The purpose of this Phase I/II, open-label, dose-escalation study is to evaluate an investigational monoclonal antibody administered as a first-line therapy to patients with acute, Grade II, III, or IV graft-versus-host disease (GVHD). Patients will be eligible for enrollment within 24 hours of beginning standard steroid treatment. The research is being conducted at up to 10 clinical research sites in the US.

A Phase I/II, open-label, dose-escalation pilot study designed to obtain preliminary safety, pharmacokinetic (PK) and efficacy information on visilizumab administered as a first-line therapy to patients with acute, Grade II, III, or IV graft-versus-host disease (GVHD). It is anticipated that up to 34 patients at up to 10 study centers could be eligible for enrollment in this study. Patients will be eligible for enrollment within 24 hours of beginning standard steroid treatment (2 mg/kg of methylprednisolone IV, or 10 mg/kg hydrocortisone IV, per day). Steroids will be rapidly tapered over one week following visilizumab administration.

• Patients who develop Grade II, III, or IV acute GVHD following allogeneic hematopoietic cell transplantation
• Patients receiving GVHD prophylaxis including cyclosporine or tacrolimus
• Clinical findings: skin thickening, joint contraction, oral ulceration, diarrhea.
• Patients with an onset date of acute GVHD that is less than or equal to 100 days posttransplant.