Negative modulation of invariant natural killer T cell responses to glycolipid antigens by p38 MAP kinase

Activation of natural killer T cells contributes to Th1 bias in the murine liver after 14 d of ethinylestradiol exposure

BACKGROUND

As the main component of oral contraceptives (OCs), ethinylestradiol (EE) has been widely applied as a model drug to induce murine intrahepatic cholestasis. The clinical counterpart of EE-induced cholestasis includes women who are taking OCs, sex hormone replacement therapy, and susceptible pregnant women. Taking intrahepatic cholestasis of pregnancy (ICP) as an example, ICP consumes the medical system due to its high-risk fetal burden and the impotency of ursodeoxycholic acid in reducing adverse perinatal outcomes.

AIM

To explore the mechanisms and therapeutic strategies of EE-induced cholestasis based on the liver immune microenvironment.

METHODS

Male C57BL/6J mice or invariant natural killer T (iNKT) cell deficiency (Jα18^-/- mice) were administered with EE (10 mg/kg, subcutaneous) for 14 d.

RESULTS

Both Th1 and Th2 cytokines produced by NKT cells increased in the liver skewing toward a Th1 bias. The expression of the chemokine/chemokine receptor Cxcr6/Cxcl16, toll-like receptors, Ras/Rad, and PI3K/Bad signaling was upregulated after EE administration. EE also influenced bile acid synthase Cyp7a1, Cyp8b1, and tight junctions ZO-1 and Occludin, which might be associated with EE-induced cholestasis. iNKT cell deficiency (Jα18^-/- mice) robustly alleviated cholestatic liver damage and lowered the expression of the abovementioned signaling pathways.

CONCLUSION

Hepatic NKT cells play a pathogenic role in EE-induced intrahepatic cholestasis. Our research improves the understanding of intrahepatic cholestasis by revealing the hepatic immune microenvironment and also provides a potential clinical treatment by regulating iNKT cells.

Role of p38 MAPK in Atherosclerosis and Aortic Valve Sclerosis

Atherosclerosis and aortic valve sclerosis are cardiovascular diseases with an increasing prevalence in western societies. Statins are widely applied in atherosclerosis therapy, whereas no pharmacological interventions are available for the treatment of aortic valve sclerosis. Therefore, valve replacement surgery to prevent acute heart failure is the only option for patients with severe aortic stenosis. Both atherosclerosis and aortic valve sclerosis are not simply the consequence of degenerative processes, but rather diseases driven by inflammatory processes in response to lipid-deposition in the blood vessel wall and the aortic valve, respectively. The p38 mitogen-activated protein kinase (MAPK) is involved in inflammatory signaling and activated in response to various intracellular and extracellular stimuli, including oxidative stress, cytokines, and growth factors, all of which are abundantly present in atherosclerotic and aortic valve sclerotic lesions. The responses generated by p38 MAPK signaling in different cell types present in the lesions are diverse and might support the progression of the diseases. This review summarizes experimental findings relating to p38 MAPK in atherosclerosis and aortic valve sclerosis and discusses potential functions of p38 MAPK in the diseases with the aim of clarifying its eligibility as a pharmacological target.

Cell Signaling Pathways That Regulate Antigen Presentation

Cell signaling pathways regulate much in the life of a cell: from shuttling cargo through intracellular compartments and onto the cell surface, how it should respond to stress, protecting itself from harm (environmental insults or infections), to ultimately, death by apoptosis. These signaling pathways are important for various aspects of the immune response as well. However, not much is known in terms of the participation of cell signaling pathways in Ag presentation, a necessary first step in the activation of innate and adaptive T cells. In this brief review, I discuss the known signaling molecules (and pathways) that regulate how Ags are presented to T cells and the mechanism(s), if identified. Studies in this area have important implications in vaccine development and new treatment paradigms against infectious diseases, autoimmunity, and cancer.

CpG oligonucleotide-mediated co-stimulation of mouse invariant natural killer T cells negatively regulates their activation status

Invariant natural killer T (iNKT) cells play important roles in antimicrobial defense and immune-regulation. We have previously shown that iNKT cells express certain toll-like receptors (TLR), and that TLR co-stimulation of iNKT cells in the presence of suboptimal concentrations of T cell receptor (TCR) agonists enhances cellular activation. In the present study, we investigated the regulatory effects of CpG oligonucleotides in mouse primary hepatic and splenic iNKT cells and in DN32.D3 iNKT cells. We show that CpG treatment of iNKT cells in the presence of higher concentrations of TCR agonists (α-GalCer or anti-CD3 mAb) results in the up-regulation of TLR9 in iNKT cells with a concurrent reduction in their cellular activation, as assessed by their production of IL-2, IL-4 and IFN-γ compared with controls. CpG-mediated down-regulation of iNKT cell activation has been found to depend, at least in part, on signaling by MyD88, a critical adapter moiety downstream of TLR9 signaling. Mechanistically, iNKT cells treated with CpG in the presence of TCR agonists show inhibition of MAPK signaling as determined by the levels of ERK1/2 and p38 MAPKs. Furthermore, CpG treatment leads to an increased induction of phosphatases, DUSP1 and SHP-1, that seem to impede MAPK and TCR signaling, resulting in the negative regulation of iNKT cell activation. Our findings therefore suggest a novel regulatory role for CpG in iNKT cells in the mediation of a negative feedback mechanism to control overactive iNKT cell responses and hence to avoid undesirable excessive immunopathology.

