Leukemia-associated changes identified by quantitative flow cytometry. II. CD5 over-expression and monitoring in B-CLL.

The CD5 antigen density on B cells was studied on fetal spleen, cord blood, and adult peripheral blood (after immunomagnetic bead purification) using an indirect immunofluorescence technique. In fetal spleen, there was a continuum in CD5 expression, whereas all cord blood and less than 20% adult peripheral blood B cells were CD5+. Mean CD5 antigen density on these normal cells was low (3-6 x 10(3) molecules/cell); eight to 20 times lower than on normal T lymphocytes. In adult blood, less than 10% B cells expressed more than 3 x 10(3) CD5 molecules/cell. In chronic malignancies, 34/35 cases had a CD5 antigen density lower than on residual T cells, but mean antigen density was higher (14.8 +/- 2.1 x 10(3) molecules/cell) than on normal B cells. Sixteen cases of chronic lymphocytic leukemia (50%) expressed a CD5 density above 10 x 10(3) molecules/cell. This aberrantly high CD5 expression was used to detect neoplastic cells after dilution in normal lymphocytes, with a limit of detection between 1:100 and 1:1000. Quantitation of the CD5 antigen allows better characterization of the B1 population and should be used for the monitoring of chronic malignancies.