Genetic deficiency of IFNγ reveals IFNγ-independent manifestations of murine Hemophagocytic Lymphohistiocytosis.

Objective Familial hemophagocytic lymphohistiocytosis (FHLH) is a complex cytokine storm syndrome caused by genetic abnormalities rendering CD8+ T cells and natural killer cells incapable of cytolytic killing. In murine models of FHLH, interferon-gamma (IFN gamma) produced by CD8+ T cells has been identified as a critical mediator of disease, and an IFN gamma-blocking antibody (emapalumab) has recently been approved by the Food and Drug Administration. However, development of hemophagocytic lymphohistiocytosis (HLH)/macrophage activation syndrome (MAS) in patients who are genetically unresponsive to IFN gamma questions the absolute necessity of IFN gamma in driving disease. This study was undertaken to determine the necessity of IFN gamma in driving HLH. Methods IFN gamma(-/-)Prf1(-/-) mice were infected with lymphocytic choriomeningitis virus (LCMV), and HLH immunopathologic features, including survival, weight loss, cytopenias, cytokine profiles, and immune cell phenotypes, were assessed. Mixed bone marrow chimeras were created to determine the immune cell-intrinsic role of IFN gamma receptor signaling. CD8+ T cell depletion and interleukin-33 (IL-33)/ST2 blockade were performed using monoclonal antibodies. Results LCMV infection of IFN gamma(-/-)Prf1(-/-) mice resulted in severe HLH-like disease. CD8+ T cells and the IL-33/ST2 axis remained essential mediators of disease; however, IFN gamma-independent HLH immunopathology correlated with a 10-15-fold increase in neutrophilia (P < 0.001) and an altered cytokine milieu dominated by IL-6, IL-1 beta, and granulocyte-macrophage colony-stimulating factor (GM-CSF) (P < 0.05). Furthermore, IFN gamma regulated CD8+ T cell expression of GM-CSF and neutrophil survival. Conclusion IFN gamma is not necessary for the development of fulminant HLH, requiring physicians to consider case-by-case treatment strategies. Use of therapies that target upstream activators of CD8+ T cells, such as IL-33/ST2 signaling, may be more universally applicable treatment options that ameliorate both IFN gamma-dependent and -independent manifestations of HLH/MAS.