Autologous hematopoietic stem cell (HSC) transplantation is an accepted therapy for many hematological malignancies. High dose chemo-radiation reduces tumor burden but also ablates lymphohematopoiesis. Subsequent infusion of cellular grafts containing HSC and mature lymphocytes “rescues” the host from this otherwise lethal ablation, and initiates immune reconstitution. In many systems, tumor-specific T cells are functionally tolerant in the presence of established tumor. Paradoxically, however, the infusion of these lymphocytes into irradiated tumor-bearing syngeneic recipients unmasks effector function manifested as prolonged progression-free survival when compared to recipients treated with lymphocytes from non-tumor bearing donors. We have recently demonstrated that this tolerant tumor-specific T cell population from mice with established tumor is in fact a heterogeneous mixture of naive, effector, and regulatory T cells (Tregs), which as a whole are rendered functionally unresponsive through dominant suppression. The apparent reversal of tolerance in the post-transplant setting prompted a more detailed examination of the fate of these individual components during immune reconstitution. Here, we show that CD4+ T cells specific for a model tumor antigen are hyporesponsive to antigen when isolated from mice harboring an established systemic B cell lymphoma. Upon transfer into irradiated lymphoma-bearing mice, however, these cells undergo robust antigen-driven clonal expansion, and their ability to produce interferon gamma (IFNγ) is restored. Notably, in spite of the presence of tumor in the transplant recipients, tolerance to tumor antigen was not established in the early post-transplant period, even for mice receiving naive T cells in the graft. Tumor-specific CD4+CD25+Foxp3+ Tregs isolated from the donors were found to undergo a modest tumor-antigen-driven expansion in transplant recipients. When isolated from recipients, such cells maintained expression of Foxp3 and their capacity to suppress naive T cells when cultured in vitro. However, the presence of tumor-specific Tregs failed to significantly inhibit the expansion of naive or effector T cells specific for tumor in vivo, when examined 2 weeks post BMT. Indeed, the expansion of tumor-specific effector T cells significantly exceeded the expansion of Tregs, resulting in a nearly five-fold increase in the effector:Treg ratio. At the ratios present during this phase of immune reconstitution, the frequency of Tregs was insufficient to suppress effector cell function (proliferation and IFNγ production) when studied in vitro. This accounts for the reversal of tolerance identified in the population as a whole and its capacity to mediate tumor rejection.

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