Conformation-specific antibodies targeting the trimer-of-hairpins motif of the human T-cell leukemia virus type 1 transmembrane glycoprotein recognize the viral envelope but fail to neutralize viral entry

Advances in preventive vaccine development against HTLV-1 infection: A systematic review of the last 35 years

Introduction

The Human T-lymphotropic virus type 1 (HTLV-1) was the first described human retrovirus. It is currently estimated that around 5 to 10 million people worldwide are infected with this virus. Despite its high prevalence, there is still no preventive vaccine against the HTLV-1 infection. It is known that vaccine development and large-scale immunization play an important role in global public health. To understand the advances in this field we performed a systematic review regarding the current progress in the development of a preventive vaccine against the HTLV-1 infection.

Methods

This review followed the Preferred Reporting Items for Systematic Reviews and Meta-analyses (PRISMA®) guidelines and was registered at the International Prospective Register of Systematic Reviews (PROSPERO). The search for articles was performed in PubMed, Lilacs, Embase and SciELO databases. From the 2,485 articles identified, 25 were selected according to the inclusion and exclusion criteria.

Results

The analysis of these articles indicated that potential vaccine designs in development are available, although there is still a paucity of studies in the human clinical trial phase.

Discussion

Although HTLV-1 was discovered almost 40 years ago, it remains a great challenge and a worldwide neglected threat. The scarcity of funding contributes decisively to the inconclusiveness of the vaccine development. The data summarized here intends to highlight the necessity to improve the current knowledge of this neglected retrovirus, encouraging for more studies on vaccine development aiming the to eliminate this human threat.

Systematic review registration

https://www.crd.york.ac.uk/prospero, identifier (CRD42021270412).

Cellular Factors Involved in HTLV-1 Entry and Pathogenicit

Human T cell leukemia virus type 1 (HTLV-1) is the causative agent of adult T cell leukemia (ATL) and HTLV-1 – associated myelopathy and tropical spastic paraparesis (HAM/TSP). HTLV-1 has a preferential tropism for CD4 T cells in healthy carriers and ATL patients, while both CD4 and CD8 T cells serve as viral reservoirs in HAM/TSP patients. HTLV-1 has also been detected other cell types, including monocytes, endothelial cells, and dendritic cells. In contrast to the limited cell tropism of HTLV-1 in vivo, the HTLV receptor appears to be expressed in almost all human or animal cell lines. It remains to be examined whether this cell tropism is determined by host factors or by HTLV-1 heterogeneity. Unlike most retroviruses, cell-free virions of HTLV-1 are very poorly infectious. The lack of completely HTLV-1-resistant cells and the low infectivity of HTLV-1 have hampered research on the HTLV entry receptor. Entry of HTLV-1 into target cells is thought to involve interactions between the env (Env) glycoproteins, a surface glycoprotein (surface unit), and a transmembrane glycoprotein. Recent studies have shown that glucose transporter GLUT1, heparan sulfate proteoglycans (HSPGs), and neuropilin-1 (NRP-1) are the three proteins important for the entry of HTLV-1. Studies using adherent cell lines have shown that GLUT1 can function as a receptor for HTLV. HSPGs are required for efficient entry of HTLV-1 into primary CD4 T cells. NRP-1 is expressed in most established cell lines. Further studies have shown that these three molecules work together to promote HTLV-1 binding to cells and fusion of viral and cell membranes. The virus could first contact with HSPGs and then form complexes with NRP-1, followed by association with GLUT1. It remains to be determined whether these three molecules can explain HTLV-1 cell tropism. It also remains to be more definitively proven that these molecules are sufficient to permit HTLV-1 entry into completely HTLV-1-resistant cells.

Molecular aspects of HTLV-1 entry: functional domains of the HTLV-1 surface subunit (SU) and their relationships to the entry receptors

The initial step in retroviral infection involves specific interactions between viral envelope proteins (Env) and specific receptors on the surface of target cells. For many years, little was known about the entry receptors for HTLV-1. During this time, however, functional domains of the HTLV-1 Env were identified by analyzing the effects of neutralizing antibodies and specific mutations in Env on HTLV-1 infectivity. More recent studies have revealed that HTLV-1 infectivity involves interactions with three different molecules: heparan sulfate proteoglycans (HSPG), the VEGF-165 receptor Neuropilin 1 (NRP-1) and glucose transporter type 1 (GLUT1). Here, we revisit previously published data on the functional domains of Env in regard to the recent knowledge acquired about this multi-receptor complex. We also discuss the similarities and differences between HTLV-1 and other deltaretroviruses in regards to receptor usage.

