Induction of humoral and cellular immune responses against the HIV-1 envelope protein using γ-retroviral virus-like particles
© Kirkegaard et al; licensee BioMed Central Ltd. 2011
Received: 12 April 2011
Accepted: 1 August 2011
Published: 1 August 2011
This study evaluates the immunogenicity of the HIV envelope protein (env) in mice presented either attached to γ-retroviral virus-like-particles (VLPs), associated with cell-derived microsomes or as solubilized recombinant protein (gp160). The magnitude and polyfunctionality of the cellular immune response was enhanced when delivering HIV env in the VLP or microsome form compared to recombinant gp160. Humoral responses measured by antibody titres were comparable across the groups and low levels of antibody neutralization were observed. Lastly, we identified stronger IgG2a class switching in the two particle-delivered antigen vaccinations modalities compared to recombinant gp160.
The induction of neutralizing antibodies remains key to developing an efficient preventive HIV vaccine. The strongest evidence in support of this comes from non-human primate studies, which demonstrate that broadly neutralizing antibodies can protect from infection [1, 2]. The functional HIV envelope protein (env) complex consists of three heterodimers of the surface subunit gp120 and the transmembrane subunit gp41 arranged in trimeric spikes . Gp120 binds in a sequential manner to CD4 and to a co-receptor (in most cases either CXCR4 or CCR5) on the target cell. These interactions promote extensive conformational changes in both gp120 and gp41, which leads to fusion of the viral and target-cell membranes. For vaccination purposes antibodies raised to monomeric gp120 antigens primarily target the oligomeric interface, which is not exposed in the functional trimeric structure . Therefore, in order to elicit neutralizing antibodies with high efficacy directed towards the native forms of env, trimeric structure, ideally in a membrane-bound form should be a crucial property of env vaccine candidates for immunization.
The production and purification of soluble recombinant env proteins having a native trimeric conformation remains challenging . As an alternative, HIV env can be presented as membrane anchored trimers on virus-like particles (VLPs) [6–8]. Several studies imply that membrane-associated trimeric env can raise higher antibody titers with increased neutralization potency compared to soluble recombinant gp120 or gp160 [9, 10], and a VLP-based vaccination strategy has previously elicited some strain-specific neutralizing activity in mice and macaques .
In order to efficiently pseudotype MLV particles with HIV env, a cytoplasmic tail-truncated HIV env is required [12, 13]. This limits the retrograde trafficking of HIV env localized on the cell surface by removing endocytosis signals within the cytoplasmic tail of gp41 , which in turn enhances env virion incorporation . Importantly, several well-characterized neutralizing antibodies are equally potent in inhibiting both native as well as C-terminal truncated env .
The aim of this study was to compare the immunogenicity of env antigens delivered via γ- retroviral-like particles consisting of murine leukemia virus (MLV) gag and HIV env (termed HIV env/gag+), a purified cell fraction containing microsomes and HIV env without a viral core (termed HIV env/gag-) and recombinant gp160 (uncleaved env precursor consisting of contiguous gp120 and gp41 domains) produced in H9 cells, detergent solubilized and immuno-affinity purified .
In conclusion, these results indicate that relative to recombinant protein immunogens, γ-retroviral-based VLPs and microsome incorporated env can stimulate quantitative and qualitative improvements in T cell responses targeting HIV env. Thus, a vaccine platform using particle-delivered env trimers appears robust and immunogenic, and also holds advantages as to vector safety. The sequence similarity between γ-retroviruses and lentiviruses is very small, reducing the potential for recombination. The microsome delivery of antigen appear indistinguishable from the γ-retroviral-based VLP in terms of immunogenicity but incorporation of env was less efficient posing challenges in terms of larger scale production. Summarized, we believe these immunogenic and safety features support further investigations of particle-delivered HIV env for vaccinations against HIV.
This work was supported by grants the Danish AIDS Foundation, the Scandinavian Society of Antimicrobial Chemotherapy, Region Midtjylland Research Foundation and the Toyota Foundation Denmark. We thank Professor T. Kitamura for the GagPol construct GagPol-IRES-bsr. FluoroSpot plates were kindly enumerated by Staffan Paulie, Mabtech, Sweden.
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