SCF on virus particles redirects binding and facilitates transduction of MO7e cells. (A) Virus-binding assays. MO7e cells cultured in GM-CSF were incubated with retroviral supernatants, washed, and stained for the presence of bound virus by incubation with anti-SU monoclonal antibody. The lighter trace represents cells alone and the heavy trace represents that obtained in the presence of virus. The ecotropic virus is shown in the left panel and the SCF-eco virus is shown in the right panel. (B) Down-regulation of c-kit expression in response to soluble SCF. MO7e cells were cultured in the presence of GM-CSF or for 24 hours in fresh medium containing SCF and then stained for expression of surface c-kit. The light trace is cells stained with a mouse IgG1 isotype control, the dotted trace is from cells grown in GM-CSF, and the heavy trace is from cells grown for 24 hours in SCF. (C) MO7e cells cultured for 24 hours in SCF were incubated with viral supernatant and stained with anti-SU, as described in panel A. The ecotropic virus is shown in the left panel and the SCF-eco virus is shown in the right panel. (D) Transduction of MO7e cells by ecotropic retrovirus incorporating surface SCF. Cells from the c-kit⁺ cell line MO7e were incubated with retroviral supernatants as indicated below and transduction was evaluated by flow cytometry to detect the presence of the EGFP reporter gene. The percentage of fluorescent cells is indicated in each panel. Mock-transduced cells (i), cells transduced with amphotropic virus (ii), cells transduced with ecotropic virus (iii), and cells transduced with SCF-eco virus (iv).