Porcine circovirus type 2 (PCV2)-induced cell fusion in blood monocyte-derived macrophages (MDM). Formation of multinucleated giant cells (MGC) (arrows) is compared between mock-inoculated MDM (A) and PCV2-inoculated MDM (B) as shown by co-labelling the nuclei and PCV2 antigen with Hochest stain and fluorescein isothiocyanate (FITC) by indirect immunofluorescence antibody staining, respectively. Note spontaneous cell fusion, mainly bi- and tri-nucleated, in mock-inoculated group (A, arrowheads) and significantly higher nuclear number in PCV2-inoculated group (B, arrows). (A color version of this figure is available at www.vetres.org.)