Characterization of blast cells in chronic granulocytic leukaemia in transformation, acute myelofibrosis and undifferentiated leukaemia II. STUDIES WITH MONOCLONAL ANTIBODIES AND TERMINAL TRANSFERASE

A panel of 19 monoclonal antibodies (McAb) and the enzyme terminal transferase (TdT) have been applied to the characterization of poorly differentiated blasts from 50 patients with chronic granulocytic leukaemia (CGL) and myelofibrosis in blast crisis (BC), acute myelofibrosis and undifferentiated leukaemia. These cells were also extensively studied by transmission electron microscopy (TEM) (see Polli et al, 1985a). McAb against platelet glycoproteins (GP) showed a high specificity for megakaryoblasts, in particular those reactive with the GPIIb/IIIa complex (J15) and GPIIIa (C15 and C1 7), which were positive in a higher proportion of blasts than the McAb to GPIb (AN51 and FMC25). Findings with these anti‐platelet McAb paralleled those of the platelet‐peroxidase (PPO) reaction in 76% of cases studied simultaneously. The PPO reaction was always positive in cases in which two or more of the McAb were reactive with the blast cells. The differences observed suggest, nevertheless, that PPO is more sensitive for megakaryoblasts than the McAb and that this TEM technique should be reserved for cases which are negative with the platelet specific McAb. Of the McAb against myeloid antigens used in this series OKM1 was positive in 50% of cases but the others failed to demonstrate early features of differentiation in myeloblasts and monoblasts. In only three cases were erythroid precursors demonstrated by TEM and these were the only ones reactive with a McAb to glycophorin‐A (LICR LON/R10). TdT and the McAb J5 helped in the identification of lymphoblasts which were seen as a ‘pure’proliferation in 23% of CGL‐BC and as case that showed frank reactivity with FMC10, 11 and 13 in this study the blasts were mast cell precursors and this may suggest that these leukaemic cells are more mature. The greater sensitivity of OKM1 to detect granulocytic/monocytic differentiation shown here confirms the findings of van der Reijden et al (1983) in AML and of Griffin et al (1983b) in CGL‐BC with a similar McAb, Mol. The demonstration of TdT and the cALL antigen by the McAb J5 showed that blast cells which may be considered undifferentiated or even myeloid on light microscopy morphology were in fact lymphoblasts. In this respect our study confirms a 20–25% incidence of pure lymphoid phenotypes in Ph1‐positive CGL‐BC (Janossy et al, 1980; Greaves et al, 1982; Griffin et al, 1983b). If we consider also the minor populations of lymphoblasts seen in the mixed cases, the incidence of cells with a lymphoid phenotype in CGL‐BC was 40% in this series. The use of McAb against early precursor cells (RFB1, FMC8 and OKIa) did not show a particular advantage for the clarification of the cell lineage of the blasts in the cases under study, although all of them were positive in lymphoblasts. It was of interest, however, that in the mixed cases these reagents usually detected the majority of blasts whilst the lineage specific McAb were reactive with some of the cell populations. A number of mixed blast cell proliferations were demonstrated in this study. TEM analysis has been valuable for distinguishing the morphology and cytochemical markers of the distinct cell populations involved (Polli et al, 1985a). Findings with the various lineage specific McAb were very often suggestive of the presence of more than one type of blast cell (Table VII). Light microscopy studies combining the demonstration of TdT in the cell nucleus with that of an antigen in the cell membrane have allowed us to distinguish megakaryoblasts (TdT negative, AN51 positive) from lymphoblasts (TdT positive, AN51 negative).