Severe cytomegalovirus infection in immunocompetent patients

Severe cytomegalovirus (CMV) infection is rare in previously healthy immunocompetent individuals; to our knowledge, only thirty-four such cases have been reported in the worldwide literature. Multiorgan involvement was associated with a high mortality rate among these patients. Disease that clinically involves only the liver or lungs could be fatal; in contrast, none of the patients with isolated central nervous system infection died. Although few patients were treated with specific antiviral therapy, five of six patients with severe infection recovered after receiving therapy with ganciclovir or foscarnet. The rarity of severe CMV disease in immunocompetent patients probably precludes the performance of a clinical trial to evaluate the efficacy of specific antiviral therapy. However, the historically poor prognosis in the absence of such therapy suggests that rapid diagnosis of CMV disease and early instigation of specific treatment may be important.

Expression of tissue factor is increased in astrocytes within the central nervous system during persistent infection with borna disease virus

Persistent tolerant infection of rats with borna disease virus (BDV) results in a central nervous system (CNS) disease characterized by behavioral abnormalities. These disorders occur without inflammation and widespread cytolysis in the CNS. Therefore, mechanisms other than virally induced destruction of brain cells may explain the CNS disturbance caused by BDV. Previously, we have shown that astrocytes in the CNS express tissue factor (TF). TF functions as the primary cellular initiator of the coagulation protease cascades, resulting in the generation of the protease thrombin. Proteases and their inhibitors play important roles in the development and physiology of the CNS, and altered protease activity has been implicated in the pathophysiology of various neurological diseases. Here, we present evidence that TF expression in the brain is markedly increased during persistent infection with BDV. Persistent infection of cultured astrocytes with BDV also increased TF expression as a result of both increased transcription of the TF gene and stabilization of TF mRNA. We speculate that increased TF expression within the brain parenchyma may lead to increased protease activity in the CNS and contribute to virus-mediated CNS functional impairment by affecting neural cell interactions.

The M22 antibody identifies highly activated reactive astrocytes responding to central nervous system disease

Astrocytes respond vigorously to diverse neurological insults. It is still not clear, however, whether this response is stereotypic following different insults or varies according to the injury. We have used a novel immunocytochemical marker of reactive astrocytes, termed M22, together with antibodies to glial fibrillary acidic protein (GFAP), to analyze region- and insult-specific differences in reactive astrocytosis in the murine central nervous system (CNS). Pathology was variously induced by (1) infectious agents, (2) transgenic overexpression of a viral glycoprotein or cytokine, or (3) focal trauma. Scrapie infection induced high levels of both GFAP and M22 epitope expression by hippocampal reactive astrocytes, but neither scrapie nor wild mouse retrovirus infection induced detectable M22 staining in reactive astrocytes of the caudal brain. Focal trauma and human immunodeficiency virus gp120 overexpression induced M22 expression only in the hippocampus, while interleukin-6 overexpression induced it in cerebellar astrocytes. Although M22 expression was limited to areas with extensive damage, GFAP expression was induced in every region of the mouse brain displaying pathology. Staining of routinely fixed human brain tissue demonstrated that M22 also labeled reactive astrocytes in chronic human CNS disease. The restriction of M22 expression to areas of strongly GFAP-positive astrocytosis suggests that the M22 antibody identified highly activated reactive astrocytes. Because of this selective staining of activated astrocytes, the M22 antibody may provide neuropathologists with a good marker for qualitative analysis of the astrocytic response to different injuries.

Activation of cerebral cytokine gene expression and its correlation with onset of reactive astrocyte and acute-phase response gene expression in scrapie

