An increasing number of clinical trials have demonstrated the usefulness of cord blood as a source of hematopoietic stem cells for reconstitution of the hematopoietic system. Nevertheless, due to a lack of convenient animal models, information about the immunological competence of umbilical cord blood cell (UCBC)-derived lymphocytes has been relatively limited. Recently, we have established a murine model of UCBC transplantation, which reconstitutes the hematopoietic system of immunodeficient mice, and studied the correct immunological functions of UCBC-derived lymphocytes generated in an allogeneic environment.
UCBC prepared from C57BL/6 (H-2b) mice were transferred into lethally irradiated BALB/c (H-2d) mice lacking T- and B-lymphocytes (RAG2 knockout). In order to evaluate the adaptive immune response in the recipient mice, rejection of skin grafts from third-party C3H/He (H-2k) mice and antibody responses to immunization with a T-dependent antigen, 2,4,6-trinitrophenyl-keyhole limpet hemocyanin (TNP-KLH), were assessed. Additionally, the responsiveness of generated T cells was verified by mixed lymphocyte reaction (MLR) assay, cytotoxic T-lymphocyte (CTL) assay and phytohemagglutinin (PHA) blast formation assay.
In this UCBC transplantation model, the overall survival of the recipient mice was dose-dependent, and all surviving recipient mice were hematopoietic chimeras possessing all lineages. These allogeneic chimeras specifically rejected skin grafts from third-party C3H/He mice (rejection time; 9–16 days) while showing tolerance to skin grafts from both donor-type and recipient type, although with a significant delay in comparison to normal BALB/c mice (8–12 days) and C57BL/6 mice (9–11 days). Although accurate antibody responses of the allogeneic chimeras against TNP-KLH immunization were not anticipated, substantial amounts of TNP-specific IgG were detected in their sera. Interestingly, the level of TNP-specific IgM was significantly higher than that of normal BALB/c mice after TNP-KLH immunization, indicating retarded immunoglobulin class-switching in the allogeneic chimeras. The mechanism responsible for the antibody production by allogeneic chimeras still remains unclear. CTL and MLR assay revealed cell-mediated third-party-specific activity of killer and helper T cells, consistent with the tendency for the skin graft rejection and antibody production capability of the allogeneic chimeras to be inferior to those of normal mice. T cells of the allogeneic chimeras also proliferated non-specifically in response to PHA stimulation. The T-cell proliferation indices of the allogeneic chimeras when those of normal mice were taken as 100 were 21 in the MLR assay and 48 in the PHA blast formation assay, suggesting relative inferiority of the recognition and response system of T cells generated in an allogeneic environment. In any event, it may be important to note that T-dependent antigen-specific antibody production and alloantigen-specific rejection were substantial in the allogeneic chimeras.
Overall, the present findings suggest that UCBC-derived lymphocytes generated in HLA-mismatched recipients are immunologically functional and competent. Regarding the relative inferiority of immune responses in the allogeneic chimeras, we predict the causes originate from narrowing of the T- and B-cell receptor repertoire by the allogeneic environment, and this possibility is now being examined.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.