Permeability of the blood-brain and blood-spinal cord barriers to interferons

Upregulation of the transport system for TNFalpha at the blood-brain barrier

The transport system for the cytokine tumor necrosis factor-alpha (TNFalpha) at the blood-brain barrier (BBB) enables an enhanced yet saturable entry of TNFalpha from blood to the CNS. This review focuses on the selective upregulation of the transport system for TNFalpha at the BBB that is specific for type of pathology, region, and time. The upregulation is reflected by increased CNS tissue uptake of radiolabeled TNFalpha after iv injection in mice and by inhibition of this increase with excess non-radiolabeled TNFalpha. (1) Spinal cord injury (SCI): upregulation of TNFalpha uptake after thoracic transection is seen in the delayed phase of BBB disruption at the lumbar spinal cord. Thoracic SCI by compression, however, has a longer lasting impact on TNFalpha transport that involves thoracic and lumbar spinal cord, in contrast to the upregulation confined to the lumbar region in lumbar SCI by compression. Regardless, the uptake of TNFalpha by spinal cord does not parallel BBB disruption as measured by the leakage of radiolabeled albumin. (2) Experimental autoimmune encephalomyelitis (EAE): the increase in the differential permeability to TNFalpha is seen in all CNS regions (brain and cervical, thoracic, and lumbar spinal cord) and has a distinct time course and reversibility. Exogenous TNFalpha has biphasic effects in modulating functional scores. The BBB, a dynamically regulated barrier, is actively involved in disease processes.

Tumor necrosis factor receptor fragment attenuates interferon-gamma-induced non-REM sleep in rabbits

Although the somnogenic actions of interferon-alpha (IFNalpha) and IFNbeta have been reported, the sleep effects of IFNgamma remained unknown. Thus, we investigated the effects of intracerebroventricular injection of human IFNgamma on sleep in rabbits. IFNgamma dose-dependently increased nonrapid eye movement sleep (NREMS), electroencephalographic slow wave activity and brain temperature (Tbr). These effects were markedly attenuated after the heat treatment of IFNgamma. IFNgamma suppressed rapid eye movement sleep if given during the light period, but not if given at dark onset. Although a tumor necrosis factor receptor fragment did not affect any sleep parameters when given at dark onset, it significantly attenuated IFNgamma-induced NREMS and Tbr. These data suggest that IFNgamma may be involved in the sleep responses during infection. Further, IFNgamma may have a synergistic interaction with intrinsic TNFalpha in the brain.

Bradykinin antagonist decreases early disruption of the blood-spinal cord barrier after spinal cord injury in mice

Bradykinin is one of the key molecules involved in the disruption of the blood-brain barrier and blood-spinal cord barrier occurring after spinal cord injury (SCI). Previously we have shown a biphasic opening of the blood-spinal cord barrier as well as increased transport of tumor necrosis factor-alpha (TNFalpha) after SCI by compression of the lumbar spinal cord in mice. To evaluate the role of bradykinin in the two phases of blood-spinal cord barrier disruption, we pretreated mice with a potent bradykinin antagonist, the decapeptide B9430, before SCI. Our results show that B9430 decreased the general blood-spinal cord barrier disruption occurring immediately after SCI but failed to affect the delayed opening of the blood-spinal cord barrier observed 72 h after SCI. By contrast, the entry of TNFalpha after SCI was not affected by B9430 treatment. We conclude that bradykinin is involved in the early phase of blood-spinal cord barrier disruption, with B9430 non-selectively blocking this early disruption without affecting the selective transport system for TNFalpha. This indicates the therapeutic potential of bradykinin antagonists in ameliorating tissue damage induced by SCI.

