CD4 dependence and avidity of pseudotypes containing wild-type, chimeric and mutant Env. Env-pseudotyped viruses were used to infect transiently-transfected QT6 cells expressing either low or high levels of CD4 and CCR5 (top) or HOS-CD4-CCR5 cells in the presence of the SK3 anti-CD4 mAb (bottom). BR, BS, Bv1v3 and BR(N283T) showed greater ability to infect cells expressing low levels of CD4 (low CD4 dependence) and were less sensitive to inhibition by the anti-CD4 mAb SK3 than SPL, SB and SPL(T283N), while Bv1v2 had an intermediate phenotype. The brain isolate-derived DS17 Env and the DS17(N283T) mutant had a mixed phenotype with low CD4 dependence but high sensitivity to the anti-CD4 mAb. Average + standard error from 4 independent experiments (top) and data from a representative experiment repeated at least 4 times with similar results (bottom) are shown.