Cyclic AMP-mediated immune regulation--overview of mechanisms of action in T cells

Regulatory T-cell-mediated inhibition of antitumor immune responses is associated with clinical outcome in patients with liver metastasis from colorectal cancer

Adaptive regulatory T cells (Tregs) contribute to an immunosuppressive microenvironment in colorectal cancer (CRC). Here, we examined whether the level of Treg-mediated inhibition of antitumor immune responses in patients with metastatic CRC (metCRC) selected for liver resection is associated with clinical outcome. Preoperatively and at follow-ups, we did flow-based phenotyping, examined antitumor immunity using peptides from carcinoembryonic antigen (CEA) protein in the presence or absence of CD4(+)CD25(+)CD127(dim/-) cells (Tregs) and determined cytokine and PGE(2) levels in patient blood samples. At 18 months post-surgery, 8 patients were disease free (7 alive and 1 dead of unrelated cause) and 10 had experienced disease recurrence (7 alive and 3 dead of metCRC). Prior to surgery, the patients demonstrated Treg-mediated suppression of TNFα and IFNγ expression that could be perturbed through the PGE(2)/cAMP pathway and the immune suppression was significantly higher in the group that later developed disease recurrence (P = 0.046). Furthermore, the post-surgery plasma PGE(2) levels were related to the clinical outcome (PGE(2) levels of 280 ± 47 vs. 704 ± 153 pg/ml (mean ± SEM) for disease free and recurrent disease, respectively). T-cell phenotyping revealed higher frequencies of COX-2(+) cells in the patients with recurrent disease. These findings support the notion that the level of Treg-mediated suppression of adaptive antitumor immune responses at the time of surgery may influence later clinical outcome of metCRC and provide valuable prognostic information.

Forced expression of stabilized c-Fos in dendritic cells reduces cytokine production and immune responses in vivo

Intracellular cyclic adenosine monophosphate (cAMP) suppresses innate immunity by inhibiting proinflammatory cytokine production by monocytic cells. We have shown that the transcription factor c-Fos is responsible for cAMP-mediated suppression of inflammatory cytokine production, and that c-Fos protein is stabilized by IKKβ-mediated phosphorylation. We found that S308 is one of the major phosphorylation sites, and that the S308D mutation prolongs c-Fos halflife. To investigate the role of stabilized c-Fos protein in dendritic cells (DCs) in vivo, we generated CD11c-promoter-deriven c-FosS308D transgenic mice. As expected, bone marrow-derived DCs (BMDCs) from these Tg mice produced smaller amounts of inflammatory cytokines, including TNF-α, IL-12, and IL-23, but higher levels of IL-10, in response to LPS, than those from wild-type (Wt) mice. When T cells were co-cultured with BMDCs from Tg mice, production of Th1 and Th17 cytokines was reduced, although T cell proliferation was not affected. Tg mice demonstrated more resistance to experimental autoimmune encephalomyelitis (EAE) than did Wt mice. These data suggest that c-Fos in DCs plays a suppressive role in certain innate and adaptive immune responses.

Quantitative phosphoproteomic analysis reveals a role for serine and threonine kinases in the cytoskeletal reorganization in early T cell receptor activation in human primary T cells

Protein phosphorylation-dephosphorylation events play a primary role in regulation of almost all aspects of cell function including signal transduction, cell cycle, or apoptosis. Thus far, T cell phosphoproteomics have focused on analysis of phosphotyrosine residues, and little is known about the role of serine/threonine phosphorylation in early activation of the T cell receptor (TCR). Therefore, we performed a quantitative mass spectrometry-based analysis of the global phosphoproteome of human primary T cells in response to 5 min of TCR activation with anti-CD3 antibody. Combining immunoprecipitation with an antiphosphotyrosine antibody, titanium dioxide phosphopeptide enrichment, isobaric tag for the relative and absolute quantitation methodology, and strong cation exchange separation, we were able to identify 2814 phosphopeptides. These unique sites were employed to investigate the site-specific phosphorylation dynamics. Five hundred and seventeen phosphorylation sites showed TCR-responsive changes. We found that upon 5 min of stimulation of the TCR, specific serine and threonine kinase motifs are overrepresented in the set of responsive phosphorylation sites. These phosphorylation events targeted proteins with many different activities and are present in different subcellular locations. Many of these proteins are involved in intracellular signaling cascades related mainly to cytoskeletal reorganization and regulation of small GTPase-mediated signal transduction, probably involved in the formation of the immune synapse.

