Patterns of membrane CD45 isoform expression by leukaemic blasts and normal mature myeloid cells

The membrane expression of CD45R isoforms by leukaemic blasts from 92 cases of acute myeloid (AML) and 12 cases of lymphoblastic leukaemia was analysed by single and two-colour flow cytometry. Compared to normal mature granulocytes, which invariably expressed UCHL1 (CD45RO) but not 2H4 (CD45RA), leukaemic myeloid blasts showed 2H4+ UCHL1- and 2H4- ++UCHL1- composite patterns of expression with the first of these phenotypes being associated in particular with the most immature AML subtype (M0). Similar analyses of blast cell fractions from monocytic AML variants, revealed wide variation in CD45R expression in cases of M4, whereas M5 leukaemias were typically 2H4+ ++UCHL1+ with minor 2H4- UCHL1- components. As most normal mature monocytes were also 2H4+ ++UCHL1+, this suggested that monocytic myeloid differentiation was primarily associated with the "acquisition" of UCHL1 and the development of 2H4/UCHL1 co-expressing cells. The expression of membrane CD45RA by leukaemic blasts is an abnormal characteristic and may be related to the maintenance of an undifferentiated state by malignant myeloid precursors.