Expression of a B-lymphoid differentiation antigen (CD 19) on acute non-lymphoblastic leukaemia cells

The expression of CD 19 (B4) surface antigen was investigated in 41 cases of acute myeloid leukaemia (AML), and 8 cases of acute undifferentiated leukaemia (AUL). Diagnosis was carried out by usual cytological and cytochemical means, and leukaemias were classified according to the FAB classification. CD 19 antigen is thought to be expressed on B lymphoid cells only. However, 2 cases out of 9 FAB M1, M2 and M3 subtypes were weakly stained (20 to 25%) by B4 antibody; 7 out of 11, and 17 out of 18 cases of M4 and M5 subtypes were markedly stained (25 to 70% of cells). 3 cases of AUL were also stained. Moreover, B4 expression was often correlated with expression of monocyte-associated antigens, as recognized by CRIS6, My9, KiM1 and UCHM1 antibodies. It is concluded that B4 antigen is probably expressed before B differentiation and is retained in early stages of monocytic differentiation.