Elevated blood-brain barrier (BBB) permeability is associated with both the protective and pathological invasion of immune and inflammatory cells into CNS tissues. Although a variety of processes have been implicated in the changes at the BBB that result in the loss of integrity, there has been no consensus as to their induction. TNF-α has often been proposed to be responsible for increased BBB permeability but there is accumulating evidence that peroxynitrite (ONOO−)-dependent radicals may be the direct trigger. We demonstrate here that enhanced BBB permeability in mice, whether associated with rabies virus (RV) clearance or CNS autoimmunity, is unaltered in the absence of TNF-α. Moreover, the induction of TNF-α expression in CNS tissues by RV infection has no impact on BBB integrity in the absence of T cells. CD4 T cells are required to enhance BBB permeability in response to the CNS infection whereas CD8 T cells and B cells are not. Like CNS autoimmunity, elevated BBB permeability in response to RV infection is evidently mediated by ONOO−. However, as opposed to the invading cells producing ONOO− that have been implicated in the pathogenesis of CNS inflammation, during virus clearance ONOO− is produced without pathological sequelae by IFN-γ-stimulated neurovascular endothelial cells.

BackgroundThe pathogenesis of rabies is associated with the inability to deliver immune effectors across the blood-brain barrier and to clear virulent rabies virus from CNS tissues. However, the mechanisms that facilitate immune effector entry into CNS tissues are induced by infection with attenuated rabies virus.Methodology/Principal FindingsInfection of normal mice with attenuated rabies virus but not immunization with killed virus can promote the clearance of pathogenic rabies virus from the CNS. T cell activity in B cell–deficient mice can control the replication of attenuated virus in the CNS, but viral mRNA persists. Low levels of passively administered rabies virus–neutralizing antibody reach infected cells in the cerebellum of B cell–deficient mice but are not sufficient to mediate virus clearance. Production of rabies virus-specific antibody by B cells invading CNS tissues is required for this process, and a substantial proportion of the B cells that accumulate in the CNS of mice infected with attenuated rabies virus produce virus-specific antibodies.Conclusions/SignificanceThe mechanisms required for immune effectors to enter rabies virus-infected tissues are induced by infection with attenuated rabies virus but not by infection with pathogenic rabies viruses or immunization with killed virus. T cell activities can inhibit rabies virus replication, but the production of rabies virus–specific antibodies by infiltrating B cells, as opposed to the leakage of circulating antibody across the BBB, is critical to elimination of the virus. These findings suggest that a pathogenic rabies virus infection may be treatable after the virus has reached the CNS tissues, providing that the appropriate immune effectors can be targeted to the infected tissues.

Poly(ADP-ribose) polymerase (PARP) activity has been implicated in the pathogenesis of several central nervous system (CNS) disorders. For example, the presence of extensive poly-(ADP)ribosylation in CNS tissues from animals with experimental allergic encephalomyelitis (EAE) indicates that PARP activity may be involved in this inflammatory disease process. Using PJ34 [N-(6-oxo-5,6-dihydrophenanthridin-2-yl)-N, N-dimethylacetamide.HCl], a selective PARP inhibitor, we studied the mechanisms through which PARP activity may contribute to the onset of acute EAE. PLSJL mice immunized with myelin antigens were treated with PJ34, and the effects on the progression of EAE and several other parameters relevant to the disease process were assessed. PJ34 exerted therapeutic effects at the onset of EAE that were associated with reduced CNS inflammation and the maintenance of neurovascular integrity. Expression of genes encoding the intercellular adhesion molecule-1 (ICAM-1) and the inflammatory mediators interferon-␥, tumor necrosis factor-␣, and inducible nitric-oxide synthase were decreased in CNS tissues from drug-treated animals. Administration of PJ34 biased the class of myelin basic protein (MBP)-specific antibodies elicited from IgG2a to IgG1 and IgG2b and modulated antigen-specific T-cell reactivity. Therefore, the mode of action of PJ34 at the onset of EAE is likely mediated by a shift in the MBP-specific immune response from a proinflammatory Th1 toward an anti-inflammatory Th2 phenotype.

