PRF1 and macrophage activation syndrome: Human fHLH has been modeled in PRF1 deficient mice infected with LCMV, and both CD8 T cells and interferon-gamma (IFNγ), a cytokine known to be the main driver of anemia in models of fHLH and fulminant MAS (32, 33), were found to be critically important mediators of mouse mortality (34).