Shedding of CD9 antigen by bone marrow cells from patients with acute lymphoblastic leukaemia

The levels of soluble CD9 antigen released into spent medium from bone marrow (BM) cells were assayed using a unique enzyme-linked immunosorbent assay. We demonstrated that a considerable amount of soluble CD9 antigen was consistently detected in the spent medium from CD9+ leukaemic blasts, but little from normal or regenerating BM cells. The sensitivity and specificity of CD9 antigen assay were tested. First, BM cells taken from patients in complete morphologic remission (CR) were studied, and the distinct level of CD9 antigen was detected in 16.3% of the cases. Second, the outcome of leukaemic patients with significant shedding of CD9 antigen from BM cells while in CR was investigated: all patients have developed systemic relapse within 5-24 (median 15.4) weeks. In contrast, 24/34 patients without the increase of CD9 antigen levels are still in CR (follow-up 19-149 weeks, median 56.2 weeks). Only 10 patients in this group have relapsed so far. No false-positive results were detected with this sensitive assay for soluble CD9 antigen, and the introduction of appropriately planned prospective studies is justified on the basis of these observations.