A Study Evaluating the Efficacy and Safety of Neihulizumab vs "Conventional Treatment" to Treat Sr-aGvHD

Inclusion Criteria: 1. Patient must be ≥18years of age, males or females; 2. Patient must have been recipients of a single allogeneic HCT; bone marrow, peripheral blood and/or umbilical cord blood recipients are allowed 3. Patients must have aGvHD without feature of classic chronic GvHD or overlap GvHD; 4. Patients must have received no prior treatment for aGvHD other than steroids; 5. Patients must have biopsy proven grade II to IV aGvHD progressing after at least 3 days, non-improving grade III to IV aGvHD persistent after at least 7 days, or non-improving grade II aGvHD persistent after at least 14 days of methylprednisolone 2mg/kg/day or equivalent; Patients with initial response but have flare of aGvHD within 14 days with methylprednisolone \> 0.5 mg/kg/day or equivalent are also eligible; 6. Patient must have an ANC of \> 500/mm3 and no evidence of HCT graft failure or multi-organ failure; 7. Patient must have Karnofsky Performance Status (KPS) ≥50%; 8. Patient must give informed consent and sign an approved consent form prior to any study procedures; 9. Females of childbearing potential must have a negative pregnancy test result prior to enrollment. Males and females of childbearing potential must agree to use a highly effective method of birth control during the study. Exclusion Criteria: 1. Uncontrolled infections not responsive to antimicrobial therapy or requiring intensive critical care or vasopressors; 2. Evidence of end-organ infection due to CMV; 3. HIV infection or a known HIV-related malignancy (NOTE: patients positive for hepatitis B or hepatitis C are not excluded, and may be evaluated on a case by case basis). 4. Tuberculosis, history of tuberculosis or a known positive Quantiferon test for tuberculosis 5. Donor lymphocyte infusion for residual or relapsed disease or mixed chimerism. DLI as part of the planned HCT protocol are allowed 6. Relapsed disease after transplant or progressive malignant disease, including post-transplant lymphoproliferative disease; any secondary malignancy diagnosed since HCT 7. Renal failure requiring hemodialysis 8. Need ICU care, with life expectancy of less than 28 days, with ongoing or unresolved veno-occlusive disease, with unstable hemodynamics, with evidence of current or previous clinically significant disease, medical condition or finding of the medical examination (including vital signs and ECG), that in the opinion of the Investigator, would compromise the safety of the patient or the quality of the data 9. History of allergy/hypersensitivity to a systemically administered biologic agent or its excipients 10. Pregnant or nursing