Phenotypic characterization of immature lymphoid cells in human umbilical cord blood

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.