Design, development, and interpretation of HIV neutralization assays

Analysis of the cross-reactive anti-gp120 antibody population in human immunodeficiency virus-infected asymptomatic individuals

This study was undertaken to analyze the specificity and neutralizing properties of cross-reactive anti-gp120 antibodies (Abs) in the sera of two human immunodeficiency virus (HIV)-infected asymptomatic individuals. Two panels of murine monoclonal anti-idiotype Abs (anti-id MAbs) were established against cross-reactive polyclonal anti-gp120 Abs purified from HIV+ sera by sequential affinity chromatography using gp120SF2- and gp120IIIB-Sepharose columns. These panels of anti-id MAbs were then used to affinity purify idiotype-positive (Id+) anti-gp120 Abs from HIV+ sera. The recovery of each of these Id+ Abs by purification indicated that several idiotypically distinct cross-reactive anti-gp120 Abs are present in sera over a wide range of concentrations. Immunological and biological studies showed that although all of the Id+ Abs were reactive against gp120SF2 and gp120IIIB, they exhibited unique epitope specificities and distinct neutralizing activities. Most of the Id+ Abs were directed against epitopes in the CD4 attachment site (CD4 site epitopes) of gp120 and exhibited a spectrum of broadly neutralizing activities. On the other hand, a minor population of Id+ Abs showed specificity for the V3 region of gp120 and exhibited limited cross-neutralizing activities. Together, these studies indicate that the CD4 site epitope-specific Abs are heterogeneous with respect to their clonality, neutralizing activity, and concentration in sera. This heterogeneity suggests that anti-gp120 Abs to the CD4 attachment site are developed in response to multiple overlapping epitopes present on the original virus isolate and/or epitopes on mutated variants which emerged over time.

Anti-idiotype monoclonal antibody elicits broadly neutralizing anti-gp120 antibodies in monkeys

Murine monoclonal antibodies (mAbs) were raised against human, polyclonal, anti-gp120 antibodies (Ab1) and were selected for binding to broadly neutralizing anti-gp120 antibodies in sera positive for human immunodeficiency virus (HIV). One anti-idiotype mAb (Ab2), 3C9, was found to be specific for human anti-gp120 antibodies directed against an epitope around the conserved CD4 attachment site of gp120. The 3C9 reactive human anti-gp120 antibodies (3C9+ Ab) neutralized MN, IIIB, RF, and four primary isolates of HIV type 1 (HIV-1). Cynomolgus monkeys were immunized with 3C9 in adjuvant to test whether this anti-idiotype mAb could induce neutralizing anti-gp120 antibodies. The results show that purified anti-anti-idiotype antibodies (Ab3) from 3C9 immune sera bind to an epitope around the CD4 attachment site of gp120SF and gp120IIIB. Furthermore, purified gp120-specific Ab3 neutralize MN, IIIB, and RF isolates. These results demonstrate that primates immunized with an anti-idiotype mAb produce broadly neutralizing anti-HIV-1 antibodies. Since this anti-idiotype mAb was selected by identifying a clonotypic marker, its biological activity can be explained as the results of clonotypic B-cell stimulation.