Aberrant STAT signaling drives T cell dysregulation in a targetable pediatric sepsis endotype.

Sepsis is a leading cause of morbidity and mortality in critically ill children, yet heterogeneity in immune responses complicates the development of targeted therapies. Although immune dysregulation is associated with poor outcomes in sepsis, it remains unclear which host immune factors contribute causally to sepsis morbidity and mortality. To address this gap, we integrated deep immune phenotyping, plasma proteomics, single-cell transcriptomics, and phosphoflow cytometry in a prospective cohort of 88 critically ill children to elucidate the immunologic mechanisms which underly disease heterogeneity. Unsupervised clustering of plasma cytokines identified three immunologic subgroups, including a high-severity group ("Group C") characterized by marked hypercytokinemia, driven primarily by IL-6 and IFN-γ. Group C exhibited distinct alterations in immune cell frequency and activation status, along with a strong association between hyperinflammatory signaling and lymphocyte dysfunction. Single-cell RNA sequencing revealed transcriptional signatures of T cell activation and metabolic stress, and identified widespread suppression of a lymphoid protective gene program across CD8⁺ T cell subsets. In the setting of increased expression of activation markers, T cell receptor repertoire analysis revealed no dominant clonotypes, consistent with a bystander mechanism of T cell activation. Using phosphoflow cytometry, we demonstrated baseline hyperactivation of STAT1 and STAT3 in CD8⁺ T cells from patients in Group C, and these cells failed to respond to aCD3/aCD28 stimulation. Together, these findings define IL-6/IFN-γ-driven T-cell dysfunction as a distinct endotype of immune dysregulation in pediatric sepsis, highlighting the JAK/STAT axis as a potential future target for immunomodulatory therapy.