Antibodies to the envelope glycoprotein of human T cell leukemia virus type 1 robustly activate cell-mediated cytotoxic responses and directly neutralize viral infectivity at multiple steps of the entry process

Infection of human cells by human T cell leukemia virus type 1 (HTLV-1) is mediated by the viral envelope glycoproteins. The gp46 surface glycoprotein binds to cell surface receptors, including heparan sulfate proteoglycans, neuropilin 1, and glucose transporter 1, allowing the transmembrane glycoprotein to initiate fusion of the viral and cellular membranes. The envelope glycoproteins are recognized by neutralizing Abs and CTL following a protective immune response, and therefore, represent attractive components for a HTLV-1 vaccine. To begin to explore the immunological properties of potential envelope-based subunit vaccine candidates, we have used a soluble recombinant surface glycoprotein (gp46, SU) fused to the Fc region of human IgG (sRgp46-Fc) as an immunogen to vaccinate mice. The recombinant SU protein is highly immunogenic and induces high titer Ab responses, facilitating selection of hybridomas that secrete mAbs targeting SU. Many of these mAbs recognize envelope displayed on the surface of HTLV-1-infected cells and virions and several of the mAbs robustly antagonize envelope-mediated membrane fusion and neutralize pseudovirus infectivity. The most potently neutralizing mAbs recognize the N-terminal receptor-binding domain of SU, though there is considerable variation in neutralizing proficiency of the receptor-binding domain-targeted mAbs. By contrast, Abs targeting the C-terminal domain of SU tend to lack robust neutralizing activity. Importantly, we find that both neutralizing and poorly neutralizing Abs strongly stimulate neutrophil-mediated cytotoxic responses to HTLV-1-infected cells. Our data demonstrate that recombinant forms of SU possess immunological features that are of significant utility to subunit vaccine design.

Highly specific inhibition of leukaemia virus membrane fusion by interaction of peptide antagonists with a conserved region of the coiled coil of envelope

Background

Human T-cell leukaemia virus (HTLV-1) and bovine leukaemia virus (BLV) entry into cells is mediated by envelope glycoprotein catalyzed membrane fusion and is achieved by folding of the transmembrane glycoprotein (TM) from a rod-like pre-hairpin intermediate to a trimer-of-hairpins. For HTLV-1 and for several virus groups this process is sensitive to inhibition by peptides that mimic the C-terminal α-helical region of the trimer-of-hairpins.

Results

We now show that amino acids that are conserved between BLV and HTLV-1 TM tend to map to the hydrophobic groove of the central triple-stranded coiled coil and to the leash and C-terminal α-helical region (LHR) of the trimer-of-hairpins. Remarkably, despite this conservation, BLV envelope was profoundly resistant to inhibition by HTLV-1-derived LHR-mimetics. Conversely, a BLV LHR-mimetic peptide antagonized BLV envelope-mediated membrane fusion but failed to inhibit HTLV-1-induced fusion. Notably, conserved leucine residues are critical to the inhibitory activity of the BLV LHR-based peptides. Homology modeling indicated that hydrophobic residues in the BLV LHR likely make direct contact with a pocket at the membrane-proximal end of the core coiled-coil and disruption of these interactions severely impaired the activity of the BLV inhibitor. Finally, the structural predictions assisted the design of a more potent antagonist of BLV membrane fusion.

Conclusion

A conserved region of the HTLV-1 and BLV coiled coil is a target for peptide inhibitors of envelope-mediated membrane fusion and HTLV-1 entry. Nevertheless, the LHR-based inhibitors are highly specific to the virus from which the peptide was derived. We provide a model structure for the BLV LHR and coiled coil, which will facilitate comparative analysis of leukaemia virus TM function and may provide information of value in the development of improved, therapeutically relevant, antagonists of HTLV-1 entry into cells.

Resistance to neutralization by antibodies targeting the coiled coil of fusion-active envelope is a common feature of retroviruses

The human T-cell leukemia virus transmembrane glycoprotein (TM) is a typical class 1 membrane fusion protein and a subunit of the viral envelope glycoprotein complex. Following activation, the TM undergoes conformational transitions from a native nonfusogenic state to a fusion-active pre-hairpin intermediate that subsequently resolves to a compact trimer-of-hairpins or six-helix bundle. Disruption of these structural transitions inhibits membrane fusion and viral entry and validates TM as an anti-viral and vaccine target. To investigate the immunological properties of fusion-active TM, we have generated a panel of monoclonal antibodies that recognize the coiled-coil domain of the pre-hairpin intermediate. Antibody reactivity is highly sensitive to the conformation of the coiled coil as binding is dramatically reduced or lost on denatured antigen. Moreover, a unique group of antibodies are 100-1000-fold more reactive with the coiled coil than the trimer-of-hairpins form of TM. The antibodies recognize virally expressed envelope, and significantly, some selectively bind to envelope only under conditions that promote membrane fusion. Most importantly, many of the antibodies potently block six-helix bundle formation in vitro. Nevertheless, viral envelope was remarkably resistant to neutralization by antibodies directed to the coiled coil. The data imply that the coiled coil of viral envelope is poorly exposed to antibody during membrane fusion. We suggest that resistance to neutralization by antibodies directed to fusion-associated structures is a common property of retroviral TM and perhaps of other viral class I fusion proteins. These observations have significant implications for vaccine design.