Characterization of immunological responses in patients with severe fever with thrombocytopenia syndrome: A cohort study in China

Background: The immunological responses of patients with severe fever with thrombocytopenia syndrome (SFTS) remain largely unknown. We aim to study the magnitude and sustainability of host immune responses and their correlation with clinical, virological and hematological parameters. Methods: A longitudinal cohort study was performed in a SFTS reference hospital. The sequential immunological evaluation was determined for SFTSV infected patients, including anti-SFTSV IgM, IgG antibodies and the lymphocyte subsets. Results: Altogether 298 laboratory-confirmed SFTS cases were analyzed, from whom 55 patients were followed after convalescence. SFTSV specific IgM antibody could be detected at medium of 9 days, surged to peak levels by 4 weeks, and remained persistent until 6 months after disease onset. SFTSV specific IgG antibody could be detected at medium of 6 weeks; surged to peak levels by 6 months, and remained positive in most of the patients even at 3 years after infection. SFTS patients experienced obvious T cell, B cell and NK cells loss during the first week of infection, which was rapidly restored to normal levels. A significantly lower level of humoral immunity was identified concurrently from severe disease, especially in acute phase of the infection. These abnormalities can be used as a potential indicator in the prediction of an adverse clinical outcome. Conclusions: Information gained from this study have clinical significance in enhancing our understanding of SFTS immunological characteristics and the disease pathogenesis.