Primary human erythroid cells express both receptor and coreceptor for parvovirus B19 and internalize the virus. (A) Flow cytometric analyses of surface expression of P antigen (left), α5 (middle), and β1 (right) integrins in primary human erythroid progenitor cells. Control cells stained with secondary antibodies only are shown in black. Cell surface expression of P antigen, α5, and β1 integrins was analyzed by flow cytometry as described in “Materials and methods.” (B) Southern blot analysis of low M_r DNA isolated from uninfected (lane 1) and wt parvovirus B19-infected primary human erythroid progenitor cells (lane 2), which showed the presence of internalized single-stranded (ss) viral genomes. Inhibitory β1 integrin antibodies (lane 3) and parvovirus B19 anti-VP2 antibodies (lane 5) abrogated viral entry, whereas cross-linking of β1 integrin-inhibitory antibodies with secondary antibodies (JB1a+sec. Ab) had no significant effect (lane 4).