Transmembrane glycoprotein gp41 of the Human Immunodeficiency Virus Type I : gene synthesis, recombinant expression and immunological characterization
HIV, the causative agent of AIDS, is one of the first retroviruses to infect humans. Controlling this epidemic is extremely challenging. Studies that focus on understanding the biology, biochemistry and structural biology of HIV as well as the interactions between viral components and new drug candidates are required to determine an effective strategy to combat HIV/AIDS. Gp41 might be of interest for vaccine development because of its antigenic neutralizing epitope sites previously discovered. Here, the ecto- and endo-domains of HIV-1 gp41 were studied. The endo-domain was truncated into three gene fragments namely gp41T3 (405 bp), gp41T2 (591 bp) and gp41T1(696 bp), all containing the Kennedy domain located on the C-terminal of gp41. The 80 amino acid fragment gp41(80) of the ectodomain, located on the N-terminal of gp41 was also constructed. The pMALgp41T3, pMAL-gp41(80) and gp41T1 constructs were expressed in either E. coli or P. pastoris and purified. Both the pMAL-gp41T3 and the pMAL-gp41(80) peptides proved to be antigenic towards HIV positive patient sera determined by ELISA, but in the case of gp41T1 no antigenicity was found. Gp41(80) showed increasing antibody-antigen binding activity as the CD4 T cell count decreased, i.e. as HIV infection progressed towards AIDS, while gp41T3 showed no correlation between the CD4 count and the antibody-antigen binding activity. Therefore, irrespective of the decline in the CD4 counts of the patients there is a constant antibody-antigen binding activity. It was also indicated that the hydrophobicity of pMAL-gp41T3 was higher than for pMAL-gp41(80). The high levels of hydrophobicity of this peptide may lead to its presentation by antigen presenting cells on CD1 as a “lipid like” peptide to CD4/CD8 double negative T-cell or NKT cells. Auto-immunity may be of some concern, because antibodies and other forms of immunity induced by the CD4/CD8 double negative T-cell population are often involved in auto-immunity. Many viruses have, as part of their invasion strategy, the ability to modulate the apoptotic pathways of the host. Preliminary evidence shows that the pMAL-gp41T3 construct has apoptotic properties. Considerable homology was illustrated by the sequence alignment of gp41T3 with apoptotic TNFα, FasL, TRAIL, APRIL, TRANCE and VEGI. Gp41 may be considered as a vaccine target due to its antigenicity shown in this study, but its apoptotic and auto-immune potential argue for care. The ecto-domain seems to elicit antibodies that do not prevent the progression to AIDS. Even if no vaccine can be made from this, the antibody response to the endo-domain of gp41 was found not to be perturbed by the progression towards AIDS. It may therefore, find use as a surrogate marker for HIV infection that may be useful in diagnosis. The immune properties of pMALgp41( 80) and pMAL-gp41T3 may be exploited to steer immunity away from AIDS, e.g. by selective suppression of pathological auto-immune antibody activity by idiotypic vaccines or suppression of peptide induced apoptosis of CD4 T-cells by pharmaceutical or biological means.
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