Localization of CD4(+) T cell epitope hotspots to exposed strands of HIV envelope glycoprotein suggests structural influences on antigen processing

The spectrum of immunogenic epitopes presented by the H2-IA(b) MHC class II molecule to CD4(+) T cells has been defined for two different (clade B and clade D) HIV envelope (gp140) glycoproteins. Hybridoma T cell lines were g enerated from mice immunized by a sequential prime and boost regime with DN A, recombinant vaccinia viruses, and protein. The epitopes recognized by re active T cell hybridomas then were characterized with overlapping peptides synthesized to span the entire gp140 sequence. Evidence of clonality also w as assessed with antibodies to T cell receptor V alpha and V beta chains. A total of 80 unique clonotypes were characterized from six individual mice. Immunogenic peptides were identified within only four regions of the HIV e nvelope. These epitope hotspots comprised relatively short sequences ( appr oximate to 20-80 aa in length) that were generally bordered by regions of h eavy glycosylation. Analysis in the context of the gp120 crystal structure showed a pattern of uniform distribution to exposed, nonhelical strands of the protein. A likely explanation is that the physical location of the pept ide within the native protein leads to differential antigen processing and consequent epitope selection.