Macrophage Activation Syndrome (MAS) is a rare, life-threatening condition characterized by uncontrolled hyperinflammation which may develop on the background of systemic Juvenile Idiopathic Arthritis (sJIA). NI-0501 (Emapalumab) is a monoclonal antibody neutralizing interferon-gamma (IFN-gamma), a key cytokine which contributes to the inflammation and tissue damage seen in MAS. The purpose of this study is to assess the safety, tolerability and efficacy of NI-0501 in sJIA patients developing MAS, presenting an inadequate response to high dose glucocorticoid treatment.