ICOS is a key costimulatory receptor facilitating differentiation and function of follicular helper T cells and inflammatory T cells. Rheumatoid arthritis patients were shown to have elevated levels ...of ICOS
T cells in the synovial fluid, suggesting a potential role of ICOS-mediated T cell costimulation in autoimmune joint inflammation. In this study, using ICOS knockout and knockin mouse models, we found that ICOS signaling is required for the induction and maintenance of collagen-induced arthritis (CIA), a murine model of rheumatoid arthritis. For the initiation of CIA, the Tyr
-based SH2-binding motif of ICOS that is known to activate PI3K was critical for Ab production and expansion of inflammatory T cells. Furthermore, we found that Tyr
-dependent ICOS signaling is important for maintenance of CIA in an Ab-independent manner. Importantly, we found that a small molecule inhibitor of glycolysis, 3-bromopyruvate, ameliorates established CIA, suggesting an overlap between ICOS signaling, PI3K signaling, and glucose metabolism. Thus, we identified ICOS as a key costimulatory pathway that controls induction and maintenance of CIA and provide evidence that T cell glycolytic pathways can be potential therapeutic targets for rheumatoid arthritis.
Hippo signaling pathways are evolutionarily conserved signal transduction mechanisms mainly involved in organ size control, tissue regeneration, and tumor suppression. However, in mammals, the ...primary role of Hippo signaling seems to be regulation of immunity. As such, humans with null mutations in
STK4
(mammalian homologue of
Drosophila Hippo
; also known as
MST1
) suffer from recurrent infections and autoimmune symptoms. Although dysregulated T cell homeostasis and functions have been identified in MST1-deficient human patients and mouse models, detailed cellular and molecular bases of the immune dysfunction remain to be elucidated. Although the canonical Hippo signaling pathway involves transcriptional co-activator Yes-associated protein (YAP) or transcriptional coactivator with PDZ motif (TAZ), the major Hippo downstream signaling pathways in T cells are YAP/TAZ-independent and they widely differ between T cell subsets. Here we will review Hippo signaling mechanisms in T cell immunity and describe their implications for immune defects found in MST1-deficient patients and animals. Further, we propose that mutual inhibition of Mst and Akt kinases and their opposing roles on the stability and function of forkhead box O and β-catenin may explain various immune defects discovered in mutant mice lacking Hippo signaling components. Understanding these diverse Hippo signaling pathways and their interplay with other evolutionarily-conserved signaling components in T cells may uncover molecular targets relevant to vaccination, autoimmune diseases, and cancer immunotherapies.
Human patients with homozygous null mutations in the ICOS gene suffer from recurrent infections due to humoral immune defects. Studies on human patients and mouse models have shown that inducible ...T‐cell co‐stimulator (ICOS)‐deficient individuals cannot form T follicular helper (Tfh) cells, a group of CD4 T cells that migrate into B cell follicles and facilitate germinal center (GC) reactions. ICOS‐induced phosphoinositide 3‐kinase signaling pathways have been shown to play critical roles in Tfh programming, migration of Tfh cells into the GC, and delivery of T cell help during Tfh‐GC B cell conjugation. These processes are also assisted by ICOS‐mediated intracellular calcium mobilization and TANK‐binding kinase 1 signaling. However, ICOS signaling also has stimulatory roles in T regulatory cells and innate lymphoid cells (ILCs), providing another layer of complexity. In this review, we discuss cell‐type‐specific signaling mechanisms utilized by ICOS in Tfh cells, T regulatory cells, and ILCs. Whenever relevant, we compare the roles and signaling pathways of ICOS and CD28. Understanding ICOS signal transduction mechanisms used by distinct immune subsets at different stages of immune responses or disease progression may help improve vaccination protocols, treat autoimmune diseases, and enhance cancer immunotherapy.
