Anomalous phenotype in thymic acute lymphoblastic leukaemia

Abstract

Malignant blast cells in the various forms of acute leukaemia seem to be ‘frozen’ at early stages of haematopoietic and lymphoid cell maturation. These blasts express relatively stable morphological, enzymatic and antigenic phenotypes which may largely represent the normal gene products of the corresponding precursor cells^1–4. A number of findings support these suggestions. The phenotype of the common form of acute lymphoblastic leukaemia (cALL) corresponds to the phenotype of normal small non-T, non-B cells of lymphoid morphology detected in low numbers in the normal and regenerating infant bone marrow^4,5. Also, the phenotype of blast cells in Thy-ALL corresponds approximately to that of cortical thymocytes or their immediate precursors^6–9. Leukaemia-specific (virus-induced or mutant) changes or aberrant expression of normal gene products may also occur in leukaemia. To describe such phenomena, detailed single cell studies comparing leukaemic cells with their appropriate normal (frequently very rare) counterparts are needed. In this study we have performed such a comparative study and find that the human Thy-ALL blasts express unexpectedly high amounts of HLA-A, -B and -C antigens: the sensitive single cell assays used failed to find normal cells (in infant and fetal thymus, and in bone marrow) which express the exact phenotype of Thy-ALL blasts.