Schematic representation of envelope expression constructs. The human IL-2 cDNA was fused to the 5′ end of the amphotropic MLV env gene at the first codon of the SU envelope subunit. A schematic diagram of the IL2-SU polypeptide encoded by this chimeric gene is shown. A 7-amino acid-long interdomain spacer was inserted between the cytokine and amphotropic receptor binding domain to reduce steric hindrances and optimize the display of IL-2. In the IL2-SUX chimeric envelope derived from IL2-SU, mutations were introduced in the SU/TM cleavage site to prevent cleavage of the envelope precursor and, thus, SU shedding. The amino acid sequence of the interdomain spacer between IL-2 and the SU protein as well as that of the SU/TM cleavage site are shown. L, leader-signal-peptide; IL2, IL-2; SU, surface envelope subunit; TM, transmembrane envelope subunit.