Defective thymocyte development and perturbed homeostasis of T cells in STAT-induced STAT inhibitor-1/suppressors of cytokine signaling-1 transgenic mice

Suppressors of cytokine signaling proteins are differentially expressed in Th1 and Th2 cells: implications for Th cell lineage commitment and maintenance

Positive regulatory factors induced by IL-12/STAT4 and IL-4/STAT6 signaling during T cell development contribute to polarized patterns of cytokine expression manifested by differentiated Th cells. These two critical and antagonistic signaling pathways are under negative feedback regulation by a multimember family of intracellular proteins called suppressor of cytokine signaling (SOCS). However, it is not known whether these negative regulatory factors also modulate Th1/Th2 lineage commitment and maintenance. We show here that CD4(+) naive T cells constitutively express low levels of SOCS1, SOCS2, and SOCS3 mRNAs. These mRNAs and their proteins increase significantly in nonpolarized Th cells after activation by TCR signaling. We further show that differentiation into Th1 or Th2 phenotype is accompanied by preferential expression of distinct SOCS mRNA transcripts and proteins. SOCS1 expression is 5-fold higher in Th1 than in Th2 cells, whereas Th2 cells contain 23-fold higher levels of SOCS3. We also demonstrate that IL-12-induced STAT4 activation is inhibited in Th2 cells that express high levels of SOCS3 whereas IL-4/STAT6 signaling is constitutively activated in Th2 cells, but not Th1 cells, with high SOCS1 expression. These results suggest that mutually exclusive use of STAT4 and STAT6 signaling pathways by differentiated Th cells may derive in part, from SOCS3- or SOCS1-mediated repression of IL-12/STAT4- or IL-4/STAT6 signaling in Th2 and Th1 cells, respectively. Given the strong correlation between distinct patterns of SOCS expression and differentiation into the Th1 or Th2 phenotype, SOCS1 and SOCS3 proteins are therefore Th lineage markers that can serve as therapeutic targets for immune modulation therapy.

Impaired NK cell development in an IFN-gamma transgenic mouse: aberrantly expressed IFN-gamma enhances hematopoietic stem cell apoptosis and affects NK cell differentiation

Aberrant expression of IFN-gamma has been demonstrated to cause a wide variety of alterations in cell function and development. Previously we reported that constitutive expression of IFN-gamma in bone marrow (BM) and thymus results in a total absence of B cells and a substantial decrease in the number of hematopoietic progenitor cells. In this study, we demonstrate a severe deficiency of NK1.1(+)CD3(-) cells in this transgenic mouse model. Compared with normal control littermates, we found a pronounced reduction of NK cells in IFN-gamma transgenic mouse spleen and liver despite maintenance of normal function. In addition, we observed a reduced number of BM cells in the IFN-gamma transgenic mouse despite normal expression of hematopoietic growth factors in the BM. Interestingly, these cells were less responsive to stem cell factor (SCF) despite c-kit expression on hematopoietic stem cells (HSCs). We observed that addition of exogenous IFN-gamma inhibited proliferation of HSCs and differentiation of NK precursors from HSCs in normal mice in response to SCF, IL-7, fms-like tyrosine kinase 3 ligand, and IL-15. Furthermore, we found that HSCs express the IFN-gammaRalpha subunit and undergo apoptosis in response to exogenous IFN-gamma. Thus, we have demonstrated the occurrence of a severe deficiency of NK cells and lower numbers of BM cells in an IFN-gamma transgenic mouse model. Furthermore, because exogenous IFN-gamma affects the responsiveness to hematopoietic growth factors such as SCF in vitro, our results indicate that chronic expression of IFN-gamma in vivo leads to widespread immune system defects, including alterations in NK cell differentiation.

