Odile Burlen-Defranoux

On the ontogeny and physiology of regulatory T cells

Summary: Lymphocytes can interfere with the activity of other lymphocytes in a thousand and one ways. A particular subset of so‐called regulatory CD4+ T cells is capable of controlling the activity of other lymphocytes in yet another way. Their function is primarily defined by the ability to protect the integrity of tissues and organs in vivo. This was demonstrated in experimental models of natural tolerance to peripheral tissues, transplantation tolerance and the regulation of immune responses promoted by exogenous antigens at the level of the intestinal mucosa. Moreover, regulatory T cells also play a major role in the systemic homeostatic mechanisms that control total lymphocyte numbers. There is good evidence to support the contention that a significant fraction of the naturally occurring regulatory T cells is generated in the thymus following selection mediated by high avidity T‐cell receptor/ligand interactions. Symbolically, self‐reactive regulatory T cells do represent the breakthrough of concepts challenging the long‐lasting Burnetian dogma that all autoreactive cells should be eliminated or inactivated. Although clonal deletion of self‐reactive cells is a fundamental process in T‐cell development, controlled autoreactivity is part of the physiology of the immune system. Thus, autoreactive regulatory T cells also protect immunologists from the desperate hunting for the evil of horror autotoxicus.

Immature hematopoietic stem cells undergo maturation in the fetal liver

Hematopoietic stem cells (HSCs), which are defined by their capacity to reconstitute adult conventional mice, are first found in the dorsal aorta after 10.5 days post coitus (dpc) and in the fetal liver at 11 dpc. However, lympho-myeloid hematopoietic progenitors are detected in the dorsal aorta from 9 dpc, raising the issue of their role in establishing adult hematopoiesis. Here, we show that these progenitors are endowed with long-term reconstitution capacity, but only engraft natural killer (NK)-deficient Rag2γc–/– mice. This novel population, called here immature HSCs, evolves in culture with thrombopoietin and stromal cells, into HSCs, defined by acquisition of CD45 and MHC-1 expression and by the capacity to reconstitute NK-competent mice. This evolution occurs during ontogeny, as early colonization of fetal liver by immature HSCs precedes that of HSCs. Moreover, organ culture experiments show that immature HSCs acquire, in this environment, the features of HSCs.

Origin, trafficking, and intraepithelial fate of gut-tropic T cells

Tropism to the small intestinal epithelium is a general property of unconventional and conventional recent thymic emigrants, but for both cell types only GALT-related cycling thoracic duct lymphocytes are the precursors of cytotoxic intraepithelial lymphocytes.

Two waves of distinct hematopoietic progenitor cells colonize the fetal thymus

The generation of T cells depends on the migration of hematopoietic progenitor cells to the thymus throughout life. The identity of the thymus-settling progenitor cells has been a matter of considerable debate. Here we found that thymopoiesis was initiated by a first wave of T cell lineage–restricted progenitor cells with limited capacity for population expansion but accelerated differentiation into mature T cells. They gave rise to αβ and γδ T cells that constituted Vγ3+ dendritic epithelial T cells. Thymopoiesis was subsequently maintained by less-differentiated progenitor cells that retained the potential to develop into B cells and myeloid cells. In that second wave, which started before birth, progenitor cells had high proliferative capacity but delayed differentiation capacity and no longer gave rise to embryonic γδ T cells. Our work reconciles conflicting hypotheses on the nature of thymus-settling progenitor cells.

Notchless-dependent ribosome synthesis is required for the maintenance of adult hematopoietic stem cells

Conditional deletion of Notchless leads to rapid deletion and exhaustion of HSCs and early progenitor cells, whereas committed progenitor cells survive as a result of differences in ribosomal biogenesis.

Extrathymic induction of Foxp3 + regulatory T cells declines with age in a T-cell intrinsic manner

Extrathymically induced Foxp3+ regulatory T (Treg) cells contribute to the pool of Treg cells and are implicated in the maintenance of immune tolerance at environmental interfaces. The impact of T‐cell senescence on their generation and function is, however, poorly characterized. We report here that steady‐state induction of Foxp3 is impaired in aged T cells in vivo. In vitro assays further revealed that this defective generation of Treg cells was independent from the strength of TCR stimulation and arose before T‐cell proliferation. Importantly, they also revealed that this impairment of Foxp3 induction is unrelated to known age‐related T‐cell defects, such as IL‐2 secretion impairment, accumulation of activated T‐cell populations, or narrowing of the T‐cell repertoire. Finally, a loss of extrathymic induction of Foxp3 and tolerance to minor‐mismatched skin graft were observed in aged mice treated by nondepleting anti‐CD4 antibody. The T‐cell intrinsic impairment of Treg‐cell generation revealed here highlights age as a key factor to be considered in immune tolerance induction.

