ANTIGENIC AND ENZYMATIC PHENOTYPES OF THE PRE-B SUBCLASS OF ACUTE LYMPHOBLASTIC-LEUKEMIA

Leukaemias of immature B cells (pre-B cells) can be identified by the presence of cytoplasmic IgM in the absence of detectable cell surface immunoglobulin. One hundred and thirty-one cases of acute lymphoblastic leukaemia were assessed for this marker and 29 including both childhood and adult cases were positive. Twenty-eight of these had the phenotype of the major or common (c) ALL subclass, i.e. cALL antigen+, p28,33 antigen+, and were indistinguishable in presenting clinical and haematological features from IgM negative cALL. Variable proportions (10-95%) of leukaemic lymphoblasts contained detectable cytoplasmic IgM indicating differential levels of maturation arrest and a probable close developmental relationship between IgM+ cALL and IgM- cALL. Twenty out of 22 of these pre-B ALLs tested had elevated levels of terminal deoxynucleotidyl transferase, double antibody fluorescence tests with anti-TdT and anti-IgM revealing that individual cells contained nuclear TdT and cytoplasmic IgM. In contrast, four cases of the infrequent B-ALL subgroup had the phenotype p28,33+, cALL- and TdT negative indicating that the latter enzyme is a correlate of immaturity in both the T and B cell lineage. Eight out of 19 cases of pre-B ALL also had elevated levels of the I (intermediate) isoenzyme peak of hexosaminidase in common with the majority of cases of non-T, non-B or common ALL. These observations provide further insight into the possible target cells and levels of maturation arrest in ALL. © 1980.