Proposed model of the cellular mechanisms leading to T-cell reconstitution following haplo-HSCT and pt-Cy. Naive (T_N) and memory T cells (T_MEM) are infused in the recipient with the BM. Allogeneic antigens (allo-Ags), as well as inflammatory/homeostatic cytokines, the availability of which increases after chemotherapy, induce T-cell activation. Proliferating (HLA-DR⁺, Ki-67⁺) T cells uniformly acquire an effector phenotype, irrespective of their original differentiation status, and are preferentially depleted by Cy, given at day 3 and 4 after HSCT. T stem cell memory (T_SCM) is the dominant peripheral T-cell subset at day 7, likely originating from T_N that survived Cy. In the following weeks, naive-derived T_SCM generate memory cells in response to exogenous antigens and, presumably, homeostatic cytokines. Adoptively transferred T_MEM, which have survived Cy, expand to detectable levels in the circulation only in the presence of the cognate antigen. Whether T-cell memory can persist in the haplo-HSCT individual in the absence of the cognate antigen is currently unknown.