Patient description: The patient is a 17-year-old boy complaining of severe migratory pain in the right lower quadrant of his abdomen that started four days ago. The pain is accompanied by nausea and vomiting. He was febrile with tenderness, rebound tenderness and guarding on palpation. His WBC was elevated with dominant neutrophils. CT scan showed evidence of acute perforated appendicitis with free fluid in the pelvis. Diagnostic laparoscopy revealed phlegmon with no other abdominal abnormalities. He is now a candidate for emergent laparoscopic appendectomy under general anesthesia.

Clinical trial eligibility criteria: eligible ages (years): 0.5-24.0,  Acute Lymphoblastic Leukemia (ALL) ≥ 6 months and < 25 years of age 2. Histologically confirmed diagnosis of B-lineage ALL. Verification of CD22 expression is not required 3. Bone marrow in morphologic remission (any remission number) defined as < 5% blasts (M1 classification) performed in local institution lab 4. CNS 1 (< 5/μL WBCs in CSF and cytospin negative for blasts) 5. Evidence of bone marrow MRD defined as ≥ 0.01% by flow cytometry performed in the study central lab 6. Candidate committed to HCT independent of participation in this study, with the following requirements Meets local transplant center requirements for HCT In the opinion of the HCT center will be ready to begin pre-transplant conditioning within 6 weeks of trial enrollment from a medical and psychosocial standpoint Has an available HCT donor or identified cord blood unit. Related and unrelated donors, and bone marrow, peripheral blood, or cord blood stem cell sources allowed 7. Adequate organ function including the following Hepatic function: Total bilirubin < 1.5 × upper limit of normal (ULN) (except in the case of subjects with known Gilbert's disease: < 5 × ULN) and transaminases (alanine aminotransferase (ALT) and aspartate aminotransferase (AST)) < 3 × ULN based on age and institution specific laboratory-specific normal ranges Renal function: Age-adjusted normal serum creatinine is required. A 24-hour creatinine clearance value > 60 mL/min/1.73 m2 (updated Schwartz formula or nuclear GFR) must be obtained if serum creatinine is elevated Hematologic function: Absolute neutrophil count (ANC) > 500/μL and platelet count > 25,000/μL without transfusion Oxygen saturation at rest or with exercise > 88% as measured by pulse oximeter or PaO2 > 55 mm Hg without need for supplemental oxygen at rest or with activity Serum albumin > 2 g/dL 8. Performance status  Active extramedullary disease at any site. (Note: Definitive therapy of known previous sites of extramedullary disease is allowed) 2. Females who are breast-feeding or pregnant 3. Subjects with known 11q23 MLL rearrangement are excluded. 4. Prior therapy Prior treatment with CAT-3888 (BL22), moxetumomab pasudotox (CAT-8015, HA22), any pseudomonas-exotoxin-containing compound, or any anti-CD22 directed therapy at any time in the past Prior allogeneic or autologous HCT or adoptive cellular therapies, including T-cell chimeric antigen receptor (CAR) therapy Chemotherapy < 2 weeks prior to starting study drug with the following exception: There is no time restriction in regard to prior intrathecal chemotherapy provided there is complete recovery from any acute toxic effects of such Monoclonal antibody therapy < 30 days from study enrollment Other investigational agents currently or within 30 days prior to study enrollment 5. Subjects with an absolute contraindication to corticosteroid administration 6. HIV infection (due to increased risk of severe infection and unknown interaction of moxetumomab pasudotox with antiretroviral drugs) 7. Active hepatitis B or C infection as defined by seropositivity for hepatitis B (hepatitis B surface antigen [HbsAg]) or hepatitis C (hepatitis C antibody) and elevated liver transaminases (defined as above the ULN per the institution normal ranges) 8. Second malignancy other than non-basal cell carcinoma of the skin or in situ carcinoma of the cervix, unless the tumor was treated with curative intent at least two years previously and subject is in remission 9. Subject with clinical or laboratory evidence of active DIC 10. Subject with prior history of thrombotic microangiopathy or HUS within 3 months prior to enrollment 11. History of known congenital hypercoagulable condition 12. Previous life-threatening anaphylactic reactions to prior monoclonal antibody-based immunotherapy or any component of the moxetumomab pasudotox formulation 13. Subjects who will be or are currently being treated with high dose estrogen (high dose is defined as >0.625mg daily as conjugated estrogens or equivalent) within 7 days prior to study enrollment