Inclusion Criteria:

• New onset acute GVHD Ann Arbor score 2 or 3 following allogeneic BMT. Any clinical severity (Glucksberg grade I-IV) is eligible. Patients with prior or existing diagnosis of GVHD without any treatment are eligible. Patients given only topical corticosteroids for skin GVHD are eligible.
• Any donor type (e.g., related, unrelated) or stem cell source (bone marrow, peripheral blood, cord blood). Recipients of non-myeloablative and myeloablative transplants are eligible.
• No prior systemic treatment for acute GVHD except for a maximum of 3 days of prednisone (or IV methylprednisolone) ≤2 mg/kg/day. Topical skin steroid treatment, non-absorbable oral steroid treatment for GI GVHD, and resumption of GVHD prophylaxis agents (e.g., calcineurin inhibitors) are permissible. Patients enrolled in BMT CTN 1501 who randomized to sirolimus are also eligible.  
  • Exceptions can be made on a case-by-case basis with Sponsor-Investigator approval.
• Age 18 years or older.
• Direct bilirubin must be <2 mg/dL unless the elevation is known to be due to Gilbert syndrome or aGVHD within 3 days of enrollment.
• ALT/SGPT and AST/SGOT must be <5 x the upper limit of the normal range within 3 days of enrollment unless the elevation is due to liver GVHD.
• If the patient is a woman of child-bearing potential, the patient and their sexual partner must agree to practice effective contraception.
• Written informed consent from patient or legal representative.
• Biopsy of acute GVHD target organ is strongly recommended but not required. Enrollment should not be delayed for biopsy or pathology results. Patients who do not enroll within 3 days of initiation of systemic steroid treatment for acute GVHD are not permitted to participate.

Exclusion Criteria:

• Patients with malignancy that is suspected or proven to have progressed, relapsed, or be persistent since BMT.
• Uncontrolled active infection.
• Patients with chronic GVHD only. Patients diagnosed with overlap syndrome are still eligible.
• History of or current diagnosis of progressive multifocal leukoencephalopathy (PML).
• Known hypersensitivity to natalizumab.
• Pregnant or nursing (lactating) women.
• Use of other drugs for the treatment of acute GVHD.
• Steroid therapy for indications other than GVHD at doses >0.5 mg/kg/d of methylprednisolone or equivalent within 7 days prior to initiation of GVHD treatment.
• Patients on dialysis.
• Patients requiring ventilator support.
• Investigational agent within 30 days of enrollment without approval from the Sponsor-Investigator (PI).
• History of allergic reaction to natalizumab.