Abstract
ICOS is induced in activated T cells and its main role is to boost differentiation and function of effector T cells. ICOS is also constitutively expressed in a subpopulation of Foxp3+ ...regulatory T cells under steady-state condition. Studies using ICOS germline knockout mice or ICOS-blocking reagents suggested that ICOS has supportive roles in regulatory T (Treg) cell homeostasis, migration, and function. To avoid any compounding effects that may arise from ICOS-deficient non–Treg cells, we generated a conditional knockout system in which ICOS expression is selectively abrogated in Foxp3-expressing cells (ICOS FC mice). Compared to Foxp3-Cre control mice, ICOS FC mice showed a minor numerical deficit of steady-state Treg cells but did not show any signs of spontaneous autoimmunity, indicating that tissue-protective Treg populations do not heavily rely on ICOS costimulation. However, ICOS FC mice showed more severe inflammation in oxazolone-induced contact hypersensitivity, a model of atopic dermatitis. This correlated with elevated numbers of inflammatory T cells expressing IFN-γ and/or TNF-α in ICOS FC mice compared with the control group. In contrast, elimination of ICOS in all T cell compartments negated the differences, confirming that ICOS has a dual positive role in effector and Treg cells. Single-cell transcriptome analysis suggested that ICOS-deficient Treg cells fail to mature into T-bet+CXCR3+ “Th1-Treg” cells in the draining lymph node. Our results suggest that regimens that preferentially stimulate ICOS pathways in Treg cells might be beneficial for the treatment of Th1-driven inflammation.
Abstract
The Inducible Costimulator (ICOS) is a T cell costimulatory receptor critical for humoral immunity. ICOS-deficient patients suffer from recurrent infections due to lack of protective ...antibodies. However, some patients also display signs of antibody-mediated autoimmunity. These findings may reflect a dual role of ICOS: facilitating the differentiation and function of T follicular helper (Tfh) and T follicular regulatory (Tfr) cells. While Tfh cells are known to provide help to B cells to produce high affinity antibodies, the main role of Tfr cells seems to be preventing autoantibody generation. Using Foxp3-cre-mediated ICOS knockout (ICOS FC) mice, we show that T regulatory (Treg)-specific ICOS deletion drastically reduces the number of Tfr cells without altering Treg cell numbers. Single cell RNA sequencing further revealed shifts in transitory Tfr precursor populations in immunized ICOS FC mice. Importantly, we observed a lowered ratio of antigen-specific germinal center B (GCB) cells and increased anti-nuclear antibodies in ICOS FC mice, suggesting a rise in autoreactive GCB cells. We also noted variations in isotype composition of total and virus-specific antibodies in infected ICOS FC mice. Mechanistically, our data suggests that ICOS could promote the Treg-to-Tfr transition by regulating CXCR5 expression. Thus, our study demonstrates that ICOS is critical for Tfr cell generation and supports the role of Tfr cells in preventing generation of autoantibodies during germinal center reactions.
Abstract
The inducible T-cell costimulator (ICOS) is a key costimulatory receptor facilitating differentiation and function of follicular helper T cells and inflammatory T cells. Rheumatoid arthritis ...patients were shown to have elevated levels of ICOS+ T cells in the synovial fluid suggesting a potential role of ICOS-mediated T cell costimulation in autoimmune joint inflammation. In this study, using ICOS knockout and knockin mouse models, we found that ICOS-mediated phosphoinositide 3-kinase (PI3K) signaling is required for the induction and maintenance of collagen-induced arthritis (CIA), a murine model of rheumatoid arthritis. For the initiation of CIA, ICOS-mediated PI3K signaling was critical for antibody production and expansion of inflammatory T cells. Further, we found that ICOS-PI3K signaling is important for maintenance of CIA in an antibody independent manner. Importantly, we found that a small molecule inhibitor of glycolysis, 3-bromopyruvate, ameliorates established CIA suggesting a potential link between ICOS-PI3K signaling and glucose metabolism. Thus, we identified ICOS-PI3K axis as a key signaling pathway that controls induction and maintenance of CIA and provide evidence that T cell glycolytic pathways can be potential therapeutic targets for rheumatoid arthritis.
•A conserved KKKY motif in the cytoplasmic tail of ICOS is necessary and sufficient for ICOS-mediated calcium flux.•ICOS-calcium and ICOS-PI3K signaling can be induced independently of each ...other.•ICOS activates both PLCγ1 and actin remodeling machineries to trigger release of calcium from the intracellular pool.
