Giamila Fantuzzi

Leptin in the regulation of immunity, inflammation, and hematopoiesis

Leptin, the product of the ob gene, is a pleiotropic molecule that regulates food intake as well as metabolic and endocrine functions. Leptin also plays a regulatory role in immunity, inflammation, and hematopoiesis. Alterations in immune and inflammatory responses are present in leptin‐ or leptin‐receptor‐deficient animals, as well as during starvation and malnutrition, two conditions characterized by low levels of circulating leptin. Both leptin and its receptor share structural and functional similarities with the interleukin‐6 family of cytokines. Leptin exerts proliferative and anti‐apoptotic activities in a variety of cell types, including T lymphocytes, leukemia cells, and hematopoietic progenitors. Leptin also affects cytokine production, the activation of monocytes/macrophages, wound healing, angiogenesis, and hematopoiesis. Moreover, leptin production is acutely increased during infection and inflammation. This review focuses on the role of leptin in the modulation of the innate immune response, inflammation, and hematopoiesis.

Interleukin-18 binding protein: a novel modulator of the Th1 cytokine response.

An interleukin-18 binding protein (IL-18BP) was purified from urine by chromatography on IL-18 beads, sequenced, cloned, and expressed in COS7 cells. IL-18BP abolished IL-18 induction of interferon-gamma (IFNgamma), IL-8, and activation of NF-kappaB in vitro. Administration of IL-18BP to mice abrogated circulating IFNgamma following LPS. Thus, IL-18BP functions as an inhibitor of the early Th1 cytokine response. IL-18BP is constitutively expressed in the spleen, belongs to the immunoglobulin superfamily, and has limited homology to the IL-1 type II receptor. Its gene was localized on human chromosome 11q13, and no exon coding for a transmembrane domain was found in an 8.3 kb genomic sequence. Several Poxviruses encode putative proteins highly homologous to IL-18BP, suggesting that viral products may attenuate IL-18 and interfere with the cytotoxic T cell response.

Interleukin-18 (IFNgamma-inducing factor) induces IL-8 and IL-1beta via TNFalpha production from non-CD14+ human blood mononuclear cells.

IL-18 is synthesized as a precursor molecule without a signal peptide but requires the IL-1beta converting enzyme (ICE, caspase-1) for cleavage into a mature peptide. Human precursor IL-18 was expressed, purified, and cleaved by ICE into a 18-kD mature form. Mature IL-18 induced IL-8, macrophage inflammatory protein-1alpha, and monocyte chemotactic protein-1 in human peripheral blood mononuclear cells in the absence of any co-stimuli. Blocking IL-1 with IL-1 receptor antagonist resulted in a 50% reduction in IL-8. Neutralization of TNF with TNF binding protein resulted in a 66% reduction in IL-1beta, an 80% reduction of IL-8, and an 88% reduction in mean TNFalpha mRNA. In purified CD14+ cells but not CD3+/CD4+, IL-18 induced gene expression and synthesis of IL-8 and IL-1beta. TNFalpha production was induced in the non-CD14+ population and there was no induction of TNFbeta by IL-18. In purified natural killer cells, IL-18 induced IL-8 that was also inhibited by TNF binding protein. IL-18 did not induce antiinflammatory cytokines, IL-1Ra, or IL-10, although IL-18 induction of TNFalpha was inhibited by IL-10. In the presence of IFNgamma, IL-18-induced TNFalpha was enhanced and there was an increase in the mature form of IL-1beta. We conclude that IL-18 possesses proinflammatory properties by direct stimulation of gene expression and synthesis of TNFalpha from CD3+/CD4+ and natural killer cells with subsequent production of IL-1beta and IL-8 from the CD14+ population.

Increasing Levels of Interleukin (IL)-1Ra and IL-6 During the First 2 Days of Hospitalization in Unstable Angina Are Associated With Increased Risk of In-Hospital Coronary Events

