Antigenic domains of the HIV-1 vif protein as recognized by human sera and murine monoclonal antibodies

Replacement of feline foamy virus bet by feline immunodeficiency virus vif yields replicative virus with novel vaccine candidate potential

BACKGROUND

Hosts are able to restrict viral replication to contain virus spread before adaptive immunity is fully initiated. Many viruses have acquired genes directly counteracting intrinsic restriction mechanisms. This phenomenon has led to a co-evolutionary signature for both the virus and host which often provides a barrier against interspecies transmission events. Through different mechanisms of action, but with similar consequences, spumaviral feline foamy virus (FFV) Bet and lentiviral feline immunodeficiency virus (FIV) Vif counteract feline APOBEC3 (feA3) restriction factors that lead to hypermutation and degradation of retroviral DNA genomes. Here we examine the capacity of vif to substitute for bet function in a chimeric FFV to assess the transferability of anti-feA3 factors to allow viral replication.

RESULTS

We show that vif can replace bet to yield replication-competent chimeric foamy viruses. An in vitro selection screen revealed that an engineered Bet-Vif fusion protein yields suboptimal protection against feA3. After multiple passages through feA3-expressing cells, however, variants with optimized replication competence emerged. In these variants, Vif was expressed independently from an N-terminal Bet moiety and was stably maintained. Experimental infection of immunocompetent domestic cats with one of the functional chimeras resulted in seroconversion against the FFV backbone and the heterologous FIV Vif protein, but virus could not be detected unambiguously by PCR. Inoculation with chimeric virus followed by wild-type FFV revealed that repeated administration of FVs allowed superinfections with enhanced antiviral antibody production and detection of low level viral genomes, indicating that chimeric virus did not induce protective immunity against wild-type FFV.

CONCLUSIONS

Unrelated viral antagonists of feA3 cellular restriction factors can be exchanged in FFV, resulting in replication competence in vitro that was attenuated in vivo. Bet therefore may have additional functions other than A3 antagonism that are essential for successful in vivo replication. Immune reactivity was mounted against the heterologous Vif protein. We conclude that Vif-expressing FV vaccine vectors may be an attractive tool to prevent or modulate lentivirus infections with the potential option to induce immunity against additional lentivirus antigens.

An improved method for purification and refolding of recombinant HIV Vif expressed in Escherichia coli

Virion infectivity factor (Vif) is a 23 kDa protein that protects HIV-1 from deamination of its proviral DNA by APOBEC3G. The active form of Vif is a multimer that interacts simultaneously with CBF-beta, the elongin B and C subunits, Cullin 5, and APOBEC3G to form a ubiquitin ligase complex targeting the latter for degradation. Vif clearly represents an attractive target for developing novel antiviral drugs for the therapy of HIV/AIDS, and this goal requires a source of well folded, readily available protein. For that purpose, we have cloned Vif in the pET28a expression vector, expressing the resulting His-tagged recombinant protein in the BL21(DE3) Escherichia coli strain. After lysis, Vif was solubilized from the insoluble fraction with 6 M guanidinium chloride and purified by denaturing immobilized-metal affinity chromatography, refolding the protein afterwards by dialysis. The use of 2-(N-morpholino)ethanesulfonic acid buffer at pH 6.2 and the presence of EDTA improved Vif refolding yields by reducing the formation of insoluble aggregates. The purified protein was bound by two monoclonal antibodies against sequential and conformational epitopes located at the C and N terminus, respectively.

HIV-Specific Antibody Responses in HIV-Infected Patients: From a Monoclonal to a Polyclonal View

HIV infections represent a major global health threat, affecting more than 35 million individuals worldwide. High infection rates and problems associated with lifelong antiretroviral treatment emphasize the need for the development of prophylactic and therapeutic immune intervention strategies. It is conceivable that insights for the design of new immunogens capable of eliciting protective immune responses may come from the analysis of HIV-specific antibody responses in infected patients. Using sophisticated technologies, several monoclonal neutralizing antibodies were isolated from HIV-infected individuals. However, the majority of polyclonal antibody responses found in infected patients are nonneutralizing. Comprehensive analyses of the molecular targets of HIV-specific antibody responses identified that during natural infection antibodies are mainly misdirected towards gp120 epitopes outside of the CD4-binding site and against regions and proteins that are not exposed on the surface of the virus. We therefore argue that vaccines aiming to induce protective responses should include engineered immunogens, which are capable of focusing the immune response towards protective epitopes.

A new approach to an AIDS vaccine: creating antibodies to HIV vif will enable apobec3G to turn HIV-infection into a benign problem

For a decade, attempts to produce a vaccine that prevents HIV infection have been fruitless, and fresh approaches are required. Apobec3G is a natural defensin and a cytidine deaminase. Apobec3G induces a high rate of dC to dU mutation in the first minus strand of cDNA, causing degradation throughout the HIV genome that renders the virus effete. The viral infectivity factor (vif) of HIV is essential for efficient replication of that virus. Vif binds to apobec3G and induces its polyubiquitination, which enables HIV to evade apobec3G. This suggests that a vif-based vaccine which induced anti-vif antibodies, would prevent the neutralizing action of vif upon apobec3G. Then, with HIV-vif ineffective, apobec3G could act without hindrance to create a less aggressive, non-lethal HIV infection. Mutated vif impedes HIV infection. Slow progressors with vif 132S had 4-fold lower viral loads than those with vif 132R; and introducing vif 132S into HIV-1 caused a 5-fold decrease in viral replication. And in the absence of vif, HIV virions accumulate multiple defects in structural, enzymatic, and regulatory viral proteins. The success of a vif-based vaccine depends upon (1) a vif-antibody response, and (2) vif antibodies entering the cells that harbor HIV. First, antibodies to vif have been seen in frequencies ranging between 25% and 100% in patients infected with HIV-1. Second, transport of anti-vif antibodies into cells might occur via several mechanisms. Likeliest is that in viremic persons, antibodies would attach to cell-free virions which would piggyback the antibodies into CD4+ cells. Alternatively, a fusion protein between vif and a cell-surface receptor, e.g., CD4 or CCR5, might be used as vaccine antigen. Also, anti-vif antibodies might internalize after ligation of HIV virions budding on the cell surface, in the same way as monoclonal antibodies against porcine pseudorabies virus induced viral glycoproteins on the cell surface to internalize. Finally, monoclonal antibodies, using unknown mechanisms to enter cells, have been effective against several other intracellular pathogens. In summary, HIV-vif might be effective in a vaccine intended to ameliorate either preexisting or subsequent HIV infection.

