Objectives:The biologic phenotype of HIV-1 primary isolates obtained from ˜50% of patients who progress to AIDS switches from non-syncytium-inducing (NSI) to syncytium-inducing (SI). We evaluated possible associations between virus coreceptor usage, sensitivity to inhibition by &bgr;-chemokines, and disease progression of patients who continue to yield NSI isolates after developing AIDS.Study Design/Methods:Sequential virus isolates were analyzed for biologic phenotype using the MT-2 cell assay, for sensitivity to &bgr;-chemokines using RANTES inhibition, and for coreceptor usage using U87.CD4 and GHOST.CD4 cells expressing different chemokine/orphan receptors or donor peripheral blood mononuclear cells (PBMC) defective in CCR5 expression. In addition, the env V3 region was sequenced and the length of the V2 region determined.Results:All NSI isolates, regardless of patient status at time of isolation, were dependent on CCR5 expression for cell entry. Furthermore, there was no indication of broadened coreceptor usage of NSI isolates obtained from persons with latestage AIDS. A majority of NSI isolates remained RANTES sensitive; however, virus variants with reduced sensitivity were observed. The V2 lengths and the V3 sequences exhibited no or minor changes at analysis of sequential NSI isolates.Conclusions:Our data suggest that NSI isolates obtained from AIDS patients remain CCR5 dependent (ie, R5) and, in many cases, also remain sensitive to RANTES inhibition. However, virus variants with decreased sensitivity to RANTES inhibition may evolve during disease progression, not only as a result of a switch from NSI to SI but also in patients who develop AIDS while continuing to maintain R5 isolates.GenBank Accession Numbers:AF199032-AF199043.Journal of Human Virology 1999;2:325-338 © Lippincott Williams & Wilkins, Inc.