Project Summary Pediatric Acute Liver Failure (PALF) is a rare, devastating condition that affects an estimated 250 children per year in North America, causing death in approximately 15% and the need for liver transplantation (LT) in an additional 20-30%. In the majority of cases, a specific cause of the liver injury is never determined. Recent research conducted in these patients supports the theory that many of these patients have liver injury related to a hyperinflammatory immune response to everyday infections or environmental exposures. Studies have recently implicated T lymphocytes as the central drivers of the inflammatory process. Physicians caring for PALF patients are searching for therapies that quiet this T cell response, dampen the inflammatory cascade and allow the patient’s remaining liver cells to heal and regenerate. Clinical experience in children with abnormal hyperinflammatory immune responses associated with other disease states such as Systemic Juvenile Idiopathic Arthritis and Acquired Aplastic Anemia has demonstrated that both high dose corticosteroids and equine anti- thymocyte globulin can suppress immune responses and reverse progressive tissue damage. The goal of the Pediatric Acute Liver Failure Immune Response Network (PALF IRN) is to support a treatment trial of immunosuppressive therapy using high dose corticosteroids or equine anti-thymocyte globulin to reverse harmful inflammatory immune responses in children with PALF to prevent further disease progression and reduce mortality and LT in this population. We propose the TReatment for ImmUne Mediated PathopHysiology (TRIUMPH) trial, a double-blind, three arm, randomized, placebo controlled trial of high dose methylprednisolone or equine anti-thymocyte globulin. We will test the hypothesis that survival with native liver at 21 days post randomization will be significantly higher in patients receiving immunosuppressive therapy as compared to patients that receive supportive care alone. We will also determine the safety of immunosuppressive therapy in PALF patients and define the balance between risk of side-effects and treatment benefit. Our outcomes will include not only clinical end-points such as patient survival, time to resolution of disease and adverse health events, but also measures of patient reported outcomes during rehabilitation from the illness. Trial participants will provide blood and liver samples that will be examined to better understand their immune responses, especially that of T lymphocytes, in both the circulation and in the liver. Samples will be stored in a repository to support future studies exploring new aspects of this rare disease.