Frequent expression of human Thy-1 antigen on pre-B cell acute lymphoblastic leukemia with t(9;22)

Thy-1 (CDw90) is a phosphatidylinositol-anchored protein, and is expressed on human pluripotential hematopoietic stem cells. The expression pattern of this antigen on leukemia cells is still controversial. In this study, 72 adult patients with pre-B cell acute lymphoblastic leukemia (pre-B ALL) were examined for the expression pattern of Thy-1 by using indirect immunofluorescence and reversed transcription polymerase chain reaction (RT-PCR) methods. Twelve cases were judged positive on the basis of conventional immunophenotype criteria. Thirteen cases showed a weak clonal shift on the fluorogram, even though their positive percentages were from 6.7% to 14.9%. Thy-1 gene transcripts were detected in all of the 13 cases showing a weak clonal shift. The study of antibody binding capacity, which was calculated by the mean fluorescence intensity of the test sample on the basis of a calibration curve using standard beads, showed that cases with more than 150 sites/cell could be positive. Thy-1 positivity in pre-B ALL was not associated with the expression of B-cell differentiation antigens. Thy-1 expression was significantly higher in pre-B ALL cases with karyotypic abnormalities than in those with normal karyotype (p = 0.0071). The t(9;22) abnormality was found in 18 of the 25 Thy-1+ cases. Simultaneous expression of transcriptional factors, GATA-2 and SCL, was frequently detected in the Thy-1+ cases. bcr-abl and GATA-2 are thought to play important roles in the proliferation of immature hematopoietic cells. Indeed, cell-cycle analysis showed that the cell population in the S/G2/M phase of the present Thy-1+ cases was less than that in the Thy-1- cases (p = 0.001770). Our data suggest that Thy-1 expression indicates the proliferative status of the leukemia cells, not their phenotypic immaturity.