Claire F. Evans

Primary demyelination in transgenic mice expressing interferon-γ

Ever since the use of interferon-γ to treat patients with multiple sclerosis resulted in enhanced disease, the role of IFN-γ in demyelination has been under question. To address this issue directly, transgenic mice were generated that expressed the cDNA of murine IFN-γ in the central nervous system by using an oligodendrocyte-specific promoter. Expression of the transgene occurred after 8 weeks of age, at which time the murine immune and central nervous systems are both fully developed. Directly associated with transgene expression, primary demyelination occurred and was accompanied by clinical abnormalities consistent with CNS disorders. Additionally, multiple hallmarks of immune-mediated CNS disease were observed including upregulation of MHC molecules, gliosis and lymphocytic Infiltration. These results demonstrate a direct role for IFN-γ as an inducer of CNS demyelination leading to disease and provide new opportunities for dissecting the mechanism of demyelination.

In Vivo Expression of Major Histocompatibility Complex Molecules on Oligodendrocytes and Neurons during Viral Infection

Demyelination in multiple sclerosis and in animal models is associated with infiltrating CD8+ and CD4+ T cells. Although oligodendrocytes and axons are damaged in these diseases, the roles T cells play in the demyelination process are not completely understood. Antigen-specific CD8+ T cell lysis of target cells is dependent on interactions between the T cell receptor and major histocompatibility complex (MHC) class I-peptide complexes on the target cell. In the normal central nervous system, expression of MHC molecules is very low but often increases during inflammation. We set out to precisely define which central nervous system cells express MHC molecules in vivo during infection with a strain of murine hepatitis virus that causes a chronic, inflammatory demyelinating disease. Using double immunofluorescence labeling, we show that during acute infection with murine hepatitis virus, MHC class I is expressed in vivo by oligodendrocytes, neurons, microglia, and endothelia, and MHC class II is expressed only by microglia. These data indicate that oligodendrocytes and neurons have the potential to present antigen to T cells and thus be damaged by direct antigen-specific interactions with CD8+ T lymphocytes.

Using Transgenic Mouse Models to Dissect the Pathogenesis of Virus‐Induced Autoimmune Disorders of the Islets of Langerhans and the Central Nervous System

Viruses have often been associated with autoimmune diseases. One mechanism by which self-destruction can be triggered is molecular mimicry. Many examples of cross-reactive immune responses between pathogens and self-antigens have been described. This review presents two transgenic models of autoimmune disease induced by a virus through activation of anti-self lymphocytes. Viral antigens are expressed as transgenes either in beta-cells of the pancreas or in the oligodendrocytes of the CNS. Infection by a virus encoding the same gene activated autoreactive T cells that cleared the viral infection, and as a consequence of transgene expression resulted in organ-specific autoimmune disease. In both transgenic mouse models, autoreactive lymphocytes that escaped thymic negative selection were present in the periphery. Several factors are described that play a role in the regulation of the self-reactive process precipitated by a viral infection. These include the quantity of activated autoreactive T cells, the affinity of these T cells, the number of memory T cells generated following primary infection, costimulation by accessory molecules, and the types and locations of cytokines produced. In addition, unique barriers exist in target tissues that prevent or suppress autoreactive responses and define to a large extent the outcome of disease. Restimulation of autoreactive memory lymphocytes may be required to bypass these barriers and enhance autoimmune disease. Therapy directed at modifying these factors can reduce and even prevent autoimmune disease after it has been initiated.

Virus-induced autoimmune disease : transgenic approach to mimic insulin-dependent diabetes mellitus and multiple sclerosis

