Modification of retroviral tropism by display of IGF-I

We have displayed insulin-like growth factor I (IGF-I) as an N-terminal extension of 4070A (amphotropic) retroviral envelope protein. Western blot demonstrated that chimaeric envelope proteins were incorporated into retroviral particles. Interaction between the displayed IGF-I and cell-surface receptors impaired gene delivery. The magnitude of this inhibitory effect was smallest on NIH 3T3 cells, greater on NIH 3T3 cells over-expressing insulin receptor, and greatest on NIH 3T3 cells over-expressing human type-I IGF receptor. Hence, both the number of ligand receptors and their affinity for the displayed ligand influenced the level of gene delivery. The inhibitory effect was abrogated by cleaving the displayed domain from the underlying envelope protein with factor Xa protease, and by the addition of free ligand to the infection. Addition of IGF-I or insulin caused a dose-dependent increase in titre. Possible mechanisms for receptor-mediated inhibition of gene delivery by IGF-displaying vectors are discussed, together with the implications of these results for practical applications of retroviral display and for understanding the mechanism of virus entry.