Analysis of a human immunodeficiency virus type 1 R5 strain that exhibits an expanded tropism to CD4+ T-cell lines

Abstract

Infection by human immunodeficiency virus type 1 (HIV-1) requires the functional interaction of two viral envelope (env) gene products with the target cell. The surface (gp120) glycoprotein first binds to CD4 and then to a chemokine receptor/coreceptor (mainly CCR5 and CXCR4). The transmembrane (gp41) glycoprotein interacts with the target cell membrane through its fusion domain and in so doing, promotes lipid mixing and viral entry. Extensive genetic variability among HIV-1 populations, especially in env, results in differences in coreceptor use, fusion kinetics, and neutralization sensitivity among HIV-1 isolates, which may have relevance in vivo as these differences can lead to an increase in drug resistance, the ability to infect new cell populations, and an increase in pathogenic potential. In an attempt to identify genotypic/phenotypic changes that might have the capacity to alter CCR5 usage, I have constructed a library of mutant HIV-1 envelope genes produced using error prone PCR, and analyzed the phenotypic characteristics of viruses containing these envelopes relative to their coreceptor usage. One mutant (HXBaLM 1.2) maintained dependence on CCR5 for infection; however, it could use extremely low levels of CCR5 for entry, in contrast with the parental virus. Unexpectedly, determinants for this adaptation mapped to the cytoplasmic tail of gp41. Further characterization of HXBaLM 1.2 and its cloned derivative, HXBaLM 6133, demonstrated that these viruses could also enter cells in a CD4-independent manner, and were more sensitive to neutralization by sCD4 and an antibody against conserved epitopes that overlap the coreceptor-binding site of gp120, compared to the parental virus. These results suggest that the envelope proteins of HXBaLM 1.2 and its infectious molecular clone, HXBaLM 6133, exist in a partially triggered state in which the conserved coreceptor-binding site of gp120 is exposed. Notably, this characteristic has not before been linked to sequence changes in the cytoplasmic tail of gp41. The capacity of CCR5-using strains, such as HXBaLM 1.2, to adapt to use low levels of CCR5 could have significance in vivo by allowing viruses to infect low CCR5-expressing cell populations currently believed to be relatively poor hosts for HIV-1, and could either make the virus resistant to the new generation of drugs that target CCR5 or result in selection of resistant variants by these drugs.