Shedding of CD9 antigen in acute lymphoblastic leukemia

A leukemia-associated CD9 glycoprotein antigen released into the extracellular milieu from acute lymphoblastic leukemia cells has been detected using a unique lectin-monoclonal antibody immunoassay. It has been demonstrated that the release of CD9 antigen is an active process and is associated with active cell growth. In addition, the difference of carbohydrate moiety, and hence glycosylation, in the CD9 antigen derived from lymphoblasts and neuroblasts was verified using lectin affinity chromatography. The lectin affinity of the carbohydrate moiety of lymphoblast CD9 antigen would indicate the presence of N-linked oligosaccharide chains having groups of N-acetyl glucosamine residues, a mannose core and a terminal D-galactose. The soluble CD9 antigen is specifically detected in plasma from ALL patients at the time of diagnosis, in cerebrospinal fluid from patients with central nervous system involvement, and spent medium from CD9-positive leukemic blasts obtained at the time of diagnosis. Interestingly when bone marrow cells taken from patients in complete remission were studied, a distinct amount of CD9 antigen was released into spent medium in some of the cases. All of these patients have subsequently developed hematological relapse. The present data suggest that shedding of CD9 antigen by leukemic cells may enable the clinical monitoring of residual leukemic cell burden.