NK cells constitute a major reservoir of HTLV‐2 infection in vivo

HTLV‐2 is a deltaretrovirus, possibly associated with neurological manifestations. Initially believed to be restricted to CD8+ lymphocytes, the virus has been shown to infect also CD4+ T‐cells and B cells. Here we show for the first time that HTLV‐2 also infects NK cells in vivo . HTLV‐2‐infected subjects were enrolled. PBMCs isolated by Ficoll density gradient were submitted to sequential isolation using MicroBeads (Miltenyi Biotech), in the following order: CD4+, CD56+, CD8+ and CD19+. For flow cytometry sorting in a BD FACSAria, cells were stained with a combination of anti‐human surface markes (all from BD Biosciences). DNA from the subsets and from total PBMCs was extracted. Proviral load (PVL) measurement was done by a multiplex real‐time PCR assay, targeting a fragment of HTLV‐2 Tax gene and a fragment of human albumin gene. 6 samples had undetectable PVL (bellow 1 Tax copy/ 100 PBMCs). A comparison CD8+ T cells showed the highest proviral burden, followed by CD56+ cells, CD19+ cells and CD4+ cells (mean PVLs= 17.84, 11.7, 7.27 and 0.47 respectively). To further exclude that the PVL detected in the NK subset was due to contaminants, total PBMCs from 2 samples were stained for FACS sorting. Both the ‘classic’ NK cells and NK cells expressing CD8+ were found to be infected. This study provides for the first time evidence that NK cells are naturally infected by HTLV‐2, and constitute a major reservoir of infection. Potentially, infected NK cells might be functionally impaired, resulting in viral escape from the innate immune surveillance. A clearer assessment of the in vivo spectra of the infection is essential for understanding virus – host interactions and the mechanisms of virus persistence.