HUMAN MXA PROTEIN CONFERS RESISTANCE TO SEMLIKI-FOREST-VIRUS AND INHIBITS THE AMPLIFICATION OF A SEMLIKI FOREST VIRUS-BASED REPLICON IN THEABSENCE OF VIRAL STRUCTURAL PROTEINS

Mx proteins form a small family of interferon (IFN)-induced GTPases wi th potent antiviral activity against various negative-strand RNA virus es, To examine the antiviral spectrum of human MxA in homologous cells , we stably transfected HEp-2 cells with a plasmid directing the expre ssion of MxA cDNA. HEp-2 cells are permissive for many viruses and are unable to express endogenous MxA in response to IFN, Experimental inf ection with various RNA and DNA viruses revealed that MxA-expressing H Ep-2 cells were protected not only against influenza virus and vesicul ar stomatitis virus (VSV) but also against Semliki Forest virus (SFV), a togavirus with a single-stranded RNA genome of positive polarity, I n MxA-transfected cells, viral yields were reduced up to 1,700-fold, a nd the degree of inhibition correlated well with the expression level of MxA. Furthermore, expression of MxA prevented the accumulation of 4 9S RNA and 26S RNA, indicating that SFV was inhibited early in its rep lication cycle, Very similar results were obtained with MxA-transfecte d cells of the human monocytic cell line U937, The results demonstrate that the antiviral spectrum of MxA is not restricted to negative-stra nd RNA viruses but also includes SFV, which contains an RNA genome of positive polarity, To test whether MxA protein exerts its inhibitory a ctivity against SFV in the absence of viral structural proteins, we to ok advantage of a recombinant vector based on the SFV replicon, The ve ctor contains only the coding sequence for the viral nonstructural pro teins and the bacterial LacZ gene, which was cloned in place of the vi ral structural genes, Upon transfection of vector-derived recombinant RNA, expression of the beta-galactosidase reporter gene was strongly r educed in the presence of MxA. This finding indicates that viral compo nents other than the structural proteins are the target of MxA action.