Expansion of granulocytic myeloid-derived suppressor cells in patients with severe fever with thrombocytopenia syndrome.

Severe fever with thrombocytopenia syndrome (SFTS), caused by novel bunyavirus (SFTSV), is a hemorrhagic fever with a high mortality rate of over 10%. We have previously shown that granulocytic myeloid-derived suppressor cell (gMDSC) might affect arginine metabolism, which was associated with decreased platelet count and T lymphocyte dysfunction in this disease. The study was designed to investigate the expression of the gMDSCs subsets in SFTS patients, and to evaluate its association with disease severity. A prospective study was performed on 166 confirmed SFTSV infected patients. Sequential blood samples were collected during hospitalization and after recovery. SFTSV RNA was quantified by real-time RT-PCR. The gMDSCs and NK cells were determined by flow cytometry analysis, which were associated with disease severity. Elevation of the activated gMDSC was observed in SFTS patients at the acute phase, with a significantly higher level of gMDSC attained in 79 severe and 29 fatal SFTS patients than in the mild patients. The NK cells were depleted at the early infection and not restored to normal level until 4 months after the disease. The expansion of gMDSC was accompanied by the elevated expressions of CD3-ζ of NK and Arginase-1, in contrast with the decreased reactive oxygen species (ROS) in gMDSC. The levels of NK, CD3-ζ of NK, viral load, and platelet count were significantly associated with the level of gMDSC. Expansion of gMDSC was demonstrated in SFTS, which was associated with severe disease and suppressed antiviral NK cell via other mechanisms than Arginase-1 or ROS.