Activated cell surface β1 integrins (α5β1) mediate recombinant parvovirus B19 entry independent of fibronectin binding. (A) Parvovirus B19 entry and nuclear localization in PMA-treated K562 cells were inhibited by inhibitory (P4C10) and increased by high-affinity conformation-stabilizing (N29, 21C8) β1 integrin antibodies. K562 cells were infected with Cy3-labeled recombinant parvovirus B19 for 30 minutes, fixed, and nuclei stained with SYTO 16 green fluorescent nucleic acid stain. Original magnification, × 630 for all panels. (B) Transduction of PMA-treated K562 cells with recombinant parvovirus B19-Luc vectors was inhibited by inhibitory α5 and β1 integrin antibodies (α5, JB1a) but was independent of disruption of β1 integrin interaction with fibronectin by RGD peptides (RGD). β1 integrin activation induced either by stabilization of high-affinity β1 integrins (N29) or by cross-linking of antibody-ligated β1 integrins with goat antimouse secondary antibodies (JB1a+sec. Ab) resulted in an increase in transduction, whereas ligand binding without cross-linking (RGD+sec. Ab) had no effect. Data are representative of 3 independent experiments; error bars represent standard deviations (SDs). Firefly luciferase activity was detected in cell extracts 24 hours after infection as described in “Materials and methods.” ^*P < .05; ^**P < .001.