Heparin-binding capacity of the HIV-1 NEF-protein allows one-step purification and biochemical characterization

Immunogenicity of chloroplast-derived HIV-1 p24 and a p24-Nef fusion protein following subcutaneous and oral administration in mice

High-level expression of foreign proteins in chloroplasts of transplastomic plants provides excellent opportunities for the development of oral vaccines against a range of debilitating or fatal diseases. The HIV-1 capsid protein p24 and a fusion of p24 with the negative regulatory protein Nef (p24-Nef) accumulate to ∼4% and ∼40% of the total soluble protein of leaves of transplastomic tobacco (Nicotiana tabacum L.) plants. This study has investigated the immunogenicity in mice of these two HIV-1 proteins, using cholera toxin B subunit as an adjuvant. Subcutaneous immunization with purified chloroplast-derived p24 elicited a strong antigen-specific serum IgG response, comparable to that produced by Escherichia coli-derived p24. Oral administration of a partially purified preparation of chloroplast-derived p24-Nef fusion protein, used as a booster after subcutaneous injection with either p24 or Nef, also elicited strong antigen-specific serum IgG responses. Both IgG1 and IgG2a subtypes, associated with cell-mediated Th1 and humoral Th2 responses, respectively, were found in sera after subcutaneous and oral administration. These results indicate that chloroplast-derived HIV-1 p24-Nef is a promising candidate as a component of a subunit vaccine delivered by oral boosting, after subcutaneous priming by injection of p24 and/or Nef.

Plant-based strategies aimed at expressing HIV antigens and neutralizing antibodies at high levels. Nef as a case study

The first evidence that plants represent a valid, safe and cost-effective alternative to traditional expression systems for large-scale production of antigens and antibodies was described more than 10 years ago. Since then, considerable improvements have been made to increase the yield of plant-produced proteins. These include the use of signal sequences to target proteins to different cellular compartments, plastid transformation to achieve high transgene dosage, codon usage optimization to boost gene expression, and protein fusions to improve recombinant protein stability and accumulation. Thus, several HIV/SIV antigens and neutralizing anti-HIV antibodies have recently been successfully expressed in plants by stable nuclear or plastid transformation, and by transient expression systems based on plant virus vectors or Agrobacterium-mediated infection. The current article gives an overview of plant expressed HIV antigens and antibodies and provides an account of the use of different strategies aimed at increasing the expression of the accessory multifunctional HIV-1 Nef protein in transgenic plants.

Anchorage to the cytosolic face of the endoplasmic reticulum membrane: a new strategy to stabilize a cytosolic recombinant antigen in plants

The levels of accumulation of recombinant vaccines in transgenic plants are protein specific and strongly influenced by the subcellular compartment of destination. The human immunodeficiency virus protein Nef (negative factor), a promising target for the development of an antiviral vaccine, is a cytosolic protein that accumulates to low levels in transgenic tobacco and is even more unstable when introduced into the secretory pathway, probably because of folding defects in the non-cytosolic environment. To improve Nef accumulation, a new strategy was developed to anchor the molecule to the cytosolic face of the endoplasmic reticulum (ER) membrane. For this purpose, the Nef sequence was fused to the C-terminal domain of mammalian ER cytochrome b5, a long-lived, tail-anchored (TA) protein. This consistently increased Nef accumulation by more than threefold in many independent transgenic tobacco plants. Real-time polymerase chain reaction of mRNA levels and protein pulse-chase analysis indicated that the increase was not caused by higher transcript levels but by enhanced protein stability. Subcellular fractionation and immunocytochemistry indicated that Nef-TA accumulated on the ER membrane. Over-expression of mammalian or plant ER cytochrome b5 caused the formation of stacked membrane structures, as observed previously in similar experiments performed in mammalian cells; however, Nef-TA did not alter membrane organization in tobacco cells. Finally, Nef could be removed in vitro by its tail-anchor, taking advantage of an engineered thrombin cleavage site. These results open up the way to use tail-anchors to improve foreign protein stability in the plant cytosol without perturbing cellular functions.