The inducible costimulator (ICOS) is a T cell costimulatory receptor that plays crucial roles in T cell differentiation and function. So far, ICOS has been shown to activate three signaling components: phosphoinositide 3-kinase (PI3K), intracellular calcium mobilization, and TANK binding kinase 1 (TBK1). By generating a knock-in strain of mice in which the ICOS gene is modified such that the ICOS-mediated PI3K pathway is selectively abrogated while the capacity of ICOS to mobilize intracellular calcium remains intact, we have shown that ICOS-mediated PI3K activation is required for some but not all T cell responses. This suggests that the ICOS-calcium signaling axis may explain some of the PI3K-independent ICOS functions. Further, a recent in vivo imaging study indicated that ICOS-dependent intracellular calcium flux facilitates cognate T cell-B cell interactions within the germinal center. However, how ICOS promotes TCR-mediated calcium flux has not been clear. Here we identified a membrane proximal motif in the cytoplasmic tail of ICOS that is essential for ICOS-assisted calcium signaling and demonstrate that ICOS can induce calcium flux independently of other signaling motifs. We also provide evidence that ICOS potentiates phospholipase Cγ1 (PLCγ1) activation to enhance calcium release from the intracellular pool. In parallel, acute ligation of ICOS without TCR co-engagement leads to activation of small GTPases, RhoA and Cdc42, consistent with the capacity of ICOS to induce actin remodeling. Importantly, interruption of actin dynamics during acute TCR or TCR-ICOS co-ligation severely impairs calcium flux in T cells even in the presence of activated PLCγ1. Thus, ICOS potentiates TCR-induced calcium flux by enhancing PLCγ1 activation and actin remodeling in a coordinated manner.
Protein kinases carry out important functions in cells both by phosphorylating substrates and by means of regulated non-catalytic activities. Such non-catalytic functions have been ascribed to many ...kinases, including some members of the Ste20 family. The Drosophila Ste20 kinase Slik phosphorylates and activates Moesin in developing epithelial tissues to promote epithelial tissue integrity. It also functions non-catalytically to promote epithelial cell proliferation and tissue growth. We carried out a structure-function analysis to determine how these two distinct activities of Slik are controlled. We find that the conserved C-terminal coiled-coil domain of Slik, which is necessary and sufficient for apical localization of the kinase in epithelial cells, is not required for Moesin phosphorylation but is critical for the growth-promoting function of Slik. Slik is auto- and trans-phosphorylated in vivo. Phosphorylation of at least two of three conserved sites in the activation segment is required for both efficient catalytic activity and non-catalytic signaling. Slik function is thus dependent upon proper localization of the kinase via the C-terminal coiled-coil domain and activation via activation segment phosphorylation, which enhances both phosphorylation of substrates like Moesin and engagement of effectors of its non-catalytic growth-promoting activity.
Background: Slik kinase has catalytic activity-dependent and -independent functions.
Results: Mutation of activation segment phosphorylation sites abolishes both catalytic and non-catalytic activities; non-catalytic function also depends upon localization via the C-terminal domain.
Conclusion: Slik is regulated by both localization and phosphorylation.
Significance: Conformational activation can control not only catalytic but also non-catalytic activities of kinases.
The protein kinase Mst1 is a key component of the evolutionarily conserved Hippo pathway that regulates cell survival, proliferation, differentiation, and migration. In humans, Mst1 deficiency causes ...primary immunodeficiency. Patients with MST1-null mutations show progressive loss of naive T cells but, paradoxically, mildly elevated serum Ab titers. Nonetheless, the role of Mst1 in humoral immunity remains poorly understood. In this study, we found that early T cell-dependent IgG1 responses in young adult Mst1-deficient mice were largely intact with signs of impaired affinity maturation. However, the established Ag-specific IgG1 titers in Mst1-deficient mice decayed more readily because of a loss of Ag-specific but not the overall bone marrow plasma cells. Despite the impaired affinity and longevity of Ag-specific Abs, Mst1-deficient mice produced plasma cells displaying apparently normal maturation markers with intact migratory and secretory capacities. Intriguingly, in immunized Mst1-deficient mice, T follicular helper cells were hyperactive, expressing higher levels of IL-21, IL-4, and surface CD40L. Accordingly, germinal center B cells progressed more rapidly into the plasma cell lineage, presumably forgoing rigorous affinity maturation processes. Importantly, Mst1-deficient mice had elevated levels of CD138
Blimp1
splenic plasma cell populations, yet the size of the bone marrow plasma cell population remained normal. Thus, overproduced low-affinity plasma cells from dysregulated germinal centers seem to underlie humoral immune defects in Mst1-deficiency. Our findings imply that vaccination of Mst1-deficient human patients, even at the early stage of life, may fail to establish long-lived high-affinity humoral immunity and that prophylactic Ab replacement therapy can be beneficial to the patients.
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