ABSTRACT HIV is naturally transmitted as a virus swarm, which establishes infection via only one or few transmitted/founder (T/F) viruses. Several observations strongly indicate that T/F viruses are non-randomly favored for productive transmission via shared signature genotypic and phenotypic characteristics. Current knowledge regarding Env- coreceptor interactions suggest innovative concepts to explain the T/F phenotype. T/F virus envelopes of all subtypes preferentially use the CCR5 coreceptor for entry in vitro and in vivo. Yet cell surface CCR5 is expressed in multiple functional and antigenic forms with differential sensitivity to the antiretroviral CCR5 antagonist maraviroc (MVC); affinity for natural chemokine ligands, and/or reactivity with anti-CCR5 monoclonal antibodies (mAbs). Further, the abilities of various anti-CCR5 mAbs to inhibit CCR5 interactions with either tagged soluble gp120s or pseudoviruses vary according to virus strain and target cell-type. Collectively, these findings prompt the central hypothesis for this exploratory project: as a class, T/F viruses utilize signature CCR5 subpopulation(s) that potentially favor more efficient entry and virus replication. Tests of this hypothesis will exploit how active infection downregulates a “footprint” in the surface CCR5 population, distinguishing which subsets have been engaged by HIV. We recently reported how innovative superresolution optical imaging methods reflect CI virus use of distinct antigenic CCR5 forms. The two specific aims of this project will employ similar approaches to generate an unprecedented view of T/F coreceptor usage and entry on primary cells (human monocytes, lymphocytes and CD4+ T cells). The definition of “T/F CCR5” usage will impact HIV prevention/treatment efforts by informing the development of more potent and selective CCR5-based interventions and by elucidating complementary aspects of Env structure/function that confer coreceptor selectivity and the T/F phenotype. Such features can be explored by molecular methods and translated to HIV vaccine design.