Computational Design of Epitope-Enriched HIV-1 Gag Antigens with Preserved Structure and Function for Induction of Broad CD8+ T Cell Responses

Abstract

The partially protective phenotype observed in HIV-infected long-term-non-progressors is often associated with certain HLA alleles, thus indicating that cytotoxic T lymphocyte (CTL) responses play a crucial role in combating virus replication. However, both the vast variability of HIV and the HLA diversity impose a challenge on elicitation of broad and effective CTL responses. Therefore, we conceived an algorithm for the enrichment of CD8+ T cell epitopes in HIV’s Gag protein, respecting functional preservation to enable cross-presentation. Experimentally identified epitopes were compared to a Gag reference sequence. Amino-acid-substitutions (AAS) were assessed for their impact on Gag’s budding-function using a trained classifier that considers structural models and sequence conservation. Experimental assessment of Gag-variants harboring selected AAS demonstrated an apparent classifier-precision of 100%. Compatible epitopes were assigned an immunological score that incorporates features such as conservation or HLA-association in a user-defined weighted manner. Using a genetic algorithm, the epitopes were incorporated in an iterative manner into novel T-cell-epitope-enriched Gag sequences (TeeGag). Computational evaluation showed that these antigen candidates harbor a higher fraction of epitopes with higher score as compared to natural Gag isolates and other artificial antigen designs. Thus, these designer sequences qualify as next-generation antigen candidates for induction of broader CTL responses.