Bert A. 't Hart

Metabolic activation of natural phenols into selective oxidative burst agonists by activated human neutrophils.

Phenols isolated from the traditional medicinal plant Picrorhiza kurroa inhibit the release of superoxide anion (O2-) by activated human neutrophils, but leave the phagocytotic capacity intact. Resting neutrophils and resting or activated human lymphocytes are insensitive to these agents. The underlying mechanism of this highly selective activity is investigated. A critical event is the reaction of the phenols with secretory products from the activated neutrophils. The reaction products interfere with the assembly of a functional NADPH-oxidase in the membrane. Analysis of the mode of activation of the phenols reveals two possible pathways. Catechols react directly with reactive oxygen species (ROS) from the oxidative burst. For the activation of the orthomethoxy-substituted catechols the combined activity of ROS and myeloperoxidase (MPO) is obligatory. Catechols with a dimethoxy substitution cannot be activated metabolically by neutrophil-derived ROS.

Prevention of Experimental Autoimmune Encephalomyelitis in Common Marmosets Using an Anti-IL-12p40 Monoclonal Antibody

The experimental autoimmune encephalomyelitis (EAE) model in the common marmoset approximates recognized features of the human disease multiple sclerosis (MS) with regard to its clinical presentation as well as neuropathological and radiological aspects of the lesions in brain and spinal cord. IL-12 is a proinflammatory cytokine that is produced by APC and promotes differentiation of Th1 effector cells. IL-12 is produced in the developing lesions of patients with MS as well as in EAE-affected animals. Previously it was shown that interference in IL-12 pathways effectively prevents EAE in rodents. In this study we report that in vivo neutralization of IL-12p40 using a novel Ab has beneficial effects in the myelin-induced EAE model in common marmosets. The Ab was injected i.v. at 7-day intervals starting well after immunization (day 14) and was continued until the end of the study (day 86). Stable levels of the Ab were measured 3 days after each injection throughout the study period. During this period anti-Ab responses could not be detected. We demonstrate that anti-IL-12p40 treatment has a protective effect on the neurological dysfunction as well as on neuropathological changes normally observed in the brain and spinal cord of EAE-affected individuals.

Histopathological Characterization of Magnetic Resonance Imaging-Detectable Brain White Matter Lesions in a Primate Model of Multiple Sclerosis: A Correlative Study in the Experimental Autoimmune Encephalomyelitis Model in Common Marmosets (Callithrix jacchus)

Experimental autoimmune encephalomyelitis in the common marmoset, a nonhuman primate species (Callithrix jacchus), is a new model for multiple sclerosis. Given the close immunological relationship between marmosets and humans, it is an attractive model for investigating immunopathological pathways relevant to multiple sclerosis and to evaluate new treatments for the disease. Unlike in the originally documented model, experimental autoimmune encephalomyelitis induced without the use of Bordetella pertussis led to a chronic disease of moderate severity. The clinical course of experimental autoimmune encephalomyelitis in the present model was mainly chronic and progressive, but periods of incomplete remission did occur. At the chronic stage of the disease, actively demyelinating lesions were found together with inactive demyelinated and remyelinated (shadow) plaques. Before immunization and during clinically active experimental autoimmune encephalomyelitis, T1- and T2-weighted magnetic resonance brain images were obtained. Correlation of the data from the magnetic resonance images and the neuropathology analysis revealed that the hyperintense regions in T2-weighted images represented both active and inactive remyelinating lesions. Quantification showed that the number of lesions in T2-weighted magnetic resonance images equalled those found by pathological examination, and thus T2-weighted magnetic resonance imaging can be used to discern the total lesion load. Extravasation of gadolinium-diethylenetriamine-penta-acetic acid (triple dose) was found only in lesions, which by histopathology were shown to be engaged in the process of active demyelination.

Antiarthritic activity of the newly developed neutrophil oxidative burst antagonist apocynin

The plant-phenol 4-hydroxy-3-methoxyacetophenone (trivial name apocynin) is a strong inhibitor of neutrophil superoxide anion (O2-) release in vitro. In vitro the inhibitory effect of apocynin is restricted to cells with the capacity to release peroxidase and reactive oxygen species (ROS). Peroxidase deficient cells are insensitive to apocynin. In the present study the antiinflammatory activity of apocynin was tested in collagen-induced arthritis in rats. Collagen-immunized rats were treated with different doses of apocynin in the drinking water starting at the onset of joint-swelling and terminating 14 days later, at the time when joint swelling in the control group was maximal. Apocynin-treated animals had a normal plasma level of collagen-specific antibodies, but showed a significant reduction of the joint swelling. Also the plasma IL-6 level in apocynin-treated animals was substantially lower than in control animals. No flare-up of joint swelling after termination of the treatment was observed in the apocynin-treated groups.

Effects of apocynin, a drug isolated from the roots of Picrorhiza kurroa, on arachidonic acid metabolism

Apocynin is a constituent of root extracts of the medicinal herb Picrorhiza kurroa and has been shown to possess anti‐inflammatory properties. We investigated the effects of apocynin on the production of arachidonic acid‐derived inflammatory mediators by guinea pig pulmonary macrophages. Apocynin concentration‐dependently inhibited the formation of thromboxane A2, whereas the release of prostaglandins E2 and F2α was stimulated. Apocynin potently inhibited arachidonic acid‐induced aggregation of bovine platelets, possibly through inhibition of thromboxane formation. The present results suggest that apocynin might, beside its therapeutic effects in inflammatory conditions when given in a root extract of P. kurroa, also be a valuable tool in the development of new anti‐inflammatory or anti‐thrombic drugs.

