Background:  Previous studies demonstrated that a severe genetic bottleneck selects for viruses with compact, glycan-restricted envelope (Env) glycoproteins during heterosexual transmission of subtype C HIV-1. We hypothesized that the compact hyper-variable domains of the newly transmitted Env could enable the virus to use low levels of CD4 and CCR5, which could be important for transmission across a mucosal surface.

Methods:  NL4-3 based GFP-reporter proviral plasmids were constructed to express multiple donor and recipient Env from 5 heterosexual transmission pairs and the viruses expressed from these constructs were evaluated for their ability to enter into a panel of HeLa cell lines engineered to express different levels of CD4 and CCR5. 

Results:  Initial results from 2 of the 5 transmission pairs show that donor and recipient Env entered into cells expressing high levels of CD4 and CCR5 with equal efficiency, but their infectivity declined as CCR5 levels decreased. Neither donor nor recipient Env could mediate entry when CD4 levels were limiting or CCR5 was absent. In contrast, a virus that expressed the CXCR4-tropic NL43 Env successfully entered all cells expressing both low and high levels of CD4, independent of CCR5 level. Overall, there was no significant difference in the efficiency of entry between the donor and recipient Env in the cell lines where infection occurred.

Conclusions:  There appears to be no difference in the efficiency of receptor utilization between the donor and recipient Env in these HeLa-derived cell lines. The requirement for high levels of CD4 and CCR5 by both newly transmitted Env and those from chronic infection suggests that the observed genetic bottleneck does not involve the ability to utilize low levels of CD4 or CCR5 or both.