Human T-cell leukemia virus type 1 (HTLV-1) was the first human retrovirus discovered. Studies on HTLV-1 have been instrumental for our understanding of the molecular pathology of virus-induced cancers. HTLV-1 is the etiological agent of an adult T-cell leukemia (ATL) and can lead to a variety of neurological pathologies, including HTLV-1-associated-myelopathy/tropical spastic paraparesis (HAM/TSP). The ability to treat the aggressive ATL subtypes remains inadequate. HTLV-1 replicates by (1) an infectious cycle involving virus budding and infection of new permissive target cells and (2) mitotic division of cells harboring an integrated provirus. Virus replication initiates host antiviral immunity and the checkpoint control of cell proliferation, but HTLV-1 has evolved elegant strategies to counteract these host defense mechanisms to allow for virus persistence. The study of the molecular biology of HTLV-1 replication has provided crucial information for understanding HTLV-1 replication as well as aspects of viral replication that are shared between HTLV-1 and human immunodeficiency virus type 1 (HIV-1). Here in this review, we discuss the various stages of the virus replication cycle—both foundational knowledge as well as current updates of ongoing research that is important for understanding HTLV-1 molecular pathogenesis as well as in developing novel therapeutic strategies.

A critical aspect of viral replication is the assembly of virus particles, which aresubsequently released as progeny virus. While a great deal of attention has beenfocused on better understanding this phase of the viral life cycle, many aspects of themolecular details remain poorly understood. This is certainly true for retroviruses,including that of the human immunodeficiency virus type 1 (HIV-1) as well as for humanT-cell leukemia virus type 1 (HTLV-1). This review discusses the retroviral Gag proteinand its interactions with itself, the plasma membrane and the role of lipids in targetingGag to virus assembly sites Recent progress using sophisticated biophysicalapproaches to investigate – in a comparative manner – retroviral Gag-Gag and Gagmembraneinteractions are discussed. Differences among retroviruses in Gag-Gag andGag-membrane interactions imply dissimilar molecular aspects of the viral assemblypathway, including the interactions of Gag with lipids at the membrane.

The Gag protein is the main retroviral structural protein, and its expression alone is usually sufficient for production of virus-like particles (VLPs). In this study, we sought to investigate—in parallel comparative analyses—Gag cellular distribution, VLP size, and basic morphological features using Gag expression constructs (Gag or Gag-YFP, where YFP is yellow fluorescent protein) created from all representative retroviral genera:Alpharetrovirus,Betaretrovirus,Deltaretrovirus,Epsilonretrovirus,Gammaretrovirus,Lentivirus, andSpumavirus. We analyzed Gag cellular distribution by confocal microscopy, VLP budding by thin-section transmission electron microscopy (TEM), and general morphological features of the VLPs by cryogenic transmission electron microscopy (cryo-TEM). Punctate Gag was observed near the plasma membrane for all Gag constructs tested except for the representativeBeta- andEpsilonretrovirusGag proteins. This is the first report ofEpsilonretrovirusGag localizing to the nucleus of HeLa cells. While VLPs were not produced by the representativeBeta- andEpsilonretrovirusGag proteins, the other Gag proteins produced VLPs as confirmed by TEM, and morphological differences were observed by cryo-TEM. In particular, we observedDeltaretrovirus-like particles with flat regions of electron density that did not follow viral membrane curvature,Lentivirus-like particles with a narrow range and consistent electron density, suggesting a tightly packed Gag lattice, andSpumavirus-like particles with large envelope protein spikes and no visible electron density associated with a Gag lattice. Taken together, these parallel comparative analyses demonstrate for the first time the distinct morphological features that exist among retrovirus-like particles. Investigation of these differences will provide greater insights into the retroviral assembly pathway.IMPORTANCEComparative analysis among retroviruses has been critically important in enhancing our understanding of retroviral replication and pathogenesis, including that of important human pathogens such as human T-cell leukemia virus type 1 (HTLV-1) and HIV-1. In this study, parallel comparative analyses have been used to study Gag expression and virus-like particle morphology among representative retroviruses in the known retroviral genera. Distinct differences were observed, which enhances current knowledge of the retroviral assembly pathway.

Human T-cell leukemia virus type 1 (HTLV-1) particle structure is poorly characterized. Here, we have used cryo-electron tomography to analyze HTLV-1 particle morphology. Particles produced from MT-2 cells were polymorphic, roughly spherical, and varied in size. Capsid cores, when present, were typically poorly defined polyhedral structures with at least one curved region contacting the inner face of the envelope membrane. However, most particles observed lacked a defined capsid core, which likely impacts HTLV-1 particle infectivity.

The Gag polyprotein is the main retroviral structural protein and is essential for the assembly and release of virus particles. In this study, we have analyzed the morphology and Gag stoichiometry of human T-cell leukemia virus type 1 (HTLV-1)-like particles and authentic, mature HTLV-1 particles by using cryogenic transmission electron microscopy (cryo-TEM) and scanning transmission electron microscopy (STEM). HTLV-1-like particles mimicked the morphology of immature authentic HTLV-1 virions. Importantly, we have observed for the first time that the morphology of these virus-like particles (VLPs) has the unique local feature of a flat Gag lattice that does not follow the curvature of the viral membrane, resulting in an enlarged distance between the Gag lattice and the viral membrane. Other morphological features that have been previously observed with other retroviruses include: (1) a Gag lattice with multiple discontinuities; (2) membrane regions associated with the Gag lattice that exhibited a string of bead-like densities at the inner leaflet; and (3) an arrangement of the Gag lattice resembling a railroad track. Measurement of the average size and mass of VLPs and authentic HTLV-1 particles suggested a consistent range of size and Gag copy numbers in these two groups of particles. The unique local flat Gag lattice morphological feature observed suggests that HTLV-1 Gag could be arranged in a lattice structure that is distinct from that of other retroviruses characterized to date.