Impairment of the blood-nerve and blood-brain barriers in apolipoprotein e knockout mice

Apolipoprotein E (apoE) is well characterized as a plasma lipoprotein involved in lipid and cholesterol metabolism. Recent studies implicating apoE in Alzheimer's disease and successful recovery from neurological injury have stimulated much interest in the functions of apoE within the brain. To explore the functions of apoE within the nervous system, we examined apoE knockout (KO) mice. Previously, we showed that apoE KO mice have a delayed response to noxious thermal stimuli associated with a loss and abnormal morphology of unmyelinated fibers in the sciatic nerve. From these data, we hypothesized that apoE KO mice could have an impaired blood-nerve barrier (BNB). In this report, we demonstrate functionally impaired blood-nerve and blood-brain barriers (BBB) in apoE KO mice using immunofluorescent detection of serum protein leakage into nervous tissue as a diagnostic for decreased BNB and BBB integrity. Extensive extravasation of serum immunoglobulin G (IgG) is detected in the sciatic nerve, spinal cord, and cerebellum of apoE KO but not WT mice. In a subpopulation of apoE KO mice, IgG also extravasates into discrete cortical and subcortical locations, including hippocampus. Loss of BBB integrity was additionally confirmed by the ability of exogenously supplied Evans blue dye to penetrate the BBB and to colocalize with IgG immunoreactivity in CNS tissue. These observations support a role for apoE in maintaining the integrity of the BNB/BBB and suggest a novel relationship between apoE and neural injury.

Changing the chemokine gradient: CINC1 crosses the blood-brain barrier

Chemokines are a large family of small, inducible, secreted, chemoattractant cytokines that are involved in inflammatory processes. It is well known that systemic and CNS infections cause disruption of the blood-brain barrier (BBB); however, it is not clear how chemokines are involved in this process. We studied the pharmacokinetics of the passage of the chemokine cytokine-induced neutrophil chemoattractant-1 (CINC1) from blood to brain after i.v. bolus injection and its efflux out of the brain after i.c.v. injection. Radiolabeled CINC1 was injected i.v. into mice, and the results were determined by multiple-time regression analysis. Using HPLC, we detected intact CINC1 in brain homogenate and blood after i.v. administration. CINC1 accumulated in the cerebral vasculature but also crossed the BBB completely and rapidly. No saturation of the influx was found, suggesting that either CINC1 crossed the BBB by simple diffusion or the dynamic interactions of binding and internalization precluded the self-inhibition typical of a transport system. Furthermore, there was no efflux system, with CINC1 exiting the brain at the same rate as reabsorption of CSF. The CINC1 injected into blood or CSF did not cause any breakdown of the BBB during the course of the experiments. Thus, the influx of CINC1 may alter the "chemokine gradient" across the BBB and therefore affect inflammatory reactions involving the CNS.

Saturable entry of leukemia inhibitory factor from blood to the central nervous system

Leukemia inhibitory factor (LIF) is a neurotrophic cytokine now under clinical investigation for its effects on the CNS. We studied its passage across the blood-brain barrier (BBB) from blood to brain and spinal cord. Although a large amount of LIF was reversibly associated with the cerebral vasculature, intact LIF did reach brain parenchyma. Multiple-time regression analysis showed ready access of LIF to the CNS at a rate much faster than that of the vascular marker albumin. Excess LIF inhibited the entry of 125I-LIF after administration i.v. or by in-situ perfusion in blood-free buffer. Efflux of LIF from brain to blood was slower than reabsorption by CSF bulk flow, indicating that LIF tended to be retained in the brain. Although ciliary neurotrophic factor (CNTF) and LIF bind to the same receptor complex, CNTF did not cross-inhibit the entry of LIF into the CNS. A monoclonal antibody to LIF, however, abolished the entry of LIF. Our results show that peripherally administered LIF readily enters the brain and spinal cord by a saturable transport system across the BBB that may have biological implications.

Endocrine and neurological adverse effects of the therapeutic interferons

There is experimental evidence that the nervous central and the neuroendocrine systems can influence the immune system, which can in turn influence the brain activity. Endogenous cytokines are known to play a critical role in the pathophysiology of many diseases. The recently acquired experience on the adverse effects of therapeutic cytokines, particularly neurological and endocrine adverse effects, are further illustrative of these interferences. Interferons-alpha have been used in thousands of patients, so that the information accumulated with this group of closely related products is essential to delineate the potential and severity for non-immunological, but largely immune-mediated adverse effects to develop in patients treated with immuno-activating agents.

