Belimumab After Rituximab in Resistant Primary Juvenile SS

Inclusion criteria

• Age 5-18 years old.
• Meets SS diagnostic AECG criteria or Japan criteria.
• classification for "resistant patients": Application of two or more immunosuppressants or prednisone +immunosuppressive therapy for more than 3 months. One of the following conditions still exists: a) systemic involvement: polyarthritis, vasculitis, autoimmune cytopenia or involvement of skin, kidney, lung, nerve, and liver. b) constituted B cell activation: elevated IgG, light chain, high β 2MG, C4 decrease, cryoglobulinemia, monoclonal antibody c) sustained increased inflammatory markers, such as ESR
• Agree to receive the treatment of rituximab combined with belimumab

Exclusion criteria

• Previously treated with rituximab within six months, or previously treated with other biologics, including belimumab or Telitacicept
• Participate in other clinical trials within 6 months
• eGFR\<30ml/min
• Active infections, including but not limited to: -- Current or past infection with hepatitis B or C as defined by: Hepatitis B surface antigen positive. Hepatitis B surface antibody positive and hepatitis B core antibody positive. Hepatitis C antibody positive. -- Historically positive HIV test or test positive at screening for HIV. -- Active tuberculosis.
• Infection history: -- Currently on any suppressive therapy for a chronic infection (such as tuberculosis, pneumocystis, cytomegalovirus, herpes simplex virus, herpes zoster and atypical mycobacteria) -- Hospitalisation for treatment of infection within 60 days of Day 0. -- Use of parenteral (intravenous or intramuscular) antibiotics (anti-bacterials, antivirals, anti-fungals or anti-parasitic agents) within 30 days of Day 0. -- Receipt of a live-attenuated vaccine within 3 months of Day 0. -- In the investigator's opinion, participants that are at high risk for infection (including but not limited to in dwelling catheter, dysphagia with aspiration, decubitus ulcer, history of prior aspiration pneumonia or recurrent severe urinary tract infection).
• Primary immunodeficiency
• History of malignant neoplasm
• Severe, progressive or uncontrolled renal, hepatic, haematological, gastrointestinal, pulmonary, cardiac or neurological disease or, in the investigator's opinion, any other concomitant medical condition or significant abnormal laboratory value that places the participant at risk by participating in this trial with the exception of diseases or conditions related to active SS
• Comorbidities not SS related currently requiring systemic corticosteroid therapy.
• Within 10 days before the first administration of Belimumab, IgG\<4g/L or IgA\<0.1g/L
• WBC\<1.5 × 109/L within 10 days before the first administration of Belimumab or neutrophils\<1 × 109/L