Acute lymphoblastic leukaemia associated antigen. IV Expression on non-leukaemic ‘lymphoid’ cells

The cellular specificity of the common ALL (cALL) membrane (gp100) antigen and the p28, 33 (Ia-like) antigen has been further explored. Non-leukaemic cells with a variable and usually weak expression of the cALL antigen can be demonstrated in foetal haemopoietic tissue, in bone marrow of normal children, in the marrow of children with a variety of non-leukaemic haematologic and non-haematologic disorders. In normal children the cALL antigen positive cells appear to be restricted to the bone marrow. These cells are regularly detected in the marrow of many leukaemic (ALL, AML) and non-leukaemic children with post-chemotherapy associated lymphocytosis, in marrow transplant recipients, and in neonatal ‘leukaemoid’ reactions. When the number of cALL + cells is monitored in children following cessation of maintenance chemotherapy for ALL, some prognostic correlation can be observed; in general, however, the numbers of ‘lymphoid’ cells bearing these leukaemia associated antigens have no clear predictive value with respect to relapse. Analysis by simultaneous markers coupled with flow microfluorimetry and sorting indicates that the majority of cALL antigen + cells (> 90%) are positive for the p28, 33 (Ia-like) antigen but do not express markers of mature lymphocytes (T antigen, E sheep, E mouse rosettes, surface immunoglobulin), binding sites for complement and IgG or a myeloid membrane antigen. More than 90% of pre-B cells (i.e. cyt IgM +, SmIg −) are p28, 33 positive but only a small proportion (5–15%) are cALL antigen positive. These and additional data support the view that the cALL and p28, 33 (Ia) antigens are normal ‘early’ differentiation antigens of the lymphoid lineage.