Sirolimus Monotherapy in the Treatment of Antiphospholipid Antibody Related Thrombocytopenia

Inclusion Criteria: * persistent positive of antiphospholipid antibody (either lupus anticoagulant, anti-cardiolipin antibody, or anti-b2GP1 antibody, at least two times with 12 weeks apart) * persistent thrombocytopenia (30-100×10\^9/L, at least for 2 weeks) Eligible concomitant treatment: * prednisone or equivalent dose less than 10mg per day is allowed, and dose should be stable for more than 2 weeks * hydroxychloroquine less than 400mg per day is allowed, and dose should be stable for more than 1 month * anti-platelet and/or anti-coagulant therapy is allowed, and strength should be the stable for 1 week * these following therapies should be discontinued for more than 5 half-lives before the enrollment, including thrombopoietin or thrombopoietin receptor antagonist, intravenous immunoglobulin, immunosuppressants, B cell inhibitors (Belimumab or Talitacicept) and B cell depletion therapy (Rituximab or Obinutuzumab). Exclusion Criteria: * fulling the criteria of other connective tissue disease other than antiphospholipid syndrome * received oral/intravenous antibiotics within 2 weeks before the enrollment. * new onset of thrombosis within 4 weeks before the enrollment. * apparent bleeding tendency. * life or organ threatening manifestations, includes but not limit to catastrophic antiphospholipid syndrome and thrombotic microangiopathy. * liver and renal dysfunction: ALT or AST more than three times of upper limit of normal range; eGFR\<40mL/min/1.73m\^2 * hematocytopenia: WBC\<3.0×10\^9/L, Hb\<100g/L. * uncontrollable hyperlipidemia: low density lipoprotein cholesterol\>3.1 mmol/L, triglycerides\>2.3 mmol/L after lipid lowering therapy. * current active infection * women in pregnancy and postpartum period