Cytotoxic T lymphocyte-associated antigen-4 inhibits integrin-mediated stimulation

NFAT signaling dysregulation in cancer: Emerging roles in cancer stem cells

The nuclear factor of activated T cells (NFAT) was first identified as a transcriptional regulator of activated T cells. The NFAT family is involved in the development of tumors. Furthermore, recent evidence reveals that NFAT proteins regulate the development of inflammatory and immune responses. New discoveries have also been made about the mechanisms by which NFAT regulates cancer progression through cancer stem cells (CSC). Here, we discuss the role of the NFAT family in the immune system and various cancer types.

Radiotherapy, PARP Inhibition, and Immune-Checkpoint Blockade: A Triad to Overcome the Double-Edged Effects of Each Single Player

Radiotherapy and, more recently, PARP inhibitors (PARPis) and immune-checkpoint inhibitors represent effective tools in cancer therapy. Radiotherapy exerts its effects not only by damaging DNA and inducing tumor cell death, but also stimulating anti-tumor immune responses. PARPis are known to exert their therapeutic effects by inhibiting DNA repair, and they may be used in combination with radiotherapy. Both radiotherapy and PARPis modulate inflammatory signals and stimulate type I IFN (IFN-I)-dependent immune activation. However, they can also support the development of an immunosuppressive tumor environment and upregulate PD-L1 expression on tumor cells. When provided as monotherapy, immune-checkpoint inhibitors (mainly antibodies to CTLA-4 and the PD-1/PD-L1 axis) result particularly effective only in immunogenic tumors. Combinations of immunotherapy with therapies that favor priming of the immune response to tumor-associated antigens are, therefore, suitable strategies. The widely explored association of radiotherapy and immunotherapy has confirmed this benefit for several cancers. Association with PARPis has also been investigated in clinical trials. Immunotherapy counteracts the immunosuppressive effects of radiotherapy and/or PARPis and synergies with their immunological effects, promoting and unleashing immune responses toward primary and metastatic lesions (abscopal effect). Here, we discuss the beneficial and counterproductive effects of each therapy and how they can synergize to overcome single-therapy limitations.

Phosphorylation: A Fast Switch For Checkpoint Signaling

Checkpoint signaling involves a variety of upstream and downstream factors that participate in the regulation of checkpoint expression, activation, and degradation. During the process, phosphorylation plays a critical role. Phosphorylation is one of the most well-documented post-translational modifications of proteins. Of note, the importance of phosphorylation has been emphasized in aspects of cell activities, including proliferation, metabolism, and differentiation. Here we summarize how phosphorylation of specific molecules affects the immune activities with preference in tumor immunity. Of course, immune checkpoints are given extra attention in this book. There are many common pathways that are involved in signaling of different checkpoints. Some of them are integrated and presented as common activities in the early part of this chapter, especially those associated with PD-1/PD-L1 and CTLA-4, because investigations concerning them are particularly abundant and variant. Their distinct regulation is supplementarily discussed in their respective section. As for checkpoints that are so far not well explored, their related phosphorylation modulations are listed separately in the later part. We hope to provide a clear and systematic view of the phosphorylation-modulated immune signaling.

Regulation of cell adhesion: a collaborative effort of integrins, their ligands, cytoplasmic actors, and phosphorylation

Integrins are large heterodimeric type 1 membrane proteins expressed in all nucleated mammalian cells. Eighteen α-chains and eight β-chains can combine to form 24 different integrins. They are cell adhesion proteins, which bind to a large variety of cellular and extracellular ligands. Integrins are required for cell migration, hemostasis, translocation of cells out from the blood stream and further movement into tissues, but also for the immune response and tissue morphogenesis. Importantly, integrins are not usually active as such, but need activation to become adhesive. Integrins are activated by outside-in activation through integrin ligand binding, or by inside-out activation through intracellular signaling. An important question is how integrin activity is regulated, and this topic has recently drawn much attention. Changes in integrin affinity for ligand binding are due to allosteric structural alterations, but equally important are avidity changes due to integrin clustering in the plane of the plasma membrane. Recent studies have partially solved how integrin cell surface structures change during activation. The integrin cytoplasmic domains are relatively short, but by interacting with a variety of cytoplasmic proteins in a regulated manner, the integrins acquire a number of properties important not only for cell adhesion and movement, but also for cellular signaling. Recent work has shown that specific integrin phosphorylations play pivotal roles in the regulation of integrin activity. Our purpose in this review is to integrate the present knowledge to enable an understanding of how cell adhesion is dynamically regulated.