Fully functional, naturally occurring and C-terminally truncated variant human immunodeficiency virus (HIV) Vif does not bind to HIV Gag but influences intermediate filament structure

A variant human immunodeficiency virus type 1 (HIV-1) vif gene, vifA45-2, which encodes a protein lacking 19 amino acids at the C terminus but which is fully functional in supporting HIV replication in non-permissive cells has been described previously. By employing newly generated anti-VifA45 serum, further properties of VifA45 and its full-length counterpart, VifA45open, in comparison to Vif from HIV strain BH10 are reported in permissive HeLa and COS-7 cells. The results obtained using confocal microscopic localization studies and in vitro binding assays do not support a requirement for the direct interaction of HIV Gag with Vif. Furthermore and in contrast to previous conclusions, detergent solubility analyses do not demonstrate a role for the C terminus of Vif in mediating localization to the fraction containing cellular membrane proteins. Localization of Vif from HIV strain BH10 to perinuclear aggregates in a small fraction (about 10%) of transfected HeLa cells has been previously reported. The intermediate filament protein vimentin colocalizes to these structures. In contrast, VifA45 and VifA45open form perinuclear aggregates in nearly all transfected HeLa cells; vimentin as well as the cytoskeletal-bridging protein plectin, but not the microtubular protein tubulin, become relocalized to these structures. Interestingly, in COS-7 cells, all of the functional Vif proteins tested (Vif from strain BH10, VifA45 and VifA45open) predominantly localize in the cytoplasm but still induce dramatic aggregation of vimentin and plectin, i.e. in these cells the respective Vif proteins are influencing intermediate filament structure in the absence of colocalization.

Development of genetic vaccines for pathogenic genes: construction of attenuated vif DNA immunization cassettes

OBJECTIVE

To develop a putative immunization cassette using HIV-1 vif accessory gene derived from HIV-1 clinical specimens as a component of a DNA vaccine for HIV-1.

METHODS

vif genes were cloned from HIV-1-infected patients and the sequence variation present within the patients was analyzed. Prototypic genetic variants were selected and the ability of these clones to induce humoral and cellular immune responses was studied in animals. The selected protective genetic variants were biologically characterized through transcomplementation assays using primary cells infected with a vif-defective HIV-1 proviral clone.

RESULTS

Analysis of vif variants from different patients revealed that vif is highly conserved with the open reading frame remaining intact in vivo. It was shown that attenuated vif clones from HIV-1-infected subjects can effectively induce both humoral and cellular responses against Vif protein in mice. Evaluation of the cellular responses in vitro using human cellular targets infected with a clinical HIV-1 isolate showed that vif clones could induce cellular responses capable of destroying the virus.

CONCLUSIONS

The vif variants developed in this study exhibited non-productive phenotypes, yet were capable of inducing specific immune responses against HIV-1. These constructs could be used as part of a DNA vaccine strategy for HIV-1. This vaccine adaptation strategy could be used for the development of immunogens for any pathogen resulting in cross-reactive immunity and attenuated gene pathogenesis.

Prevalence of anti-vif antibodies in HIV-1 infected individuals assessed using recombinant baculovirus expressed vif protein

A 630 base pair fragment of the HIV-1 genome encompassing the entire vif open reading frame has been produced by the polymerase chain reaction and cloned into the baculovirus transfer vector pAcYM1. Extracts from insect cells infected with a recombinant baculovirus expressing the HIV-1 vif gene product were used in a radioimmunoassay to analyse 238 sera from HIV infected individuals for the presence of anti-vif antibodies. The overall prevalence of anti-vif antibodies in this group of patients was 25.3%. Stratification of the group according to CD4 levels showed that anti-vif antibodies were more prevalent in patients with CD4 counts below the median of the group (155 x 10(6) cells/L; P = 0.005). A significant increase in anti-vif antibodies was observed in patients with CD4 levels less than 280 x 10(6) cells/L (P < 0.01) and in patients with symptomatic HIV infection (P = 0.0003). However, there was no significant difference in the prevalence of anti-vif antibodies in patients stratified according to p24 antigen status. The implications of these findings in the context of HIV replication are discussed.

Sequence analysis of HIV-1 vif gene in Spanish isolates

We have determined the nucleotide sequence of the HIV-1 vif gene in viruses obtained from symptomatic patients of distinct risk groups in Madrid. The genetic diversity among the isolates was estimated in 4.6% (+/- 1.4 standard deviation), a similar value to that obtained for the gag gene 3.9% (+/- 0.8 standard deviation) and env 4.1% (+/- 1 standard deviation) (Rojas et al., Virus Res 31, 331-342, 1994). Amino acid sequence analysis revealed the presence of hypermutable residues at positions 101 and 167, close to antigenically relevant sequential epitopes (comprising amino acids 87-94 and 172-178). Phylogenetic analysis supports the existence of two virus lineages circulating preferentially within different risk groups.