Evaluate Efficacy, Safety and Tolerability, PK and PD of Emapalumab in Children and Adults With MAS in Still's or SLE

Study NI-0501-14 is a two-cohort trial that enrolls subjects who are diagnosed with sHLH/MAS (MAS being a form of secondary HLH) and who are presenting an inadequate response to high doses of GCs. These subjects will be enrolled in 2 cohorts as per their background disease. The cohorts are defined as follows: * Cohort 1: MAS in the context of sJIA and AOSD. * Cohort 2: MAS in the context of pediatric and adult SLE. The study has the objectives to investigate the efficacy, safety and tolerability, for 8 weeks, and PK and PD, QoL and immunogenicity in these 2 cohorts for up to 1 year after last dose of of emapalumab. Macrophage Activation Syndrome (MAS) Secondary Hemophagocytic Lymphohistiocytosis (sHLH) systemic Juvenile Idiopathic Arthritis (sJIA) Adult-onset Still's Disease (AOSD) Systemic Lupus Erythematosus (SLE)