Thymosin induces helper function in OKT3-positive, E-rosette-negative human cord blood T cells

E-rosette-negative lymphocytes (TE-), which express monoclonal T-cell markers (OKT3+) were found in higher frequencies in human cord blood (CB) (30% by phenotype and 11.7% by sequential rosette isolation) than in adult peripheral blood lymphocytes (13% by phenotype and 3.7% by isolation). TE- cells had very low levels of 5'-nucleotidase, a lymphocyte maturational marker low in thymocytes. Like other CB T cells, TE- cells possessed marked suppressor capability. Irradiated TE- cells from two cord blood samples demonstrated helper activity for Ig production by adult B cells. In six cord blood samples, however, irradiated TE- cells lacked helper activity. When incubated with thymosin, these TE- cells E+ and differentiated into inducer-helper T cells. These observations confirm previous assumptions that CB T cells are not fully differentiated. Furthermore, T cells from different CB samples may be at various stages of ontogeny. We have also shown that thymosin fraction 5 is able to confer a new immunoregulatory function.