Immunological study of the nef protein from HIV-1 by polyclonal and monoclonal antibodies

Monoclonal antibodies recognize at least five epitopes on the SIV Nef protein and identify an in vitro-induced mutation

Eleven monoclonal antibodies (MAbs) to SIV Nef were produced and characterized. Five antibody-binding sites on SIV Nef were identified on the basis of the reactivity of the antibodies with recombinant proteins. Two of the five epitopes were defined using overlapping peptides. A further three epitopes could not be defined with peptides but all antibodies reacted in Western blot, suggesting that the epitopes were at least partially conformation dependent. Antibodies in two of the five epitope groups were further differentiated by competition analysis. The panel of MAbs described is able to distinguish between a number of recombinant Nef proteins currently under investigation in vivo in macaques. Two of the MAbs described are able to distinguish between the Nef protein from pathogenic (J5) and attenuated (C8) strains of SIV, thus providing useful tools for studying the relevance of the Nef protein in the pathogenesis of SIV infection. In FACScan analysis two of the MAbs, KK70 and KK75, were used to identify an in vitro-induced mutation in J5 Nef grown in C8166 cells. Sequence analysis of the phenotypic variants identified a mutation of the tryptophan (TGG) at amino acid 214 to a stop codon (TGA), thus truncating the Nef protein. The functional significance of this observation remains unclear but highlights the need to interpret data with caution if virus has been cultured in vitro even for a short period of time.

Association of human immunodeficiency virus Nef protein with actin is myristoylation dependent and influences its subcellular localization

Human immunodeficiency virus (HIV) Nef functions are thought to be mediated via interactions with cellular proteins. Utilizing zone velocity sedimentation in glycerol gradients we found that recombinant HIV-1 Nef non-covalently associates with actin forming a high-molecular-mass complex of 150-300 kDa. This Nef/actin complex was present in human B and T lymphocytes but not in insect cells and was dependent on the N-terminal myristoylation of Nef, whereas the SH3-binding proline motif of Nef was not involved. Despite being myristoylated, HIV-2 Nef did not associate with actin. This might reflect differences in the subcellular localization of Nef since cell-fractionation experiments revealed that HIV-1 Nef was virtually exclusively localized in the cytoskeletal (detergent-insoluble) fraction whereas HIV-2 Nef had significantly reduced affinity for the cytoskeleton. Colocalization experiments in HIV-1-infected CD4+ fibroblasts revealed that Nef/actin complexes may also exist in HIV-infected cells. This novel interaction of HIV-1 Nef with actin provides insight into the association of Nef with cellular structures and reveals general differences in the interactions of the Nef proteins from HIV-1 and HIV-2.

Cytotoxicity resulting from addition of HIV-1 Nef N-terminal peptides to yeast and bacterial cells

The Nef protein of human immunodeficiency type 1 (HIV-1) has been implicated in diverse intracellular functions; however, extracellular functions have been less studied. Nef and the N-terminus of Nef possess membrane-perturbing and fusogenic activities in artificial membranes that also cause cytotoxicity to human cells, including lymphocytes. The present study investigates the toxicity of HIV-1 Nef peptides employing yeast and bacterial cells. The N-terminal portion of Nef was found to cause cell killing in Escherichia coli and in a variety of yeast cells. This activity was enhanced by myristylation of the Nef N-terminus, a modification that did not lead to toxicity in a control peptide. Cell death in yeast was due to permeabilization of the cell membrane as determined by the propidium iodide uptake of peptide-treated cells. Extracellular Nef, or its breakdown products, may have effects similar to the Nef peptides described here and could be responsible, at least in part, for the death of cells in lymphoid tissues during AIDS. Assays using yeast or bacteria are convenient, inexpensive, and robust and should be useful in further analysis and screening of inhibitors of this activity associated with HIV-1 Nef.

Subcloning of HIV-2 nef genes in E. coli and immunological reactivity of expressed fusion proteins

The nef gene located in the 3' region of the HIV-2 genome encodes an N-terminally myristylated protein of 27-35 kD, likely to be involved in the regulation of viral transcription. The nef genes of HIV-2 isolates GH-1, ROD, ST, BEN, and D194.17 were inserted into E. coli pEX vectors and expression of Nef beta-galactosidase fusion proteins was detected in stained gels. All fusion proteins specifically reacted with a rabbit serum raised against bacterially expressed Nef from HIV-2D194.17. Sera from monkeys inoculated with HIV-2BEN or SIVMAC251 recognized the Nef proteins of only certain HIV-2 isolates. No cross-reactivity of these sera with HIV-1 Nef and of a rabbit anti-HIV-1-Nef serum with the described HIV-2 Nef fusion proteins was observed.

