PAG/Cbp suppression reveals a contribution of CTLA-4 to setting the activation threshold in T cells

The potency of lncRNA MALAT1/miR-155/CTLA4 axis in altering Th1/Th2 balance of asthma

Objectives: The present study examined if the metastasis-associated lung adenocarcinoma transcript 1 (MALAT1)/miR-155/CTLA-4 axis was involved in modifying Th1/Th2 balance, a critical indicator for asthma progression. Methods: Altogether 772 asthma patients and 441 healthy controls were recruited, and their blood samples were collected to determine expressional levels of MALAT1, miR-155, CTLA-4, T-bet, GATA3, Th1-type cytokines and Th2-type cytokines. The CD⁴⁺ T cells were administered with pcDNA3.1-MALAT1, si-MALAT1, miR-155 mimic and miR-155 inhibitor to assess their effects on cytokine release. The luciferase reporter gene assay was also adopted to evaluate the sponging relationships between MALAT1 and miR-155, as well as between miR-155 and CTLA-4. Results: Over-expressed MALAT1 and under-expressed miR-155 were more frequently detected among asthma patients who showed traits of reduced forced expiratory failure volume in 1 s (FEV1), FEV1/forced vital capacity (FVC) and FEV1% of predicted (P<0.05). Moreover, MALAT1 expression was negatively expressed with the Th1/Th2 and T-bet/GATA3 ratios, yet miR-155 expression displayed a positively correlation with the ratios (P<0.05). Additionally, the IFN-γ, IL-2 and T-bet levels were reduced under the influence of pcDNA3.1-MALAT1 and miR-155 inhibitor, while levels of IL-4, IL-10 and GATA3 were raised under identical settings (P<0.05). Furthermore, MALAT1 constrained expression of miR-155 within CD⁴⁺ T cells by sponging it, and CTLA-4 could interfere with the effects of MALAT1 and miR-155 on Th1/Th2 balance and T-bet/Gata3 ratio (P<0.05). Conclusion: MALAT1 sponging miR-155 was involved with regulation of Th1/Th2 balance within CD⁴⁺ T cells, which might aid to develop therapies for amelioration of asthmatic inflammation.

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.

Overexpression of Csk-binding protein/phosphoprotein associated with glycosphingolipid-enriched microdomains induces cluster of differentiation 59-mediated apoptosis in Jurkat cells

Csk-binding protein/phosphoprotein associated with glycosphingolipid-enriched microdomains (CBP/PAG) is a membrane-bound adaptor protein that downregulates the activation of Src family kinases present in lipid rafts. To elucidate the role of CBP/PAG in human T cell activation, a cell line overexpressing CBP/PAG was constructed and the function of CBP/PAG in Jurkat cells was examined. The present study revealed that increased CBP/PAG expression in T cells significantly enhanced their apoptosis and reduced cellular activation and proliferation. Overexpression of CBP/PAG suppressed the growth of Jurkat cells by recruiting c-Src and its negative regulator, C-terminal Src kinase (CSK), to lipid rafts. The negative regulation of CBP/PAG was enhanced in the presence of anti-cluster of differentiation (CD)59 monoclonal antibodies. In addition, a significant association was revealed between the location of CBP/PAG and CD59, which were co-expressed in the same region of the cell membrane, implicating a potential overlap of the elicited signaling pathways. These results indicate that CBP/PAG functions as a negative regulator of cell signal transduction and suggest that CD59 may strengthen the role of negative feedback regulation.

Long-Range Modulation of PAG1 Expression by 8q21 Allergy Risk Variants

The gene(s) whose expression is regulated by allergy risk variants is unknown for many loci identified through genome-wide association studies. Addressing this knowledge gap might point to new therapeutic targets for allergic disease. The aim of this study was to identify the target gene(s) and the functional variant(s) underlying the association between rs7009110 on chromosome 8q21 and allergies. Eight genes are located within 1 Mb of rs7009110. Multivariate association analysis of publicly available exon expression levels from lymphoblastoid cell lines (LCLs) identified a significant association between rs7009110 and the expression of a single gene, PAG1 (p = 0.0017), 732 kb away. Analysis of histone modifications and DNase I hypersensitive sites in LCLs identified four putative regulatory elements (PREs) in the region. Chromosome conformation capture confirmed that two PREs interacted with the PAG1 promoter, one in allele-specific fashion. To determine whether these PREs were functional, LCLs were transfected with PAG1 promoter-driven luciferase reporter constructs. PRE3 acted as a transcriptional enhancer for PAG1 exclusively when it carried the rs2370615:C allergy predisposing allele, a variant in complete linkage disequilibrium with rs7009110. As such, rs2370615, which overlaps RelA transcription factor (TF) binding in LCLs and was found to disrupt Foxo3a binding to PRE3, represents the putative functional variant in this locus. Our studies suggest that the risk-associated allele of rs2370615 predisposes to allergic disease by increasing PAG1 expression, which might promote B cell activation and have a pro-inflammatory effect. Inhibition of PAG1 expression or function might have therapeutic potential for allergic diseases.

