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Activation-induced cell death (AICD) of lymphocytes is an apoptotic pathway involved in the control of T-cell homeostasis. The magnitude of graft-vs.-host disease (GVHD) following allogeneic hematopoietic cell transplantation may be attenuated by the enhancement of AICD. The aim of the present study was to clarify the effect of T cell dose upon the fate (proliferation or apoptosis) of individual activated T cells in a murine GVHD model. To this end, we investigated the kinetics of the proliferation and apoptosis of donor T cells in recipient spleens in the early stage of a fully major histocompatibility complex (MHC)-mismatched murine transplantation model from C57BL/6 (H-2(b)) to lethally-irradiated (8.5 Gy) BALB/c (H-2(d)) mice. To track the behavior of alloreactive lymphocytes in vivo, we used the fluorescent cytoplasmic dye carboxyfluorescein diacetate succinimidyl ester in combination with flow cytometry. Engraftment of donor T cells to recipient spleens was almost completed within 24 h after transfer. After that, at higher doses of transferred cells, the donor T cells actively divided for up to 72 h resulting in a 30-fold increase in cell number at the maximum cell dose (2.0 x 10(7)). As the transferred cell dose decreased, the proliferation of T cells tended to be suppressed. At cell doses of 0.5 x 10(7) or less, the proliferation of T cells was profoundly suppressed, ultimately resulting in little proliferation of donor T cells observed from 24 to 72 h at the minimum cell dose (0.1 x 10(7)). The frequency of Annexin-V-positive cells was found to increase gradually as the transferred cell dose decreased. Thus, an increase in apoptotic events appeared to play an important role in the suppression of the proliferation of T cells at lower splenocyte doses. Further analyses revealed that Fas ligand (FasL)-positive T cells were observed exclusively among T cells that divided at least 5 times, and that all of them were positive for Annexin-V, indicating that they were in the process of apoptosis. Together with our finding that the frequency of apoptosis increased with the progression of cell division, these findings strongly suggest that AICD occurred through the Fas/FasL system and that AICD increased as the dose of donor T cells participating in the allogeneic response decreased. When relatively small numbers of T cells are confronted with an excess of antigen, they disappear. This process is called clonal exhaustion-deletion. Our results support the idea that AICD is involved in the process of clonal exhaustion-deletion. Relevant to the clinical aspects of hematopoietic cell transplantation, our findings indicate that AICD may be associated with tolerance induction in T-cell-depleted transplantation from HLA-mismatched donors, in which T cells contaminating marrow grafts do not need to be completely removed for achieving tolerance between donors and recipients. Furthermore, our results indicate that a small change in the quantitative balance between antigens and T cells responding to them leads to a large difference in the fate of T cells activated in response to MHC-incompatible antigens. Thus, the size of the T cell dose is one of the important considerations in tolerance induction, GVHD and rejection.