Both soluble and membrane-associated TNF activate brain microvascular endothelium: relevance to multiple sclerosis

Platelets potentiate brain endothelial alterations induced by Plasmodium falciparum

Brain lesions of cerebral malaria (CM) are characterized by a sequestration of Plasmodium falciparum-parasitized red blood cells (PRBC) and platelets within brain microvessels, as well as by blood-brain barrier (BBB) disruption. In the present study, we evaluated the possibility that PRBC and platelets induce functional alterations in brain endothelium. In a human brain endothelial cell line, named HBEC-5i, exhibiting most of the features demanded for a pathophysiological study of BBB, tumor necrosis factor (TNF) or lymphotoxin alpha (LT-alpha) reduced transendothelial electrical resistance (TEER), enhanced the permeability to 70-kDa dextran, and increased the release of microparticles, a recently described indicator of disease severity in CM patients. In vitro cocultures showed that platelets or PRBC can have a direct cytotoxic effect on activated, but not on resting, HBEC-5i cells. Platelet binding was required, as platelet supernatant had no effect. Furthermore, platelets potentiated the cytotoxicity of PRBC for TNF- or LT-alpha-activated HBEC-5i cells when they were added prior to these cells on the endothelial monolayers. This effect was not observed when platelets were added after PRBC. Both permeability and TEER were strongly affected, and the apoptosis rate of HBEC-5i cells was dramatically increased. These findings provide insights into the mechanisms by which platelets can be deleterious to the brain endothelium during CM.

Adrenergic mediation of TNF alpha-stimulated ICAM-1 expression on human brain microvascular endothelial cells

Adrenergic innervation derived from locus ceruleus has been implicated in regulating BBB permeability and inflammatory responses associated with neurological disorders. This report demonstrates that adrenergic agents attenuate the tumor necrosis factor-alpha (TNF alpha)-induced expression of intercellular adhesion molecule-1 (ICAM-1) on cerebral microvascular endothelial cells (HBMEC) derived from human brains. HBMEC were incubated with isoproterenol (1-10 microM) alone or in the presence of propranolol (10 microM) for 30 min followed by the addition of various concentrations of TNF alpha. ICAM-1 expression on cultured HBMEC was dose-dependently upregulated by TNF alpha. Incubation with isoproterenol significantly reduced levels of ICAM-1 expression indicating the possible involvement of adrenergic agents on ICAM-1 expression. Treatment with propranolol (beta 1/beta 2-adrenergic antagonist) and butoxamine (beta 2-adrenergic antagonist), but not atenolol (beta 1-adrenergic antagonist) reversed this inhibitory effect. Isoproterenol also dose-dependently stimulated cAMP production (assayed by RIA) by HBMEC; propranolol treatment abolished this effect. These data show that the beta 2-adrenergic receptor/cAMP pathway may be partly involved in TNF alpha-stimulated ICAM-1 expression and indicate the possible involvement of adrenergic mediation of capillary function including BBB integrity.

Outside-to-inside signal through the membrane TNF-alpha induces E-selectin (CD62E) expression on activated human CD4+ T cells

The membrane TNF-alpha is known to serve as a precursor of the soluble form of TNF-alpha. Although it has been reported the biological functions of the membrane TNF-alpha as a ligand, the outside-to-inside (reverse) signal transmitted through membrane TNF-alpha is poorly understood. Here we report a novel function mediated by outside-to-inside signal via membrane TNF-alpha into the cells expressing membrane TNF-alpha. Activation by anti-TNF-alpha Ab against membrane TNF-alpha on human T cell leukemia virus (HTLV) I-infected T cell line, MT-2, or PHA-activated normal human CD4(+) T cells resulted in the induction of an adhesion molecule, E-selectin (CD62E), on the cells with the peak of 12-24 h, which completely disappeared by 48 h. When wild-type or mutant membrane TNF-alpha (R78T/S79T) resistant to proteolytic cleavage was introduced into Jurkat or HeLa cells, E-selectin was induced by the treatment with anti-TNF-alpha Ab with the similar kinetics. Membrane TNF-alpha-expressing Jurkat cells also up-regulated E-selectin when brought into cell-to-cell contact with TNF receptor-expressing HeLa cells. Northern blot analysis and RT-PCR analysis showed that the membrane TNF-alpha-mediated E-selectin expression was up-regulated at the level of transcription. These results not only confirmed our previous findings of reverse signaling through membrane TNF-alpha, but also presented evidence that E-selectin was inducible in cell types different from endothelial cells. It is strongly suggested that membrane TNF-alpha is a novel proinflammatory cell surface molecule that transmits bipolar signals in local inflammation.

Circulating cytokines as mediators of fever

The febrile response is thought to be mediated by endogenous mediators, generically called "endogenous pyrogens." In the classical model of pathogenesis, induction of fever is mediated by the release of pyrogenic cytokines such as tumor necrosis factor (TNF), interleukin (IL)-1, IL-6, and interferons into the bloodstream in response to exogenous pyrogens. These mediators act at the level of the organum vasculosum of the lamina terminalis in the central nervous system (CNS), inducing synthesis of prostaglandins, which are the central mediators of the coordinated responses leading to fever. However, analysis of recent data suggests that multiple pathways may be involved in the induction of fever by cytokines, such as local cytokine production leading to signaling through vagal fibers, release of cytokine-induced circulating mediators at the tissue level, the use of membrane-bound cytokines as mediators, or the local release of cytokines in the hypothalamus by circulating activated monocytes. In addition, certain bacterial products can stimulate cytokine production directly at the level of hypothalamus, probably by activation of Toll-like receptors. A multipathway mechanism for the induction of fever is therefore suggested.

Do only circulating pyrogenic cytokines act as mediators in the febrile response? A hypothesis

The classical model of the pathogenesis of fever suggests that pyrogenic cytokines, produced by leucocytes in the bloodstream in response to exogenous pyrogens, represent the distal mediators of the febrile response. They are recognized at the level of the organum vasculosum of the lamina terminalis in the central nervous system, where they induce synthesis of prostaglandins representing the central mediators of the coordinated responses leading to fever. This classical model is challenged by studies showing inconsistencies between the febrile response and the cytokine pattern, as well as by data demonstrating paradoxical hyperfebrile reactions in knock-out mice lacking cytokines or cytokine receptors. Moreover, no measurable cytokine concentrations are to be found in a variety of specific patients groups with febrile conditions. There are recent data in the literature suggesting that alternative pathways may be involved in the induction of fever, ranging from the use of vagal fibres to transmit the signals leading to fever, to local production of cytokines at the level of the hypothalamus, or the use of membrane-bound cytokines as mediators. A multipathway mechanism for the induction of fever is suggested.

Monoclonal antibodies against vascular endothelial antigens expressed in normal human brain

A panel of monoclonal antibodies reactive with human-brain vessels was raised by immunizing BALB/c mice with homogenate of whole human brain, obtained from temporal lobectomies. Hybridoma supernates were screened by immunohistochemical methods on frozen sections of human brain, liver and spleen and 16 clones were isolated. The pattern of immunoreactivity varied with respect to the type of brain blood vessels predominantly labelled and to tissue specificity. Some antibodies cross-reacted with cow or squirrel monkey forebrain microvessels with an intensity equal to that shown by human brain. The immunoreactivity patterns reflected antigenic heterogeneity among different subsets of vascular endothelial cells in human brain.