Abstract: The present invention to methods for producing mononuclear cells overexpressing IL-10. The method comprises the ex vivo introduction of an expression construct comprising a nucleotide sequence encoding a polypeptide having IL-10 activity into peripheral blood mononuclear cells of a subject. The thus obtained peripheral blood mononuclear cells have an altered phenotype as a result of the expression of an introduced IL-10 transgene. In particular the invention relates to CD4+ T cells that functionally behave as regulatory T cells as a result of the expression of an IL-10 transgene. The IL-10 transgenic mononuclear cells may be used to treat a variety of inflammatory diseases, particularly T helper 1-mediated inflammatory diseases.