Recognition by human monoclonal antibodies of free and complexed peptides representing the prefusogenic and fusogenic forms of human immunodeficiencyvirus type 1 gp41

Human immunodeficiency virus type 1 (HIV-1) entry into target cells appears to be triggered when two heptad repeat regions in the ectodomain of gp41 a ssociate, converting the prefusogenic form of gp41 to a fusogenic form. Pep tides from these two heptad repeat regions, designated N51 and C43, form a coiled coil consisting of an alpha-helical trimer of heterodimers which app roximates the core of the fusogenic form of gp41. To understand the antigen ic structures of gp41 in these two configurations, and to examine the speci ficity of anti-gp41 antibodies produced by HIV-l-infected individuals. huma n anti-gp41 monoclonal antibodies (MAbs) were tested for their reactivity a gainst N51, C43, and the complex formed by these peptides, Of 11 MAbs, 7 re acted with the complex but with neither of the parent peptides, These MAbs reacted optimally with the N51-C43 complex prepared at a 1:1 ratio and appe ared to recognize the fusogenic Form of gp41 in which the two heptad repeat regions are associated to form the coiled coil. The existence of antibodie s from HIV-infected humans that exclusively recognize the N51-C43 complex c onstitutes the first proof that the coiled-coil conformation of gp41 exists in vivo and is immunogenic. Two of the 11 MAbs were specific for the hydro philic Loop region of gp41 and failed to react with either peptide alone or with the peptide complex, while the remaining 2 MAbs reacted with peptide C43, One of these two latter MAbs, 98-6, also reacted well with the equimol ar N51-C43 complex, while reactivity with C43 by the other MAb, 2F5, was in hibited by even small amounts of N51, suggesting that the interaction of th ese peptides occludes or disrupts the epitope recognized hy MAb 2F5, MAbs 9 8-6 and 2F5 are also unusual among the MAbs tested in their ability to neut ralize multiple primary HIV isolates, although 2F5 displays more broad and potent activity. The data suggest that anti-gp41 neutralizing activity is a ssociated with specificity for a region in C43 which participates in comple x formation with N51.