Patients with acute liver failure (ALF) have defects in innate immune responses to microbes (immuneparesis) and are susceptible to sepsis. Cytotoxic T-lymphocyte associated protein 4 (CTLA4), which interacts with the membrane receptor B7 (also called CD80 and CD86), is a negative regulator of T-cell activation. We collected T cells from patients with ALF and investigated whether inhibitory signals downregulate adaptive immune responses in patients with ALF.