The efficiency of cell targeting by recombinant retroviruses depends on the nature of the receptor and the composition of the artificial cell-virus linker.

Using streptavidin-bound antibodies specific for both viral and cell membrane epitopes, we have reported previously that human cells may be infected by murine ecotropic retroviruses through an interaction with major histocompatibility complex class I and class II antigens, and thus have demonstrated that cell targeting by recombinant retroviruses is feasible. We report here that (i) growth factor or hormone receptors, such as those for epidermal growth factor (EGF) and insulin, can also mediate infection of human cells; (ii) a biotinylated cytokine or hormone can substitute for the anti-cell antibody in bispecific antibody complexes, thus extending the versatility of the method; (iii) although yields are low in our assay, infection efficiency clearly appears to depend upon the biochemical composition of molecular bridges because bi-functional antibody complexes are more efficient than cytokine-antibody complexes in the case of the EGF receptor. Finally, our study indicates that different cell membrane molecules are not equally efficient in allowing infection of human cells because targeting of the transferrin, high density lipoprotein and galactose receptors, as well as that of various membrane glycoconjugates, by murine ecotropic retroviruses did not lead to the establishment of a proviral state.