We show that the lymphoid hyperplasia observed in IL-2Rα- and IL-2-deficient mice is due to the lack of a population of regulatory cells essential for CD4 T cell homeostasis. In chimeras reconstituted with bone marrow cells from IL-2Rα-deficient donors, restitution of a population of CD25+CD4+ T cells prevents the chaotic accumulation of lymphoid cells, and rescues the mice from autoimmune disease and death. The reintroduction of IL-2-producing cells in IL-2-deficient chimeras establishes a population of CD25+CD4+ T cells, and restores the peripheral lymphoid compartments to normal. The CD25+CD4+ T cells regulated selectively the number of naive CD4+ T cells transferred into T cell-deficient hosts. The CD25+CD4+/naive CD4 T cell ratio and the sequence of cell transfer determines the homeostatic plateau of CD4+ T cells. Overall, our findings demonstrate that IL-2Rα is an absolute requirement for the development of the regulatory CD25+CD4+ T cells that control peripheral CD4 T cell homeostasis, while IL-2 is required for establishing a sizeable population of these cells in the peripheral pools.

Purpose: The Vd1 þ subset of gd T lymphocytes is a promising candidate for cancer immunotherapy, but the lack of suitable expansion/differentiation methods has precluded therapeutic application. We set out to develop and test (preclinically) a Vd1 þ T-cell-based protocol that is good manufacturing practice compatible and devoid of feeder cells for prompt clinical translation. Experimental design: We tested multiple combinations of clinical-grade agonist antibodies and cytokines for their capacity to expand and differentiate (more than 2-3 weeks) Vd1 þ T cells from the peripheral blood of healthy donors and patients with chronic lymphocytic leukemia (CLL). We characterized the phenotype and functional potential of the final cellular product, termed Delta One T (DOT) cells, in vitro and in vivo (xenograft models of CLL).Results: We describe a very robust two-step protocol for the selective expansion (up to 2,000-fold in large clinicalgrade cell culture bags) and differentiation of cytotoxic Vd1

To fulfill its mission, the immune system must maintain a complete set of different cellular subpopulations that play specific roles in immune responses. We have investigated the mechanisms regulating CD4+CD25+ regulatory T (Treg) cell homeostasis. We show that the expression of the high-affinity IL-2Rα endows these cells with the capacity to explore the IL-2 resource, ensuring their presence while keeping their number tied to the number of CD4+ T cells that produce IL-2. We show that such a homeostatic mechanism allows the increased expansion of T cells without causing disease. The indexing of Treg cells to the number of activated IL-2-producing cells may constitute a feedback mechanism that controls T cell expansion during immune responses, thus preventing autoimmune or lymphoproliferative diseases. The present study highlights that maintenance of proportions between different lymphocyte subsets may also be critical for the immune system and are under strict homeostatic control.

Haematopoiesis is a developmental cascade that generates all blood cell lineages in health and disease. This process relies on quiescent haematopoietic stem cells capable of differentiating, self renewing and expanding upon physiological demand. However, the mechanisms that regulate haematopoietic stem cell homeostasis and function remain largely unknown. Here we show that the neurotrophic factor receptor RET (rearranged during transfection) drives haematopoietic stem cell survival, expansion and function. We find that haematopoietic stem cells express RET and that its neurotrophic factor partners are produced in the haematopoietic stem cell environment. Ablation of Ret leads to impaired survival and reduced numbers of haematopoietic stem cells with normal differentiation potential, but loss of cell-autonomous stress response and reconstitution potential. Strikingly, RET signals provide haematopoietic stem cells with critical Bcl2 and Bcl2l1 surviving cues, downstream of p38 mitogen-activated protein (MAP) kinase and cyclic-AMP-response element binding protein (CREB) activation. Accordingly, enforced expression of RET downstream targets, Bcl2 or Bcl2l1, is sufficient to restore the activity of Ret null progenitors in vivo. Activation of RET results in improved haematopoietic stem cell survival, expansion and in vivo transplantation efficiency. Remarkably, human cord-blood progenitor expansion and transplantation is also improved by neurotrophic factors, opening the way for exploration of RET agonists in human haematopoietic stem cell transplantation. Our work shows that neurotrophic factors are novel components of the haematopoietic stem cell microenvironment, revealing that haematopoietic stem cells and neurons are regulated by similar signals.