Extracellular HIV-1 Nef increases migration of monocytes

Infiltration of human immunodeficiency virus type 1 (HIV-1)-infected and uninfected monocytes/macrophages in organs and tissues is a general phenomenon observed in progression of acquired immunodeficiency syndrome (AIDS). HIV-1 protein Nef is considered as a progression factor in AIDS, and is released from HIV-1-infected cells. Here, we show that extracellular Nef increases migration of monocytes. This effect is (i) concentration-dependent, (ii) reaches the order of magnitude of that induced by formyl-methyonyl-leucyl-proline (fMLP) or CC chemokine ligand 2 (CCL2)/monocyte chemotactic protein (MCP)-1, (iii) inhibited by anti-Nef monoclonal antibodies as well as by heating, and (iv) depends on a concentration gradient of Nef. Further, Nef does not elicit monocytic THP-1 cells to express chemokines such as CCL2, macrophage inhibitory protein-1alpha (CCL3) and macrophage inhibitory protein-1beta (CCL4). These data suggest that extracellular Nef may contribute to disease progression as well as HIV-1 spreading through affecting migration of monocytes.

Genetic immunization with multiple HIV-1 genes provides protection against HIV-1/MuLV pseudovirus challenge in vivo

Superinfection by HIV-1 of a cell line containing the complete murine leukemia virus (MuLV) genome was shown to give rise to pseudotyped HIV-1/MuLV. Such superinfection was successful with certain strains of HIV-1 subtypes A-D. Primary spleen cells and cells of the peritoneal cavity of immunocompetent mice of the C57Bl/6 strain were infectable with the pseudotype HIV-1/MuLV and secreted HIV-1 in vitro and in vivo. In contrast, the murine cell lines, NIH 3T3, myeloma cell line Sp2/0, and two murine hybridoma cell lines were relatively resistant to infection and produced no or little HIV. After primary murine spleen cells had been infected with pseudotyped HIV-1 and transferred to C57Bl/6 mice, replication-competent HIV-1 was obtained from the peritoneal cavity for at least 10-14 days. High amounts (> 10(5) vRNA copies/ml) of HIV-1 vRNA could be measured in the peritoneal fluid. Presence of HIV-1 proviral DNA was detectable in cells from the peritoneal cavity for up to 24 days after infected cell transfer. Active reverse transcriptase representing both HIV-1 and C-type murine retroviruses was detected in the peritoneal washes. The HIV-infected spleen cells injected into the peritoneal cavity elicited HIV-1-specific cellular immune responses to p24gag, gp160Env, Nef, Tat and Rev. Mice immunized with HIV-1 DNA, but not with HIV-1 protein, cleared their HIV-1-infected cells within 10-14 days after challenge with HIV-1/MuLV-infected syngeneic spleen cells. This novel model system of primarily cellular reactivity to HIV-1-infected cells in vivo may become useful for assaying experimental HIV-1 immunization schedules.

HIV-1 Tat increases the adhesion of monocytes and T-cells to the endothelium in vitro and in vivo: implications for AIDS-associated vasculopathy

HIV-1-infected patients exhibit severe damages of the aortic endothelium, develop angioproliferative lesions such as Kaposi's sarcoma (KS), and have an increased risk of cardiovascular diseases and atherosclerosis. An increased adhesion of leukocytes to the endothelium is a common pathogenic parameter of AIDS-associated vascular diseases. Here we show that the HIV-1 Tat protein, a regulatory protein of HIV-1 released by infected cells, and TNF-alpha, a cytokine increased in sera and tissues of HIV-1-infected patients, activate synergistically the adhesion of leukocytes to endothelial cells both in vitro and in vivo. This effect is selectively mediated by HIV-1 Tat, since HIV-1 Nef, another HIV-1 regulatory protein, and the HIV-1 envelope protein gp41, had no effect. In vitro adhesion assays with PBMC and quantitative cell type analysis of adherent cells by FACS demonstrated that HIV-1 Tat selectively activates the adhesion of T-cells and monocytes but not of B-cells. Intravital microscopic studies in mice confirmed the synergistic activity of HIV-1 Tat and TNF-alpha on leukocyte adhesion to the endothelium in vivo. These data indicate that HIV-1 Tat in cooperation with TNF-alpha may contribute to the vascular damage and cardiovascular diseases observed in AIDS patients but also to the prominent extravasation of T-cells and monocytes which is a key process in the formation and progression of KS lesions.

