T cell differentiation in allogeneic bone marrow chimeras

K Onoe MD PhD^a, K Iwabuchi MD PhD^a, RA Good MD^b

^aDivision of Immunobiology, Institute for Genetic Medicine, Hokkaido University, Sapporo, Japan, ^bDepartment of Pediatrics, University of South Florida/All Children's Hospital, FL, Tampa

AIM: To investigate the molecular and cellular basis of differentiation and selection of cells of T-lineage in the thymus using allogeneic bone marrow (BM) chimeras. METHODS: Allogeneic BM chimeras were prepared using various combinations of donor and host strains of mice. Generation of the T-lineage cells and their sequential changes of surface phenotypes and functions were analyzed. RESULTS: T cells acquired immune functions during expansion and differentiation in the thymus. However, helper T cells from MHC incompatible chimeras (i.e. [B10, H-2b→AKR, H-2k] were unable to generate antibody response s to a T-dependent antigen (Ag). It was shown that T cells acquired self-MHC recognition system (self-MHC restriction specificity) a posteriori in the presence of MHC (H-2k) expressed on the cortical epithelial cells in the host thymus. Thus, helper T cells derived from donor BM cells could not cooperate with Ag presenting cells (APC) expressing donor MHC (H-2b). On the other hand, T cells from allogeneic chimeras generated no aggressive responses to both donor and host Ag. Using a super Ag system (Mls-1a), it was shown that clonal elimination of Vb6+ T cells that are reactive to Mls-1a plus I-E, an MHC class II molecule, was induced by the Mls-1a plus I-E on the BM-derived APC in the thymic medulla. Then, differentiation of NK-T cell, a new member of T -lineage cells, was analyzed using BM chimeras prepared by aly/aly donor and b2m knockout host mice, both of which showed severe deficiency in NK-T cell generation. We found that NK-T cells were normally generated in [aly/aly →beta2m knockout] chimeras. It was shown that both CD1, a non-polymorphic MHC I-like molecule, on aly/aly-derived thymocytes and medullary epithelial cells in the thymus of beta2m knockout host mice were indispensable for generation of NK-T cells. CONCLUSIONS: Differentiation and functions of cells of T-lineag e were not genetically determined but acquired a posteriori by positive and negative selection in the thymus in harmony with the functional maturation. Allogeneic BM chimera systems have been a useful strategy to elucidate mechanisms underlying the positive and negative selection of these T cells.

Paper presented at the International Symposium on Predictive Oncology and Intervention Strategies; Nice, France; February 7 - 10, 2004; in plenary session 901 (Immunobiology).