HUMAN-ANTIBODY RESPONSES TO HIV TYPE-1 GLYCOPROTEIN 41 CLONED IN PHAGE DISPLAY LIBRARIES SUGGEST 3 MAJOR EPITOPES ARE RECOGNIZED AND GIVE EVIDENCE FOR CONSERVED ANTIBODY MOTIFS IN ANTIGEN-BINDING

A large panel of human Fab fragments against the gp41 subunit of the H IV-1 envelope glycoprotein was isolated by panning six phage-displayed antibody libraries against recombinant gp41. The libraries were prepa red from HIV-1-seropositive donors, Twenty-three Fabs recognizing conf ormation-dependent determinants on gp41 were isolated, Further selecti on of libraries against (1) gp41 ligated with Fabs from the initial se lection and against (2) a recombinant gp41-containing gp140 protein yi elded five additional Fabs, Competition of members of the Fab panel wi th one another and with previously described antibodies revealed a ser ies of overlapping specificities that were conveniently grouped into t hree major epitope clusters. The majority of Fabs recognized epitopes involving residues 649-668 (previously known as the cluster II region) , numbered using the Los Alamos LAI sequence, A second set of Fabs rea cted with an epitope involving residues 584-609 (known as the cluster I region), Another set of Fabs appeared to recognize a third conformat ional epitope that has been termed the cluster III region. This third Fab epitope group demonstrated some overlap with both clusters I and I I in binding assays, None of the Fabs neutralized HIV-1 laboratory str ains at biologically significant concentrations, This tends to support the opinion that a vaccine based on the gp41 molecule has the drawbac k that neutralizing epitopes of gp41 are rare and/or unfavorably prese nted to the immune system, Analysis of heavy chain sequences revealed common CDR3 motif sequences in several antibodies, which appears to be an interesting consequence of a persistent immune response to conserv ed antigen structures.