Modulation of Kv1.3 channels by protein kinase A I in T lymphocytes is mediated by the disc large 1-tyrosine kinase Lck complex

The cAMP/PKA signaling system constitutes an inhibitory pathway in T cells and, although its biochemistry has been thoroughly investigated, its possible effects on ion channels are still not fully understood. K(V)1.3 channels play an important role in T-cell activation, and their inhibition suppresses T-cell function. It has been reported that PKA modulates K(V)1.3 activity. Two PKA isoforms are expressed in human T cells: PKAI and PKAII. PKAI has been shown to inhibit T-cell activation via suppression of the tyrosine kinase Lck. The aim of this study was to determine the PKA isoform modulating K(V)1.3 and the signaling pathway underneath. 8-Bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP), a nonselective activator of PKA, inhibited K(V)1.3 currents both in primary human T and in Jurkat cells. This inhibition was prevented by the PKA blocker PKI(6-22). Selective knockdown of PKAI, but not PKAII, with siRNAs abolished the response to 8-BrcAMP. Additional studies were performed to determine the signaling pathway mediating PKAI effect on K(V)1.3. Overexpression of a constitutively active mutant of Lck reduced the response of K(V)1.3 to 8-Br-cAMP. Moreover, knockdown of the scaffolding protein disc large 1 (Dlg1), which binds K(V)1.3 to Lck, abolished PKA modulation of K(V)1.3 channels. Immunohistochemistry studies showed that PKAI, but not PKAII, colocalizes with K(V)1.3 and Dlg1 indicating a close proximity between these proteins. These results indicate that PKAI selectively regulates K(V)1.3 channels in human T lymphocytes. This effect is mediated by Lck and Dlg1. We thus propose that the K(V)1.3/Dlg1/Lck complex is part of the membrane pathway that cAMP utilizes to regulate T-cell function.

Underlying mechanisms of cAMP- and glucocorticoid-mediated inhibition of FasL expression in activation-induced cell death

Glucocorticoids (GCs) and cAMP-dependent signaling pathways exert diverse and relevant immune regulatory functions, including a tight control of T cell death and homeostasis. Both of these signaling molecules inhibit TCR-induced cell death and FasL expression, but the underlying mechanisms are still poorly understood. Therefore, to address this question, we performed a comprehensive screening of signaling pathways downstream of the TCR, in order to define which of them are targets of cAMP- and GC-mediated inhibition. We found that cAMP inhibited NF-κB and ERK pathways through a PKA-dependent mechanism, while Dexamethasone blocked TCR-induced NF-κB signaling. Although GCs and cAMP inhibited the induction of endogenous FasL mRNA expression triggered by TCR activation, they potentiated TCR-mediated induction of FasL promoter activity in transient transfection assays. However, when the same FasL promoter was stably transfected, the facilitatory effect of GCs and cAMP became inhibitory, thus resembling the effects on endogenous FasL mRNA expression. Hence, the endogenous chromatinization status known to occur in integrated or genomic vs. episomic DNA might be critical for proper regulation of FasL expression by cAMP and GCs. Our results suggest that the chromatinization status of the FasL promoter may function as a molecular switch, controlling cAMP and GC responsiveness and explaining why these agents inhibit FasL expression in T cells but induce FasL in other cell types.

Divergent requirement for Gαs and cAMP in the differentiation and inflammatory profile of distinct mouse Th subsets

cAMP, the intracellular signaling molecule produced in response to GPCR signaling, has long been recognized as an immunosuppressive agent that inhibits T cell receptor activation and T cell function. However, recent studies show that cAMP also promotes T cell-mediated immunity. Central to cAMP production downstream of GPCR activation is the trimeric G protein Gs. In order to reconcile the reports of divergent effects of cAMP in T cells and to define the direct effect of cAMP in T cells, we engineered mice in which the stimulatory Gα subunit of Gs (Gαs) could be deleted in T cells using CD4-Cre (Gnas(ΔCD4)). Gnas(ΔCD4) CD4(+) T cells had reduced cAMP accumulation and Ca2(+) influx. In vitro and in vivo, Gnas(ΔCD4) CD4(+) T cells displayed impaired differentiation to specific Th subsets: Th17 and Th1 cells were reduced or absent, but Th2 and regulatory T cells were unaffected. Furthermore, Gnas(ΔCD4) CD4(+) T cells failed to provoke colitis in an adoptive transfer model, indicating reduced inflammatory function. Restoration of cAMP levels rescued the impaired phenotype of Gnas(ΔCD4) CD4(+) T cells, reinstated the PKA-dependent influx of Ca2(+), and enhanced the ability of these cells to induce colitis. Our findings thus define an important role for cAMP in the differentiation of Th subsets and their subsequent inflammatory responses, and provide evidence that altering cAMP levels in CD4(+) T cells could provide an immunomodulatory approach targeting specific Th subsets.

