Summary. Analysis of differentiation antigens on leukaemic blasts is routinely done for diagnostic purposes, i.e. determination of stage of differentiation and lineage assignment. Acute lymphoblastic leukaemias are also frequently characterized by a leukaemia-associated immunophenotype (LAIP), either the coexpression of differentiation antigens physiologically restricted to other stages of differentiation (asynchronous LAIP) or cell lineages (aberrant LAIP). We defined LAIP in 241 consecutive unselected B-lineage (n ¼ 193) and T-lineage (n ¼ 48) ALL by three-colour flow cytometry using directly conjugated monoclonal antibodies. The incidence of LAIP was found to be 91·7%. In 63% of patients two to six leukaemia-associated expression patterns were detected. In order to study the specificity of LAIP in a therapy-relevant setting, remission bone marrow samples from patients with B-lineage ALL were analysed for the expression of T-lineage-associated phenotypes on the normal bone marrow cells and vice versa. The frequency of all Tlineage LAIP þ cells and all aberrant B-lineage LAIP þ cells was <1% in regenerating bone marrow samples at different timepoints. The incidence and clinical significance of LAIP þ cells was studied in 196 remission marrows of 70 ALL patients (55 remaining in CCR, 14 with bone marrow relapse, one with isolated CNS relapse). The presence of >1% LAIP þ at two consecutive timepoints predicted 5/8 bone marrow relapses in B-lineage ALL. The occurrence of LAIP þ cells >1% in T-lineage ALL after induction therapy predicted relapse in 7/7 cases. In conclusion, flow cytometric detection of LAIP þ cells appears to be a powerful tool for the prediction of outcome in ALL.

Summary. Analysis of differentiation antigens on leukaemic blasts is routinely done for diagnostic purposes, i.e. determination of stage of differentiation and lineage assignment. Acute lymphoblastic leukaemias are also frequently characterized by a leukaemia-associated immunophenotype (LAIP), either the coexpression of differentiation antigens physiologically restricted to other stages of differentiation (asynchronous LAIP) or cell lineages (aberrant LAIP). We defined LAIP in 241 consecutive unselected B-lineage (n ¼ 193) and T-lineage (n ¼ 48) ALL by three-colour flow cytometry using directly conjugated monoclonal antibodies. The incidence of LAIP was found to be 91·7%. In 63% of patients two to six leukaemia-associated expression patterns were detected. In order to study the specificity of LAIP in a therapy-relevant setting, remission bone marrow samples from patients with B-lineage ALL were analysed for the expression of T-lineage-associated phenotypes on the normal bone marrow cells and vice versa. The frequency of all Tlineage LAIP þ cells and all aberrant B-lineage LAIP þ cells was <1% in regenerating bone marrow samples at different timepoints. The incidence and clinical significance of LAIP þ cells was studied in 196 remission marrows of 70 ALL patients (55 remaining in CCR, 14 with bone marrow relapse, one with isolated CNS relapse). The presence of >1% LAIP þ at two consecutive timepoints predicted 5/8 bone marrow relapses in B-lineage ALL. The occurrence of LAIP þ cells >1% in T-lineage ALL after induction therapy predicted relapse in 7/7 cases. In conclusion, flow cytometric detection of LAIP þ cells appears to be a powerful tool for the prediction of outcome in ALL.

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