Critical Role of TCR Specificity in the Development of Vγ1Vδ6.3+ Innate NKTγδ Cells

A large fraction of innate NKTγδ T cells uses TCRs composed of a semi-invariant Vδ6.3/6.4-Dδ2-Jδ1 chain together with more diverse Vγ1-Jγ4 chains. To address the role of γδTCR specificity in their generation, we analyzed their development in mice transgenic (Tg) for a Vγ1-Jγ4 chain frequently expressed by NKTγδ cells (Tg-γ) and in mice Tg for the same Vγ1-Jγ4 chain together with a Vδ6BDδ2Jδ1 chain not usually found among NKTγδ cells (Tg-γδ). Surprisingly, both promyelocytic leukemia zinc finger (PLZF)+ and NK1.1+ NKTγδ cells were found in the thymus of Tg-γδ albeit at lower numbers than in Tg-γ mice, and virtually all of them expressed the Tg TCR. However, the PLZF+ subset, but not the NK1.1+ subset, also expressed an endogenous Vδ6.3/6.4 chain, and its size was severely reduced in TCRδ−/− Tg-γδ mice. These results could suggest that the PLZF+ and the NK1.1+ subsets are developmentally unrelated. However, PLZF+ and NK1.1+ NKTγδ cells express identical Vδ6.3/6.4 chains, and NK1.1+ cells can be obtained upon intrathymic injection of sorted PLZF+ cells, thus indicating their developmental relationship. In fact, the NK1.1+ γδ thymocytes present in Tg-γδ mice correspond to a small subset of NK1.1+ γδ thymocytes in wild-type animals, which express a more diverse repertoire of TCRs and can be recognized by the expression of the CD62L Ag. Collectively, our data demonstrated that TCR specificity is essential for the development of most NKTγδ T cells and revealed a developmental heterogeneity in γδ T cells expressing the NK1.1 marker.

Neonatal Tolerance to Alloantigens is Induced by Enriched Antigen‐Presenting Cells

Spleen and bone marrow cells from normal or Rag‐1‐deficient donors are equally competent in their ability to induce neonatal transplantation tolerance in semi‐allogeneic hosts, and the latter are also capable of tolerizing fully allogeneic recipients. Both types of donor cells resulted in comparable levels of haemopoietic chimerism in tolerant animals. Lymphoid hyperactivity, however, was absent in animals tolerized with Rag‐1‐deficient cells. The authors conclude that neonatal tolerance induced with haemopoietic cells requires no donor lymphocytes, and is thus not the result of deficient antigen presentation. Furthermore, the state of tolerance can be dissociated from the lymphoid hyperactivity that requires donor lymphocytes and is regularly scored in conventionally tolerized animals.

17-P017 Notchless regulates adult hematopoietic stem cell homeostasis

The enteric nervous system (ENS) is a complex network of neurons and glia within the gut wall which originate from neural crest cells. Self-renewing, multipotential ENS progenitors have been isolated from the gut of foetal as well as adult rodents, however, the identity of the ENS progenitor and the regulation of its neurogenic potential invivo, are currently unknown. Sox10 is an HMG-containing transcriptional regulator expressed in ENS progenitors and in glia but not in neurons. To lineally mark the progeny of Sox10-expressing cells, we used Sox10Cre; R26ReYFP transgenics. Our analysis shows that both the Sox10 neurons and the Sox10 glia cells are derived from Sox10-expressing progenitors. To examine the temporal regulation of the neurogenic potential of Sox10 ENS progenitors, we generated Sox10iCreER transgenics. Analysis of Sox10iCreER; R26ReYFP transgenics exposed to tamoxifen at different time points showed that the neurogenic potential of Sox10-expressing progenitors decreases progressively during embryogenesis and is undetectable at some point between P30 and P84. These findings raise the question of the origin of multilineage ENS progenitors isolated from cultures of post-neurogenic gut. To address whether such progenitors originate from Sox10-expressing glial cells, we cultured dissociated myenteric plexus of Sox10iCreER; R26ReYFP transgenics exposed to tamoxifen at p84. In such cultures, glial cells proliferate, and at least a subset of them can give rise to nNos, VIP and NPY neurons. Taken together, our data suggest that, although Sox10expressing cells in the ENS of adult animals loose their neurogenic capacity invivo, they can be activated to generate self renewing, multipotential progenitors.