Phenotypic characterization of immature lymphoid cells in human umbilical cord blood

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Abstract
The antigenic phenotype of neonatal lymphoid cells isolated from umbilical cord blood was investigated using monoclonal antibodies and flow cytometry. Although the majority of cells expressed mature T or B cell differentiation antigens, small subpopulations of phenotypically immature lymphocytes were detected. A small proportion (mean 2·8%) of cells 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‐1) was detected on a very small subset of cord lymphoid cells, 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 immunofluorescence 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 cell 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 of the CD‐1+ cells is unclear, although the absence of CD‐1+ TdT+ double labelled cells mitigates against the possible presence of immature thymus‐processed lymphocytes in these samples.