The eradication of infectious virus from the central nervous system (CNS) following infection with the neurotropic JHM strain of mouse hepatitis virus (JHMV) is thought to be immune-mediated. Furthermore, a significant decrease of infectious virus coincides with the appearance of prominent inflammatory infiltrates in the brain and spinal cord. In the present study, mononuclear cells infiltrating the brain during JHMV infection were isolated and characterized. While all subsets of immune cells were present, there appeared to be a temporal relationship between the peak incidence of CD8+ T cells (40% of total isolated cells) and reduction of virus at day 7 post-infection. Cells with the natural killer (NK) phenotype (at least 30%) were also present throughout infection. These data suggest that CD8+ T cells and NK cells are prominent among cells which infiltrate the brain during JHM virus infection and may have important roles in reduction of virus within the CNS.

The identification of eukaryotic DNA topoisomerases I and I1 as the targets of a large number of anticancer drugs has opened biochemical approaches to the study of the cytotoxicity and clinical applications of these drugs.'-5 The genetics and cell biology afforded by the yeast Saccharomyces cerevisiae offers an additional set of tools for the study of these anti-cancer agents. Yeast is still the only eukaryotic organism that has defined, viable, topoisomerase mutants.6-8 The ability to carry out targeted gene replacement in this organism has made it possible to carry out a range of studies relating to the specificity of particular drugs, to demonstrate processes that are important for cell killing, and to identify mutations that confer hypersensitivity or drug resistance.A key aspect of anti-topoisomerase drug action is the notion that most antitopoisomerase drugs act by stabilizing an intermediate of the topoisomerase reaction. This intermediate consists of the enzyme covalently bound to DNA by a phosphotyrosine linkage. In this intermediate the DNA strand scission has occurred, hence the intermediate is referred to as a cleavable or cleavage complex. It has been demonstrated that camptothecin inhibits the religation reaction of topoisomerase I, which leads to elevated levels of cleavage c o m p l e~e s .~~'~ For topoisomerase 11, either increased cleavage or decreased religation can result in higher levels of the covalent complex. " Stabilization of a covalent intermediate, thereby converting the topoisomerase into a cellular poison is the most plausible way that camptothecin can kill yeast cells, since topoisomerase I is not essential for yeast cell ~i a b i l i t y . '~. '~ Although topoisomerase I1 is essential for ~i a b i l i t y , '~ cell killing by topoisomerase I1 targeting agents such as etoposide, amsacrine, mitoxantrone, and fluoroquinolones also mainly depends on converting topoisomerase I1 into a cellular toxin. This was demonstrated by showing that cells that overexpress topoisomerase I1 become drug hypersensiti~e,'~ while cells with reduced topoisomerase I1 activity become drug r e~i s t a n t .~. '~~'~ a