Comprehensive Analysis of Contributions from Protein Conformational Stability and Major Histocompatibility Complex Class II-Peptide Binding Affinity to CD4⁺Epitope Immunogenicity in HIV-1 Envelope Glycoprotein

Abstract

Helper T-cell epitope dominance in human immunodeficiency virus type 1 (HIV-1) envelope glycoprotein gp120 is not adequately explained by peptide binding to major histocompatibility complex (MHC) proteins. Antigen processing potentially influences epitope dominance, but few, if any, studies have attempted to reconcile the influences of antigen processing and MHC protein binding for all helper T-cell epitopes of an antigen. Epitopes of gp120 identified in both humans and mice occur on the C-terminal flanks of flexible segments that are likely to be proteolytic cleavage sites. In this study, the influence of gp120 conformation on the dominance pattern in gp120 from HIV strain 89.6 was examined in CBA mice, whose MHC class II protein has one of the most well defined peptide-binding preferences. Only one of six dominant epitopes contained the most conserved element of the I-A k binding motif, an aspartic acid. Destabilization of the gp120 conformation by deletion of single disulfide bonds preferentially enhanced responses to the cryptic I-A k motif-containing sequences, as reported by T-cell proliferation or cytokine secretion. Conversely, inclusion of CpG in the adjuvant with gp120 enhanced responses to the dominant CD4 ؉ T-cell epitopes. The gp120 destabilization affected secretion of some cytokines more than others, suggesting that antigen conformation could modulate T-cell functions through mechanisms of antigen processing. IMPORTANCE CD4؉ helper T cells play an essential role in protection against HIV and other pathogens. Thus, the sites of helper T-cell recognition, the dominant epitopes, are targets for vaccine design; and the corresponding T cells may provide markers for monitoring infection and immunity. However, T-cell epitopes are difficult to identify and predict. It is also unclear whether CD4 ؉ T cells specific for one epitope are more protective than T cells specific for other epitopes. This work shows that the three-dimensional (3D) structure of an HIV protein partially determines which epitopes are dominant, most likely by controlling the breakdown of HIV into peptides. Moreover, some types of signals from CD4 ؉ T cells are affected by the HIV protein 3D structure; and thus the protectiveness of a particular peptide vaccine could be related to its location in the 3D structure.

Env Exceptionalism: Why Are HIV-1 Env Glycoproteins Atypical Immunogens?

“…Specifically, the 378-418 mutation redirected the Ab specificity toward the CD4bs (Mirano-Bascos et al, 2010). However, in CBA mice, in which MHC-II has well-defined binding properties, disrupting a single gp120 disulfide bond redirected responses to cryptic CD4 + T cell epitopes (Li et al, 2014). Thus, manipulating THCE epitopes in an immunogen as complex as a gp120 monomer can have unpredictable and un-desired outcomes.…”

Neutralizing Antibody Induction by HIV-1 Envelope Glycoprotein SOSIP Trimers on Iron Oxide Nanoparticles May Be Impaired by Mannose Binding Lectin

“…We also assessed whether exogenous TCHEs could improve SOSIP trimer immunogenicity. The availability of the few TCHEs in the HIV-1 Env sequence for MHC class II presentation may be highly restricted by glycans and disulfide bonds; glycans can be present within a TCHE sequence or interfere with Env protein processing and hence the liberation of TCHE-containing peptides (35)(36)(37)(38)(39)(40)(41)(42). As the topic is under-researched for modern Env protein designs, we incorporated a TCHE into SOSIP trimers as a C-terminal PADRE-tag flanked by cathepsin-S cleavage sites intended to facilitate its release.…”

Protection of Mice against Experimental Cryptococcosis by Synthesized Peptides Delivered in Glucan Particles

“…A chimeric recombinant protein ( 21 ) containing multiple peptide antigens could be designed so that it stimulated Th-dependent responses among a broad range of MHC class II alleles found in the human population. However, care would need to be taken to avoid creating spurious neoepitopes at peptide junctions and/or dominance hierarchy issues ( 33 , 34 ).…”