Characterization of interferon-induced proteins and their role in the antiviral, anticellular and antimicrobial activities of interferons

Interferons (IFNs) are a family of proteins with antiviral, antiproliferative, antitumor, antimicrobial, and immunomodulatory activities that are dependent on de novo mRNA and protein synthesis. More than 30 proteins have been found to be induced in response to IFN treatment. Characterization of the mRNA encoding the 56 kD protein demonstrated that IFN-$\alpha$ induced a transient induction of this mRNA, while IFN-$\gamma$ caused a sustained accumulation, mirroring the time kinetics of induction of the protein. Sequencing revealed greater than 99% homology with a human tryptophanyl tRNA synthetase and 87% homology with a eukaryotic release factor. tRNA synthetase assays showed an up-regulation of tryptophanyl tRNA synthetase activity in IFN-treated cells. Release factor activity could not be demonstrated. The cDNA encoding the IFN-induced 67 kD protein was found to have greater than 99% homology with an IFN-induced guanylate binding protein. The cDNA was inserted into a eukaryotic expression vector and transfected into human cell lines. Constitutive expression of 67 kD protein resulted in cells that grew more slowly than the parental cell line, suggesting a possible role in IFN\u27s anticellular effect. Constitutive expression also resulted in a weak antiviral effect against VSV and EMCV, while an inability to make 67 kD protein in IFN-treated cells, as a result of transfection with an antisense construct, resulted in a partial abrogation of IFN\u27s antiviral activity. The 67 kD protein thus may also have a role in IFN\u27s antiviral action. The cDNA encoding the 42 kD protein was sequenced and found to have 100% homology with an IFN-induced indoleamine 2,3-dioxygenase (IDO). Transfection studies resulted in clones that constitutively expressed as much 42 kD protein and had as much IDO activity as cells treated with 300 U/mL IFN-$\gamma$. Cells constitutively expressing 42 kD protein grew at a slower rate than the parental cells, suggesting that this protein is partially responsible for IFN\u27s anticellular activity. Constitutive expression of IDO provided no antimicrobial capability against Toxoplasma gondii or Chlamydia psittaci, demonstrating that the IDO alone is not responsible for IFN\u27s antimicrobial action