Accumulation of NFAT mediates IL-2 expression in memory, but not naïve, CD4+ T cells.

In contrast to naïve CD4+ T cells, memory CD4+ T cells rapidly express high levels of effector cytokines in response to antigen stimulation. The molecular mechanism for this specific behavior is not well understood. The nuclear factor of activated T cells (NFAT) family of transcription factors plays an important role in the transcription of many cytokine genes. Here we show that memory CD4+ T cells rapidly induce NFAT-mediated transcription upon T cell receptor ligation whereas NFAT activation in naïve CD4+ T cells requires longer periods of stimulation. The difference in kinetics correlates with the low levels of NFATc1 and NFATc2 proteins present in naïve CD4+ T cells and their high levels in memory CD4+ T cells. Accordingly, IL-2 expression requires NFAT activation only in memory CD4+ T cells whereas it is NFAT-independent in naïve CD4+ T cells. Thus, the accumulation of NFATc1 and NFATc2 in memory CD4+ T cells represents a previously uncharacterized regulatory mechanism for the induction of early gene expression after antigen stimulation.

p38 mitogen-activated protein kinase mediates the Fas-induced mitochondrial death pathway in CD8+ T cells.

The p38 mitogen-activated protein kinase (MAPK) signaling pathway can be activated by a variety of stress stimuli such as UV radiation and osmotic stress. The regulation and role of this pathway in death receptor-induced apoptosis remain unclear and may depend on the specific death receptor and cell type. Here we show that binding of Fas ligand to Fas activates p38 MAPK in CD8+ T cells and that activation of this pathway is required for Fas-mediated CD8+ T-cell death. Active p38 MAPK phosphorylates Bcl-xL and Bcl-2 and prevents the accumulation of these antiapoptotic molecules within the mitochondria. Consequently, a loss of mitochondrial membrane potential and the release of cytochrome c lead to the activation of caspase 9 and, subsequently, caspase 3. Therefore, the activation of p38 MAPK is a critical link between Fas and the mitochondrial death pathway and is required for the Fas-induced apoptosis of CD8+ T cells.

Regulation of p38 MAP kinase in CD4+ lymphocytes by infliximab therapy in patients with rheumatoid arthritis.

Tumor necrosis factor alpha (TNFalpha) plays a key role in the pathogenesis of rheumatoid arthritis (RA) and most patients treated with anti-TNFalpha agents show significant improvement in both signs and symptoms. While TNFalpha inhibitors rapidly reduce joint inflammation, the mechanisms by which these agents exert their long-term effects remain unclear. The p38 MAP kinase pathway is one of the signaling pathways triggered by TNFalpha and pharmacological inhibitors of this kinase are being developed for use in RA. Since p38 MAP kinase is involved in interferon gamma (IFNgamma) production by CD4+ T helper 1 (Th1) cells and a Th1 immune response has been associated with RA, we investigated whether anti-TNFalpha therapy could affect the activation of this signaling pathway in the CD4+ T cells of RA patients. We show that in five patients, treatment with infliximab caused a marked reduction of activated p38 MAP kinase levels in CD4+ T cells, without affecting the total levels of p38 MAPK. In contrast to T cells, infliximab therapy did not affect the levels of active p38 MAP kinase in macrophages from the same patients. The selective effect of anti-TNFalpha therapy on the p38 MAP kinase signaling pathway of CD4+ T cells in patients with RA suggests that prolonged benefit with these agents may be mediated by their effect on CD4+ T cells.

Inhibition of NFAT specifically in T cells prevents allergic pulmonary inflammation.

NFAT is a family of transcription factors important in the regulation of cytokine genes and is widely expressed in different lymphoid and nonlymphoid tissues. Consequently, the role of NFAT in CD4+ T cells during an in vivo immune response is not completely clear. In this study, we use transgenic mice expressing a dominant negative NFAT mutant exclusively in T cells to address the role of NFAT in T cells during a Th2 immune response in a model of allergic airway inflammation. We have observed that inhibition of NFAT in T cells results in a reduction of Ag-specific Th2 Ab levels and IL-4 production by CD4+ T cells. The accumulation of eosinophils in the bronchoalveolar lavage is delayed in dominant negative NFAT-transgenic mice. These mice are also more resistant to the development of lung pathology in response to allergen exposure. We, therefore, conclude that activation of NFAT in CD4+ T cells is required for the development of a Th2 immune response in vivo and allergic airway inflammation.

c-Jun NH(2)-terminal kinase (JNK)1 and JNK2 have distinct roles in CD8(+) T cell activation.

The c-Jun NH(2)-terminal kinase (JNK) signaling pathway is induced by cytokines and stress stimuli and is implicated in cell death and differentiation, but the specific function of this pathway depends on the cell type. Here we examined the role of JNK1 and JNK2 in CD8(+) T cells. Unlike CD4(+) T cells, the absence of JNK2 causes increased interleukin (IL)-2 production and proliferation of CD8(+) T cells. In contrast, JNK1-deficient CD8(+) T cells are unable to undergo antigen-stimulated expansion in vitro, even in the presence of exogenous IL-2. The hypoproliferation of these cells is associated with impaired IL-2 receptor alpha chain (CD25) gene and cell surface expression. The reduced level of nuclear activating protein 1 (AP-1) complexes in activated JNK1-deficient CD8(+) T cells can account for the impaired IL-2 receptor alpha chain gene expression. Thus, JNK1 and JNK2 play different roles during CD8(+) T cell activation and these roles differ from those in CD4(+) T cells.

Granulocyte macrophage-colony stimulating factor (GM-CSF) recruits immune cells to the pancreas and delays STZ-induced diabetes.

Granulocyte macrophage-colony stimulating factor (GM-CSF) is one of the most widely used growth factors for enhancing immune responses and is known to recruit and activate antigen-presenting cells (APCs). This study hypothesized that overexpression of this cytokine within the pancreatic beta-cells would recruit, expand, and activate APCs. The question was whether this would lead to tolerance or autoimmunity to pancreatic antigens. This possibility was tested by preparing transgenic mice (ins-GM-CSF) whose islets expressed murine GM-CSF. By 6-8 weeks of age, these mice developed a profound mononuclear cell infiltration that often overwhelmed the exocrine pancreas, although no changes in enzyme or hormone function were apparent. The majority of the mononuclear infiltrate within the pancreas was identified as F4/80+ macrophages. Transgenic ins-GM-CSF mice had splenomegaly due to a massive increase in the macrophage population. Additionally, mononuclear cells were found within the livers of transgenic mice, with F4/80+ cells also identified within the infiltrate, indicating that GM-CSF-activated mononuclear cells circulated to organs other than the pancreas. To assess the disease potential, this study tested whether macrophage recruitment to the pancreas might accelerate or protect the islets from diabetes. It was found that the induction of diabetes by low-dose streptozotocin (STZ) was delayed and reduced within ins-GM-CSF transgenic mice, in comparison with negative littermates. Together, these data highlight the role of GM-CSF in recruiting APCs such as macrophages. Advanced cellular infiltration does not overtly harm, and may even protect, pancreatic function, as seen with the delay in chemically induced diabetes.