HIV-1–cellular interactions analyzed by single virus tracing

Single virus tracing (SVT) allows the direct investigation of the entry pathway of viruses into living cells. Using fluorescently labeled virus-like particles (VLPs) and SVT, we have studied the interaction between human immunodeficiency virus type 1 (HIV-1) and the plasma membrane of living cells. From the trajectories of freely diffusing VLPs in solution, we established that the particle preparation was homogeneous and the particles had a hydrodynamic radius of 86 +/- 5 nm, consistent with the size of single HI viruses. The VLPs that come in contact with the cell surface either become immobilized or rapidly dissociate from the cell surface. The fraction of virions that become immobilized on the plasma membrane correlates with the surface heparan sulfate linked proteoglycans (HSPG) concentration of the cell line tested. The particles that are not immobilized make an average of 1.5 contacts with the cell surface before diffusing away. For most cell lines investigated, the contact duration follows an exponential distribution with a lifetime between 20 and 50 ms depending on the cell type