Mechanisms of rapid induction of interleukin-22 in activated T cells and its modulation by cyclosporin a

IL-22 is an immunoregulatory cytokine displaying pathological functions in models of autoimmunity like experimental psoriasis. Understanding molecular mechanisms driving IL-22, together with knowledge on the capacity of current immunosuppressive drugs to target this process, may open an avenue to novel therapeutic options. Here, we sought to characterize regulation of human IL22 gene expression with focus on the established model of Jurkat T cells. Moreover, effects of the prototypic immunosuppressant cyclosporin A (CsA) were investigated. We report that IL-22 induction by TPA/A23187 (T/A) or αCD3 is inhibited by CsA or related FK506. Similar data were obtained with peripheral blood mononuclear cells or purified CD3(+) T cells. IL22 promoter analysis (-1074 to +156 bp) revealed a role of an NF-AT (-95/-91 nt) and a CREB (-194/-190 nt) binding site for gene induction. Indeed, binding of CREB and NF-ATc2, but not c-Rel, under the influence of T/A to those elements could be proven by ChIP. Because CsA has the capability to impair IκB kinase (IKK) complex activation, the IKKα/β inhibitor IKKVII was evaluated. IKKVII likewise reduced IL-22 induction in Jurkat cells and peripheral blood mononuclear cells. Interestingly, transfection of Jurkat cells with siRNA directed against IKKα impaired IL22 gene expression. Data presented suggest that NF-AT, CREB, and IKKα contribute to rapid IL22 gene induction. In particular the crucial role of NF-AT detected herein may form the basis of direct action of CsA on IL-22 expression by T cells, which may contribute to therapeutic efficacy of the drug in autoimmunity.

T cell targeting by anthrax toxins: two faces of the same coin

Bacillus anthracis, similar to other bacterial pathogens, has evolved effective immune evasion strategies to prolong its survival in the host, thus ensuring the unchecked spread of the infection. This function is subserved by lethal (LT) and edema (ET) toxins, two exotoxins produced by vegetative anthrax bacilli following germination of the spores. The structure of these toxins and the mechanism of cell intoxication are topics covered by other reviews in this issue. Here we shall discuss how B. anthracis uses LT and ET to suppress the immune defenses of the host, focusing on T lymphocytes, the key players in adaptive immunity. We shall also summarize recent findings showing that, depending on its concentration, ET has the ability not only to suppress T cell activation but also to promote the polarization of CD4(+) T cells to the Th2 and Th17 subsets, highlighting the potential use of this toxin as an immunomodulator.

Mice with disrupted type I protein kinase A anchoring in T cells resist retrovirus-induced immunodeficiency

Type I protein kinase A (PKA) is targeted to the TCR-proximal signaling machinery by the A-kinase anchoring protein ezrin and negatively regulates T cell immune function through activation of the C-terminal Src kinase. RI anchoring disruptor (RIAD) is a high-affinity competitor peptide that specifically displaces type I PKA from A-kinase anchoring proteins. In this study, we disrupted type I PKA anchoring in peripheral T cells by expressing a soluble ezrin fragment with RIAD inserted in place of the endogenous A-kinase binding domain under the lck distal promoter in mice. Peripheral T cells from mice expressing the RIAD fusion protein (RIAD-transgenic mice) displayed augmented basal and TCR-activated signaling, enhanced T cell responsiveness assessed as IL-2 secretion, and reduced sensitivity to PGE(2)- and cAMP-mediated inhibition of T cell function. Hyperactivation of the cAMP-type I PKA pathway is involved in the T cell dysfunction of HIV infection, as well as murine AIDS, a disease model induced by infection of C57BL/6 mice with LP-BM5, a mixture of attenuated murine leukemia viruses. LP-BM5-infected RIAD-transgenic mice resist progression of murine AIDS and have improved viral control. This underscores the cAMP-type I PKA pathway in T cells as a putative target for therapeutic intervention in immunodeficiency diseases.