Identification of exposed epitopes on the envelope glycoproteins of human T-cell lymphotropic virus type I (HTLV-I)

Biofilm-like extracellular viral assemblies mediate HTLV-1 cell-to-cell transmission at virological synapses

Human T cell leukemia virus type 1 (HTLV-1) is a lymphotropic retrovirus whose cell-to-cell transmission requires cell contacts. HTLV-1-infected T lymphocytes form 'virological synapses', but the mechanism of HTLV-1 transmission remains poorly understood. We show here that HTLV-1-infected T lymphocytes transiently store viral particles as carbohydrate-rich extracellular assemblies that are held together and attached to the cell surface by virally-induced extracellular matrix components, including collagen and agrin, and cellular linker proteins, such as tetherin and galectin-3. Extracellular viral assemblies rapidly adhere to other cells upon cell contact, allowing virus spread and infection of target cells. Their removal strongly reduces the ability of HTLV-1-producing cells to infect target cells. Our findings unveil a novel virus transmission mechanism based on the generation of extracellular viral particle assemblies whose structure, composition and function resemble those of bacterial biofilms. HTLV-1 biofilm-like structures represent a major route for virus transmission from cell to cell.

HTLV-1 uses HSPG and neuropilin-1 for entry by molecular mimicry of VEGF165

Human T-cell lymphotropic virus type 1 (HTLV-1) entry involves the interaction between the surface (SU) subunit of the Env proteins and cellular receptor(s). Previously, our laboratories demonstrated that heparan sulfate proteoglycans (HSPGs) and neuropilin-1 (NRP-1), a receptor of VEGF(165), are essential for HTLV-1 entry. Here we investigated whether, as when binding VEGF(165), HSPGs and NRP-1 work in concert during HTLV-1 entry. VEGF(165) binds to the b domain of NRP-1 through both HSPG-dependent and -independent interactions, the latter involving its exon 8. We show that VEGF(165) is a selective competitor of HTLV-1 entry and that HTLV-1 mimics VEGF(165) to recruit HSPGs and NRP-1: (1) the NRP-1 b domain is required for HTLV-1 binding; (2) SU binding to target cells is blocked by the HSPG-binding domain of VEGF(165); (3) the formation of Env/NRP-1 complexes is enhanced by HSPGs; and (4) the HTLV SU contains a motif homologous to VEGF(165) exon 8. This motif directly binds to NRP-1 and is essential for HTLV-1 binding to, internalization into, and infection of CD4(+) T cells and dendritic cells. These findings demonstrate that HSPGs and NRP-1 function as HTLV-1 receptors in a cooperative manner and reveal an unexpected mimicry mechanism that may have major implications in vivo.

Structural and immunogenicity analysis of chimeric B-cell epitope constructs derived from the gp46 and gp21 subunits of the envelope glycoproteins of HTLV-1

B-cell epitopes were selected from the gp21 and gp46 subunits of the envelope glycoprotein of human T-cell lymphotropic virus type 1 (HTLV-1) by computer-aided analyses of protein antigenicity. Molecular modeling was used to design and synthesize the epitopes as chimeric constructs with promiscuous T-helper epitopes derived either from the tetanus toxoid (amino acids 947-967) or measles virus fusion protein (amino acids 288-302). Circular dichroism measurements revealed that the peptides had a secondary structure that correlated well with the crystal structure data or predicted structure. The chimeric peptides were then evaluated for their immunogenicity in rabbits or mice. Antibodies against one of the epitopes derived from the gp21 subunit were found to be neutralizing in its ability to inhibit the formation of virus-induced syncytia. These studies underscore the importance of the gp21 transmembrane region for the development of vaccine candidates. The applicability of a chimeric approach is discussed in the context of recent findings regarding the role of gp21 transmembrane region in the viral fusion process.