Oligomerization of the Nef protein from human immunodeficiency virus (HIV) type 1

The nef genes, derived from two different human immunodeficiency-virus-type-1 (HIV-1) strains, were expressed in procaryotic cells (Escherichia coli) and in eucaryotic cells (insect cells infected with nef-containing baculovirus). The oligomerization of recombinant Nef protein was studied by NMR spectroscopy and immunoblotting under various experimental conditions. 1H-NMR spectroscopy shows that native folded protein has the tendency to polymerize under low-salt conditions. These oligomers become covalently linked by disulfide bonds after decreasing the reduction potential, a process which is fully reversible. Cross-linking studies with bis(sulfo-succinimidyl)suberate and alkylation with iodoacetic acid under non-reducing and reducing conditions document for the first time that Nef can also form homomeric structures including monomers, dimers, trimers and tetramers in cell lysates and intact cells. We found disulfide-linked as well as non-covalently associated oligomers. Since the Nef molecules are not exclusively found in the cytoplasm of HIV infected cells and since the reduced glutathione concentration in lymphocytes of virus infected persons is known to be unusually low, it might be possible that these Nef oligomers have a biological function in vivo as well.

Fine mapping of HIV-1 Nef-epitopes by monoclonal antibodies

A panel of newly isolated murine monoclonal antibodies is described which are specific for the Nef protein of the human immunodeficiency virus type 1 (HIV-1). Epitope mapping using recombinant Nef-related proteins, synthetic peptides and lipopeptides showed 3 independent antigenic determinants located within the regions of amino acids 83-93, 175-190 and 86-166 of the Nef protein. None of the monoclonal antibodies reacted with recombinant Nef proteins of HIV-2.

DNA damage in human T-lymphoblastoid cell line Molt-3 induced by reactive oxygen species

In the course of studying the effect of reactive oxygen species such as superoxide anion (O2-), hydrogen peroxide (H2O2) on oxidant-sensitive human T-lymphoblastoid cell line Molt-3, O2- has been generated by the interaction of xanthine with xanthine oxidase (XOD). To confirm that H2O2 is a key intermediate for inducing DNA single-strand breaks in Molt-3 cells, studies have been carried out with pure H2O2. In the presence of xanthine + XOD or H2O2 the amount of DNA single-strand breaks has been found to increase as a function of the O2- and H2O2 concentration. Data from studies with antioxidants such as superoxide dismutase and catalase supports a mechanism of DNA damage dependent on the presence of H2O2 in Molt-3 cells. Molt-3 cells are CD4+ and sensitive to reactive oxygen stress and therefore, could be an ideal cell line for determining the relationship between oxidative stress and various diseases such as acquired immunodeficiency syndrome (AIDS).

Expression and cellular localization of the Nef protein from human immunodeficiency virus-1 in stably transfected B-cells

Nef protein, encoded by the regulatory nef gene of human immunodeficiency virus type 1 (HIV-1), was expressed in the B-cell line Raji. The cells were stably transfected with plasmids containing the nef transcriptional cassette. They expressed Nef with an Mr of 27,000; the yield could be augmented by incubation with the tumor promoter 12-O-tetradecanoylphorbol-13-acetate. The intracellular localization of Nef was analyzed applying immunofluorescence microscopy using a confocal laser scanning microscope. The antigen was stained with a monoclonal antibody directed against the N-terminal part of Nef. The experiments revealed that in non-dividing cells Nef is present both in the cytoplasm and the nucleus while in dividing cells the viral protein is present in the cytoplasm and at the nuclear membrane.

Expression of the HIV-1 Nef protein in the baculovirus system: investigation of anti-Nef antibodies response in human sera and subcellular localization of Nef

The nef gene of HIV-1 was expressed in insect cells using the eucaryotic baculovirus system. The recombinant Nef protein frequently reacted with seropositive sera of HIV-1 and HIV-2 infected patients. Anti-Nef antibodies in HIV-1 seronegative high risk groups individuals were only occasionally seen. Confocal laser scanning microscopy demonstrated that Nef is present both in the cytoplasm and in the nucleus, indicating that Nef might directly function on gene expression.