Paul W. Wu

The ikaros gene is required for the development of all lymphoid lineages

The Ikaros gene encodes a family of early hematopoietic- and lymphocyte-restricted transcription factors. Mice homozygous for a germline mutation in the Ikaros DNA-binding domain lack not only T and B lymphocytes and natural killer cells but also their earliest defined progenitors. In contrast, the erythroid and myeloid lineages were intact in these mutant mice. We propose that Ikaros promotes differentiation of pluripotential hematopoietic stem cell(s) into the lymphocyte pathways. In the absence of a functional Ikaros gene, these stem cells are exclusively diverted into the erythroid and myeloid lineages.

A dominant mutation in the Ikaros gene leads to rapid development of leukemia and lymphoma

The Ikaros gene is essential for lymphoid lineage specification. As previously reported, mice homozygous for a mutation in the Ikaros DNA-binding domain fail to generate mature lymphocytes as well as their earliest described progenitors. In addition, our studies with mice heterozygous for this mutation establish the Ikaros gene as an essential regulator of T cell proliferation. Thymocytes display augmented TCR-mediated proliferative responses, and peripheral T cells are autoproliferative. A general lymphoproliferation precedes the T cell leukemia and lymphoma that rapidly develop in all heterozygotes. The first step toward leukemic transformation occurs within the maturing thymocyte population and is demarcated by clonal expansions and loss of the single Ikaros wild-type allele. From these studies, we propose that within developing and mature T lymphocytes, distinct thresholds of Ikaros activity are required to regulate proliferation. A decrease in Ikaros activity below the first threshold causes the rapid accumulation of T lymphoblasts, whereas a further decrease leads to neoplastic transformation.

Aiolos, a lymphoid restricted transcription factor that interacts with Ikaros to regulate lymphocyte differentiation

Development of the lymphoid system is dependent on the activity of zinc finger transcription factors encoded by the Ikaros gene. Differences between the phenotypes resulting from a dominant‐negative and a null mutation in this gene suggest that Ikaros proteins act in concert with another factor with which they form heterodimers. Here we report the cloning of Aiolos, a gene which encodes an Ikaros homologue that heterodimerizes with Ikaros proteins. In contrast to Ikaros_which is expressed from the pluripotent stem cell to the mature lymphocyte_Aiolos is first detected in more committed progenitors with a lymphoid potential and is strongly up‐regulated as these differentiate into pre‐T and pre‐B cell precursors. The expression patterns of Aiolos and Ikaros, the relative transcriptional activity of their homo‐ and heteromeric complexes, and the dominant interfering effect of mutant Ikaros isoforms on Aiolos activity all strongly suggest that Aiolos acts in concert with Ikaros during lymphocyte development. We therefore propose that increasing levels of Ikaros and Aiolos homo‐ and heteromeric complexes in differentiating lymphocytes are essential for normal progression to a mature and immunocompetent state.

Cutting Edge: Identification of GL50, a Novel B7-Like Protein That Functionally Binds to ICOS Receptor

By the genetic selection of mouse cDNAs encoding secreted proteins, a B7-like cDNA clone termed mouse GL50 (mGL50) was isolated encoding a 322-aa polypeptide identical with B7h. Isolation of the human ortholog of this cDNA (hGL50) revealed a coding sequence of 309 aa residues with 42% sequence identity with mGL50. Northern analysis indicated GL50 to be present in many tissues including lymphoid, embryonic yolk sac, and fetal liver samples. Of the CD28, CTLA4, and ICOS fusion constructs tested, flow cytometric analysis demonstrated only mouse ICOS-IgG binding to mGL50 cell transfectants. Subsequent phenotyping demonstrated high levels of ICOS ligand staining on splenic CD19+ B cells and low levels on CD3+ T cells. These results indicate that GL50 is a specific ligand for the ICOS receptor and suggest that the GL50-ICOS interaction functions in lymphocyte costimulation.

