Successful implementation of bone marrow transplantation for hematopoletic reconstitution is limited by the lack of suitably IELA-matched donors and by the occurrence of graft-versus-host disease that frequently accompanies this procedure. Recent clinical reports have implied that the use of umbilical cord blood as a source of transplantable stem cells may solve these problems. To date, definitive experiments have not been performed to assess the immunological potential of T cells found in umbilical cord blood, which could mediate graft-versus-host disease. In the present study we have observed that umbilical cord blood contains T lymphocytes that appear to be phenotypically immature. In addition, umbilical cord blood lymphocytes appeared to be functionally immature as shown by minimal responses to stimulation with interleukin 2, phytohemagglutinin, or alloantigens. Thus, umbilical cord blood may be more suitable for allogeneic transplantation than bone marrow in that these cord blood cells may not be as capable of mediating graft-versus-host disease.Bone marrow transplantation plays an integral part in the treatment of various cancers and inherited blood disorders. The ideal donor for bone marrow transplantation is an identical twin, the next best donor being an HLA-matched sibling. As 1 ± 2 8 ± 4 5 ± 5 7 ± 3 2 ± 2 10 + 5 4 ± 3 2 ± 3 5 ± 4 9 ± 5 3 ± 2 84 ± 6 PBL (n = 13) 62 + 11 38 ± 9 22 ± 6 63 ± 8 3 ± 3 11 ± 5 13 ± 8 8 ± 2 6 ± 5 14 ± 6 6 ± 3 17 ± 6 0 ± 0 0 ± 0 1 ± 0.4 53 ± 20 HUCB and PBL mononuclear cells were isolated and analyzed by flow cytometry. The mean percentage ofcells positive for each ofthe indicated antigens is shown, as well as the standard deviation for each population of cells. The data were derived from independent analysis of 50 HUCB and 13 PBL samples. Data were analyzed by t test, as reported in the text. TCR, T-cell antigen receptor.CD45RA phenotype. More importantly, JfUCB T cells expressed the CD3 antigen at lower levels than jlid PBL T cells (Fig. 1). The average mean fluorescence of d>3 expression on T cells of five randomly selected HUCB samples was 599 + 9.8 (mean ± SEM) versus 659 ± 6.7 for the CD3 expression of T cells from five randomly selected PBL samples.In addition, percentages of B cells (CD19+ and CD24+), monocytes (Macl+) and NK cells (CD16+) were comparable to those in PBL samples, but s50% of the B cells in cord blood were of the CD5+ phenotype, indicative of immature lymphocytes. Relatively few cells in HUCB expressed class II HLA antigens, and cells expressing the HLA-DP antigens were barely detectable. Those HUCB cells that did express class II HLA antigens did so at a level of expression that was at least an order of magnitude lower than that found on PBLs (as determined by the mean channel fluorescence) (data not shown). Functional Analysis of T Lymphocytes from HUCB. HUCB was analyzed immediately upon isolation for the ability ofthe cells to mediate cytolysis. No lytic activity was detectable in fresh HUCB cells, neither NK cell-like lysis (as measured a...

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