Inflammatory activation of CD8⁺ T cells can, when left unchecked, drive severe immunopathology. Hyperstimulation of CD8⁺ T cells through a broad set of triggering signals can precipitate hemophagocytic lymphohistiocytosis (HLH), a life-threatening systemic inflammatory disorder.

The mechanism linking CD8⁺ T-cell hyperactivation to pathology is controversial, with excessive production of IFN-γ and, more recently, excessive consumption of IL-2, which are proposed as competing hypotheses. We formally tested the proximal mechanistic events of each pathway in a mouse model of HLH.

In addition to reporting a complete autosomal recessive IFN-γ receptor 1–deficient patient with multiple aspects of HLH pathology, we used the mouse model of perforin (Prf1)^KO mice infected with lymphocytic choriomeningitis virus to genetically eliminate either IFN-γ production or CD25 expression and assess the immunologic, hematologic, and physiologic disease measurement.

We found a striking dichotomy between the mechanistic basis of the hematologic and inflammatory components of CD8⁺ T cell–mediated pathology. The hematologic features of HLH were completely dependent on IFN-γ production, with complete correction after loss of IFN-γ production without any role for CD8⁺ T cell–mediated IL-2 consumption. By contrast, the mechanistic contribution of the immunologic features was reversed, with no role for IFN-γ production but substantial correction after reduction of IL-2 consumption by hyperactivated CD8⁺ T cells. These results were complemented by the characterization of an IFN-γ receptor 1–deficient patients with HLH-like disease, in whom multiple aspects of HLH pathology were observed in the absence of IFN-γ signaling.

These results synthesize the competing mechanistic models of HLH pathology into a dichotomous pathogenesis driven through discrete pathways. A holistic model provides a new paradigm for understanding HLH and, more broadly, the consequences of CD8⁺ T-cell hyperactivation, thereby paving the way for clinical intervention based on the features of HLH in individual patients.