Taro Fukao

PI3K and negative regulation of TLR signaling

Excessive immune responses are detrimental to the host and negative feedback regulation is crucial for the maintenance of immune-system integrity. Recent studies have shown that phosphoinositide 3-kinase (PI3K) is an endogenous suppressor of interleukin-12 (IL-12) production triggered by Toll-like receptor (TLR) signaling and limits excessive Th1 polarization. Unlike IRAK-M (IL-1 receptor-associated kinase-M) and SOCS-1 (suppressor of cytokine signaling-1) that are induced by TLR signaling and function during the second or continuous exposure to stimulation, PI3K functions at the early phase of TLR signaling and modulates the magnitude of the primary activation. Thus, PI3K, IRAK-M and SOCS-1 have unique roles in the gate-keeping system, preventing excessive innate immune responses.

PI3K-mediated negative feedback regulation of IL-12 production in DCs

Although interleukin 12 (IL-12) production by dendritic cells (DCs) confers protection against harmful invasions by regulating both innate and adaptive immunity, its dysregulation may have detrimental effects on the host. We show here that phosphoinositide 3-kinase (PI3K) negatively regulates IL-12 synthesis by DCs. We found that numerous stimuli that induced IL-12 production concomitantly elicited PI3K activation in DCs, but both PI3K−/− and PI3K inhibitor–treated DCs showed increased IL-12 production. Accordingly, an enhanced T helper type 1 (TH1) response was observed upon Leishmania major infection in PI3K−/− mice. Our findings indicate that a negative feedback mechanism exists that regulates IL-12 production during DC activation and may help prevent the excessive TH1 polarization that causes undesirable immune responses.

IFN-γ production by antigen-presenting cells: mechanisms emerge

Abstract The suggestion that antigen-presenting cells (APCs) produce interferon γ (IFN-γ) is controversial because it conflicts with the initial paradigm in which the production of IFN-γ was restricted to lymphoid cells. However, some answers to this skepticism have been provided by recent findings of high-level production and intracellular expression of IFN-γ by interleukin-12 (IL-12)-stimulated macrophages and dendritic cells. New data are now emerging to explain the mechanism of production of IFN-γ vby APCs. As in lymphoid cells, IL-12-induced IFN-γ production in APCs requires signal transducer and activator of transcription 4 (STAT4), although the precise molecular events that govern the transcription of the gene encoding IFN-γ are enigmatic still. Understanding these processes in lymphoid, and now nonlymphoid, cells remains an important challenge.

Synergistic Effects of IL-4 and IL-18 on IL-12-Dependent IFN-γ Production by Dendritic Cells

Mouse splenic dendritic cells (DCs) produce IFN-γ in response to IL-12. In the present study, we analyzed effects of Th1 and Th2 cytokines on IFN-γ production by DCs. IL-18 produced by DCs and macrophages acts in an autocrine manner and augments IL-12-induced IFN-γ production by DCs as also observed in T and NK cells. Surprisingly, IL-4, a Th2 cytokine, also acts synergistically with IL-12 on IFN-γ production by DCs. In addition, IL-4 markedly enhances IFN-γ production when DCs are stimulated through CD40 or MHC class II. These results indicate that both Th1 and Th2 cytokines act on DCs during T cell-DC interaction upon Ag presentation. p38 mitogen-activated protein kinase is constitutively activated in mature DCs and is required for IFN-γ production by DCs. IL-18 but not IL-4 or IL-12 further activates the p38 mitogen-activated protein kinase activity, suggesting that IL-4 and IL-18 enhance IFN-γ production through distinct intracellular signal transduction pathways in DCs.

Inducible Expression of Stat4 in Dendritic Cells and Macrophages and Its Critical Role in Innate and Adaptive Immune Responses

Autocrine activation of APC by IL-12 has recently been revealed; we demonstrate here that inducible expression of Stat4 in APC is central to this process. Stat4 is induced in dendritic cells (DC) in a maturation-dependent manner and in macrophages in an activation-dependent manner. Stat4 levels directly correlate with IL-12-dependent IFN-γ production by APC as well as IFN-γ production by DC during Ag presentation. The Th2 cytokines IL-4 and IL-10 suppress Stat4 induction in DC and macrophages when present during maturation and activation, respectively, diminishing IFN-γ production. In contrast, IL-4 has no effect on Stat4 levels in mature DC and actually augments IFN-γ production by DC during Ag presentation, indicating that IL-4 acts differently in a spatiotemporal manner. The functional importance of Stat4 is evident in Stat4−/− DC and macrophages, which fail to produce IFN-γ. Furthermore, Stat4−/− macrophages are defective in NO production in response to IL-12 and are susceptible to Toxoplasma. Autocrine IL-12 signaling is required for high-level IFN-γ production by APC at critical stages in both innate and adaptive immunity, and the control of Stat4 expression is likely an important regulator of this process.

Expression of functional IL-2 receptors on mature splenic dendritic cells.

We report here the expression of functional IL‐2 receptor (IL‐2R) on mature splenic dendritic cells (DC) and synergistic effect of IL‐2 on IFN‐γ production by DC. IL‐2 augmented IL‐12‐dependent IFN‐γ production by DC purified from both splenocytes of wild‐type and anti‐asialoGM1 Ab‐treated Rag‐2–/– splenocytes devoid of T, B, NK and NKT cells. A neutralizing mAb against IL‐2Rα blocked such enhancing effect of IL‐2 on IFN‐γ production, indicating the presence of functional IL‐2R on DC. Synergistic effects of IL‐2 were also observed on IFN‐γ production by DC stimulated through CD40 or MHC class II, suggesting that T cell‐derived IL‐2 can act on DC during antigen presentation. Furthermore, we provide evidence that DC produce IFN‐γ during interaction with allogeneic CD4+ T cells from IFN‐γ–/– mice. These results suggest that IL‐2 produced by naive T cells upon antigen stimulation is an important factor during Th0 to Th1 differentiation by inducing IFN‐γ from DC.

Role of phosphoinositide 3-kinase signaling in mast cells: new insights from knockout mouse studies.

Phosphoinositide 3-kinases (PI3Ks) are a family of lipid kinases essential for diverse physiological reactions. In recent years a series of gene-targeted mice lacking different types of PI3Ks and related molecules have been generated which enable us to understand the role of PI3K pathways, particularly class I members, in vivo. Analyses of such gene-targeted mice have led to major discoveries in the physiological roles of PI3K signaling in mast cell biology. In particular the role of PI3Ks has been extensively studied in signaling through the high-affinity IgE receptor (FcεRI), since mast cells are the main effector cells in type I allergic reaction associated with IgE-dependent mechanisms. Furthermore, the knockout mice have provided significant information concerning the role of PI3K signals in mast cell differentiation. This review presents several new insights into mast cell biology, which have been elucidated by the analyses of these knockout mice.

Dendritic-cell-based anticancer vaccination: has it matured?

Abstract The International Cancer Vaccine Symposium: Current Achievement in Dendritic-Cell Vaccines was held in Tokyo, Japan on 2 December 2001.