Cytokine-cytokine interactions and the brain

Cytokine-cytokine interactions play a role in health and are crucial during immunological and inflammatory responses in disease. Cytokine interactions can result in additive, antagonist, or synergistic activities in maintaining physiological functions such as feeding, body temperature, and sleep, as well as in anorectic, pyrogenic, and somnogenic neurological manifestations of acute and chronic disease. These interactions involve signaling homology, convergence of signaling pathways, and/or positive or negative feedbacks within and among cytokine systems. The interplay of cytokines with neurotransmitters, peptides/neuropeptides, and hormones also influence cytokine action in the brain. Interactive chemical cascades involving cytokines are consistent with the homeostatic physiological mechanisms and with the multi-humoral, pleiotropic, and redundant processes that occur during acute and chronic disease.

TGFalpha and the blood-brain barrier: accumulation in cerebral vasculature

Transforming growth factor alpha (TGFalpha) is a cytokine that belongs to the epidermal growth factor (EGF) family of growth factors. EGF has a fast and saturable entry from blood to brain that is inhibitable by TGFalpha (18). In this report, we studied the passage of TGFalpha from blood to brain after an i.v. bolus injection. Using radioactively labeled peptide, we found that TGFalpha had an apparent rate of entry of 0.7 microl/g/min. However, most of the TGFalpha was trapped in the capillary endothelial cells of the cerebral vasculature rather than entering the brain parenchyma. No saturation was detected. TGFalpha was relatively stable in blood for 20 min after i.v. injection, but dissociation of the isotope 125I was more evident in brain. The accumulation of TGFalpha in the cerebral vasculature was similar to that of amyloid-beta protein1-40. Therefore, we conclude that TGFalpha from the periphery interacts with the blood-brain barrier without substantial uptake into brain parenchyma. This raises the possibility that TGFalpha might be involved in intracranial vascular disorders such as angiopathy.

Peptides crossing the blood-brain barrier: some unusual observations

An interactive blood-brain barrier (BBB) helps regulate the passage of peptides from the periphery to the CNS and from the CNS to the periphery. Many peptides cross the BBB by simple diffusion, mainly explained by their lipophilicity and other physicochemical properties. Other peptides cross by saturable transport systems. The systems that transport peptides into or out of the CNS can be highly specific, transporting MIF-1 but not Tyr-MIF-1, PACAP38 but not PACAP27, IL-1 but not IL-2, and leptin but not the smaller ingestive peptides NPY, orexin A, orexin B, CART (55-102[Met(O)(67)]), MCH, or AgRP(83-132). Although the peptides EGF and TGF-alpha bind to the same receptor, only EGF enters by a rapid saturable transport system, suggesting that receptors and transporters can represent different proteins. Even the polypeptide NGF enters faster than its much smaller subunit beta-NGF. The saturable transport of some compounds can be upregulated, like TNF-alpha in EAE (an animal model of multiple sclerosis) and after spinal cord injury, emphasizing the regulatory role of the BBB. As has been shown for CRH, saturable transport from brain to blood can exert effects in the periphery. Thus, the BBB plays a dynamic role in the communication of peptides between the periphery and the CNS.

Upregulation of tumor necrosis factor alpha transport across the blood-brain barrier after acute compressive spinal cord injury

Tumor necrosis factor alpha (TNF) is a cytokine that is involved in the inflammatory process after CNS injury and is implicated in neuroregeneration. A saturable transport system for TNF located at the blood-brain barrier (BBB) is responsible for the limited entry of TNF from blood to the CNS in normal mice. After partial disruption of the BBB by compression of the lumbar spinal cord, permeability to TNF was increased not only in the lumbar spinal cord but also in brain and distal spinal cord segments, where the BBB remained intact. The increase in the entry of TNF to the CNS followed a biphasic temporal pattern, with a first peak immediately after injury and a second peak starting on day 3; these changes lasted longer than the mere disruption of the BBB. The increased entry of TNF was abolished by addition of excess unlabeled TNF, showing that the transport system for TNF remained saturable after spinal cord injury (SCI) and providing evidence that the enhanced entry of TNF could not be explained by diffusion or leakage. This study adds strong support for our concept that the saturable transport system for TNF across the BBB can be upregulated in the diseased state, and it suggests that the BBB is actively involved in the modulation of the processes of degeneration and regeneration after SCI.

