• To evaluate the safety, tolerability and immunogenicity of sc administration of ACZ885 in pediatric subjects with active Systemic Juvenile Idiopathic Arthritis (SJIA). • To assess the initial efficacy profile (% responders to treatment and time to relapse) of sc dosing of ACZ885 in pediatric subjects with active SJIA according to the modified American College of Rheumatology (ACR) pediatric 30 definitions and the ability of ACZ885 to control the systemic manifestations of SJIA such as fever and rash. • To assess the PK of ACZ885 administered sc in children. • To assess PK and PD relationships in order to derive a dose and dosing regimen for Phase III (dose and dosing frequency required for relief of signs and symptoms and response according to at least ACR 30 definition).