SPONTANEOUS REVERSION OF HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 NEUTRALIZATION-RESISTANT VARIANT HXB2THR582 - IN-VITRO SELECTION AGAINST CYTOPATHICITY HIGHLIGHTS GP120-GP41 INTERACTIVE REGIONS

Spontaneous revertants of the immune-selected variant HXB2thr582, whic h resists neutralization by certain conformationally dependent antibod ies specific for the CD4-binding site on gp120 (such as F105), appeare d after long-term culture in the absence of immune-selecting serum, Mo lecular analysis showed some of the viruses in the revertant stock con tained a simple back mutation, whereas others retained the Thr-582 cod on but contained a substitution of serine for phenylalanine in gp41 at position 673. Neutralization sensitivity to the selecting serum and t o F105 of infectious clones containing either the back mutation or the compensatory mutation, HXB2thr582ser673, was confirmed. HXB2thr582-in fected cells have a greater propensity for syncytium formation and Sin gle cell killing than do either the parental HXB2 or the revertant HXB 2thr582ser673. This suggests that the revertant arose by selection in vitro for a less cytopathic virus. Our results link three envelope reg ions shown to influence virus-cell fusion as well as neutralization by antibody: the CD4-binding region, the leucine zipper domain, and a re gion hidden to antipeptide antibodies upon envelope oligomerization. T aken together they illustrate the functional importance of the gp120-g p41 interaction and emphasize the impact of the interplay between enve lope regions on overall conformation and function and on recognition b y neutralizing antibodies.