Penetration of neurotrophins and cytokines across the blood-brain/blood-spinal cord barrier

Now that peptides are no longer considered too large to cross the blood-brain barrier, attention has turned to the possibility that larger substances like polypeptides might also enter the central nervous system (CNS). This review summarizes evidence showing that many cytokines and neurotrophins not only enter the brain but also enter the spinal cord, sometimes faster than into the brain.

Saturable entry of ciliary neurotrophic factor into brain

Ciliary neurotrophic factor (CNTF), like tumor necrosis factor-alpha (TNF) and granulocyte-macrophage colony-stimulating factor (GM-CSF), is a cytokine with neurotrophic properties. Since all three cytokines are found in the periphery as well as brain, and since TNF and GM-CSF cross the blood-brain barrier (BBB) by a saturable mechanism, we investigated whether CNTF also saturably enters the brain from the blood. We found that CNTF crosses the BBB rapidly, with a rate of entry (Ki) of 4.60 (+/-0.78) x 10(-4) ml/g min, considerably faster than that of the 99mTc-albumin control. The Ki was reduced more than 3-fold by addition of excess unlabeled CNTF. The results indicate that CNTF is saturably transported across the BBB from blood to brain.

Transport of brain-derived neurotrophic factor across the blood-brain barrier

Brain-derived neurotrophic factor (BDNF) is a potential therapeutic agent for degenerative disorders of the central nervous system. In this report, we investigated the ability of BDNF to cross the blood-brain barrier (BBB). BDNF was stable in blood up to 60 min after i.v. injection, with evidence for aggregation, and had an early, rapid influx into brain. By 10 min, most of the BDNF sequestered by the cerebral cortex was associated with the parenchyma rather than with the endothelial cells, demonstrating complete passage across the BBB. A small dose of unlabeled BDNF enhanced the entry of 125I-BDNF from blood to brain after an i.v. bolus injection, whereas larger doses had no effect. In contrast, a large dose of unlabeled BDNF inhibited the influx of 125I-BDNF during in situ brain perfusion. After intracerebroventricular injection, the efflux of BDNF from brain to blood occurred at a rate similar to that for reabsorption of cerebrospinal fluid, and no evidence for self-inhibition was found. Therefore, we conclude that intact BDNF in the peripheral circulation crosses the BBB by a high-capacity, saturable transport system.

Alzheimer's beta-amyloid peptides induce inflammatory cascade in human vascular cells: the roles of cytokines and CD40

Accumulating evidence suggests that beta-amyloid (Abeta)-induced inflammatory reactions may partially drive the pathogenesis of Alzheimer's disease (AD). Recent data also implicate similar inflammatory processes in cerebral amyloid angiopathy (CAA). To evaluate the roles of Abeta in the inflammatory processes in vascular tissues, we have tested the ability of Abeta to trigger inflammatory responses in cultured human vascular cells. We found that stimulation with Abeta dose-dependently increased the expression of CD40, and secretion of interferon-gamma (IFN-gamma) and interleukin-1beta (IL-1beta) in endothelial cells. Abeta also induced expression of IFN-gamma receptor (IFN-gammaR) both in endothelial and smooth muscle cells. Characterization of the Abeta-induced inflammatory responses in the vascular cells showed that the ligation of CD40 further increased cytokine production and/or the expression of IFN-gammaR. Moreover, IL-1beta and IFN-gamma synergistically increased the Abeta-induced expression of CD40 and IFN-gammaR. We have recently found that Abeta induces expression of adhesion molecules, and that cytokine production and interaction of CD40-CD40 ligand (CD40L) further increase the Abeta-induced expression of adhesion molecules in these same cells. These results suggest that Abeta can function as an inflammatory stimulator to activate vascular cells and induces an auto-amplified inflammatory molecular cascade, through interactions among adhesion molecules, CD40-CD40L and cytokines. Additionally, Abeta1-42, the more pathologic form of Abeta, induces much stronger effects in endothelial cells than in smooth muscle cells, while the reverse is true for Abeta1-40. Collectively, these findings support the hypothesis that the Abeta-induced inflammatory responses in vascular cells may play a significant role in the pathogenesis of CAA and AD.