SIVmac251 susceptibility to raltegravir in tissue culture. The effective concentrations at 50%, 90% and 95% (respectively, EC50, EC90, and EC95) are presented (means ± SEM from at least two independent experiments) for inhibition of: lentiviral cythopathogenicity in MT-4 cells (Panel A), viral core antigen release in supernatants of acutely infected MT-4 cells (Panel B), syncytium formation in acutely infected CEMx174 cells (Panel C), viral core antigen release in supernatants of acutely SIVmac251-infected rhesus peripheral blood mononuclear cells (PBMCs) and enriched CD4+ T-cell fractions (Panel D). In panel A, the inhibitory concentrations were determined by the methyl tetrazolium (MTT) method when the majority of control infected cells (in the absence of drug treatment) were dead at light microscopy examination. In panel B, values were derived by quantifying, using antigen-capture ELISA assays, SIVmac251 p27 and HIV-1 p24 in supernatants from five-day old cultures. In panel C, values were calculated on the basis of the numbers of syncytia per well at five days post-infection, Syncytia were counted in triplicate on three different occasions by light microscopy. In panel D, values are representative for supernatants of primary cells from three different donors at Day 5 post-infection.