Experimental allergic encephalomyelitis (EAE) is an inflammatory demyelinating disease of the CNS that is used to model certain parameters of multiple sclerosis. To establish the relative contributions of T cell reactivity, the loss of blood-brain barrier (BBB) integrity, CNS inflammation, and lesion formation toward the pathogenesis of EAE, we assessed the incidence of EAE and these parameters in mice lacking NF-B, TNF-␣, IFN-␣␤ receptors, IFN-␥ receptors, and inducible nitric oxide synthase. Although increased myelin oligodendrocyte glycoprotein-specific T cell reactivity was generally associated with a more rapid onset or increased disease severity, the loss of BBB integrity and cell accumulation in spinal cord tissues was invariably associated with the development of neurological disease signs. Histological and real-time RT-PCR analyses revealed differences in the nature of immune/inflammatory cell accumulation in the spinal cord tissues of the different mouse strains. On the other hand, disease severity during the acute phase of EAE directly correlated with the extent of BBB permeability. Thus, the loss of BBB integrity seems to be a requisite event in the development of EAE and can occur in the absence of important inflammatory mediators.gene knockout mice ͉ multiple sclerosis

experimental allergic encephalomyelitis ͉ neuroimmunology ͉ peroxynitrite-dependent radicals ͉ rabies virus ͉ CNS inflammation C ontact between circulating cells and factors and CNS tissues is restricted by specializations in the neurovasculature collectively referred to as the BBB. To respond to an infection or other stimulus in the CNS tissues, immune/inflammatory cells in the circulation must be provided access across the BBB. Similarly, the proinflammatory cytokines and chemokines produced by CNS-resident cells that attract immune/inflammatory cells to the site of infection or insult must be able to reach their targets. Whether a stimulus for cell infiltration into the CNS tissues or a consequence of this process, changes in the integrity of the BBB are invariably associated with a CNS inflammatory response.Elevations in BBB permeability to circulating markers are generally observed and investigated in the context of a pathological CNS inflammatory response. The classical example is multiple sclerosis (MS), where the leakage of gadolinium derivatives across compromised regions of the BBB is used to detect active CNS lesions (1). In experimental allergic encephalomyelitis (EAE), an animal model of autoimmune CNS inflammation that shares certain characteristics with MS, there is a clear correlation between the extent of BBB permeability to a fluid phase marker and the severity of clinical signs of disease (2-6). Similarly, the lethal outcome of Borna disease virus (BDV) infection in rats is a consequence of a neuroinflammatory response associated with the loss of BBB integrity; as long as BBB permeability changes and the CNS inflammatory response are inhibited, BDV-infected rats survive (7). Notwithstanding the general association between the loss of BBB integrity and the development of neurological disease, changes in BBB permeability are also seen in protective CNS immune responses. This is the case for infection with the attenuated strain of the neurotrophic rabies virus CVS-F3, where clearance of the virus from the mouse CNS follows increased BBB permeability and the infiltration of T and B cells into CNS tissues (8). Apart from a transient reduction in body weight that is seen when the virus is administered intranasally (i.n.), normal mice show no signs of the infection and clear the virus without incident despite its spread throughout CNS tissues and immune cell invasion (8,9). Elevations in BBB permeability and cell invasion in animals clearing CVS-F3 are predominantly found in the cerebellum, which may limit pathological changes due to edema formation (8). This observation has led us to hypothesize that the restricted location of BBB permeability changes may be at least partly responsible for the dissociation between elevated BBB permeability and disease. Other possible contributors include limitations in the extent of the loss of BBB integrity or differences in the characteristics of the cells infiltrating the CNS tissues in CVS-F3 infection as opposed to a neuroinflammatory disease.Understanding how ...