The pathogenesis of scrapie, a transmissible subacute spongiform encephalopathy, is unclear. However, certain aspects of the known cellular and molecular neuropathology in scrapie led us to hypothesize that cytokines could mediate cerebral pathological changes in this neurodegenerative disease. Therefore, expression of multiple cytokine genes in the brain and peripheral organs of scrapie-infected mice was examined. Late in the course of scrapie, expression of tumor necrosis factor alpha (TNF-alpha), interleukin-1 alpha (IL-1 alpha), and IL-1 beta mRNA was markedly increased in the brain but not the spleen, kidneys, or liver. In time course studies, scrapie-infected mice exhibited increased cerebral expression of the TNF-alpha, IL-1 alpha, and IL-1 beta mRNAs by week 15 postinoculation--a time point that coincided with the onset of clinical symptoms. Thereafter, the levels of these cytokine transcripts increased progressively to the terminal stages of of the disease (week 25). To determine the relationship of the increased cerebral expression of the cytokine mRNAs to the development of pathological changes in scrapie, we examined the expression of the glial fibrillary acidic protein gene (a marker for astrocytosis) and the murine acute-phase response gene homologous to the alpha 1-antichymotrypsin gene (designated EB22/5.3). Markedly increased expression of both the glial fibrillary acidic protein and EB22/5.3 mRNAs was observed in the brain but not the peripheral organs of scrapie-infected mice. The increased expression of both these gene products also occurred at week 15 of infection and, thereafter, increased progressively to the terminal stages of the disease. Therefore, infection of mice with scrapie resulted in significant increases in the expression of the TNF-alpha, IL-1 alpha, and IL-1 beta gene products, whose pattern correlated with the onset and development of molecular and clinical pathological changes. Since scrapie is known not to evoke an immune response, the present findings strongly suggest the existence of a localized cerebral host response to the agent during which proinflammatory cytokines could be key pathogenic mediators.

Cytotoxic T lymphocytes cleanse viral gene products from individually infected neurons and lymphocytes in mice persistently infected with lymphocytic choriomeningitis virus

Lymphocytes and/or monocytes/macrophages carry viral genetic information in most, if not all, persistent and latent viral infections, and serve as potential reservoirs for maintaining or reintroducing the infection. Similarly, neurons can be persistently infected by several DNA and RNA viruses whose continued presence can alter the physiologic function of these cells, leading to disorders in neurotransmitters and disease. Here, we document that adoptive transfer of virus-specific cytotoxic T lymphocytes clears virus and viral nucleic acid sequences, in vivo, from individually infected lymphocytes, macrophages, and neurons. By plaquing, infectious center, Northern blot, and in situ hybridization at the single cell level, virus was efficiently removed from these cells.

Molecular mimicry accompanying HIV-1 infection: human monoclonal antibodies that bind to gp41 and to astrocytes

Monoclonal antibodies that bound to HIV gp41 and cross-reacted with astrocytes were recovered from the blood of three patients infected with HIV-1. Mapping of the specificity of these monoclonal antibodies, using synthetic gp41 peptides, located their epitope to amino acids 644-663 and established their conformation dependence. Six other human monoclonal anti-HIV antibodies were found to bind to HIV gp41 or gp120 but not to reactive astrocytes in brain tissue. Sharing of linear or conformational protein determinants between disparate viral and host proteins is termed molecular mimicry. The consequences of such mimicry by anti-viral antibodies interacting with astrocytes may play a role in the dementia of AIDS patients since a major function of astrocytes is to maintain the appropriate milieu for neuronal function. The finding of such cross-reactive antibodies adds to the evidence for a possible autoimmune pathogenesis in some of the disease manifestations accompanying HIV infection.

Astrocytes in infectious and immune-mediated diseases of the central nervous system

The central nervous system (CNS) can be invaded and damaged by a variety of microbes. The host response to such injury involves CNS cells, and in many cases hematogenous cells also. Recent experiments indicate that astrocytes, macroglial resident cells of the CNS, play key roles in this process. The astroglial production of trophic factors and elimination of neurotoxins are likely to fulfill important protective and reparative functions during CNS infection. In addition, astrocytes could, in concert with microglial cells, regulate the recruitment and activity of infiltrating hematogenous cells through their expression of cytokines, proteases, protease inhibitors, adhesion molecules, and extracellular matrix components. Although previous experiments suggested that astrocytes might initiate inflammatory demyelinating disease by presenting CNS antigens to autoreactive immune cells, current evidence points against such a detrimental activity. In view of the generally beneficial role of astrocytes, impairments of astroglial function by microbes or host-derived factors have the potential to contribute to neurologic disease. Diseases in which this pathogenetic process may be relevant include HIV-1-associated cognitive/motor complex and spongiform encephalopathies.

