A major entry route for the gammaretrovirus amphotropic murine leukemia virus (A-MLV) into NIH 3T3 fibroblasts is via caveola-dependent endocytosis. However, during the
infection time, few viral particles can be observed intracellularly. Analyzing the dynamics of the A-MLV
infection process by using total internal reflection fluorescence microscopy, we show that the majority of viruses are extracellular and bound to the
fibronectin matrix. Moreover, the amounts of bound virus and of
fibronectin correlated. Using confocal microscopy, nanoparticles targeted to
fibronectin by a III1C-fibronectin fragment or anti-
fibronectin antibody were detected intracellularly in NIH 3T3 cells; unconjugated nanoparticles neither bound to cells nor were detectable intracellularly. Furthermore, A-MLV colocalized intracellularly with the
fibronectin-targeted nanoparticles, suggesting that they were taken up by the same cellular pathway. Both A-MLV entry and
fibronectin turnover depend on caveolar endocytosis, and we found that inhibiting viral binding to the extracellular NIH 3T3
fibronectin-matrix dramatically reduced A-MLV
infection, indeed, showing an active role of
fibronectin in
infection. We suggest that binding to the cellular
fibronectin matrix provides a new mechanism by which viruses can enter cells.