BACKGROUND A growing body of evidence suggests a role for inflammation in acute coronary syndromes. The aim of this study was to assess the role of proinflammatory cytokines, their time course, and their association with prognosis in unstable angina. METHODS AND RESULTS We studied 43 patients aged 62+/-8 years admitted to our coronary care unit for Braunwald class IIIB unstable angina. In each patient, serum levels of interleukin-1 receptor antagonist (IL-1Ra), interleukin-6 (IL-6) (which represent sensitive markers of biologically active IL-1beta and tumor necrosis factor-alpha levels, respectively), and troponin T were measured at entry and 48 hours after admission. Troponin T-positive patients were excluded. Patients were divided a posteriori into 2 groups according to their in-hospital outcome: group 1 comprised 17 patients with an uneventful course, and group 2 comprised 26 patients with a complicated in-hospital course. In group 1, mean IL-1Ra decreased at 48 hours by 12%, and IL-6 diminished at 48 hours by 13%. In group 2, IL-1Ra and IL-6 entry levels were higher than in group 1 and increased respectively by 37% and 57% at 48 hours (P<0.01). CONCLUSIONS These findings indicate that although they receive the same medical therapy as patients who do not experience an in-hospital event, patients with unstable angina and with complicated in-hospital courses have higher cytokine levels on admission. A fall in IL-1Ra and IL-6 48 hours after admission was associated with an uneventful course and their increase with a complicated hospital course. These findings may suggest novel therapeutic approaches to patients with unstable angina.

The antitumor histone deacetylase inhibitor suberoylanilide hydroxamic acid exhibits antiinflammatory properties via suppression of cytokines

Suberoylanilide hydroxamic acid (SAHA) is a hydroxamic acid-containing hybrid polar molecule; SAHA specifically binds to and inhibits the activity of histone deacetylase. Although SAHA, like other inhibitors of histone deacetylase, exhibits antitumor effects by increasing expression of genes regulating tumor survival, we found that SAHA reduces the production of proinflammatory cytokines in vivo and in vitro. A single oral administration of SAHA to mice dose-dependently reduced circulating TNF-α, IL-1-β, IL-6, and IFN-γ induced by lipopolysaccharide (LPS). Administration of SAHA also reduced hepatic cellular injury in mice following i.v. injection of Con A. SAHA inhibited nitric oxide release in mouse macrophages stimulated by the combination of TNF-α plus IFN-γ. Human peripheral blood mononuclear cells stimulated with LPS in the presence of SAHA released less TNF-α, IL-1-β, IL-12, and IFN-γ (50% reduction at 100–200 nM). The production of IFN-γ stimulated by IL-18 plus IL-12 was also inhibited by SAHA (85% at 200 nM). However, SAHA did not affect LPS-induced synthesis of the IL-1-β precursor, the IL-1 receptor antagonist, or the chemokine IL-8. In addition, IFN-γ induced by anti-CD3 was not suppressed by SAHA. Steady-state mRNA levels for LPS-induced TNF-α and IFN-γ in peripheral blood mononuclear cells were markedly decreased, whereas IL-8 and IL-1-β mRNA levels were unaffected. Because SAHA exhibits antiinflammatory properties in vivo and in vitro, inhibitors of histone deacetylase may stimulate the expression of genes that control the synthesis of cytokines and nitric oxide or hyperacetylate other targets.

Interleukin-18 and Interleukin-1β: Two Cytokine Substrates for ICE (Caspase-1)

This special article deals with the role of processing enzymes in the generation of bioactive cytokines, particularly IL-1β and the novel cytokine IL-18, which was formerly called IFNγ-inducing factor (IGIF). The “classical” pathways of cytokine processing are described, as well as the importance of alternative cleavage enzymes. The topic of this review also concerns the biology of IL-18. The regulation of IL-18 production, the IL-18 receptor complex, and the biological effects of this novel cytokine are described.

Impaired IL-18 processing protects caspase-1–deficient mice from ischemic acute renal failure

We sought to determine whether mice deficient in the proinflammatory caspase-1, which cleaves precursors of IL-1 beta and IL-18, were protected against ischemic acute renal failure (ARF). Caspase-1(-/-) mice developed less ischemic ARF as judged by renal function and renal histology. These animals had significantly reduced blood urea nitrogen and serum creatinine levels and a lower morphological tubular necrosis score than did wild-type mice with ischemic ARF. Since caspase-1 activates IL-18, lack of mature IL-18 might protect these caspase-1(-/-) mice from ARF. In wild-type animals, we found that ARF causes kidney IL-18 levels to more than double and induces the conversion of the IL-18 precursor to the mature form. This conversion is not observed in caspase-1(-/-) ARF mice or sham-operated controls. We then injected wild-type mice with IL-18-neutralizing antiserum before the ischemic insult and found a similar degree of protection from ARF as seen in caspase-1(-/-) mice. In addition, we observed a fivefold increase in myeloperoxidase activity in control mice with ARF, but no such increase in caspase-1(-/-) or IL-18 antiserum-treated mice. Finally, we confirmed histologically that caspase-1(-/-) mice show decreased neutrophil infiltration, indicating that the deleterious role of IL-18 in ischemic ARF may be due to increased neutrophil infiltration.

