DESCRIPTION (provided by candidate): The V3 loop of gp120 is both the principal neutralizing determinant of HIV-1 and the main determinant of the virus' cellular tropism. Over the course of host infection, the almost exclusive early use of CCR5 for entry shifts as CXCR4-using viral populations associated with greater morbidity and mortality emerge in approximately 50% of patients over the first 5 years of infection. The advent of antiretrovirals targeting the gp120-CCR5 interaction, coupled with the observation in clinical trials that changes in coreceptor usage represent the major in vivo pathway of HIV escape from these drugs, have re-emphasized the need to improve our understanding of coreceptor usage at the sequence population level. The ability to quantify sequence diversity at V3 and other loci, however, has been limited by the technical and practical constraints of conventional sequencing. We hypothesize that viral coreceptor usage across the HIV quasispecies is more complex than previously appreciated and consists of a multitude of co-circulating CXCR4- and CCR5- using viruses. Viral variants from this wealth of diversity could then be selected under a coreceptor antagonist drug pressure and contribute to the failure of antiretroviral therapy. We further hypothesize that minority CXCR4-using variants are less fit than their CCR5-using counterparts, and will investigate the viral determinants of fitness of minor CXCR4-using viral populations identified by deep sequencing. We propose to 1) Investigate the diversity of coreceptor usage across the HIV quasispecies, identify unique V3 variants, and quantify shifts in their proportions during VCV treatment, 2) characterize the population dynamics of V3 loop sequences during VCV treatment, and 3) investigate the fitness of CXCR4-using minor variants. The candidate is currently an Instructor in the Division of Infectious Diseases at Massachusetts General Hospital and seeks further training in both bench and clinical research skills that will allow him to develop into and independent clinical research scientist. This plan will be conducted under the mentoring of Dr. Daniel Kuritzkes. RELEVANCE: The proposed studies will illuminate the effects of CCR5 antagonist therapy on viral coreceptor usage and V3 loop sequence evolution across the entire quasispecies within an infected host. These studies will contribute to a better understanding of the role these changes may play in disease pathogenesis and inform the most optimal clinical use of HIV-1 CCR5 antagonists.