibrosis is controversial, since opposing studies suggest either an antifibrotic or a profibrotic role for IL-4. Izbicki et al. used an IL-4 knockout and IL-4 transgenic bleomycin mouse model to demonstrate the effect that the presence or absence of IL-4 has in causing fibrosis. In bleomycin-treated IL-4 mice, an increase in lung fibrosis was observed, whereas in mice overexpressing IL-4 lung fibrosis was decreased, suggesting an antifibrotic function of IL-4. Huaux et al. demonstrated the pleiotropic characteristics of IL-4 using an IL-4 bleomycin-treated mouse model and examining the extent of pulmonary fibrosis. In this model, an increase in lymphocytes was observed in the bronchoalveolar lavage fluid but pulmonary fibrosis was decreased. However, Huaux et al. also showed that stimulation of pulmonary fibroblasts with IL-4 did not induce proliferation of these cells. In contrast to these observations, it has been shown that stimulation of healthy dermal fibroblasts with IL-4 results in an increase in collagen production by these cells due to an activation of the ERK pathway. In addition, Postlethwaite et al. demonstrated that stimulation of dermal fibroblasts with recombinant IL-4 induces collagen production, which is inhibited by the addition of IL-4 antibody. Furthermore, use of the tight skin model of SSc implicates a role for IL-4 in causing dermal fibrosis. In IL-4 tight skin models, a decrease in skin fibrosis is observed. Further to this, it was found that the stimulation of tight skin dermal fibroblasts with IL-4 induced collagen production, which was attenuated with the addition of anti-IL-4 antibody.
IL-6 is another pleiotropic cytokine that is increased in the serum of SSc patients. IL-6 signaling requires the expression of two receptors: IL-6R and gp130. IL-6R is only expressed on hepatocytes, neutrophils, monocytes and leukocytes, whereas gp130 is ubiquitously expressed. gp130 is a critical shared component of IL-6-related cytokine ligands. A soluble form of the receptor (sIL-6R) also exists, owing to the cleaving of IL-6R from activated lymphocytes or hepatocytes by the enzyme ADAM17. IL-6 signaling can occur via classical signaling, in which two IL-6 molecules bind two IL-6R chains, this in turn recruits two gp130 molecules; or via trans-signaling, in which two IL-6 molecules bind to two sIL-6R molecules. This IL-6–sIL-6R complex then binds to gp130, which contains the phosphorylation domains, resulting in signaling via the JAK–STAT and RAS–MAPK pathways. Thus, trans-signaling enables IL-6 to signal to cells that do not usually express IL-6R. As previously stated, IL-6 is increased in the serum of SSc patients and correlates with modified skin thickness score and internal organ fibrosis in diffuse SSc. The same study demonstrated that IL-6 induction of collagen production was dependent on the JAK2–STAT3 signaling cascades and that inhibiting these pathways attenuates collagen production. It has also been shown that stimulated T cells from SSc patients, incubated with TNF-α, produced higher levels of IL-6 and sIL-6R. These T cells also induced an increase in collagen production by healthy dermal fibroblasts, which can be inhibited with the addition of anti-IL-6 antibody, thus implicating trans-signaling in causing fibrosis. Duncan et al. demonstrated that addition of recombinant IL-6 to healthy dermal fibroblasts results in an increase in collagen production, which can be inhibited by the addition of an anti-IL-6 antibody. A bleomycin mouse model of fibrosis shows that there is an increase in IL-6 in the serum and IL-6 mRNA in these mice. Treatment of bleomycin mice with MR16-1 (an anti-IL-6R antibody) results in a decrease in dermal thickening, myofibroblast proliferation and αSMA production. In addition, in an IL-6 bleomycin mouse model, a decrease in dermal sclerosis and αSMA was observed (Figure 2).
(Enlarge Image)
Figure 2.
In systemic sclerosis patients, there is a skewed T-cell repertoire, with an increase in Th2 and Th17 cells observed.
Cytokines, including IL-4, -6, -13 and -17, are secreted by T cells and can interact with fibroblasts, resulting in activation of the cell and its differentiation into a myofibroblast. This results in an increase in extracellular matrix components disposition, resulting in the characteristic fibrosis that is observed in systemic sclerosis patients. gp130 is the common cytokine shared subunit for IL-6 and associates with IL-6R to form a functional receptor complex. The box indicates downstream effects of T-cell products in dermal fibroblasts.
R: Receptor.
IL-13 is a 33 amino acid-long cytokine that has similar properties to IL-4 and shares a common receptor subunit. IL-13 binds the IL-13R and signals via JAK kinases to activate STAT6 signaling. IL-13 can stimulate fibroblasts to increase ECM production; however, conflicting studies have suggested that IL-13 stimulated fibrosis may be TGF-β dependent or independent. Fichtner-Feigl et al. demonstrated that IL-13 and TNF-α induced TGF-β production via IL-13Rα, which resulted in fibrosis. However, Kaviratne et al. suggested that liver fibrosis caused by IL-13 was independent of TGF-β. This study used IL-13 mice and showed a decrease in fibrosis of the liver after infection with Schistosoma mansoni. Fuschiotti et al. implicated a role of CD8 T cells in the production of IL-13, as CD8 IL-13-secreting cells were found elevated in the skin of patients with SSc. Culturing these CD8 T cells with fibroblasts resulted in an increase in ECM production, which correlated with IL-13 levels. In addition, inhibition of IL-13 with an IL-13 antibody decreased the collagen mRNA production by these cells. Although CD8 T cells are best known for their ability to lyse virally infected cells, it is now known that they have a much broader role in the immune system.
TNF-α is another pleiotropic cytokine involved in the pathogenesis of SSc. TNF-α has two forms; a membrane-bound (mTNF-α)form and a soluble (sTNF-α)form. TNF-α can signal via two receptors. TNFR1 is a ubiquitously expressed receptor, which both sTNF-α and mTNF-α can bind to, whereas TNFR2 only binds mTNF-α and is only expressed by immune cells. TNF-α is increased in patients with SSc and T cells from SSc patients have increased expression of TNFR1 and 2 on their surface and this correlates with skin thickening. In addition, activation of these T cells prior to stimulation of TNF-α increases IL-6 and IL-13 secretion, resulting in an increase in collagen production by dermal fibroblasts. In addition, as previously stated, DNAX-1 has been shown to be involved in the development of dermal thickness and T-cell infiltration into the skin. DNAX-1 knockout mice have decreased TNF-α expression, which is thought to decrease T-cell infiltration and dermal thickening.
IL-17 is a proinflammatory cytokine produced by Th17 cells that has been found to be increased in the serum, bronchoalveolar lavage fluid, skin lesions and lungs of patients with SSc. IL-17 expression correlates with interstitial lung disease in SSc patients. Kurasawa et al. showed that IL-17 mRNA is increased in SSc patients with lung fibrosis and when IL-17 is incubated with dermal fibroblasts from both healthy controls and SSc patients, proliferation of these cells is observed. However, it was not shown that IL-17 increases collagen production by these cells. More recently it has been shown that the presence of IL-17-expressing T cells and mast cells, in SSc patients, colocalizes with areas of myofibroblast proliferation. However, Truchetet et al. also confirmed that IL-17 does not induce collagen production, rather IL-17 seems to inhibit myofibroblast generation.
