Complementary HLH Susceptibility Factors Converge on CD8 T-cell Hyperactivation

•Excess IL-18 and complete or partial perforin deficiency cooperate to drive clonal CD8 T-cell expansion•Concurrent terminal exhaustion and effector programs define CD8 T-cell hyperactivation Hemophagocytic Lymphohistiocytosis (HLH) and Macrophage Activation Syndrome (MAS) are life-threatening hyperinflammatory syndromes. Familial HLH is caused by genetic impairment of granule-mediated cytotoxicity (e.g., perforin-deficiency). MAS is linked to excess activity of the inflammasome-activated cytokine IL-18. Though individually tolerated, mice with dual susceptibility (Prf1-/-Il18tg; DS) succumb to spontaneous, lethal hyperinflammation. We hypothesized that understanding how these susceptibility factors synergize would uncover key pathomechanisms in the activation, function, and persistence of hyperactivated CD8 T-cells. In IL-18 transgenic (Il18tg) mice, IL-18 effects on CD8 T-cells drove MAS following a viral (LCMV), but not innate (TLR9), trigger. In vitro, CD8 T-cells also required T-cell receptor (TCR) stimulation to fully respond to IL-18. IL-18 induced, but perforin deficiency impaired, immunoregulatory Restimulation-Induced Cell Death (RICD). Paralleling hyperinflammation, dual susceptibility mice displayed massive post-thymic oligoclonal CD8 T-cell hyperactivation in their spleens, livers, and bone marrow as early as 3 weeks. These cells increased proliferation and IFNg production contrasted with increased expression of receptors and transcription factors associated with exhaustion. Broad-spectrum antibiotics and anti-retrovirals failed to ameliorate disease. Attempting to genetically “fix” TCR antigen-specificity instead demonstrated the persistence of spontaneous HLH and hyperactivation chiefly on T-cells that had evaded TCR fixation. Thus, drivers of HLH may preferentially act on CD8 T-cells: IL-18 amplifies activation and demand for RICD, whereas perforin supplies critical immunoregulation. Together, these factors promote a terminal CD8 T-cell activation state combining features of exhaustion and effector function. Therefore, susceptibility to hyperinflammation may converge on a unique, unrelenting, and antigen-dependent state of CD8 T-cell hyperactivation.