We have recently shown that expression of the enzyme indoleamine 2,3-dioxygenase (IDO) during murine pregnancy is required to prevent rejection of the allogeneic fetus by maternal T cells. In addition to their role in pregnancy, IDO-expressing cells are widely distributed in primary and secondary lymphoid organs. Here we show that monocytes that have differentiated under the influence of macrophage colony-stimulating factor acquire the ability to suppress T cell proliferation in vitro via rapid and selective degradation of tryptophan by IDO. IDO was induced in macrophages by a synergistic combination of the T cell–derived signals IFN-γ and CD40-ligand. Inhibition of IDO with the 1-methyl analogue of tryptophan prevented macrophage-mediated suppression. Purified T cells activated under tryptophan-deficient conditions were able to synthesize protein, enter the cell cycle, and progress normally through the initial stages of G1, including upregulation of IL-2 receptor and synthesis of IL-2. However, in the absence of tryptophan, cell cycle progression halted at a mid-G1 arrest point. Restoration of tryptophan to arrested cells was not sufficient to allow further cell cycle progression nor was costimulation via CD28. T cells could exit the arrested state only if a second round of T cell receptor signaling was provided in the presence of tryptophan. These data reveal a novel mechanism by which antigen-presenting cells can regulate T cell activation via tryptophan catabolism. We speculate that expression of IDO by certain antigen presenting cells in vivo allows them to suppress unwanted T cell responses.

No abstract

The nuclear protein FOG-1 binds transcription factor GATA-1 to facilitate erythroid and megakaryocytic maturation. However, little is known about the function of FOG-1 during myeloid and lymphoid development or how FOG-1 expression is regulated in any tissue. We used in situ hybridization, gain-and loss-of-function studies in zebrafish to address these problems. Zebrafish FOG-1 is expressed in early hematopoietic cells, as well as heart, viscera, and paraspinal neurons, suggesting that it has multifaceted functions in organogenesis. We found that FOG-1 is dispensable for endoderm specification but is required for endoderm patterning affecting the expression of latestage T-cell markers, independent of GATA-1. The suppression of FOG-1, in the presence of normal GATA-1 levels, induces severe anemia and thrombocytopenia and expands myeloid-progenitor cells, indicating that FOG-1 is required during erythroid/myeloid commitment. To functionally interrogate whether GATA-1 regulates FOG-1 in vivo, we used bioinformatics combined with transgenic assays. Thus, we identified 2 cis-regulatory elements that control the tissue-specific gene expression of FOG-1. IntroductionTranscription factor GATA-1 is the founding member of a small family of nuclear proteins that bind the DNA consensus sequences [(T/A)GATA(A/G)] 1 in a variety of tissues. GATA-1 was initially discovered as a nuclear protein that binds numerous GATA consensus motifs distributed throughout enhancers and promoters of erythroid-specific genes. Moreover, GATA-1 is a key regulator of erythropoiesis, as demonstrated by genetic studies in zebrafish and mice. 2-5 GATA-1 null mice showed complete ablation of primitive and definitive erythropoiesis resulting from arrested maturation 4,5 and apoptosis of erythroid cells. 2 In addition, numerous gain-and loss-of-function experiments show that GATA-1 is important for the development of megakaryocytes, 6,7 eosinophils, and mast cells. [8][9][10][11] In addition to driving the maturation of several hematopoietic lineages, GATA-1 also inhibits the formation of alternate lineages by interacting with the myeloid transcription factor PU.1. 11 Specifically, studies in zebrafish and mice demonstrate that GATA-1 and PU.1 proteins antagonize each other during hematopoietic lineage specification. [12][13][14][15] GATA-1 contains 2 zinc finger domains, which are highly conserved throughout vertebrate evolution. The C-terminal zinc finger is required for DNA binding, whereas the N-terminal finger stabilizes DNA binding and facilitates physical interaction with numerous proteins. Interactions between the GATA-1 N-finger and the multitype zinc finger protein, FOG-1 (Friend-of-GATA-1, zfpm1), appear to be particularly important. 16,17 GATA-1/FOG-1 complexes can function as activators for several erythroid and megakaryocytic genes and as repressors for others. 18 In both mice and humans, GATA-1 missense mutations that disrupt FOG-1 interaction cause severe anemia and thrombocytopenia, partially recapitulating loss of GATA-1 phenotypes. ...

Much of our current understanding of the function of genes modulating the normal process of embryonic development has come from mutant analysis. The availability of thousands of mutant lines in zebrafish that allows for identification of novel genes regulating various aspects of embryogenesis has been instrumental in establishing zebrafish as a robust and reliable genetic system. With the advances in genomic sequencing, the construction of several genetic maps, and cloning of hundreds of ESTs, positional cloning experiments in zebrafish have become more approachable. An increasing number of mutant genes have been cloned. Several zebrafish mutants are representative of known forms of human genetic diseases. The success of morpholino antisense technology in zebrafish potentially opens the door for modeling nearly any inherited developmental defect. This review highlights the strengths and limitations of using the zebrafish as an organism for elucidation of the genetic etiology of human disease. Additionally a survey of current and future zebrafish models of human disease is presented.

Growth Factor Independence(Gfi) transcription factors play essential roles in hematopoiesis, differentially activating and repressing transcriptional programs required for hematopoietic stem/progenitor cell (HSPC) development and lineage specification. In mammals, Gfi1a regulates hematopoietic stem cells (HSC), myeloid and lymphoid populations, while its paralog, Gfi1b, regulates HSC, megakaryocyte and erythroid development. In zebrafish, gfi1aa is essential for primitive hematopoiesis; however, little is known about the role of gfi1aa in definitive hematopoiesis or about additional gfi factors in zebrafish. Here, we report the isolation and characterization of an additional hematopoietic gfi factor, gfi1b. We show that gfi1aa and gfi1b are expressed in the primitive and definitive sites of hematopoiesis in zebrafish. Our functional analyses demonstrate that gfi1aa and gfi1b have distinct roles in regulating primitive and definitive hematopoietic progenitors, respectively. Loss of gfi1aa silences markers of early primitive progenitors, scl and gata1. Conversely, loss of gfi1b silences runx-1, c-myb, ikaros and cd41, indicating that gfi1b is required for definitive hematopoiesis. We determine the epistatic relationships between the gfi factors and key hematopoietic transcription factors, demonstrating that gfi1aa and gfi1b join lmo2, scl, runx-1 and c-myb as critical regulators of teleost HSPC. Our studies establish a comparative paradigm for the regulation of hematopoietic lineages by gfi transcription factors.