Enzyme and membrane markers in leukaemia: recent developments.

Terminal deoxynucleotidyl transferase (TdT) assay has proved a valuable test for distinguishing lymphoblastic from myeloblastic leukaemias, particularly in adults whose blast cells are often negative for the c-ALL antigen. The immunofluorescence assay, particularly when used in combination with antisera to surface membrane antigens, has proved a sensitive technique for detecting small numbers of lymphoblasts in extramedullary sites, for example, testis or cerebrospinal fluid, or of residual Thy-ALL blasts in the marrow, which might otherwise be difficult to recognise. Differences in concentration of several enzymes concerned in purine metabolism have been detected between the blast cells in the various acute leukaemias. Adenosine deaminase (ADA) concentrations tend to be higher in Thy-ALL than in other forms of leukaemia, but the wide overlap reduces the diagnostic value of this assay. Thy-ALL blasts, however, appear to be selectively and exquisitely susceptible to inhibition of ADA by the drug deoxycoformycin, which has now been used sucessfully in a number of other wise resistant patients with Thy-ALL to obtain a complete remission. The recently introduced technique for the production of monoclonal antibodies has substantially widened the reagents available for analysing the membrane characteristics of bone marrow stem cells and of cell lineages derived from them. These have revealed previously unsuspected heterogeneity among different cases of acute lymphoblastic leukaemia, for example, among Thy-ALL blasts from different patients, and they have also delineated minor populations of immature thymocytes from which these leukaemic cells are derived. The potential use of these antibodies to prevent graft-versus-host disease by selective removal of T-lymphocytes from donor bone marrow before allogeneic bone marrow transplantation, or to prevent recurrence of Thy-ALL and other lymphoblastic leukaemias or lymphomas by selective removal of leukaemic or lymphoma malignant cells before autologous transplantation, is reviewed.