Initial binding of murine leukemia virus particles to cells does not require specific Env-receptor interaction

The initial step of virus-cell interaction was studied by immunofluorescenc e microscopy. Single particles of murine leukemia virus (MLV) vectors and h uman immunodeficiency virus (HIV) were visualized by immunofluorescence. Fl uorescent dots representing single virions could be localized by staining o f capsid proteins (CA) or surface envelope proteins (SU) after fixation of virus supernatants. This technique can be used to determine particle concen tration in viral supernatants and also to study virus-cell interaction. We investigated the role of the Env-receptor interaction for the initial bindi ng event between the cell and the viral particles. Ecotropic MLV vector par ticles were shown to bind to human cells which do not express the specific viral receptor. In addition, MLV particles defective for Fm were shown to b ind the cells similarly to infectious MLV. Time course experiments of virus -cell binding and dissociation showed identical profiles for infectious and Env-defective MW particles and suggested that MLV Env is not involved in t he early phases of attachment of virus to cells. The possible implication o f cellular factors in enhancing viral binding and infectivity is discussed.