The partial molecular characterization of multiple sclerosis (MS)-associated retrovirus (MSRV), a novel retrovirus previously called LM7, is reported. MSRV has been isolated repeatedly from leptomeningeal, choroid plexus and from Epstein-Barr virus-immortalized B cells of MS patients. A strategy based on reverse transcriptase PCR with RNA-purified extracellular virions yielded an initial pol fragment from which other regions of the retroviral genome were subsequently obtained by sequence extension. MSRVspecific PCR primers amplified a pol region from RNA present at the peak of reverse transcriptase activity, coinciding with extracellular viral particles in sucrose density gradients. The same sequence was detected in noncellular RNA from MS patient plasma and in cerebrospinal f luid from untreated MS patients. MSRV is related to, but distinct from, the endogenous retroviral sequence ERV9. Whether MSRV represents an exogenous retrovirus with closely related endogenous elements or a replication-competent, virion-producing, endogenous provirus is as yet unknown. Further molecular epidemiological studies are required to determine precisely the apparent association of virions containing MSRV RNA with MS.

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A leptomeningeal cell line (LM7) harbouring an unknown retrovirus was recently isolated from the cerebrospinal fluid of a patient with multiple sclerosis. However, spontaneous expression of the LM7 retrovirus in this primary culture is quite low. We present results showing that in vitro infection of LM7 cells with herpes simplex virus type 1 (HSV-1), but not that of control cells, results in (i) potent stimulation of the specific reverse transcriptase (RT) activity detected in the culture supernatant and (ii) co-expression of both typical HSV-1 virions and abundant retrovirus-like particles. Transfection of LM7 cells with plasmids expressing HSV-1 immediate early (IE) ICP0 and ICP4 proteins produced a similar enhancement of RT activity in culture supernatants with retrovirus-like particles being identifiable by electron microscopy. These effects were not observed with a plasmid expressing ICP27 or with the parental plasmid in LM7 cells, nor with any of these four plasmids in control cells. These results show that HSV IE trans-activating proteins strongly enhance the expression of the latent retrovirus present in LM7 cells. The possible role in vivo of herpesviruses as 'triggering' cofactors in the retrovirus hypothesis for multiple sclerosis aetiology is also discussed.

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