Severe fever thrombocytopenia syndrome (SFTS) is an emerging, tick-borne infectious disease caused by SFTS virus, characterized by high fever, leukopenia, thrombocytopenia, and multiple organ dysfunctions, including the lung, liver, kidney and so forth [1–3]. SFTSV is recognized as Phlebovirus genus in the Bunyaviridae family, with a high mortality up to 30% at early epidemic period [3, 4]. Since the first report of SFTS in China, the incidence of SFTS has expanded to at least 15 provinces in China; and other Asian countries such as South Korea, Japan and Vietnam have also discovered this disease one after another, indicating that the epidemic area is extending [5–7]. Therefore, due to the worldwide spread of SFTSV, high mortality rate and human communicable nature of the virus, the World Health Organization (WHO) has listed the SFTS virus as one of the most in need of attention pathogens [8]. Nonetheless, to date, there are no available effective antiviral drugs or vaccines against SFTSV, and more diagnostic tools for detecting SFTSV are needed [9].
Using clinical manifestation to diagnose SFTSV is non-specific, since SFTS is difficult to distinguish from many other diseases with similar clinical features [6]. Although virus isolation from blood of viremic patients is a reliable method to diagnose SFTSV infection, this way is time-consuming and requires high security biocontainment facility [10, 11]. There are several methods to detect SFTSV genome. The reverse transcription polymerase chain reaction (RT-PCR) [11–13], quantitative real-time RT-PCR [11, 14, 15], and reverse transcription loop-mediated isothermal amplification assay (RT-LAMP) [16–18] have been used for detecting SFTSV genome. Studies have shown that these methods are only suitable for acute onset period of 1-6 days [1, 10, 19]. Hence, complement methods for the diagnosis of SFTSV are essential.
SFTSV is a single-stranded, negative RNA virus with three segments, designated L, M, and S [20]. The L segment encodes the RNA-dependent RNA polymerase (RaRp); the M segment encodes the glycoprotein precursor (GCP), including Gc and Gn glycoprotein; and the S segment encodes nucleocapsid protein (NP) and nonstructural protein (NSs) via using ambisense coding [21, 22]. NP is highly conserved and immunogenic in Bunyaviridae family. It has been shown that NP plays an important role in the virus replication, transcription, packaging of genomic RNA into ribonucleoprotein [23, 24]. Our previous study demonstrated that recombinant SFTSV nucleocapsid (rSFTSV-N) protein based indirect ELISA assay systems has been established to detect specific human IgG and IgM antibodies, respectively. However, rSFTSV-N protein based indirect IgM ELISA missed to detect several patients [25].
In the present study, three monoclonal antibodies (MAbs: 5G12, 4A10, 1C3) against rSFTSV-N protein were successfully developed, and the MAb based IgG sandwich ELISA and IgM capture ELISA system were established. Serum samples, collected from clinically-suspected SFTS patients, were used to evaluate the newly established systems and results were compared with the total antibody detecting sandwich ELISA system and the indirect ELISA systems.