Transmembrane adaptor protein PAG/CBP is involved in both positive and negative regulation of mast cell signaling

The transmembrane adaptor protein PAG/CBP (here, PAG) is expressed in multiple cell types. Tyrosine-phosphorylated PAG serves as an anchor for C-terminal SRC kinase, an inhibitor of SRC-family kinases. The role of PAG as a negative regulator of immunoreceptor signaling has been examined in several model systems, but no functions in vivo have been determined. Here, we examined the activation of bone marrow-derived mast cells (BMMCs) with PAG knockout and PAG knockdown and the corresponding controls. Our data show that PAG-deficient BMMCs exhibit impaired antigen-induced degranulation, extracellular calcium uptake, tyrosine phosphorylation of several key signaling proteins (including the high-affinity IgE receptor subunits, spleen tyrosine kinase, and phospholipase C), production of several cytokines and chemokines, and chemotaxis. The enzymatic activities of the LYN and FYN kinases were increased in nonactivated cells, suggesting the involvement of a LYN- and/or a FYN-dependent negative regulatory loop. When BMMCs from PAG-knockout mice were activated via the KIT receptor, enhanced degranulation and tyrosine phosphorylation of the receptor were observed. In vivo experiments showed that PAG is a positive regulator of passive systemic anaphylaxis. The combined data indicate that PAG can function as both a positive and a negative regulator of mast cell signaling, depending upon the signaling pathway involved.

Focal adhesion kinase negatively regulates Lck function downstream of the T cell antigen receptor

Focal adhesion kinase (FAK) is a critical regulator of signal transduction in multiple cell types. Although this protein is activated upon TCR engagement, the cellular function that FAK plays in mature human T cells is unknown. By suppressing the function of FAK, we revealed that FAK inhibits TCR-mediated signaling by recruiting C-terminal Src kinase to the membrane and/or receptor complex following TCR activation. Thus, in the absence of FAK, the inhibitory phosphorylation of Lck and/or Fyn is impaired. Together, these data highlight a novel role for FAK as a negative regulator TCR function in human T cells. These results also suggest that changes in FAK expression could modulate sensitivity to TCR stimulation and contribute to the progression of T cell malignancies and autoimmune diseases.

CTLA-4 and autoimmunity: new insights into the dual regulator of tolerance

Cytotoxic T-Lymphocye Antigen 4 (CTLA-4) or CD152 is an inhibitory molecule that plays a critical role in maintenance of tolerance to self-antigens. CTLA-4 is structurally as well as functionally related to CD28, since it shares 31% of homology and binds the B7 family molecules CD80 and CD86 with higher affinity. Nevertheless, CTLA-4 has opposing effects on T cell activation and current evidence shows that its inhibitory role goes beyond the ligand-binding interaction. CTLA-4 competes with CD28 in binding to B7, interacts within the immunological synapsis elements and with clathrin adaptor proteins and tyrosine phosphatases through its cytoplasmic domain to regulate cell trafficking and to set the activation threshold within T cells. Moreover, we have learned from the knock out model that CTLA-4 plays a key role in regulatory T cells and in central tolerance. Because of its importance in maintenance of peripheral tolerance, CTLA-4 has been implicated in several autoimmune diseases, such as systemic lupus erythematosus. Multiple single-nucleotide polymorphisms have been located to human Ctla-4 gene, and their association with autoimmune disease is still a matter of controversy. Despite the promising results of abatacept or CTLA-4-Ig in rheumatoid arthritis and murine lupus nephritis, more clinical randomized trials and standardization of outcomes are needed to prove its efficacy and safety in human lupus nephritis.