Figure 4.

Down‐regulation of human leukocyte antigen (HLA) class I expression at relapse, and restoration by interferon (IFN)‐γ. In one patient with acute myeloid leukemia (case 39), freshly isolated bone marrow cells were obtained both (A) at initial diagnosis and (B) at relapse after transplantation. In a sample obtained at initial diagnosis, HLA class I molecule expression is preserved. After multiple courses of chemotherapy, the patient received an allogeneic peripheral blood stem cell transplant from her uncle. A 19‐month complete remission ended in relapse. A sample obtained at relapse showed loss of HLA‐A2 expression by leukemic blasts. Chimerism analysis confirmed that these blasts were of recipient, not donor, origin. (C) Cell incubation with 200 U/mL of IFN‐γ for 48 h completely restored HLA‐A2 expression.