Vaccination against drug-resistant HIV

Abstract

Combinations of antiretroviral drugs against human immunodeficiency virus type 1 (HIV-1) have effectively postponed the progression to acquired immunodeficiency syndrome (AIDS). However, an effective vaccine against HIV-1 would undisputedly be the optimal protective strategy against the virus, especially in resource-poor settings. Because of HIV s unique ability to adapt to environmental pressure, drug-resistant viral strains develop during treatment.

In this thesis, we have evaluated vaccine strategies targeting drug-resistant HIV-1. Such a vaccine, together with antiretroviral drugs, would potentially act synergistically against the virus. The drugs would limit viral replication, and the immune pressure specific for resistance mutations would prevent mutant virus from evolving.

Epitopes that commonly mutate during therapy and are restricted to HLA-A0201 were selected as potential vaccine components. Synthetic peptides, representing the epitopes, were evaluated for binding to HLA-A0201 and HLA-A2402 allelic proteins. We found that some of the mutant epitope variants had an enhanced binding capacity over their wild type to HLA-A0201; a few epitopes also cross-bound to the HLA-A2402 protein. Next, we linked the nucleotide sequences of five epitopes, and assessed the immunogenicity of the DNA construct in HLA-A0201 transgenic mice. Contrary to our expectations, the strongest immune response was induced when we immunized mice with the wild type construct. This response was found to cross-react with mutated variants of the epitope.

In addition, we explored the possibility to enhance the immune response to mutant peptides by either bridging an HIV-1 protease derived peptide to erythrocytes and use those for vaccination, or by genetically conjugating different epitopes (two of which are presented here) with the B subunit of Cholera toxin (CTB). The expressed fusion proteins were used as immunogens. A weak immune response was measured with the peptide linked to erythrocytes ten weeks after the last immunization. This response was significantly stronger than by giving the peptide alone; despite a 500-fold higher dose of the unconjugated peptide.

Conjugation of the epitope to CTB strongly enhanced the immune response to the epitope. The response was cross-reactive with the wild type epitope, was long-lived and sustained over a four-month period. Interestingly, we observed a correlation of binding capacity of the fusion protein to the natural receptor of CTB, and the adjuvant effect of CTB. The stronger the binding, the better the immune response. We also investigated the potential use of the HIV-1 reverse transcriptase (RT) gene and a multi resistant RT variant. The proteasomal degradation of the proteins was increased by fusing them to ornithine decarboxylase (ODC) or the degradation signal of ODC. After immunization, an inflammatory response was observed in all groups. The RT-specific immune response was relatively weak. The most potent response was detected when RT was fused to the degradation signal of ODC.

In conclusion, we evaluated strategies to target drug-resistant HIV by a vaccine. By using epitopes harbouring drug-resistance mutations as vaccines components, we have consistently detected epitope-specific immune responses that were cross-reactive to wild type sequences. Similar observations were found using wild type epitopes as immunogens. However, the homologous epitope responses were always stronger than, or as strong as, the heterologous epitope responses. This suggests that mutated epitopes representing drug-induced changes are desirable when targeting drug-resistant HIV.