However, injection of this immunodominant peptide 7 days post-TMEV infection results in increased astrocyte activation, alteration of BBB tight junctions, severe CNS vascular permeability, and morbidity within 48 hours. This peptide induced fatal syndrome is dependent on virus-specific CD8 T cells and perforin expression. Perforin is a pore forming protein that plays an important role in controlling viral infections and tumors. Perforin has also been shown to play a BEZ235 critical role in an inducible mouse model of seizures, as mice deficient in perforin displayed reduced BBB disruption. When analyzing the effector functions of CD8 T cells in our PIFS model system, we found that perforin, but not Fas ligand, was required for pathology associated with PIFS to develop. In these experiments, we determined C57BL/6 perforin2/2 mice are resistant to PIFS and are devoid of CNS vascular permeability as measured by magnetic resonance imaging analysis and leakage of FITC-albumin into the CNS parenchyma. Astrocyte activation, as measured by glial fibrillary acidic protein expression, was also found to be dependent on perforin expression in the PIFS model. Events indicative of BBB disruption are dependent on perforin expression. However, the cellular source of perforin required for promoting BBB disruption is unknown. In addition to CD8 T cells, natural killer cells and cd T cells express perforin and have been shown to use perforin-mediated cytotoxicity during viral infections. Neutrophils have also recently been shown to express perforin to regulate immune responses in allergic contact dermatitis. Therefore, while we have previously demonstrated that both CD8 T cells and perforin are critical factors causing BBB disruption, it remained unknown the extent other perforinexpressing immune cell types assisted in the development of PIFS. Since PIFS is initiated by class I-restricted virus antigen, we hypothesized that CD8 T cells directly use perforin to cause BBB disruption independent of other immune cell types. We tested this hypothesis using adoptive transfer techniques to isolate the CD8 T cell as the sole perforin-expressing cell type in the PIFS model. After reconstituting perforin2/2 mice with perforin competent CD8 T cells, mice were intracranially infected with TMEV and administered either PIFS-inducing VP2121–130 peptide or mock E7 peptide 7 days post-infection. Mice were then evaluated for activation of astrocytes, disruption of the tight junction organization, and CNS vascular permeability in order to determine whether perforin competent CD8 T cells alone are sufficient to cause BBB disruption. Investigating the underlying molecular mechanisms leading to pathology associated with BBB disruption is of critical importance for the development of therapeutic approaches to treat diseases characterized by CNS vascular permeability. Using the PIFS model system, our lab has previously demonstrated critical roles for CD8 T cells and perforin in promoting activation of astrocytes, loss of linear organization of the tight junction, and extensive CNS vascular permeability. We have also shown that inhibiting the functions of these critical players improves pathology and survival. Furthermore, we have demonstrated that other molecular players implicated in causing BBB disruption, such as GR-1+ neutrophils, CD4 T cells, TNF-a, IFN-c, LTbR, and IL-1, do not contribute to lethality in CD8 T cell-initiated BBB disruption. However, whether CD8 T cells are the cellular source of perforin required for promoting BBB disruption was not known.