Dysregulated B Cell Responses in Severe Fever With Thrombocytopenia Syndrome Revealed by Single-Cell RNA Sequencing.

Severe fever with thrombocytopenia syndrome virus (SFTSV) infection is associated with poor clinical outcomes and defective humoral immunity yet the immunometabolic mechanisms underlying B cell dysfunction remain incompletely defined. Through integrated single-cell RNA sequencing and B cell receptor (BCR) repertoire profiling of peripheral B cells from SFTS patients, we dissected the molecular signatures between survivors and fatal cases. Functional validation and metabolic flux analysis were further performed. Seven transcriptionally distinct B cell subsets were identified. Fatal cases exhibited a marked expansion of CXCR3+Ki-67+CXCR5- extrafollicular plasmablasts, coupled with depletion of naïve and memory B cells. These plasmablasts exhibited hyperactivation of interferon-response genes (IFI27, ISG15), upregulation of inflammatory mediators (S100A8/A9), and contraction of BCR diversity, with skewed usage of λ-light chains. Pseudotime trajectory analysis and metabolic scoring revealed progressive upregulation of oxidative phosphorylation, glycolysis and endoplasmic reticulum stress during terminal differentiation. In fatal cases, B cells exhibited suppressed antigen presentation capacity and impaired immunoglobulin gene expression, alongside heightened oxidative stress and elevated CD39 levels. Furthermore, analysis of SFTSV-infected versus uninfected B cells revealed that infected plasmablasts displayed enhanced inflammatory and migratory features, including upregulated CXCR3 and CCR10 expression, suggesting direct viral modulation of B cell function and trafficking. Our study reveals that dysfunctional, metabolically reprogrammed plasmablasts underlie humoral immune failure in fatal SFTS. These findings provide mechanistic insight into B cell-mediated immunopathogenesis and highlight potential targets for prognostic evaluation and immunomodulatory intervention.