De Novo Design of Peptide Immunogens That Mimic the Coiled Coil Region of Human T-cell Leukemia Virus Type-1 Glycoprotein 21 Transmembrane Subunit for Induction of Native Protein Reactive Neutralizing Antibodies

Peptide vaccines able to induce high affinity and protective neutralizing antibodies must rely in part on the design of antigenic epitopes that mimic the three-dimensional structure of the corresponding region in the native protein. We describe the design, structural characterization, immunogenicity, and neutralizing potential of antibodies elicited by conformational peptides derived from the human T-cell leukemia virus type 1 (HTLV-1) gp21 envelope glycoprotein spanning residues 347-374. We used a novel template design and a unique synthetic approach to construct two peptides (WCCR2T and CCR2T) that would each assemble into a triple helical coiled coil conformation mimicking the gp21 crystal structure. The peptide B-cell epitopes were grafted onto the epsilon side chains of three lysyl residues on a template backbone construct consisting of the sequence acetyl-XGKGKGKGCONH2 (where X represents the tetanus toxoid promiscuous T cell epitope (TT) sequence 580-599). Leucine substitutions were introduced at the a and d positions of the CCR2T sequence to maximize helical character and stability as shown by circular dichroism and guanidinium hydrochloride studies. Serum from an HTLV-1-infected patient was able to recognize the selected epitopes by enzyme-linked immunosorbent assay (ELISA). Mice immunized with the wild-type sequence (WCCR2T) and the mutant sequence (CCR2T) elicited high antibody titers that were capable of recognizing the native protein as shown by flow cytometry and whole virus ELISA. Sera and purified antibodies from immunized mice were able to reduce the formation of syncytia induced by the envelope glycoprotein of HTLV-1, suggesting that antibodies directed against the coiled coil region of gp21 are capable of disrupting cell-cell fusion. Our results indicate that these peptides represent potential candidates for use in a peptide vaccine against HTLV-1.

Epitope mapping of human monoclonal antibodies recognizing conformational epitopes within HTLV type 1 gp46, employing HTLV type 1/2 envelope chimeras

The majority of the antibody response to HTLV-1 surface glycoprotein, gp46, is directed at conformational epitopes. However, the regions of HTLV-1 gp46 that contain conformational epitopes are poorly defined. We previously reported on human monoclonal antibodies (hMAbs) to conformational epitopes within the HTLV-1 surface glycoprotein (gp46) that inhibit HTLV-1-mediated syncytium formation (Hadlock KG, Rowe J, Perkins S, et al.: J Virol 1997;71:5828-5840). To localize the conformational epitopes recognized by these antibodies, chimeric envelope proteins were constructed in which selected regions of the HTLV-1 envelope were replaced with the corresponding sequences from other members of the HTLV family of retroviruses. The chimeras were tested for reactivity with three hMAbs to conformational epitopes in HTLV-1 gp46, PRH-7A, PRH-3, and PRH-4, and one hMAb to a linear epitope, 0.5alpha. hMAb PRH-3 was specifically nonreactive with a chimera that replaced amino acids 32-36 of HTLV-1 gp46 and exhibited sharply reduced reactivity with a chimera that replaced amino acids 224-251 of HTLV-1 with the corresponding HTLV-2 sequence. hMAb PRH-4 was specifically nonreactive with a construct replacing amino acids 1-162 of HTLV-1 gp46 with the corresponding HTLV-2 sequence. Thus, HTLV-1 gp46 contains multiple conformational epitopes located in the amino-terminal portion of the protein.

Interaction of CD82 tetraspanin proteins with HTLV-1 envelope glycoproteins inhibits cell-to-cell fusion and virus transmission

The entry of retroviruses into their target cell involves interactions between the virus envelope glycoproteins and their cellular receptors, as well as accessory ligand-receptor interactions involving adhesion molecules that can also participate in fusion. We have studied the contribution of CD82 proteins to the transmission of the human T-cell leukemia virus type 1 (HTLV-1), which is greatly dependent on cell-to-cell contacts. CD82 proteins belong to a class of cell surface molecules, the tetraspanins, that can act as molecular facilitators in cellular adhesion processes. The coexpression of CD82 proteins with HTLV-1 envelope glycoproteins resulted in marked inhibition of syncytium formation, whereas CD82 proteins had no effect on syncytium formation induced by human immunodeficiency virus type 1 (HIV-1) envelope proteins. The presence of CD82 proteins also inhibited cell-to-cell transmission of HTLV-1. Coimmunoprecipitation and cocapping experiments showed that CD82 associates with HTLV-1 envelope glycoproteins, both within the cell and at the cell surface. Finally, whereas the intracellular maturation of HTLV-1 glycoproteins was not modified by the presence of CD82 proteins, HTLV-1 protein coproduction delayed the intracellular maturation of CD82 proteins. There thus seems to be a reciprocal interaction between virus and cell proteins, and the cellular proteins involved in adhesion modulate retrovirus transmission both positively, as shown in other systems, and negatively, as shown here.