Astrocytes are the primary source of tissue factor in the murine central nervous system. A role for astrocytes in cerebral hemostasis

Hemostasis in the brain is of paramount importance because bleeding into the neural parenchyma can result in paralysis, coma, and death. Consistent with this sensitivity to hemorrhage, the brain contains large amounts of tissue factor (TF), the major cellular initiator of the coagulation protease cascades. However, to date, the cellular source for TF in the central nervous system has not been identified. In this study, analysis of murine brain sections by in situ hybridization demonstrated high levels of TF mRNA in cells that expressed glial fibrillary acidic protein, a specific marker for astrocytes. Furthermore, primary mouse astrocyte cultures and astrocyte cell lines from mouse, rat, and human constitutively expressed TF mRNA and functional protein. These data indicated that astrocytes are the primary source of TF in the central nervous system. We propose that astrocytes forming the glia limitans around the neural vasculature and deep to the meninges are intimately involved in controlling hemorrhage in the brain. Finally, we observed an increase in TF mRNA expression in the brains of scrapie-infected mice. This modulation of TF expression in the absence of hemorrhage suggested that TF may function in processes other than hemostasis by altering protease generation in normal and diseased brain.

Vaccination to prevent persistent viral infection

Persistent virus infections are increasingly being recognized as a significant cause of human morbidity and mortality. To establish persistence, a virus must establish infection and evade eradication by the host immune response, in particular by cytotoxic T lymphocytes (CTL). We have studied a virus that establishes persistence in part by suppressing the CTL response of the infected host. The virus persists in many cell types, including lymphocytes and macrophages. We show that prior immunization with a vaccine designed to induce CTL (in the absence of antiviral antibody) confers complete protection against subsequent establishment of persistence in all tissues analyzed. The vaccine can be designed to express as few as 10 amino acids of a viral protein that comprise the CTL epitope. Further, two CTL epitopes for two discrete MHC haplotypes can be successfully used in a single vaccine that protects both strains of mice. Hence, a "string of CTL epitopes" (beads) concept for vaccination is feasible. Finally, the CTL vaccine provided protection against the establishment of persistence by an immunosuppressive virus.

Molecular profile of reactive astrocytes--implications for their role in neurologic disease

The central nervous system responds to diverse neurologic injuries with a vigorous activation of astrocytes. While this phenomenon is found in many different species, its function is obscure. Understanding the molecular profile characteristic of reactive astrocytes should help define their function. The purpose of this review is to provide a summary of molecules whose levels of expression differentiate activated from resting astrocytes and to use the molecular profile of reactive astrocytes as the basis for speculations on the functions of these cells. At present, reactive astrocytosis is defined primarily as an increase in the number and size of cells expressing glial fibrillary acidic protein. In vivo, this increase in glial fibrillary acidic protein-positive cells reflects predominantly phenotypic changes of resident astroglia rather than migration or proliferation of such cells. Upon activation, astrocytes upmodulate the expression of a large number of molecules. From this molecular profile it becomes apparent that reactive astrocytes may benefit the injured nervous system by participating in diverse biological processes. For example, upregulation of proteases and protease inhibitors could help remodel the extracellular matrix, regulate the concentration of different proteins in the neuropil and clear up debris from degenerating cells. Cytokines are key mediators of immunity and inflammation and could play a critical role in the regulation of the blood-central nervous system interface. Neurotrophic factors, transporter molecules and enzymes involved in the metabolism of excitotoxic amino acids or in the antioxidant pathway may help protect neurons and other brain cells by controlling neurotoxin levels and contributing to homeostasis within the central nervous system. Therefore, an impairment of astroglial performance has the potential to exacerbate neuronal dysfunction. Based on the synopsis of studies presented, a number of issues become apparent that deserve a more extensive analysis. Among them are the relative contribution of microglia and astrocytes to early wound repair, the characterization of astroglial subpopulations, the specificity of the astroglial response in different diseases as well as the analysis of reactive astrocytes with techniques that can resolve fast physiologic processes. Differences between reactive astrocytes in vivo and primary astrocytes in culture are discussed and underline the need for the development and exploitation of models that will allow the analysis of reactive astrocytes in the intact organism.