Participation of mesenchymal stem cells in the regulation of immune response and cancer development

The relevance of the microenvironment in the initiation, promotion, and progression of cancer has been postulated. Mesenchymal stem cells (MSCs) have been identified as important components of the tumor stroma, which are capable of affecting the development of cancer through various mechanisms. In particular, MSCs immunosuppressive properties play an important role. It has been shown that bone marrow-derived and other healthy tissues-derived MSCs are capable of regulating the immune response by affecting the activation, maturation, proliferation, differentiation, and effector function of cells of the immune system, such as neutrophils, macrophages, dendritic cells, natural killer cells (NK) and T-lymphocytes. Similar mechanisms have been identified in MSCs associated with different types of tumors, where they generate an immunosuppressive microenvironment by decreasing the cytotoxic activity of T-lymphocytes and NK cells, skew macrophage differentiation towards an M2 phenotype, and decrease the secretion of Th1-type cytokines. Also, the cytokines, chemokines, and factors secreted by the transformed cells or other cells from the tumor stroma are capable of modulating the functions of MSCs.

Immunoregulation by mesenchymal stem cells: biological aspects and clinical applications

Mesenchymal stem cells (MSCs) are multipotent cells capable of differentiation into mesenchymal lineages and that can be isolated from various tissues and easily cultivated in vitro. Currently, MSCs are of considerable interest because of the biological characteristics that confer high potential applicability in the clinical treatment of many diseases. Specifically, because of their high immunoregulatory capacity, MSCs are used as tools in cellular therapies for clinical protocols involving immune system alterations. In this review, we discuss the current knowledge about the capacity of MSCs for the immunoregulation of immunocompetent cells and emphasize the effects of MSCs on T cells, principal effectors of the immune response, and the immunosuppressive effects mediated by the secretion of soluble factors and membrane molecules. We also describe the mechanisms of MSC immunoregulatory modulation and the participation of MSCs as immune response regulators in several autoimmune diseases, and we emphasize the clinical application in graft versus host disease (GVHD).

Blockade of cytotoxic T-lymphocyte antigen-4 as a new therapeutic approach for advanced melanoma

INTRODUCTION

The incidence of melanoma continues to rise, and prognosis in patients with metastatic melanoma remains poor. The cytotoxic T-lymphocyte antigen-4 (CTLA-4) serves as one of the primary immune check points and downregulates T-cell activation pathways. Enhancing T-cell activation by antibody blockade of CTLA-4 provides a new approach to overcome tumor-induced immune tolerance. Recently, anti-CTLA-4 therapy demonstrated significant clinical benefits in patients with metastatic melanoma, which led to the approval of ipilimumab by the FDA in early 2011.

AREAS COVERED

The fundamental concepts underlying CTLA-4 blockade-potentiated immune activation are presented in this paper, along with the scientific rationale for and the preclinical evidence supporting CTLA-4-targeted cancer immunotherapy. It also provides an update on clinical trials with anti-CTLA-4 inhibitors and discusses the associated autoimmune toxicity.

EXPERT OPINION

Given that overall survival is the only validated end point for anti-CTLA-4 therapy, the clinical implications of the antigen or tumor-specific immunity in patients remain to be clarified. Additional research is necessary to elucidate the prognostic significance of immune-related side effects and significantly optimize the treatment regimens. An improved understanding of the mechanisms of action of CTLA-4 antibodies may also culminate in wide-ranging clinical applications of this new therapy for other tumor types.

Small GTPases and LFA-1 reciprocally modulate adhesion and signaling

Leukocyte-function-associated antigen-1 (LFA-1) is an integrin that is critical for T-cell adhesion and immunologic responses. As a transmembrane receptor and adhesion molecule, LFA-1 signals bidirectionally, whereby information about extracellular ligands is passed outside-in while cellular activation is transmitted inside-out to the adhesive ectodomain. Here, we review the role of small guanosine triphosphatases (GTPases) in LFA-1 signaling. Rap1, a Ras-related GTPase, appears to be central to LFA-1 function. Rap1 is regulated by receptor signaling [e.g. T-cell receptor (TCR), CD28, and cytotoxic T-lymphocyte antigen-4 (CTLA-4)] and by adapter proteins [e.g. adhesion and degranulation-promoting adapter protein (ADAP) and Src kinase-associated phosphoprotein of 55 kDa (SKAP-55)]. Inside-out signaling flows through Rap1 to regulator of adhesion and cell polarization enriched in lymphoid tissues (RAPL) and Rap1-GTP interacting adapter molecule (RIAM) that act in conjunction with the cytoskeleton on the cytosolic domain of LFA-1 to increase adhesion of the ectodomain. Outside-in signaling also relies on small GTPases such as Rho proteins. Vav-1, a guanine nucleotide exchange factor for Rho proteins, is activated as a consequence of LFA-1 engagement. Jun-activating binding protein-1 (JAB-1) and cytohesin-1 have been implicated as possible outside-in signaling intermediates. We have recently shown that Ras is also downstream of LFA-1 engagement: LFA-1 signaling through phospholipase D (PLD) to RasGRP1 was required for Ras activation on the plasma membrane following stimulation of TCR.

