Evaluation of new adjuvants in enhancing the immune response to a candidate of Severe Fever with Thrombocytopenia Syndrome Virus vaccine

Abstract

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging zoonotic disease caused by the tick-borne bunyavirus SFTSV. SFTSV is primarily transmitted by Haemaphysalis longicornis ticks and can cause a hemorrhagic fever-like illness characterized by high fever, thrombocytopenia, leukopenia, and multiorgan failure, with reported case-fatality rates ranging from 10% to over 30%. Despite its growing public health relevance, no licensed vaccines or specific antiviral therapies are currently available. Here, we evaluated a beta-propiolactone-inactivated SFTSV vaccine candidate based on strain YL1, formulated with benchmark adjuvants (Addavax and Alhydrogel) or experimental innate immune agonists, including Sendai virus-derived defective interfering RNA (SDI), a RIG-I agonist, and imidazoquinoline-poly(ethylene glycol)-cholesterol conjugate (IMDQ-P-C), a TLR7/8 agonist, alone or in combination. Using an extended prime-boost regimen, we examined adjuvant-dependent modulation of humoral immunity, antigen-specific cellular recall, antibody cross-reactivity, and functional protection following passive serum transfer and live-virus challenge in IFNAR-/- mice. All adjuvanted formulations induced detectable humoral responses, with Addavax and SDI eliciting the highest virus-specific binding and neutralizing antibody titers. Antigen-specific analyses identified the nucleoprotein (NP) as the dominant target of vaccine-induced immune memory. NP elicited robust antibody response and the broadest cytokine recall profile following ex vivo stimulation. This response was broader than that induced by the whole inactivated virus and was characterized by mixed Th1-associated, Th2-associated, and proinflammatory-associated features that varied across adjuvant formulations. In contrast, the head domain of the Gn glycoprotein elicited limited cellular recall responses, consistent with its relatively weak humoral immunogenicity after both priming and boosting. Nevertheless, sera from SDI and Addavax-adjuvanted groups displayed cross-reactivity with Gn from a heterologous SFTSV strain, suggesting broader antibody recognition across viral genotypes. Functionally, sera from SDI and Addavax-immunized mice conferred partial protection following passive transfer into IFNAR-/- recipients, as reflected by reduced viral loads, delayed disease onset, and prolonged survival after lethal SFTSV challenge. Together, these findings support the immunogenicity and protective potential of a BPL-inactivated SFTSV vaccine platform and identify NP as a major target of vaccine-induced immune memory. They further highlight the importance of adjuvant selection in shaping the magnitude, antigenic focus, and functional quality of vaccine-induced antiviral immunity. In brief Severe fever with thrombocytopenia syndrome virus (SFTSV) is an emerging tick-borne bunyavirus that causes a hemorrhagic fever-like illness with high fatality rates and lacks approved vaccines. Here, we developed and characterized a β-propiolactone-inactivated whole-virus SFTSV vaccine candidate formulated with benchmark and experimental adjuvants, including the RIG-I agonist SDI and the TLR7/8 agonist IMDQ-P-C. In BALB/c mice, Addavax and SDI elicited the strongest virus-specific binding and neutralizing antibody responses. Antigen-specific recall analyses identified nucleoprotein (NP) as the dominant target of vaccine-induced immune memory, eliciting robust antibody responses and the broadest cytokine recall profile after ex vivo stimulation. NP-induced recall showed mixed Th1-associated, Th2-associated, and proinflammatory-associated features that varied across vaccine formulations, whereas the head domain of the Gn glycoprotein elicited limited humoral and cellular responses. Sera from Addavax- and SDI-adjuvanted groups displayed cross-reactivity with the Gn glycoprotein of the heterologous HB29 strain and conferred partial protection following passive transfer into IFNAR-/- mice challenged with live SFTSV. These findings identify NP as a major immunological target of this inactivated vaccine platform and highlight how adjuvant selection shapes the magnitude, antigenic focus, and functional quality of antiviral immunity.