Enhancement of HIV-1 DNA immunogens

Abstract

Human immunodeficiency virus type 1 eludes control by the immune response through a high degree of variability and immune escape mechanisms. Induction of a broad specific immune response is important to clear virus-infected cells. DNA vaccination is a relatively new approach that induces both humoral and cellular immune responses in vaccinated hosts. The aim of this thesis was to enhance immune responses to different HIV-1 proteins using different DNA vaccine regimens.

The three regulatory genes tat, rev and nef of HIV-1 have been of particular interest in vaccine design. A strong cytotoxic T-lymphocyte response against these three proteins correlates to long-term non-progression of disease. The protein expression from regulatory genes was characterized from patient and laboratory strain viruses. The laboratory strain derived genes resulted in the most efficient protein expression and were used for further studies. We examined single versus combined genes and found that individual responses to each protein were strongest after single gene administration. Immune responses to several targets were induced when the three genes were used together, which is important when developing an effective HIV-1 vaccine. The strongest responses were seen to the Nef protein. However, these responses decreased when co-immunizing with the tat and rev genes, as was the case with responses to Rev. Different combinations of plasmids, different injection sites and different doses might however overcome these drawbacks.

Several immunization strategies, using DNA, recombinant modified vaccinia Ankara (MVA) vectors, protein mixed with CpG oligodeoxyribonucleotides (ODN), and a novel adjuvant, were evaluated. Different prime-boost regimes were used to enhance Nef-specific immune responses. The combination of nef DNA and MVAnef resulted in partial resistance from challenge with HIV-1/MuLV infected cells. The combination of recombinant Nef protein mixed with CpG ODN with or without a booster immunization with MVAnef also cleared HIV/MuLV infected cells. A broad response to Nef after HIV-1/MuLV challenge was apparent in the groups of mice that had received the recombinant Nef protein mixed with CpG ODN. To develop these findings, another HIV-1 gene, the reverse transcriptase (RT) gene, was used. RT gene priming followed by RT protein mixed with CpG ODN booster was used in primates. Again, strong cellular responses were induced by RT DNA followed by RT protein mixed with CpG ODN.

A combination of regulatory and structural genes might give a beneficial broad immune response. The compound imiquimod activates the Toll like receptor 7 and is used in the clinic for treating genital warts. Imiquimod was evaluated as an adjuvant with thenef, p37 (p17 and p24 genes) and RT genes of HIV-1, and was shown to potentiate cellular immune responses capable of clearing HIV-1/MuLV infected cells.

In conclusion, we were able to induce strong immune responses to all antigens tested using DNA vaccination. The responses were increased by using either adjuvants in combination with the DNA, by boosting with protein mixed with CpG ODN or by boosting with a recombinant modified vaccinia Ankara vector. The strongest cellular responses related to partial protection from challenge with HIV-1/MuLV infected cells.