Influence of amino acid substitutions on antigenicity of immunodominant regions of the HTLV type I envelope surface gylcoprotein: a study using monoclonal antibodies raised against relevant peptides

By the use of sera of human T cell leukemia virus type I (HTVL-I)-infected individuals it was shown that amino acid substitutions at positions 192 (proline to serine) and 250 (serine to proline) in major immunodominant regions (175-199 and 239-261) of the surface envelope glycoprotein (gp46) of the virus may influence the humoral response. Since human sera are polyclonal in nature, one cannot readily discriminate between an immunoglobulin-specific recognition and multiple bindings of diverse antibodies. To overcome this difficulty we generated murine monoclonal antibodies to synthetic peptides mimicking all or portions of these gp46 regions. The reactivity of some of these antibodies to synthetic peptides harboring (or not harboring) the preceding amino acid substitutions at position 192 or 250, to denatured gp46 by Western blotting, and to live (variously substituted) HTLV-I-infected cells, combined with blocking experiments with various peptides, allow us to conclude that the major epitopes (positions 183-191, 190-197, 190-199, and 246-252) in the two immunodominant regions may elicit different antibody responses according to their sequences. It is worth noting that in a reporter gene inhibition assay, it was found that a neutralizing monoclonal antibody (MF1), the epitope for which is located between residues 190 and 197, had a high level of activity when cells (2060) harboring a gp46 with proline at position 192 were used and had no activity toward cells (1010) with a serine at this position. Therefore our results establish that certain amino acid substitutions of gp46 may drastically affect the antigenicity of the molecule and the biological activity of the antibodies elicited.

Early assembly step of a retroviral envelope glycoprotein: analysis using a dominant negative assay

As for most integral membrane proteins, the intracellular transport of retroviral envelope glycoproteins depends on proper folding and oligomeric assembly in the ER. In this study, we considered the hypothesis that a panel of 22 transport-defective mutants of the human T cell leukemia virus type 1 envelope glycoprotein might be defective in ER assembly. Upon cell cotransfection with wild-type envelope, however, the vast majority of these transport-defective mutants (21 of 22) exerted a specific trans-dominant negative effect. This effect was due to random dimerization of the mutated and wild-type glycoproteins that prevented the intracellular transport of the latter. This unexpected result suggests that association of glycoprotein monomers precedes the completion of folding. The only mutation that impaired this early assembly was located at the NH2 terminus of the protein. COOH-terminally truncated, soluble forms of the glycoprotein were also trans-dominant negative provided that their NH2 terminus was intact. The leucine zipper-like domain, although involved in oligomerization of the envelope glycoproteins at the cell surface, did not contribute to their intracellular assembly. We propose that, at a step subsequent to translation, but preceding complete folding of the monomers, glycoproteins assemble via their NH2-terminal domains, which, in turn, permits their cooperative folding.

The humoral immune response to human T-cell lymphotropic virus type 1 envelope glycoprotein gp46 is directed primarily against conformational epitopes

Individuals infected with human T-cell lymphotropic virus type 1 (HTLV-1) develop a robust immune response to the surface envelope glycoprotein gp46 that is partially protective. The relative contribution of antibodies to conformation-dependent epitopes, including those mediating virus neutralization as part of the humoral immune response, is not well defined. We assess in this report the relationship between defined linear and conformational epitopes and the antibodies elicited to these domains. First, five monoclonal antibodies to linear epitopes within gp46 were evaluated for their ability to abrogate binding of three human monoclonal antibodies that inhibit HTLV-1-mediated syncytia formation and recognize conformational epitopes. Binding of antibodies to conformational epitopes was unaffected by antibodies to linear epitopes throughout the carboxy-terminal half and central domain of HTLV-1 gp46. Second, an enzyme-linked immunoadsorbent assay was developed and used to measure serum antibodies to native and denatured gp46 from HTLV-1-infected individuals. In sera from infected individuals, reactivity to denatured gp46 had an average of 15% of the reactivity observed to native gp46. Third, serum antibodies from 24 of 25 of HTLV-1-infected individuals inhibited binding of a neutralizing human monoclonal antibody, PRH-7A, to a conformational epitope on gp46 that is common to HTLV-1 and -2. Thus, antibodies to conformational epitopes comprise the majority of the immune response to HTLV-1 gp46, and the epitopes recognized by these antibodies do not appear to involve sequences in previously described immunodominant linear epitopes.