Therapeutic vaccination with MVA-HIV-1 nef elicits Nef-specific T-helper cell responses in chronically HIV-1 infected individuals

Vaccination is currently considered as an additional therapeutic approach to stimulate HIV-specific immune response in subjects that could not naturally control HIV. Ten chronically HIV infected individuals have been vaccinated with a modified vaccinia Ankara (MVA)-HIV-1(LAI)-nef vector in order to assess safety and immunogenicity. No significant adverse effects were observed during the course of vaccination indicating for the first time that the highly attenuated vaccinia-virus vector MVA is safe in HIV-1 infected individuals. We observed a CD4 T-cell response to Nef in the majority of vaccinated chronically HIV infected individuals. In two subjects CD4 T-cell response was directed to previously unidentified Nef epitopes. The strong Nef-specific CD4 T-cell response elicited by MVA-nef vaccination provides a rationale for immunotherapeutic interventions in HIV infected individuals with suppressed CD4 T-cell responses. Moreover, the CD4 T-cell response elicited was comparable with that usually detected in long-term non-progressor (LTNP) suggesting an improvement in the immunological status of the vaccinated subjects. Furthermore, the new putative CD4 epitopes described here hold promise as important tools for epitope-based vaccination.

Two-step purification of His-tagged Nef protein in native condition using heparin and immobilized metal ion affinity chromatographies

The Nef protein encoded by human immunodeficiency virus type 1 (HIV-1) has been shown to be an important factor of progression of viral growth and pathogenesis in both in vitro and in vivo. The lack of a simple procedure to purify Nef in its native conformation has limited molecular studies on Nef function. A two-step procedure that includes heparin and immobilized metal ion affinity chromatographies (IMACs) was developed to purify His-tagged Nef (His(6)-Nef) expressed in bacteria in native condition. During the elaboration of this purification procedure, we identified two closely SDS-PAGE-migrating contaminating bacterial proteins, SlyD and GCHI, that co-eluted with His(6)-Nef in IMAC in denaturing condition and developed purification steps to eliminate these contaminants in native condition. Overall, this study describes a protocol that allows rapid purification of His(6)-Nef protein expressed in bacteria in native condition and that removes metal affinity resin-binding bacterial proteins that can contaminate recombinant His-tagged protein preparation.

DNA vaccination in mice using HIV-1 nef, rev and tat genes in self-replicating pBN-vector

The immunogenicity of a self-replicating DNA-vector containing HIV-1 nef gene (pBN-Nef) was characterized using various DNA delivery methods. In addition, gene gun immunisation was used for assessing immunogenicity of two other HIV-1 genes (rev and tat) given in the same vector. The pBN-Nef was the most immunogenic raising both humoral and cell-mediated immune responses in mice; these responses lasted for up to six months. The pBN-Nef vector was immunogenic also when given intramuscularly or intradermally. The pBN-Rev construct did not elicit humoral responses but did elicit proliferative as well as CTL-response against the corresponding protein. The pBN-Tat was a poor immunogen in all respects. The antibodies elicited with various DNA delivery methods belonged to different antibody subclasses; however, two main epitopes in Nef were frequently recognized by all of them.

Human immunodeficiency virus type-1 Nef protein induces blood-brain barrier disruption in the rat: role of matrix metalloproteinase-9

We recently showed that MMP-9 activity was detectable in the cerebrospinal fluid (CSF) of about half of neurologically symptomatic HIV-infected patients. Using an experimental animal model, we detected MMP-9 activity in CSF samples from rats that had been injected intracisternally with recombinant HIV-1 Nef protein, but not after injection of heat-treated Nef, gp120, gp160 or PBS. Nef also induced a breaching of the blood-brain barrier (BBB), which could be inhibited by pretreatment with the matrix metalloproteinase (MMP) inhibitor batimastat. In vitro Nef only slightly induced MMP-9 activity in freshly isolated human peripheral blood mononuclear cells and in the murine macrophage cell line RAW 264.7, but not in endothelial, neuronal or astroglial cell lines. Taken together, our findings indicate that HIV-1 Nef protein can induce BBB disruption in the rat - presumably via MMP induction.