Transgenic technology has been used by virologists for two main purposes. One has been to evaluate cell-specific expression and function in vivo of specific viral proteins. Of the many such studies performed, one prominent example is the work of Levine and colleagues (MARKS et al. 1989) on the expression of SV40 T antigen that resulted in choroid plexus papillomas; these experiments led to identification of p53 bound to T antigen. As another example, Nerenberg et al. (1987) used transgenic expression of the TAT protein of HTLV-1 to establish the oncogenic potential of TAT. Lastly, Nelson and colleagues (unpublished data) probed expression of the immediate early 72 KDa protein of human cytomegalo- virus and located this early regulatory protein in the islets of Langerhans, smooth muscle wall of large arteries, retina and salivary gland. All of these studies were similar in that the authentic promoter of the particular viral gene being expressed was employed. The second major avenue followed by virologists utilizing transgenic technology has been to create animal models of viral pathogenesis. Examples of this application are described in separate chapters of this volume and include, first, the isolation and expression of the gene encoding the human poliovirus receptor in mice by Ren and Racaniello (1992), REN ET AL. (1990), and independently by Horie et al. (1994) and KOIKE et al. (1991), allowing study of the virus’ tropism, pathogenesis of the disease it causes, and design of new protective vaccines. Second, CHISARI et al. (1987,1989) utilized the albumin promoter to express hepatitis B virus surface antigen in the liver to study virus-induced chronic liver disease, hepatocellular carcinoma, and the role of the immune system in either clearing virus of causing immunopathology. The third example is that of TOGGAS et al. (1994) who used an astrocyte-specific promoter to express HIV-1 gp120 in transgenic mice. The brain lesions of these mice closely mimic those of AIDS neuropathology in humans including neuronal injury, microglia activation and gliosis.

Markers of central nervous system glia and neurons in vivo during normal and pathological conditions.

Cell markers are valuable tools for examining the function of cells in normal conditions as well as during disease and repair processes. In fact, our understanding of the cell types that make up the central nervous system (CNS) is very much shaped by the markers available to identify them. CNS cell types were originally identified by morphology. The discovery of various proteins specific to certain cells led to the production of cell-type-specific antibodies that have been used to identify cells in situ. An ideal marker is specific to a given cell type in normal conditions and/or during conditions involving injury or disease. As simple as these criteria sound, they are not easy to fulfill. Markers can be expressed on more that one cell type. Astrocytes and olfactory-tract-ensheathing glia both express glial fibrillary acidic protein (GFAP), even though they have clear phe-notypic, anatomical, and functional differences (Ramon-Cueto and Valverde 1995). Also, a marker that is specific for a given cell type in normal conditions can be induced or up-regulated on other cell types during conditions such as inflammation, disease, or injury. Therefore cell type markers alone do not always conclusively identify a cell type.

Limiting the available T cell receptor repertoire modifies acute lymphocytic choriomeningitis virus-induced immunopathology

The invariably fatal immunopathological disease that follows intracerebral injection of CBA/Ca (H-2k) mice with 1000 PFU of lymphocytic choriomeningitis virus (LCMV) generally fails to develop in congenic mice transgenic for a V beta 8.1D beta 2J beta 2.3C beta 2 T cell receptor (TCR) gene. The majority of these LCMV-infected TCR-transgenic mice show a substantial meningitis of delayed onset, that resolves without causing any obvious clinical impairment. This inflammatory process depends on the involvement of V beta 8+ T cells, but does not require the participation of the CD4+ subset. The cytotoxic effectors that develop in both the transgenic mice and the CBA/Ca controls are lytic for target cells infected with a vaccinia construct expressing genes encoding the putative polymerase protein of LCMV. Limiting the available TCR repertoire to lymphocytes with a single V beta phenotype (not required for the generation of potent effectors in wild-type mice) thus modifies the development of the lethal neuropathology characteristic of LCMV infection, although the CD8+ cytotoxic T lymphocyte response is not greatly compromised.

LCMV and the Central Nervous System: Uncovering Basic Principles of CNS Physiology and Virus-Induced Disease

Our understanding of the normal functions of the central nervous system (CNS) and of the mechanisms underlying neuroimmunological responses have greatly benefited from the use of lymphocytic choriomeningitis virus (LCMV) infection of its natural host, the mouse. One of the strengths of the LCMV system is its flexibility: infection can result in dramatically distinct outcomes in mice depending on variables such as host age, immunological competence, host genetic background, virus dosage, virus strain and route of inoculation (reviewed in Borrow 1997; Buchmeier and Zajac 1999). Depending on how these variables are combined, the consequences of infection range from rapid onset, immune-mediated mortality to lifelong persistent infection in the absence of overt illness. While all of these outcomes can be induced in laboratory mice, mother-to-offspring transmission resulting in persistent LCMV infection predominates in the wild.