In this study, neutralizing antibodies against SFTSV were detected in 16 of 16 recovered patients, and previous studies have shown that all 25 recovered patients retained neutralizing antibodies for at least 4 years [14]. Furthermore, neutralizing antibodies have been found to reach 100% positivity at 6 months post-illness, remain stable for 2 years, and gradually decline to 89.5% at 3 years, 78.3% at 5 years, and 66.7% at 10 years [15]. In our study, from the three samples that were collected from individuals more than 6 years since disease onset, sufficient amounts of neutralizing antibodies to inhibit SFTSV infection were detected (Fig. 3a and b), which suggests that SFTSV infection induces durable humoral immunity. Furthermore, the long-term presence of neutralizing antibodies may reduce the risk of reinfection in high-risk populations such as livestock and forestry workers, veterinarians, and pet owners. While long-term neutralizing antibody responses may aid in preventing reinfection, antibody-dependent enhancement (ADE) remains a theoretical concern in viral infections, including SFTSV. ADE occurs when sub-neutralizing or non-neutralizing antibodies facilitate Fc receptor-mediated viral entry into host cells, potentially worsening disease severity [16, 17]. However, no cases of ADE have been reported for SFTSV or closely related viruses in the bunyavirus family to date. Furthermore, the neutralization assays employed utilized Vero cells, which are deficient in Fcγ receptors. Consequently, the assessment of ADE was not feasible within the confines of this experimental system. Future studies employing FcγR-expressing cell lines or in vivo animal models are warranted to assess the potential for ADE in SFTSV infection.
Detailed epidemiological data regarding the prevalence of SFTS are limited. Between 2011 and 2021, 18,902 confirmed cases and 966 deaths were reported in China, which corresponds to a cumulative incidence of 0.13 cases per 100,000 persons [18]. Reinfection appears to be rare; however, a case involving a 42-year-old woman in Henan Province, China, infected with a different SFTSV strain, was reported in 2021 [19]. Furthermore, in a previous study, two of 25 patients who recovered showed higher neutralizing antibody titers at 4 years post-infection than at 1 year [14]. In our study, ELISA and BLI revealed that antibody titers did not show a declining trend over time since the onset of illness (Fig. 1c). Although none of these patients have had multiple episodes of SFTS, these findings suggest that some patients may experience reinfection or antigen re-exposure, thus leading to increased production or maintenance of neutralizing antibodies against SFTSV. However, sufficient evidence for this remains to be fully established.
A previous study indicated that patients with severe SFTS tended to exhibit higher IgG levels than moderately symptomatic patients and showed a slower decline in antibody titers over time [20]. Furthermore, it has been reported that disease severity positively correlates with viral load at the time of illness onset [21]. In this study, we also observed a positive correlation between the length of hospitalization and neutralizing antibody levels, which suggests that patients with more severe disease had more robust antibody responses. These findings support the hypothesis that intense immune stimulation during severe infections may lead to stronger and more durable humoral immunity in convalescent patients.
Convalescent plasma therapy has been considered for other viral diseases, such as Coronavirus disease 2019 (COVID-19) and Ebola virus disease [22, 23]. In previous studies that used an IFNAR knockout (α/β interferon receptor-deficient) mouse model highly susceptible to SFTSV, post-exposure prophylaxis with human antiserum provided complete protection against lethal infection [24]. In South Korea, convalescent plasma therapy administered to patients with SFTS has been reported to improve survival by reducing viral RNA levels in the plasma over time [25]. The long-term persistence of neutralizing antibodies in patients who recover from SFTS suggests that blood donation from a broad range of recovered individuals across generations may be feasible, thereby expanding the potential for therapeutic plasma or serum use.
In this study, we successfully used BLI to detect antibodies specific for SFTSV Gn in patients with recovered SFTS, and BLI has several advantages over ELISA. First, sample preparation was easily completed in less than 30 min, and no manual intervention was required until completion of the analysis. Second, molecular interactions can be observed in real time. Notably, BLI has been shown to be capable of quantitative antibody detection in patients infected with severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) [26]; therefore, this detection system would be versatile regardless of the virus species.
Memory B cells play a crucial role in enhancing the immune response upon re-exposure to pathogens. Upon encountering antigens, these cells rapidly differentiate into antibody-secreting cells or re-enter germinal centers for further diversification and affinity maturation [27]. Consequently, the ability to elicit virus-specific memory B cells is pivotal for the formulation of effective vaccines. In the context of COVID-19 and Ebola virus disease, neutralizing monoclonal antibodies have been developed as antivirals [28, 29]. While the development of MAb4-5 for SFTS did not contribute to the treatment of SFTS in experimental mouse models [30], Ab10, a humanized monoclonal antibody targeting SFTSV Gn, showed 80% protection when administered five days post-infection in a mouse model [30]. However, Ab10 requires a high dose of 30 mg/kg to protect against SFTSV infection in mice and was validated only in Gangwon/Korea/2012 strain [30]. In other studies, the RNA polymerase inhibitor favipiravir showed efficacy in preventing and treating SFTSV infection in animal models [31, 32] but failed to adequately reduce mortality in a multicenter, nonrandomized, uncontrolled, single-arm study of patients with resolved SFTS [8]. Currently, there are no effective treatments for SFTS. In this study, memory B cells specific for SFTSV Gn were detected in PBMCs from 75% (12/16) of patients who recovered from SFTS and persisted for up to 6 years and 7 months post-infection (Fig. 4c). Therefore, the generation of recombinant neutralizing monoclonal antibodies from memory B cells may contribute to the development of novel anti-SFTS therapies.
Memory B cells specific for SFTSV Gn were not detected in 4 of 16 patients; however, neutralizing antibodies remained present in their plasma. This observation suggests that the maintenance of antibody responses may not be solely dependent on circulating memory B cells. A previous study using non-human primates showed that the depletion of memory B cells had no impact on bone marrow plasma cell numbers and serum antibody titers for prolonged periods and indicated that long-lived plasma cells (LLPCs) can maintain humoral memory independently of memory B cells [33, 34]. Therefore, LLPCs may sustain antibody production in the bone marrow, spleen, or lymph nodes. Further investigation is warranted to characterize the anatomical niches and survival mechanisms of these LLPCs. Such insights would enhance our understanding of long-term immune protection against SFTSV and inform the development of more effective and durable vaccine strategies.
This study has several limitations. First, the patients in the acute phase of illness were not included; therefore, the data on immune responses during this phase are lacking. Second, longitudinal follow-up was not performed, which limited the ability to evaluate temporal changes in immunity. Third, the study population was small, comprising only 16 patients from a single prefecture, which reduced statistical power, particularly for subgroup analyses by sex and age. In addition, the analyses such as ELISA, BLI, and flow cytometry targeting SFTSV Gc protein were not performed, which could have provided complementary insights into humoral and cellular immune responses. Consequently, the generalizability of these findings to broader patient populations is limited. We expect that future studies with larger, more diverse cohorts, longitudinal sampling, and comprehensive immunological profiling—including both Gn and Gc-specific responses—will help to overcome these limitations and further clarify the immunological landscape of SFTSV infection.