Effect of Belimumab on Antibody Titers in Primary APS Patients

Inclusion Criteria: * Age ≥ 18 years old. * Positive for high-titer antiphospholipid antibodies according to the criteria of the "EULAR Guidelines for the Treatment of Antiphospholipid Syndrome in Adults" published in 2019. * Stable APS regimen according to EULAR recommendations for antiphospholipid syndrome. * Female patients who are not pregnant, breastfeeding, of childbearing potential, or not using contraception. Exclusion Criteria: * Patients with a history of malignant tumor in the past 5 years, except for basal cell carcinoma or squamous cell carcinoma of skin or cervical carcinoma in situ treated locally, and there is no evidence of metastasis within 3 years; * Patients with a history of primary immunodeficiency; * Serious lack of IgG (IgG level \< 400 mg/dL); * IgA deficiency (IgA level \< 10 mg/dL); * Patients with a current history of infection; * Patients with a current history of drug or alcohol abuse or dependence, or have a history of drug or alcohol abuse or dependence within 365 days before day 0; * HIV test is historically positive or HIV screening is positive; * Hepatitis status; * Patients with a history of allergic reaction caused by injection of contrast agent, human or mouse protein or monoclonal antibody; * Patients with other abnormal laboratory values with clinical significance; * If women with reproductive potential (WCBP) are included, please refer to the following special instructions; * Patients with concurrent major medical or mental illnesss; * Patients with diseases of liver, kidney, heart and other important organs,blood and Endocrine system; * Patients who have been vaccinated with live vaccine in the last month; * Patients who have participated in any clinical trial within 28 days before the initial screening and/or within 5 times of the half-life of the study compound (whichever is longer); * Patients who use B-cell targeted therapy drugs within one year, such as rituximab or epratuzumab; * Patients who use tumor necrosis factor inhibitor and interleukin receptor blocker within one year; * Patients who use Intravenous gamma globulin (IVIG) and prednisone ≥100mg/d for more than 14 days within one year or plasma exchange; * Patients with active infection (such as herpes zoster, HIV infection, active tuberculosis, etc.) during the screening period; * Patients with depression or suicidal thoughts; * Other conditions that the investigator considers would make the candidate unsuitable for the study.