Human Antibodies that Recognize Novel Immunodominant Quaternary Epitopes on the HIV-1 Env Protein.

Numerous broadly neutralizing antibodies ( Abs) target epitopesthat are formed or enhanced during mature HIV envelope formation (i.e. quaternary epitopes). Generally, it is thought that Env epitopesthat induce broadly neutralizing Absare difficult to access and poorly immunogenic because of the characteristic oligomerization, conformational flexibility, sequence diversity and extensive glycosylation of Env protein. To enhance for isolation of quaternary epitope-targeting Abs(QtAbs), we previously used HIV virus-like particles(VLPs) to bind B cells from long-term non-progressor subjects to identify a panel of monoclonal Abs. When expressed as recombinant full-length Abs, a subset of these novel Absexhibited the binding profiles of QtAbs, as they either failed to bind to monomeric Env protein or showed much higher affinity for Env trimers and VLPs. These QtAbs represented a significant proportion of the B-cell response identified with VLPs. The Abgenes of these clones were highly mutated, but they did not neutralize common HIV strains. We sought to further define the epitopestargeted by these QtAbs. Competition-binding and mapping studies revealed these Abstargeted four separate epitopes; they also failed to compete for binding by Absto known major neutralizing epitopes. Detailed epitope mappingstudies revealed that two of the four epitopeswere located in the gp41subunit of Env. These QtAbs bound pre-fusion forms of antigen and showed differential binding kinetics depending on whether oligomers were produced as recombinant gp140 trimers or as full-length Env incorporated into VLPs. Antigenic regions within gp41present unexpectedly diverse structural epitopes, including these QtAb epitopes, which may be targeted by the naturally occurring Abresponse to HIV infection.