Decreased monocyte subsets and TLR4-mediated functions in patients with acute severe fever with thrombocytopenia syndrome (SFTS)

• The roles of monocytes in the pathogenesis of acute severe fever with thrombocytopenia syndrome (SFTS) were studied.
• Cell counts and percentages of monocytes in peripheral blood were significantly reduced in acute SFTS patients.
• Despite their higher basal toll-like receptor 4 expression, monocytes from acute SFTS patients were found to be compromised regarding the production of tumor necrosis factor alpha upon lipopolysaccharide stimulation.

SummaryObjectivesThe role of a newly discovered bunyavirus, the severe fever with thrombocytopenia syndrome virus (SFTSV), in the pathogenesis of severe fever with thrombocytopenia syndrome (SFTS) is poorly understood. In this study, it was hypothesized that peripheral monocytes, which are constantly exposed to viral infection in the blood, are likely targeted by the causative virus in SFTS patients.MethodsFifty-three patients and 25 healthy volunteers were enrolled in the study. Monocyte counts in the peripheral blood of all human subjects were monitored throughout the progress of the disease. SFTSV viral load and the expression of monocyte genes were investigated by real-time RT-PCR. Cytokine production of monocytes in SFTS patients upon lipopolysaccharide (LPS) stimulation was examined by ELISA.ResultsIn comparison to SFTS patients in the convalescent stage and healthy controls, monocyte cell counts and percentages in patients at the acute stage were significantly lower. Decreased monocyte cell counts and subsets were positively correlated with SFTSV viral loads in the serum samples from SFTS patients. Despite their higher basal toll-like receptor 4 (TLR4) expression, monocytes from patients in the acute phase were shown to be compromised regarding the production of tumor necrosis factor alpha, but not interleukin 10, upon LPS stimulation.ConclusionsThese data strongly suggest that monocytes could be a major target during SFTSV infection. The decreased population and dysfunction of monocytes in acute SFTS patients may contribute to the disease severity.