Fluorescence-based quantitative methods for detecting human immunodeficiency virus type 1-induced syncytia

Cell fusion induced by human immunodeficiency virus type 1 (HIV-1) is usual ly assessed by counting multinucleated giant cells (syncytia) visualized by light microscopy. Currently used methods do not allow quantification of sy ncytia, nor do they estimate the number of cells involved in cell fusion. W e describe two fluorescence-based methods for the detection and quantificat ion of HIV-1-induced in vitro syncytium formation. The lymphoblastoid cell lines MT-2 and SupT1 were infected with syncytium-inducing (SI) HIV-I isola tes. Syncytia were detected by DNA staining with propidium iodide using flo w cytometry to determine cell size or by two-color cytoplasmic staining of infected cell populations by using fluorescence microscopy. Both methods we re able to detect and quantify HN-induced syncytia. The methods could disti nguish between SI and non-SI HIV isolates and could be used with at least t wo separate types of CD4(+) T-cell lines. Small syncytia can be readily ide ntified by the two-color cytoplasmic staining method. Both methods were als o shown to be useful for evaluating antiretroviral compounds, as demonstrat ed by the accurate assessment of HIV inhibition by azidothymidine (zidovudi ne), dideoxycytidine (zalcytibine), and hydroxyurea. These fluorescence-bas ed assays allow a rapid and practical method for measuring MN replication a nd anti-MN activity of potential inhibitory compounds.