Christian Bogdan

Nitric oxide and the immune response

During the past two decades, nitric oxide (NO) has been recognized as one of the most versatile players in the immune system. It is involved in the pathogenesis and control of infectious diseases, tumors, autoimmune processes and chronic degenerative diseases. Because of its variety of reaction partners (DNA, proteins, low–molecular weight thiols, prosthetic groups, reactive oxygen intermediates), its widespread production (by three different NO synthases (NOS) and the fact that its activity is strongly influenced by its concentration, NO continues to surprise and perplex immunologists. Today, there is no simple, uniform picture of the function of NO in the immune system. Protective and toxic effects of NO are frequently seen in parallel. Its striking inter- and intracellular signaling capacity makes it extremely difficult to predict the effect of NOS inhibitors and NO donors, which still hampers therapeutic applications.

Repetitive Injections of Dendritic Cells Matured with Tumor Necrosis Factor α Induce Antigen-specific Protection of Mice from Autoimmunity

Mature dendritic cells (DCs) are believed to induce T cell immunity, whereas immature DCs induce T cell tolerance. Here we describe that injections of DCs matured with tumor necrosis factor (TNF)-α (TNF/DCs) induce antigen-specific protection from experimental autoimmune encephalomyelitis (EAE) in mice. Maturation by TNF-α induced high levels of major histocompatibility complex class II and costimulatory molecules on DCs, but they remained weak producers of proinflammatory cytokines. One injection of such TNF/DCs pulsed with auto-antigenic peptide ameliorated the disease score of EAE. This could not be observed with immature DCs or DCs matured with lipopolysaccharide (LPS) plus anti-CD40. Three consecutive injections of peptide-pulsed TNF/DCs derived from wild-type led to the induction of peptide-specific predominantly interleukin (IL)-10–producing CD4+ T cells and complete protection from EAE. Blocking of IL-10 in vivo could only partially restore the susceptibility to EAE, suggesting an important but not exclusive role of IL-10 for EAE prevention. Notably, the protection was peptide specific, as TNF/DCs pulsed with unrelated peptide could not prevent EAE. In conclusion, this study describes that stimulation by TNF-α results in incompletely matured DCs (semi-mature DCs) which induce peptide-specific IL-10–producing T cells in vivo and prevent EAE.

Nitric oxide and the regulation of gene expression

During the past 15 years, nitric oxide (NO) and NO synthases have become an important research topic in cellular and molecular biology. NO is produced by many if not all mammalian cells and fulfils a broad spectrum of signaling functions in physiological and pathophysiological processes. In this review, recent advances in our understanding of the mechanisms by which NO regulates the expression of eukaryotic genes will be summarized. The currently available data illustrate that NO has multiple molecular targets: it can not only directly influence the activity of transcription factors but also modulates upstream signaling cascades, mRNA stability and translation, as well as the processing of the primary gene products.

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.

Postnatal acquisition of endotoxin tolerance in intestinal epithelial cells

The role of innate immune recognition by intestinal epithelial cells (IECs) in vivo is ill-defined. Here, we used highly enriched primary IECs to analyze Toll-like receptor (TLR) signaling and mechanisms that prevent inappropriate stimulation by the colonizing microflora. Although the lipopolysaccharide (LPS) receptor complex TLR4/MD-2 was present in fetal, neonatal, and adult IECs, LPS-induced nuclear factor κB (NF-κB) activation and chemokine (macrophage inflammatory protein 2 [MIP-2]) secretion was only detected in fetal IECs. Fetal intestinal macrophages, in contrast, were constitutively nonresponsive to LPS. Acquisition of LPS resistance was paralleled by a spontaneous activation of IECs shortly after birth as illustrated by phosphorylation of IκB-α and nuclear translocation of NF-κB p65 in situ as well as transcriptional activation of MIP-2. Importantly, the spontaneous IEC activation occurred in vaginally born mice but not in neonates delivered by Caesarean section or in TLR4-deficient mice, which together with local endotoxin measurements identified LPS as stimulatory agent. The postnatal loss of LPS responsiveness of IECs was associated with a posttranscriptional down-regulation of the interleukin 1 receptor–associated kinase 1, which was essential for epithelial TLR4 signaling in vitro. Thus, unlike intestinal macrophages, IECs acquire TLR tolerance immediately after birth by exposure to exogenous endotoxin to facilitate microbial colonization and the development of a stable intestinal host–microbe homeostasis.

