Immunological monitoring of residual disease in treated thymic acute lymphoblastic leukaemia

Abstract

Combinations of antisera to human differentiation-linked antigens were used in an indirect immunofluorescence system to identify residual leukaemic blast cells in bone marrow samples from 18 patients treated for the thymic variant of acute lymphoblastic leukaemia (Thy-ALL). In six patients, 0.5 to 5.0% of nucleated cells in marrow samples taken during apparent disease remission were found to have an antigenic phenotype not normally demonstrable on bone marrow cells. These cells contained terminal transferase enzyme, expressed surface human T lymphoid antigens (and in some cases cortical thymocyte antigen) but lacked Ia-like antigens. This phenotype was characteristic of Thy-ALL blasts at the time of diagnosis and at relapse indicating that these minor subpopulations were leukaemic. In the remaining 12 cases, no cells of abnormal phenotype were detected, although some of these patients later relapsed with cells of typical Thy-ALL phenotype. The above findings indicate that Thy-ALL cells may be identified during bone marrow remission by virtue of their expression of a unique combination of differentiation antigens in an anatomically inappropriate site, and that this technique is more sensitive than conventional morphological analysis. The observations also provide information about shifts of membrane marker expression in Thy-ALL.