The differential roles of mTOR, ERK, and JNK pathways in invariant natural killer T-cell function and survival

Invariant natural killer T (iNKT) cell is a critical element for both innate and adaptive immunity. The quick responses of mature iNKT cells to TCR stimulation require activation of several different signaling pathways. However, the roles of these signaling pathways in mature iNKT cell biology remain incompletely understood. To address this issue, single signaling pathway was blocked with inhibitor in iNKT cells, and the roles of these signaling pathways were estimated. Results showed that mammalian target of rapamycin (mTOR) plays an essential role for cytokine production and survival in iNKT cells. In contrast, ERK and JNK are more important for iNKT cell effector function, but not survival. Our findings delineate the distinct roles of different signaling pathways in mature iNKT cells and therefore shed new light for modulating iNKT cell function in disease conditions.

The Mycobacterium tuberculosis early secreted antigenic target of 6 kDa inhibits T cell interferon-γ production through the p38 mitogen-activated protein kinase pathway

We reported previously that the early secreted antigenic target of 6 kDa (ESAT-6) from Mycobacterium tuberculosis directly inhibits human T cell IFN-γ production and proliferation in response to stimulation with anti-CD3 and anti-CD28. To determine the mechanism of this effect, we treated T cells with kinase inhibitors before stimulation with ESAT-6. Only the p38 MAPK inhibitor, SB203580, abrogated ESAT-6-mediated inhibition of IFN-γ production in a dose-dependent manner. SB203580 did not reverse ESAT-6-mediated inhibition of IL-17 and IL-10 production, suggesting a specific effect of SB203580 on IFN-γ production. SB203580 did not act through inhibition of AKT (PKB) as an AKT inhibitor did not affect ESAT-6 inhibition of T cell IFN-γ production and proliferation. ESAT-6 did not reduce IFN-γ production by expanding FoxP3(+) T regulatory cells. Incubation of T cells with ESAT-6 induced phosphorylation and increased functional p38 MAPK activity, but not activation of ERK or JNK. Incubation of peripheral blood mononuclear cells with ESAT-6 induced activation of p38 MAPK, and inhibition of p38 MAPK with SB203580 reversed ESAT-6 inhibition of M. tuberculosis-stimulated IFN-γ production by peripheral blood mononuclear cells from subjects with latent tuberculosis infection. Silencing of p38α MAPK with siRNA rendered T cells resistant to ESAT-6 inhibition of IFN-γ production. Taken together, our results demonstrate that ESAT-6 inhibits T cell IFN-γ production in a p38 MAPK-dependent manner.

Engagement of glycosylphosphatidylinositol-anchored proteins results in enhanced mouse and human invariant natural killer T cell responses

Invariant natural killer T (iNKT) cells are a small subset of lymphocytes that recognize glycolipid antigens in the context of CD1d and consequently produce large quantities of pro-inflammatory and/or anti-inflammatory cytokines. Several transmembrane glycoproteins have been implicated in the co-stimulation of iNKT cell responses. However, whether glycosylphosphatidylinositol (GPI)-anchored proteins can function in this capacity is not known. Here, we demonstrate that antibody-mediated cross-linking of the prototype mouse GPI-anchored protein Thy-1 (CD90) on the surface of a double-negative (CD4⁻CD8⁻) iNKT cell line leads to cytokine production at both the mRNA and protein levels. In addition, Thy-1 triggering enhanced cytokine secretion by iNKT cells that were concomitantly stimulated with α-galactosylceramide (αGC), consistent with a co-stimulatory role for Thy-1 in iNKT cell activation. This was also evident when a CD4+ mouse iNKT cell line or primary hepatic NKT cells were stimulated with αGC and/or anti-Thy-1 antibody. Cross-linking Ly-6A/E, another GPI-anchored protein, could also boost cytokine secretion by αGC-stimulated iNKT cells, suggesting that the observed effects reflect a general property of GPI-anchored proteins. To extend these results from mouse to human cells, we focused on CD55, a GPI-anchored protein that, unlike Thy-1, is expressed on human iNKT cells. Cross-linking CD55 augmented αGC-induced iNKT cell responses as judged by more vigorous proliferation and higher CD69 expression. Collectively, these findings demonstrate for the first time that GPI-anchored proteins are able to co-stimulate CD1d-restricted, glycolipid-reactive iNKT cells in both mice and humans.