Attenuation of Semliki Forest Virus Neurovirulence by Micro-RNA-mediated Detargeting.

Artificial target sequences for tissue-specific miRNAs have recently been introduced as a new means for altering tissue tropism of viral replication. This approach can be used to improve the safety of oncolytic viruses for cancer virotherapy by restricting their replication in unwanted tissues, such as the liver. Semliki Forest virus (SFV) is a positive-strand RNA virus, and similar to the related alphaviruses, like Sindbis virus, has potential as a gene therapy vector and an oncolytic virotherapy agent, but this potential is limited by the neurovirulence of these alphaviruses. Here, we have generated a replicative SFV4 carrying six tandem targets for the neuron-specific miR124 between the viral nsp3 and nsp4 genes. When administered intraperitoneally into adult Balb/c mice SFV4-miRT124 displayed attenuated spread into CNS and greatly increased survival. Peripheral replication was not affected, indicating neuron-specific attenuation. Moreover, a strong protective SFV immunity was elicited in these animals. Intracranial infection of adult mice with SFV4-miRT124 showed greatly reduced infection of neurons in the brain, but lead to infection of oligodendrocytes in corpus callosum. Taken together, our data show that miR124-mediated attenuation of neurovirulence is a feasible and promising strategy for generating safer oncolytic alphavirus virotherapy agents.