The Effect of Human Immunodeficiency Virus Type-1 Coreceptor Preference on Entry and Tropism Specific Phenotypes
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During HIV type 1 (HIV-1) entry, trimers of viral gp120 proteins attach to CD4 molecules and to CCR5 or CXCR4 coreceptors on the target cell. A virus is defined as R5 tropic if it uses CCR5 coreceptors and X4 tropic if it uses CXCR4. In addition to the difference in coreceptor usage, R5 and X4 tropic viruses display other phenotypic differences. R5 virus dominates in early infection even when recipients are co-infected with both viral tropisms. As the disease progresses, the virus evolves and a tropism switch from R5 to X4 occurs in approximately 50% of patients. This study aims to more fully elucidate the mechanisms underlying the phenotypic differences between X4 and R5 virus. The stoichiometric parameters associated with HIV-1 target cell entry remain unclear and may differ depending on coreceptor usage. Important unanswered questions include: how many viral envelope trimers (or spikes) must attach to CD4 molecules, how many must bind coreceptors, and how many functional gp120 subunits per envelope trimer are required for entry? To answer these questions we performed single round infectivity assays with chimeric viruses. Theoretical relative infectivity curves were generated using mathematical models and compared to the experimental curves. Using this methodology we determined that HIV-1 entry requires only a small number (one or two) of functional envelope spikes. Our data indicate that an individual virion has between one and three envelope spikes on its surface that are both functional and able to simultaneously contact a target cell. In addition, our analysis shows that trimeric envelope spikes may function with fewer than three active gp120 subunits. However, our analysis of the entry mechanism indicates that there is no major difference in the stoichiometric requirements for CCR5 versus CXCR4-mediated HIV-1 entry into host cells. To investigate whether factors outside of viral entry machinery differentially affect the fitness of R5 and X4 tropic viruses, we used in vitro techniques to assay infection rates, target cell availability, viral burst size, and the potential negative pressure of the cytotoxic lymphocyte (CTL) response. Our study indicates that R5 virus has a kinetic advantage over X4 virus replication. Our results show that neither CTL suppression nor burst size correlates to tropism and thus is unlikely to play a role in early R5 viral dominance. Contrary to what is seen in newly infected patients, we saw consistently higher rates of infection with X4 virus. Viral growth modeling indicates that target cell availability in our in vitro system is responsible for this apparent X4 replication advantage. Infection rate constants for X4 and R5 virus, which are influenced by infection rate, burst size, and target cell availability, indicate that R5 virus has a more than two fold replication advantage over some of the X4 viral isolates. If target cell availability during early infection does not overwhelmingly favor X4 growth, then this kinetic difference could explain R5’s initial dominance. Overall, our data supports the hypothesis that credits replication rate and target cell availability for dominance of R5 tropic virus and for tropism switching respectively. We conclude that R5 virus more efficiently causes productive infection in its target cells and this gives it a replication advantage until those target cells are depleted. We conclusively show that initial advantage of R5 virus does not come from the entry machinery, from the viral burst size, or from CTL suppression of X4 infected cells.