The present invention refers to a secretion-competent mutein of the α-subunit of human Interleukin 27 and to a human heterodimeric Interleukin 27. The present invention further refers to a nucleic acid molecule comprising a nucleotide sequence encoding a secretion-competent mutein of the α-subunit of human Interleukin 27 or the human heterodimeric Interleukin 27, to a host cell containing a nucleic acid molecule comprising a nucleotide sequence encoding a secretion-competent mutein of the α-subunit of human Interleukin 27 or of the human heterodimeric Interleukin 27. The invention also refers to an immune modulator comprising a secretion-competent mutein of the α-subunit of human Interleukin 27 or of the human heterodimeric Interleukin 27, to the respective use thereof as well as to a method of producing said secretion-competent muteins and to a secretion-incompetent mutein of the α-subunit of mouse Interleukin 27 and a secretion-competent mutein of the β-subunit of mouse Interleukin 27.