TSCM are abundant early after allogeneic hematopoietic stem cell transplantation and are based on naive T cells that survived pt-Cy. haploidentical transplantation combined with pt-Cy and precede the expansion of effector cells. Transferred naive, but not TSCM or conventional memory cells preferentially survive cyclophosphamide, thus suggesting that posttransplant TSCM originate from naive precursors. Moreover, donor naive T cells specific for exogenous and self/tumor antigens persist in the host and contribute to peripheral reconstitution by differentiating into effectors. Similarly, pathogen-specific memory T cells generate detectable recall responses, but only in the presence of the cognate antigen. We thus define the cellular basis of T-cell reconstitution following pt-Cy at the antigen-specific level and propose to explore naive-derived TSCM in the clinical setting to overcome immunodeficiency. These trials were registered at www.clinicaltrials.gov as #”type”:”clinical-trial”,”attrs”:”text”:”NCT02049424″,”term_id”:”NCT02049424″NCT02049424 and #”type”:”clinical-trial”,”attrs”:”text”:”NCT02049580″,”term_id”:”NCT02049580″NCT02049580. Introduction Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is a potentially curative approach for blood cancers. Patients benefit from the graft-versus-tumor effect exerted by alloreactive T cells, although, at the same time, they may suffer from graft-versus-host disease (GVHD), especially in the context of T-replete transplantations. Immunosuppressants are, unfortunately, not selective Rabbit Polyclonal to RPL10L for alloreactive T cells and may thus limit adaptive immune responses to opportunistic infections and cancer.1 Depletion of T cells from the allograft prevents GVHD but results in delayed reconstitution and increased morbidity and mortality due to opportunistic infections and tumor relapse.2 High-dose cyclophosphamide given early posttransplant (pt-Cy) has been proposed to selectively spare bystander naive and memory T cells while depleting alloreactive T cells in vivo after infusion of unmanipulated grafts.3-8 Indeed, the latter are thought to proliferate quickly in the alloantigen-replenished environment, thus becoming susceptible to pt-Cy, while the former survive and promote reconstitution.9 In TC-E 5001 the first months, immune competence is in part restored in a thymus-independent fashion by proliferation of the T cells in response to increased levels of homeostatic TC-E 5001 cytokines or exogenous antigens.1,10 Production of new T cells occurs only later by resumed thymic output.10 The unmanipulated graft contains subsets of naive and memory T cells with defined specificities that display distinct proliferative and persistence capacities in response to lymphopenia.11,12 In particular, a population of early-differentiated human memory T cells with stem cellClike properties (the T memory stem cells [TSCM]) has been reported to preferentially reconstitute immunodeficient mice compared with more differentiated central memory (TCM) and effector memory TC-E 5001 (TEM) T cells.13 A recent study suggested that the posttransplant lymphopenic environment may favor the generation of TSCM from naive precursors.14 Nevertheless, naive or TSCM cells are relatively absent early after transplantation,3,15-17 thus rendering unclear to what extent these T-cell subsets contribute to reconstitution. The persistence and expansion of the transferred T cells would confer protection toward opportunistic infections and cancer. In this regard, whether pt-Cy differentially affects donor T-cell subsets at the polyclonal and antigen-specific levels remains undetermined. Materials and methods Patients and transplantation procedures Thirty-nine consecutive patients were treated according to the haploidentical hematopoietic stem cell transplantation (haplo-HSCT) protocol established by Luznik et al.4 All experiments display biological replicates from different patients or healthy donors who were randomly selected, unless specified (such as for the study of antigen-specific responses). Details about the transplantation procedure are available in supplemental Methods (available on the Web site). Patient characteristics are listed in supplemental Table 1. Patients and donors signed consent forms in accordance with the Declaration of Helsinki. Clinical and experimental protocols were approved by the institutional review board of Humanitas Research Hospital and Istituto Nazionale Tumori. Flow cytometry and cell sorting Monoclonal antibodies (described in supplemental Methods) were TC-E 5001 purchased from BD Biosciences and BioLegend, unless specified otherwise, or conjugated in-house (http://www.drmr.com/abcon). Frozen cells were thawed and prepared for flow cytometry as described previously.18 Chemokine receptor expression was revealed by incubating cells at 37C for 20.