Characterizing the immune response to HIV-1 using host derived epitope R7V

Abstract

Background : Host protein beta-2 microglobulin (β2m) is incorporated into the human immunodeficiency virus (HIV) -1 coat during budding. Antibodies directed to R7V, an epitope contained in β2m, increased with the duration of infection in long term non-progressor patients (LTNPs). Purified R7V antibodies neutralized HIV isolates and did not bind to human cells. These data suggested potential for R7V antibodies to be developed as therapeutic tools or prognostic markers and the R7V epitope as a vaccine candidate. However, the literature on R7V is still incomplete. For example, most published work on this epitope make no direct reference to HIV subtypes. The rationale for this study is the lack of information on whether all HIV-1 subtypes incorporate R7V and elicit immune responses to the same extent. In particular the response of HIV-1 subtype C infected individuals to R7V antigen is evaluated here. Methodology and results : A synthetic peptide of the R7V epitope of HIV-1 was synthesized and an “in-house” enzyme-linked immunosorbent assay (ELISA) developed. The peptide was able to detect antibodies generated during natural HIV-1 subtype C infection when used as antigen in the ELISA. This response was not as strong as that reported in the literature. A significantly lower ELISA response was observed for uninfected compared to infected sera (probability, p, value ≤ 0.000152), whereas no differences were noticed between antiretroviral (ARV) treated individuals compared to those who were treatment naïve or LTNPs compared to progressors. These data hold promise for the use of these antibodies as diagnostic rather than prognostic indicators. Polyclonal R7V antibodies produced in rabbits and recombinant R7V antibody fragments did not neutralize an HIV-1 subtype C isolate (Du151.2). However, the latter antibodies neutralized an HIV-1 subtype B strain (SF162), suggesting that the R7V epitope may be more exposed in this subtype. The recombinant R7V antibodies did not neutralize a vasicular stomatitis virus (VSV-G), indicating that no nonspecific neutralization occurred. Human immunodeficiency virus type 1 subtype C infected sera containing R7V antibodies (positive response in the R7V ELISA) neutralized Du151.2 while archive sera containing strong HIV-1 subtype C neutralizing antibodies did not recognize the R7V antigen ELISAs. The R7V peptide exogenously added to HIV-1 infected peripheral blood mononuclear cells (PBMCs) did not stimulate proliferation in vitro nor the production of interferon (IFN) gamma which if produced by CD8+ T-cells would have been indicative of a cellular immune response. The parent protein β2m could not initiate these responses either. Conclusion : Data collected here support a diagnostic rather than a prognostic application for R7V antibodies. R7V conjugated to keyhole limpet hemocyanin (KLH) induced non-neutralizing antibodies in rabbits, suggesting that other modifications (branching, lipid conjugation, etc.) may be needed before this epitope can be successfully utilized in vaccine studies.