Stable expression of HIV-1 Nef induces changes in growth properties and activation state of human astrocytes

OBJECTIVE

Nef was shown to be the predominant viral protein expressed in HIV-1-infected astrocytes in vivo and in vitro suggesting a distinct role of Nef in this cell type. Nef-induced activation of T cells is well described, whereas the functional activities of Nef in astrocytes are unknown. Our aim was to examine the effect of Nef on growth properties and activation of astrocytes.

DESIGN

Human Nef-expressing astrocytic cell lines were established by stable transfection with different wild-type and mutant nef genes derived from laboratory isolates and brain tissue.

METHODS

Nef-expressing astrocytes were characterized in terms of growth properties (proliferation, growth in soft agar, focus formation) and morphology. Apoptotic cell death and expression of activation markers were determined by fluorescent antibody cell sorting.

RESULTS

Astrocytic cell lines revealed persistent Nef expression--detectable at the levels of mRNA and protein--and showed altered growth properties and morphology. Elevated expression of activation markers such as glial fibrillary acidic protein and CD88 (complement receptor C5a) was observed; these are regarded as markers for inflammatory processes in the brain. This effect was independent of the nef type or the expression level of the Nef protein. In contrast with previous reports no evidence for increased apoptotic cell death was found in astrocytes expressing Nef stably.

CONCLUSIONS

Our findings suggest that Nef changes the cellular properties of astrocytes, thus contributing to astrocyte activation and induction of astrogliosis in the central nervous system of individuals with AIDS.

HIV Type 1 Nef Protein Is a Viral Factor for Leukocyte Recruitment into the Central Nervous System

Recombinant HIV-1 Nef protein, but not Tat, gp120, and gp160, provoked leukocyte recruitment into the CNS in a rat model. The strong reduction of bioactivity by heat treatment of Nef, and the blocking effect of the mAb 2H12, which recognizes the carboxy-terminal amino acid (aa) residues 171–190 (but not of mAb 3E6, an anti-Nef Ab of the same isotype, which maps the aa sequence 168–175, as well as a mixture of mAbs to CD4) provided evidence for the specificity of the observed Nef effects. Using a modified Boyden chamber technique, Nef exhibited chemotactic activity on mononuclear cells in vitro. Coadministration of the anti-Nef mAb 2H12, as well as treatment of Nef by heat inhibited Nef-induced chemotaxis. Besides soluble Nef, chemotaxis was also induced by a Nef-expressing human astrocytoma cell line, but not by control cells. These data suggest a direct chemotactic activity of soluble Nef. The detection of elevated levels of IL-6, TNF-α, and IFN-γ in rat cerebrospinal fluid 6 h after intracisternal Nef injection hint at the additional involvement of indirect mechanisms in Nef-induced leukocyte migration into rat CNS. These data propose a mechanism by which HIV-1 Nef protein may be essential for AIDS neuropathogenesis, as a mediator of the recruitment of leukocytes that may serve as vehicles of the virus and perpetrators for disease through their production of neurotoxins.

Recognition of prominent viral epitopes induced by immunization with human immunodeficiency virus type 1 regulatory genes

Mice immunized with the regulatory genes nef, rev, and tat from human immunodeficiency virus type 1 developed both humoral and cellular immune responses to the gene products Nef, Rev, and Tat. This study demonstrates that it is feasible to induce immune reactions to all of these regulatory gene products. Humoral responses were seen after DNA boosts, while potent T-cell proliferative responses were noted already after a single immunization. A Th1-directed immune response was demonstrated early after immunization. A 3- to 75-fold-stronger T-cell response was seen in animals receiving DNA epidermally compared to that in animals receiving intramuscular injections. Nef, Rev, and Tat putative B- and T-cell epitopes were clearly mapped by using peptides derived from the regulatory proteins and were similar to those which are detected in human immunodeficiency virus infection. Although immunization by the Nef, Rev, and Tat proteins raised high immunoglobulin G titers in serum, the epitope spreading appeared broader after DNA immunization. The combination of all of these regulatory genes together with two genes for structural proteins, the envelope and gag genes, demonstrated that a combined approach is feasible in that reactivities to all antigens persisted or were even augmented. No interference between plasmids was noted.