How flavonoids inhibit the generation of luminol-dependent chemiluminescence by activated human neutrophils

The mechanism by which (a panel of) flanonoids inhibit the production of luminol-dependent chemiluminescence (CLlum) by activated human neutrophils is subject to this study. CLlum is frequently used as a bio-assay to quantify the effect of xenobiotics on the production of reactive oxygen species (ROS). Most of the flavonoids decreased CLlum by inhibition of ROS production by the cells. Four selected flavonoids (Taxifolin, Eriodictyol, Hesperetin and Luteolin), inhibited myeloperoxidase (MPO) release, while two of these (Taxifolin and Eriodictyol) strongly inhibited MPO activity. Because CLlum is a MPO-dependent process these activities might mask effects of the flavonoids on ROS production. Finally, our results provide evidence that essential determinants for inhibition of O2(-)-release are the OH-groups located in the B-ring of the flavonoid molecule. Flavonoids methylated at a single OH-group in the B-ring are only inhibitory when they react with activated neutrophils in the presence of myeloperoxidase.

Metabolic activation of chlorpromazine by stimulated human polymorphonuclear leukocytes. Induction of covalent binding of chlorpromazine to nucleic acids and proteins.

Human polymorphonuclear leukocytes (PMNs) have been stimulated with either phorbol 12-myristate 13-acetate (PMA), calcium ionophore A23187 or a combination of both to induce the respiratory burst and myeloperoxidase (MPO) release. Chlorpromazine (CPZ) but not chlorpromazine sulfoxide (CPZSO) inhibited the respiratory burst as measured with lucigenin chemiluminescence. The inhibition was due to interference with processes in the cell leading to the respiratory burst and not to scavenging of produced oxygen radicals that provoke the luminescence. CPZ was metabolized by stimulated PMNs. HPLC analysis revealed formation of CPZSO and an unidentified product. Both products result from decay of chlorpromazine radical cation (CPZ+.), indicating formation of this radical intermediate in CPZSO oxidation by stimulated PMNs. CPZ conversion correlated with H2O2 production and MPO release. The largest CPZ conversion was observed with phorbol ester plus A23187 stimulation. The conversion was reduced by catalase and sodium azide, an inhibitor of MPO, with 70% and 40%, respectively. This indicates only partial involvement of extracellularly released MPO in CPZ metabolism by PMNs. Considerable covalent binding of [3H]CPZ to nucleic acids and proteins of intact stimulated PMNs was observed. This binding was larger upon co-stimulation with phorbol ester and A23187. Azide did not reduce covalent binding. This indicates that covalent binding is not mediated by extracellularly released MPO and that CPZ is probably activated intracellularly. Activation of PMNs and production of H2O2 is a prerequisite for both CPZ conversion and covalent binding. This study demonstrates that phagocytic cells might contribute to drug metabolism and drug-induced toxicity.

The major histocompatibility complex influences the ethiopathogenesis of MS-like disease in primates at multiple levels

Abstract Multiple sclerosis (MS) is a chronic inflammatory demyelinating disease primarily affecting the central nervous system. Of the many candidate polymorphic major histocompatibility complex (MHC) and non-MHC genes contributing to disease susceptibility, including those encoding effector (cytokines and chemokines) or receptor molecules within the immune system (MHC, TCR, Ig or FcR), human leukocyte antigen (HLA) class II genes have the most significant influence. In this article we put forward the hypothesis that the influence of HLA genes on the risk to develop MS is actually the sum of multiple antigen presenting cell (APC) and T-cell interactions involving HLA class I and class II molecules. This article will also discuss that, because of the genetic and immunologic similarity to humans, autoimmune models of MS in non-human primates are the experimental models “par excellence” to test this hypothesis.

Reaction products of 1-naphthol with reactive oxygen species prevent NADPH oxidase activation in activated human neutrophils, but leave phagocytosis intact

Activation of human neutrophils with opsonized particles in the presence of a nontoxic dose of 1-naphthol resulted in inhibition of superoxide anion production but not of the phagocytotic activity of the cells. In this study we have investigated the mechanism of action of 1-naphthol. The inhibition is not at the level of cellular activation since the FMLP-induced rise of intracellular free calcium was unaffected. Our results show that the (metabolic) activation of 1-naphthol to 1,4-naphthoquinone by reaction with H2O2 from the oxidative burst is a necessary event for the inhibition to occur. The study provides evidence that by its reactivity with essential thiol groups 1,4-naphthoquinone (1,4-NQ) prevents the assembly of a functional NADPH-oxidase in the neutrophil membrane.

Liposome‐mediated peptide loading of MHC‐DR molecules in vivo

Amino acid residues 3–15 of mycobacterial HSP60 define a dominant T‐cell epitope for HLA‐DR3+ve humans and Mamu‐DR3+ve rhesus monkeys. Our results show that Mamu‐DR3 molecules on PBMC can be efficiently loaded in vivo with the above‐mentioned peptides when they are intravenously injected encapsulated in liposomes, but not in the free form. Mamu‐DR3 loading is abolished by encapsulation of a nonstimulatory peptide. These results have implications for the delivery of therapeutic peptides in vivo.