Partial impairment of cytokine responses in Tyk2-deficient mice.

To assess the role of the Janus kinase (Jak) family member Tyk2, we have generated Tyk2-/- mice. In contrast to other Jaks, where inactivation leads to a complete loss of the respective cytokine receptor signal, Tyk2-/- mice display reduced responses to IFNalpha/beta and IL-12 and a selective deficiency in Stat3 activation in these pathways. Unexpectedly, IFNgamma signaling is also impaired in Tyk2-/- mice. Tyk2-/- macrophages fail to produce nitric oxide upon lipopolysaccharide induction. Tyk2-/- mice are unable to clear vaccinia virus and show a reduced T cell response after LCMV challenge. These data imply a selective contribution of Tyk2 to the signals triggered by various biological stimuli and cytokine receptors.

Type 1 Interferon (IFNα/β) and Type 2 Nitric Oxide Synthase Regulate the Innate Immune Response to a Protozoan Parasite

Abstract Type 2 nitric oxide synthase (NOS2) is required for the Th1-dependent healing of infections with intracellular microbes, including Leishmania major . Here, we demonstrate the expression and define the function of NOS2 during the innate response to L. major . At day 1 of infection, genetic deletion or functional inactivation of NOS2 abolished the IFNγ and natural killer cell response, increased the expression of TGFβ, and caused parasite spreading from the skin and lymph node to the spleen, liver, bone marrow, and lung. Induction of NOS2 was dependent on IFNα/β. Neutralization of IFNα/β mimicked the phenotype of NOS2 −/− mice. Thus, IFNα/β and NOS2 are critical regulators of the innate response to L. major.

Central role for type I interferons and Tyk2 in lipopolysaccharide-induced endotoxin shock

Toll-like receptor-4 activation by lipopolysaccharide (LPS) induces the expression of interferon-β (IFN-β) in a MyD88-independent manner. Here we report that mice devoid of the JAK protein tyrosine kinase family member, Tyk2, were resistant to shock induced by high doses of LPS. Basal and LPS-induced expression of IFN-β and IFN-α4 mRNA in Tyk2-null macrophages were diminished. However, Tyk2-null mice showed normal systemic production of nitric oxide and proinflammatory cytokines and the in vivo response to tumor necrosis factor (TNF) was unperturbed. IFN-β–null but not STAT1-null mice were also resistant to high dose LPS treatment. Together, these data suggest that Tyk2 and IFN-β are essential effectors in LPS induced lethality.

The function of type I interferons in antimicrobial immunity

Type I interferons (IFN-alpha and IFN-beta) were originally described as potent antiviral substances, which are produced upon infection of animal cells with viruses. Despite a large body of literature that has accumulated during the past 25 years, their regulatory function in the immune system is still much less appreciated. Recent studies have highlighted the production of type I IFNs, their function in the immune response to infectious agents and the target cells of these interferons. Type I IFNs clearly affect the release of proinflammatory cytokines or nitric oxide by dendritic cells and macrophages, the capacity of type II interferon (IFN-gamma) to activate phagocytes, the differentiation of T helper cells and the innate control of non-viral pathogens.

Invasion, control and persistence of Leishmania parasites

Significant advances in research on the immunopathogenesis of leishmaniasis include the discovery of novel putative evasion and survival strategies of Leishmania parasites, a more detailed understanding of the function and regulation of interleukin-12, definition of molecules involved in cognate interaction between macrophages and T cells and new ideas concerning the mechanisms of host resistance and susceptibility. The use of transgenic mice for (re)probing certain immunological aspects of leishmaniasis has yielded not only predictable and confirmatory but also unexpected and pioneering results which require critical appreciation.