INDEPENDENT DIVERGENCES IN THE CD4 BINDING-SITE AND V3 LOOP ENCODED IN 2 SEROPREVALENT UGANDAN HIV-1 CLINICAL ISOLATES

Two major epitopes expressed in HIV-1 have been recently shown to play a central role in virus neutralization. One of these important specif icities is a type-specific or group-specific, principal neutralizing d eterminant (PND) located in the V3 loop of gp120. The other is a more broadly neutralizing determinant associated with the CD4 binding site. Structural and serological studies of the variation in these epitopes have become important in vaccine research. This report describes the analysis of the DNA clones encoding a region of gp120 that overlaps th e V3 loop and the putative CD4 recognition site in two new African iso lates, UG06c and UG23c. Phylogenetic analyses of the DNA sequences sho wed that the new African isolates clustered with two very distinct sub types of HIV-1. UG06c was grouped with U455, D687, and Z321, previousl y classified as ''HIV-1 subtype A'' in the AIDS and human retroviruses database; and UG23c was grouped with MAL, JY1, NDK, ELI, and Z2Z6 cla ssified as ''HIV-1 subtype D. '' Considerable variation was apparent i n the V3 loop. The divergence included the presence of the hexapeptide s GPGRSF and GLGQAL at the cap of the loop in UG06c and UG23c, respect ively. The GPGR tetrapeptide in UG06c formed a beta-turn configuration similar to that of MN or IIIB. The beta-turn was not found to be a li kely conformation for GLGQ. The amino acids previously implicated in C D4 binding and the associated neutralizing activity were relatively co nserved. To assess a possible impact of the sequence and conformationa l variations on serological reactivity, UG06c and UG23c were subjected to neutralization assay. Each isolate was neutralized by approximatel y 40% of a panel of preselected, Western blot positive serum specimens from asymptomatic African and North American donors. A strong correla tion was observed between the divergence in the V3 loop and discordanc e in the neutralizing potency of some of the test sera. Discrete subse ts of the sera were observed to react with UG06c or UG23c. However, a third subset of the serum samples cross-neutralized UG06c, IIIB, and U G23c despite the marked variation in the primary and secondary structu res of the region analyzed. These results indicate that UG06c and UG23 c are distinct subtypes that express a relatively conserved neutralizi ng epitope recognized by certain antiviral antibodies of African and N orth American origins.