Stoichiometric parameters of HIV-1 entry

Abstract

During HIV type 1 (HIV-1) entry, trimers of gp120 bind to CD4 and either the CCR5 or CXCR4 coreceptor on the target cell. The stoichiometric parameters associated with HIV-1 entry remain unclear. Important unanswered questions include: how many trimers must attach to CD4 molecules, how many must bind coreceptors, and how many functional gp120 subunits per trimer are required for entry? We performed single round infectivity assays with chimeric viruses and compared the experimental relative infectivity curves with curves generated by mathematical models. Our results indicate that HIV-1 entry requires only a small number of functional spikes (one or two), that Env trimers may function with fewer than three active subunits, and that there is no major difference in the stoichiometric requirements for CCR5 vs. CXCR4 mediated HIV-1 entry into host cells.

Stoichiometry for entry and binding properties of the Env protein of R5 T cell-tropic HIV-1 and its evolutionary variant of macrophage-tropic HIV-1

“…In one case, the M-tropic virus YU-2 was modeled to use a single Env trimer/CD4 interaction, consistent with what we observed for M-tropic viruses using a strategy of varying CD4 density; yet in another study, the M-tropic virus JR-FL was modeled to require two Env/CD4 interactions for infection ( 28 – 30 ). However, this approach has also given discrepant values for the Env protein from the HIV-1 IIIB isolate (as found in either the pNL4-3 clone or the HXB-2 clone) with values ranging from 2 to 5, with the confounding feature being this variant was passaged extensively in culture before clones were made ( 27 , 28 ). These approaches, along with the use of the Affinofile cell assay, all measure infectivity, and thus, it is unknown what the precise step is where these differences occur (CD4 binding versus the ease of downstream conformational changes).…”

“…The question of the number of trimer/CD4 molecule interactions needed for entry has been studied previously using a different approach where heterotrimers were created on the surface of the virion using active and inactive gp120 subunits and measuring infectivity as a function of the changing ratio of active to inactive Env proteins assembled in a trimer ( 28 ). In one case, the M-tropic virus YU-2 was modeled to use a single Env trimer/CD4 interaction, consistent with what we observed for M-tropic viruses using a strategy of varying CD4 density; yet in another study, the M-tropic virus JR-FL was modeled to require two Env/CD4 interactions for infection ( 28 – 30 ). However, this approach has also given discrepant values for the Env protein from the HIV-1 IIIB isolate (as found in either the pNL4-3 clone or the HXB-2 clone) with values ranging from 2 to 5, with the confounding feature being this variant was passaged extensively in culture before clones were made ( 27 , 28 ).…”

“…Previous studies have examined the number of Env/CD4 interactions required for virus entry using a strategy of making heterotrimers with mixtures of active and inactive gp120 subunits and modeling the loss of infectivity; results from these studies have suggested that from one to seven interactions are needed to mediate infectivity, depending on the isolate ( 27 – 31 ). Two limitations of these previous studies are the lack of distinction between M-tropic and T-tropic viruses and the inclusion of tissue culture-adapted viruses (which can acquire entry properties not seen in vivo ).…”

Inhibition of Human Immunodeficiency Virus Type 1 Entry by a Keggin Polyoxometalate

“…It involves the binding of viral glycoprotein gp120 to the cellular CD4 receptor, and subsequent reconfiguration of gp120 to allow the interaction of gp120 and a cellular coreceptor, i.e., CCR5 or CXCR4. The interaction was followed by additional conformational changes in the viral envelope, resulting in exposure of gp41 transmembrane protein and ultimately driving the fusion of the viral envelope with the host membrane [ 3 , 4 , 5 , 6 , 7 , 8 ]. To date, the CCR5 antagonists maraviroc (MVC) and the fusion inhibitor targeting gp41, T20 (enfuvirtide) have been approved for clinical use, and many other inhibitors targeting various stages of viral entry are still in development [ 9 , 10 ].…”

CXCR7/ACKR3-targeting ligands interfere with X7 HIV-1 and HIV-2 entry and replication in human host cells

“…Furthermore, there are HIV strains that can bind both CCR5 and CXCR4, the so-called dual-tropic strains. R5 strains are predominant early on in HIV infection, whereas X4 strains are linked with later disease progression [ 3 , 4 , 5 , 6 , 7 ]. Several reports have now indicated an increase of HIV-1 strains that predominantly use CXCR4 as a coreceptor for virus attachment and entry.…”