Human T-cell leukemia virus type 1 (HTLV-1) is the etiological agent of adult T-cell leukemia (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). HTLV-1 cell-to-cell transmission is dependent on the release of infectious virus particles into the virological synapse. The HTLV-1 particle structure is still poorly understood, and previous studies analyzed viruses produced by transformed lymphocytic cell lines chronically infected with HTLV-1, particularly the MT-2 cell line, which harbors truncated proviruses and expresses aberrant forms of the Gag protein. In this study, we demonstrate that the chronically infected SP cell line harbors a relatively low number of proviruses, making it a more promising experimental system for the study of the HTLV-1 particle structure. We first identified the genomic sites of integration and characterized the genetic structure of the gag region in each provirus. We also determined that despite encoding a truncated Gag protein, only the full-length Gag protein was incorporated into virus particles. Cryotransmission electron microscopy analyses of the purified virus particles revealed three classes of particles based upon capsid core morphology: complete cores, incomplete cores, and particles without distinct electron densities that would correlate with the capsid region of a core structure. Observed cores were generally polygonal, and virus particles were on average 115 nm in diameter. These data corroborate particle morphologies previously observed for MT-2 cells and provide evidence that the known poor infectivity of HTLV-1 particles may correlate with HTLV-1 particle populations containing few virus particles possessing a complete capsid core structure.
Bibliographical noteFunding Information:
This work is supported by National Institutes of Health grant R01 GM098550. Morgan E. Meissner has been supported by NIH grant T32 AI083196 (Institute for Molecular Virology Training Program). The cytogenetic analyses were performed in the Cytogenomics Shared Resource at the University of Minnesota with support from comprehensive Masonic Cancer Center NIH grant P30 CA077598. We thank Shawn Hill, care of David Derse, for the anti-NC polyclonal antibody; Cynthia Pise-Masison for the SP and C91PL cells; and Masao Matsuoka and Chou-Zen Giam the ATL-T and ATL-2 cells. Cryo-TEM was carried out at the Characterization Facility, University of Minnesota. We thank Jessica L. Martin for critical reviews of the manuscript.
- Core morphology
- Cryoelectron microscopy
- Human retrovirus
- Retrovirus assembly