SOCS proteins: negative regulators of cytokine signaling

Cytokines regulate the growth and differentiation of cells by binding to cell-surface receptors and activating intracellular signal transduction cascades such as the JAK-STAT pathway. Cytokine signaling is negatively regulated with respect to both magnitude and duration, and it is now clear that the suppressor of cytokine signaling (SOCS) family of proteins (SOCS1-SOCS7 and CIS) contributes significantly to this process. Transcripts encoding CIS, SOCS1, SOCS2, and SOCS3 are upregulated in response to cytokine stimulation, and the corresponding SOCS proteins inhibit cytokine-induced signaling pathways. SOCS proteins therefore form part of a classical negative feedback circuit. SOCS family members modulate signaling by several mechanisms, which include inactivation of the Janus kinases (JAKs), blocking access of the signal transducers and activators of transcription (STATs) to receptor binding sites, and ubiquitination of signaling proteins and their subsequent targeting to the proteasome. Gene targeting has been used to generate mice lacking socs1, socs2, or socs3, in order to elucidate the physiological function of these SOCS family members. The analysis of socs1(-/-) mice has revealed that SOCS1 plays a key role in the negative regulation of interferon-gamma signaling and in T cell differentiation. Socs2(-/-) mice are 30%-40% larger than wild-type mice, demonstrating that SOCS2 is a critical regulator of postnatal growth. Additionally, the study of embryos lacking socs3 has revealed that SOCS3 is an important regulator of fetal liver hematopoiesis. The biological role of other SOCS proteins remains to be determined.

SOCS-1/SSI-1-deficient NKT cells participate in severe hepatitis through dysregulated cross-talk inhibition of IFN-gamma and IL-4 signaling in vivo

Suppressor of cytokine signaling-1 (SOCS-1), also known as STAT-induced STAT inhibitor-1 (SSI-1), is a negative feedback molecule for cytokine signaling, and its in vivo deletion induces fulminant hepatitis. However, elimination of the STAT1 or STAT6 gene or deletion of NKT cells substantially prevented severe hepatitis in SOCS-1-deficient mice, while administration of IFN-gamma and IL-4 accelerated its development. SOCS-1 deficiency not only sustained IFN-gamma/IL-4 signaling but also eliminated the cross-inhibitory action of IFN-gamma on IL-4 signaling. These results suggest that SOCS-1 deficiency-induced persistent activation of STAT1 and STAT6, which would be inhibited by SOCS-1 under normal conditions, may induce abnormal activation of NKT cells, thus leading to lethal pathological changes in SOCS-1-deficient mice.

Defective gp130-mediated signal transducer and activator of transcription (STAT) signaling results in degenerative joint disease, gastrointestinal ulceration, and failure of uterine implantation

The receptor subunit gp130 transduces multiple cell type-specific activities of the leukemia inhibitory factor (LIF)/interleukin (IL)-6 family of cytokines through the signal transducer and activator of transcription (STAT) and src homology 2 domain-bearing protein tyrosine phosphatase (SHP)-2/ras/Erk pathways. To define STAT-dependent physiological responses, we generated mice with a COOH-terminal gp130(DeltaSTAT) "knock-in" mutation which deleted all STAT-binding sites. gp130(DeltaSTAT) mice phenocopyed mice deficient for IL-6 (impaired humoral and mucosal immune and hepatic acute phase responses) and LIF (failure of blastocyst implantation). However, unlike mice with null mutations in any of the components in the gp130 signaling pathway, gp130(DeltaSTAT) mice also displayed gastrointestinal ulceration and a severe joint disease with features of chronic synovitis, cartilaginous metaplasia, and degradation of the articular cartilage. Mitogenic hyperresponsiveness of synovial cells to the LIF/IL-6 family of cyto-kines was caused by sustained gp130-mediated SHP-2/ras/Erk activation due to impaired STAT-mediated induction of suppressor of cytokine signaling (SOCS) proteins which normally limits gp130 signaling. Therefore, the joint pathology in gp130(DeltaSTAT) mice is likely to arise from the disturbance of the otherwise balanced activation of the SHP-2/ras/Erk and STAT signaling cascades emanating from gp130.

Overexpression of suppressor of cytokine signaling-1 impairs pre-T-cell receptor-induced proliferation but not differentiation of immature thymocytes

Cytokines play an essential role during early T-cell development. However, the mechanisms controlling cytokine signaling in developing thymocytes have not been elucidated. Cytokine receptor signaling can be modulated by suppressor of cytokine signaling-1 (SOCS-1), which acts as a negative regulator of Janus kinases. SOCS-1 is normally expressed throughout thymocyte development; however, retroviral-mediated overexpression of SOCS-1 in fetal liver-derived hematopoietic progenitors prevented their progression beyond the earliest stage of T-cell development. Further analysis revealed that SOCS-1 expression is transiently suppressed following pre-T-cell receptor (TCR) signaling. Moreover, constitutive expression of SOCS-1 abrogated pre-TCR- mediated expansion of immature thymocytes but did not interfere with differentiation. These findings reveal that SOCS-1 serves to regulate cytokine signaling at critical checkpoints during early T-cell development.