Pivotal advance: CTLA-4+ T cells exhibit normal antiviral functions during acute viral infection

Previous studies have shown that T cells, which are genetically deficient in CTLA-4/CD152 expression, will proliferate uncontrollably, resulting in lethal autoimmune disease. This and other evidence indicate that CTLA-4 plays a critical role in the negative regulation of effector T cell function. In contrast to expectations, BrdU incorporation experiments demonstrated that CTLA-4 expression was associated with normal or even enhanced in vivo proliferation of virus-specific CD4+ and CD8+ T cells following acute lymphocytic choriomeningitis virus or vaccinia virus infection. When compared with CTLA-4- T cells directly ex vivo, CTLA-4+ T cells also exhibited normal antiviral effector functions following stimulation with peptide-coated cells, virus-infected cells, plate-bound anti-CD3/anti-CTLA-4, or the cytokines IL-12 and IL-18. Together, this indicates that CTLA-4 does not directly inhibit antiviral T cell expansion or T cell effector functions, at least not under the normal physiological conditions associated with either of these two acute viral infections.

Cytotoxic T-lymphocyte antigen-4 inhibits GATA-3 but not T-bet mRNA expression during T helper cell differentiation

Naive CD4+ T-cell differentiation to T helper 1 (Th1) and Th2 cells is dependent on T-bet and GATA-3 factors, respectively. T-bet and GATA-3, indeed, through chromatin remodelling allow transcriptional activation of Ifngamma and Th2 cytokine (Il4, Il5, Il13) genes, respectively. We investigated the effects of the negative costimulatory receptor cytotoxic T-lymphocyte antigen-4 (CTLA-4) on GATA-3 and T-bet mRNA expression and Th cell differentiation in mouse naive CD4+ T cells. Our results show that CTLA-4 inhibits GATA-3 mRNA expression and Th2 cell differentiation. At variance, CTLA-4 does not affect T-bet mRNA expression and Th1 cell differentiation. GATA-3 mRNA expression is inhibited when CD4+ cells are stimulated under both neutral (i.e. absence of cytokines) and Th2-polarizing (i.e. presence of interleukin (IL)-4) conditions, the effect being larger under the latter condition. Hence CTLA-4 might affect the IL-4/signal transducer and activator of transcription-6 (STAT6) pathway leading to GATA-3 mRNA up-regulation. We found, indeed, that CTLA-4 engagement inhibits STAT6 activation leaving unaffected the STAT6 protein level. Moreover, CTLA-4 engagement drastically inhibits IL-4Ralpha mRNA and protein up-regulation under Th2-polarizing conditions. Thus, CTLA-4 exerts a tight control on Th2 cell differentiation by negatively regulating both the CD3/CD28 and the IL-4/STAT6 pathways.

Regulation of the small GTPase Rap1 and extracellular signal-regulated kinases by the costimulatory molecule CTLA-4

The mitogen-activated protein kinase extracellular signal-regulated kinase (ERK) is activated following engagement of the T-cell receptor and is required for interleukin 2 (IL-2) production and T-cell proliferation. This activation is enhanced by stimulation of the coreceptor CD28 and inhibited by the coreceptor CTLA-4. We show that the small G protein Rap1 is regulated in the opposite manner; it is inhibited by CD28 and activated by CTLA-4. Together, CD3 and CTLA-4 activate Rap1 in a sustained manner. To delineate T-cell function in the absence of Rap1 activity, we generated transgenic mice expressing Rap1GAP1, a Rap1-specific GTPase-activating protein. Transgenic mice showed lymphadenopathy, and transgenic T cells displayed increased ERK activation, proliferation, and IL-2 production. More significantly, the inhibitory effect of CTLA-4 on T-cell function in Rap1GAP1-transgenic T cells was reduced. We demonstrate that CTLA-4 activates Rap1, and we propose that intracellular signals from CTLA-4 antagonize CD28, at least in part, at the level of Rap1.

IL-4 Modulation of CD4+CD25+ T Regulatory Cell-Mediated Suppression

Murine CD4(+)CD25(+) T regulatory (Treg) cells were cocultured with CD4(+)CD25(-) Th cells and APCs or purified B cells and stimulated by anti-CD3 mAb. Replacement of APCs by B cells did not significantly affect the suppression of CD4(+)CD25(-) Th cells. When IL-4 was added to separate cell populations, this cytokine promoted CD4(+)CD25(-) Th and CD4(+)CD25(+) Treg cell proliferation, whereas the suppressive competence of CD4(+)CD25(+) Treg cells was preserved. Conversely, IL-4 added to coculture of APCs, CD4(+)CD25(-) Th cells, and CD4(+)CD25(+) Treg cells inhibited the suppression of CD4(+)CD25(-) Th cells by favoring their survival through the induction of Bcl-2 expression. At variance, suppression was not affected by addition of IL-13, although this cytokine shares with IL-4 a receptor chain. When naive CD4(+)CD25(-) Th cells were replaced by Th1 and Th2 cells, cell proliferation of both subsets was equally suppressed, but suppression was less pronounced compared with that of CD4(+)CD25(-) Th cells. IL-4 production by Th2 cells was also inhibited. These results indicate that although CD4(+)CD25(+) Treg cells inhibit IL-4 production, the addition of IL-4 counteracts CD4(+)CD25(+) Treg cell-mediated suppression by promoting CD4(+)CD25(-) Th cell survival and proliferation.