Frequent associations between CTL and T-Helper epitopes in HIV-1 genomes and implications for multi-epitope vaccine designs

Abstract

Background: Epitope vaccines have been suggested as a strategy to counteract viral escape and development of drug resistance. Multiple studies have shown that Cytotoxic T-Lymphocyte (CTL) and T-Helper (Th) epitopes can generate strong immune responses in Human Immunodeficiency Virus (HIV-1). However, not much is known about the relationship among different types of HIV epitopes, particularly those epitopes that can be considered potential candidates for inclusion in the multi-epitope vaccines.

Results: In this study we used association rule mining to examine relationship between different types of epitopes (CTL, Th and antibody epitopes) from nine protein-coding HIV-1 genes to identify strong associations as potent multi-epitope vaccine candidates. Our results revealed 137 association rules that were consistently present in the majority of reference and non-reference HIV-1 genomes and included epitopes of two different types (CTL and Th) from three different genes (Gag, Pol and Nef). These rules involved 14 non-overlapping epitope regions that frequently co-occurred despite high mutation and recombination rates, including in genomes of circulating recombinant forms. These epitope regions were also highly conserved at both the amino acid and nucleotide levels indicating strong purifying selection driven by functional and/or structural constraints and hence, the diminished likelihood of successful escape mutations.

Conclusions: Our results provide a comprehensive systematic survey of CTL, Th and Ab epitopes that are both highly conserved and co-occur together among all subtypes of HIV-1, including circulating recombinant forms. Several co-occurring epitope combinations were identified as potent candidates for inclusion in multi-epitope vaccines, including epitopes that are immuno-responsive to different arms of the host immune machinery and can enable stronger and more efficient immune responses, similar to responses achieved with adjuvant therapies. Signature of strong purifying selection acting at the nucleotide level of the associated epitopes indicates that these regions are functionally critical, although the exact reasons behind such sequence conservation remain to be elucidated.

Figure 1

A "multi-type" association rule involving three CTL and one Th epitope from three different genes, Gag, Pol and Nef in reference to HIV-1 genome. The corresponding amino acid coordinates (as per HIV-1 HXB2 reference sequence) and HLA allele supertypes recognizing these epitopes are also shown.

Figure 2

Relative composition of unique association rules involving multiple genes (Gag, Pol and Nef) and epitope types (Cytotoxic T Lymphocyte (CTL), T-Helper (Th) and antibody (Ab) epitopes). The 6142 unique association rules are classified according to the genes that harbor these epitopes. The pie-chart inside each segment represents the division according to the epitope region types involved. The single association rule in Nef-only category involved CTL and Th epitopes, while that in Pol-Env category involved CTL and Ab epitopes. Out of four association rules involving epitopes from Gag and Env, three belonged to CTL-Ab and one belonged to Th-Ab epitope regions types.