A HOMOLOG OF INTERLEUKIN-10 IS ENCODED BY THE POXVIRUS ORF VIRUS

A gene encoding a polypeptide with homology to interleukin-10 (IL-10) has been discovered in the genome of orf virus (OV) strain NZ2, a para poxvirus that infects sheep, goats, and humans. The predicted polypept ide sequence shows high levels of amino acid identity to IL-10 of shee p (80%), cattle (75%), humans (67%), and mice (64%), as well as IL-10- like proteins of Epstein-Barr virus (63%) and equine herpesvirus (67%) . The C-terminal region, comprising two-thirds of the OV protein, is i dentical to ovine IL-10, which suggests that this gene has been captur ed from its host sheep during the evolution of OV. The IL-10-like gene is transcribed early. Conditioned medium from COS cells transfected w ith a eukaryotic expression vector containing the OV IL-10-like gene s howed the same biological activity as ovine IL-10 in a murine thymocyt e proliferation assay. OV IL-10 is likely to be important in immune ev asion by OV, since IL-10 is a multifunctional cytokine that has inhibi tory effects on nonspecific immunity and Th1 effector function.