DESCRIPTION: DA strain and other TO subgroup strains of Theiler's murine encephalomyelitis virus (TMEV) cause a chronic demyelinating disease in mice with a persistent restricted virus infection. The demyelinating lesions in One of the goals of our work is to define the molecular determinants for the TO subgroup strains' biological activities (viz., the demyelinating activity, virus persistence, and restricted virus expression) and to characterize the mechanisms of action. We have described a protein, L*, which is synthesized in TO subgroup strains (but not in non-demyelinating TMEV strains) in addition to the polyprotein. L* is initiated 13 molecules downstream from the initiation codon for the DA polyprotein and is in a different reading frame. The presence of a viral protein that is synthesized out of frame with the polyprotein is unique among picornaviruses. Virus with a mutation in L* is no longer capable of inducing the late white matter disease, indicating that this protein is critical for the demyelinating disease. Recent unpublished data show that: L* is critical for virus persistence; L* inhibits a virus-induced apoptosis that is induced in certain cells (e.g., macrophages); L* inhibits an antiviral cytolytic (CTL) T cell response (and therefore mouse strains susceptible to the late demyelinating disease fail to raise an antiviral CTL response following DA virus infection - and therefore virus persists). There is a rather unique control of L* expression, viz., a cell-type specific regulation of translation initiation from one or the other alternative AUGs, either leading to an increased synthesis of L* (with the generation of a restricted virus expression and the inhibition of virus-induced apoptosis and of the virus-specific CTL response) or of the polyprotein (with the accumulation of capsid proteins and cell lysis). In this proposal, we seek to further delineate the function of L* protein and its role in DA-induced demyelinating disease. We will characterize the effect of L* in vitro and in vivo and whether the synthesis of L* vs. polyprotein varies in different cells.