SummaryThe antigenic phenotype of neonatal lymphoid cells isolated from umbilical cord blood was investigated using monoclonal antibodies and flow cytometry. Although the majority ofcells expressed mature T or B cell differentiation antigens, small subpopulations of phenotypically immature lymphocytes were detected. A small proportion (mean 2-8%) ofcells expressed the common acute lymphoblastic leukaemia antigen (CD-10), a significantly higher figure than that detected on adult peripheral blood lymphocytes. The cortical thymocyte antigen (CD-I) was detected on a very small subset of eord lymphoid eells, but was also present on adult lymphocytes at approximately the same frequency. The nuclear enzyme terminal deoxynucleotidyl transferase (TdT). a marker of early lymphoid differentiation, was detected by immunofluoreseenee on 0 031% of mononuclear cells in cytocentrifuge preparations, representing an approximate 10-fold increase in frequency over expression in childhood or adult blood. These circulating TdT+ cells were shown in double labelling experiments to predominantly express markers of B eell differentiation (CD-24, CD-10. MHC Class 2), although occasional cells co-expressing the T lineage marker CD-2 were also seen. These findings are consistent with the circulation of B cell precursors in neonatal blood. The nature ofthe CD-I + cells is unclear, although the absence of CD-1 ^ TdT"*" double labelled cells mitigates against the possible presenee of immature thymus-processed lymphocytes in these samples.

Blast cells from untreated cases of acute leukemia were examined for expression of terminal transferase enzyme (TdT) by immunofluorescence and for myeloid lineage commitment as demonstrated by the presence of myeloperoxidase enzyme at the ultrastructural level. In three cases, an overlapping expression of the "lymphoid-specific" marker TdT was found on peroxidase-positive myeloblasts. These results indicate either the clonal expansion of a rare TdT-positive myeloid precursor or inappropriate expression of TdT by malignant myeloblasts, and further illustrate that TdT expression must be interpreted with caution when distinguishing between lymphoid and myeloid leukemias.

A mouse monoclonal antibody, WM-15, has been developed which reacts with a human myeloid lineage-restricted cell surface antigen. WM-15, an IgG1 antibody, reacts with a mean of 3.8% of normal bone marrow mononuclear cells, identifying predominantly promyelocytes and myeloblasts, and in fluorescence-activated cell sorting experiments produces enrichment of bone marrow granulocyte-macrophage progenitor cells. Normal mature monocytes and granulocytes are weakly labelled by WM-15, but other haemopoietic cells including erythroblasts and all lymphoid cells are unreactive. The myeloid specificity of this antibody is highlighted by its reactivity with myeloid leukaemia cell lines and 65% of cases of acute myeloid leukaemia, while lymphoid cell lines and leukaemias are WM-15 negative. WM-15 appears to be a useful reagent for further investigation of normal and abnormal myelopoiesis.

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