New insights into the biology of the acute phase response.

Innate or natural immunity is a highly conserved defense mechanism against infection found in all multicellular organisms. The acute phase response is the set of immediate inflammatory responses initiated by pattern recognition molecules. These germ cell-encoded proteins recognize microbial pathogens based on shared molecular structures and induce host responses that localize the spread of infection and enhance systemic resistance to infection. Innate immunity also influences the initiation and type of adaptive immune response by regulating T cell costimulatory activity and antigen presentation by antigen presenting cells and by influencing mediator production, which affects lymphocyte function and trafficking. Acute phase protein concentrations rapidly increase after infection, and their production is controlled primarily by IL-6- and IL-1-type cytokines. The acute phase proteins provide enhanced protection against microorganisms and modify inflammatory responses by effects on cell trafficking and mediator release. For example, serum amyloid A has potent leukocyte activating functions including induction of chemotaxis, enhancement of leukocyte adhesion to endothelial cells, and increased phagocytosis. The constellation of inflammatory responses seen after endotoxin administration to humans represents an in vivo model of the acute phase response. Studies with inflammatory modifying agents, such as soluble dimeric TNF receptor and IL-10, show that these responses are not dependent on a single mediator but result from multiple overlapping inflammatory pathways. Understanding the factors that initiate and alter the magnitude and duration of the acute phase response represents an important step in the development of new therapies for infectious and inflammatory diseases.

Overview of interleukin-18: more than an interferon-gamma inducing factor.

Initially described in 1989 as interferon‐γ (IFN‐γ) inducing factor (IGIF), interleukin‐18 (IL‐18) is a novel proinflammatory cytokine that is clearly more than an inducer of IFN‐γ The cytokine possesses several biological properties such as activation of nuclear factor‐κB (NF‐κB), Fas ligand expression, the induction of both CC and CXC chemokines, and increased production of competent human immunodeficiency virus. Most activities are due to a receptor complex that recruits the IL‐1 receptor‐activating kinase (IRAK), leading to translocation of NF‐κB. This property and others support the concept that IL‐18 is related to the IL‐1 family. Indeed, one of the IL‐18 receptor chains is the IL‐1 receptor‐related protein, a member of the IL‐1R family. In addition, IL‐18 is structurally similar to IL‐1β and like IL‐1β is first synthesized as a leaderless precursor requiring the IL‐1β converting enzyme for cleavage into an active molecule. The biology of IL‐18 is reviewed in the overview and the implication for a role for this cytokine in disease is presented. J. Leukoc. Biol. 63: 658–664; 1998.

IL-1β-converting enzyme (caspase-1) in intestinal inflammation

IL-1β-converting enzyme (ICE; caspase-1) is the intracellular protease that cleaves the precursors of IL-1β and IL-18 into active cytokines. In the present study, the effect of ICE deficiency was evaluated during experimental colitis in mice. In acute dextran sulfate sodium-induced colitis, ICE-deficient (ICE KO) mice exhibited a greater than 50% decrease of the clinical scores weight loss, diarrhea, rectal bleeding, and colon length, whereas daily treatment with IL-1 receptor antagonist revealed a modest reduction in colitis severity. To further characterize the function of ICE and its role in intestinal inflammation, chronic colitis was induced over a 30-day time period. During this chronic time course, ICE KO mice exhibited a near complete protection, as reflected by significantly reduced clinical scores and almost absent histological signs of colitis. Consistently, colon shortening occurred only in dextran sulfate sodium-exposed wild-type mice but not in ICE KO mice. Protection was accompanied by reduced spontaneous release of the proinflammatory cytokines IL-18, IL-1β, and IFN-γ from total colon cultures. In addition, flow cytometric analysis of isolated mesenteric lymph node cells revealed evidence of reduced cell activation in ICE KO mice as evaluated by surface expression of CD3 CD69 and CD4 CD25. We conclude that inhibition of ICE represents a novel anti-inflammatory strategy for intestinal inflammation.