Duplication of primate and rodent B7-H3 immunoglobulin V- and C-like domains: divergent history of functional redundancy and exon loss

B7-H3 is a novel protein structurally related to the B7 family of ligands by the presence of a single set of immunoglobulin-V-like and immunoglobulin-C-like (VC) domains. By multiplex PCR, the dominantly expressed form of human B7-H3 was found to be a splice variant containing tandemly duplicated VC domains (VCVC). In contrast, mouse B7-H3 cDNA contained only one single VC form due to an exon structure corresponding to V-(pseudoexon C)-(pseudoexon V)-C. Comparisons of human, monkey, mouse, and hamster genomic B7-H3 reveal that primates, but not rodents, exhibited a higher degree of intramolecular sequence similarity between VC duplications than between molecules. Both VC and VCVC forms of human B7-H3 inhibited CD4(+) T cell proliferation and downregulated cytokine production upon TCR activation. These results suggest independent, but convergent, paths of B7-H3 active domain duplication followed by divergent histories of exon degeneration in rodents and exon maintenance by humans.

Differential Expression of Inducible Costimulator-Ligand Splice Variants: Lymphoid Regulation of Mouse GL50-B and Human GL50 Molecules

The process of immunological costimulation between APC and T cells is mediated by protein ligand:receptor interactions. To date, costimulatory receptors known to be expressed by T cells include the structurally related proteins CD28 and the inducible costimulator (ICOS). The ligands to human and mouse ICOS, human GL50 (hGL50), and mouse GL50 (mGL50) were recently cloned and demonstrated to have sequence similarity to the CD28 ligands B7-1 and B7-2. Examination of mGL50 cDNA transcripts by 3′RACE revealed an alternatively spliced form, mGL50-B, that encoded a protein product with a divergent 27-aa intracellular domain. Both mGL50- and mGL50-B-transfected cells exhibited binding to human and mouse ICOS-Ig fusion protein, indicating that the alternate cytoplasmic domain of mGL50-B does not interfere with extracellular interactions with ICOS receptor. Flow cytometric and RT-PCR analysis of BALB/c and RAG1−/− mice splenocytes demonstrate that freshly isolated B cells, T cells, macrophages, and dendritic cells express both splice variant forms of ICOS ligand. Comparative analyses with the human ICOS ligand splice variants hGL50 and B7-H2 indicate that differential splicing at the junction of cytoplasmic exon 6 and exon 7 may be a common method by which GL50-ICOS immunological costimulatory processes are regulated in vivo.

IL-22R, IL-10R2, and IL-22BP binding sites are topologically juxtaposed on adjacent and overlapping surfaces of IL-22.

Interleukin (IL) 22 is a type II cytokine that is produced by immune cells and acts on nonimmune cells to regulate local tissue inflammation. As a product of the recently identified T helper 17 lineage of CD4(+) effector lymphocytes, IL-22 plays a critical role in mucosal immunity as well as in dysregulated inflammation observed in autoimmune diseases. We used comprehensive mutagenesis combined with mammalian cell expression, ELISA cell-based, and structural methods to evaluate how IL-22 interacts with its cell surface receptor, IL-22R/IL-10R2, and with secreted IL-22 binding protein. This study identifies those amino acid side chains of IL-22 that are individually important for optimal binding to IL-22R, considerably expands the definition of IL-22 surface required for binding to IL-10R2, and demonstrates how IL-22 binding protein prevents IL-22R from binding to IL-22. The IL-22R and IL-10R2 binding sites are juxtaposed on adjacent IL-22 surfaces contributed mostly by helices A, D, and F and loop AB. Our results also provide a model for how IL-19, IL-20, IL-24, and IL-26 which are other IL-10-like cytokines, interact with their respective cell surface receptors.

Conversion of CTLA-4 from inhibitor to activator of T cells with a bispecific tandem single-chain Fv ligand.

Abs or their recombinant fragments against surface receptors of the Ig superfamily can induce or block the receptors’ native function depending on whether they induce or prevent the assembly of signalosomes on their cytoplasmic tails. In this study, we introduce a novel paradigm based on the observation that a bispecific tandem single-chain variable region fragment ligand of CTLA-4 by itself converts this inhibitory receptor into an activating receptor for primary human T lymphocytes. This reversal of function results from increased recruitment of the serine/threonine phosphatase 2A to the cytoplasmic tail of CTLA-4, consistent with a role of this phosphatase in the regulation of CTLA-4 function, and assembly of a distinct signalosome that activates an lck-dependent signaling cascade and induces IL-2 production. Our data demonstrate that the cytoplasmic domain of CTLA-4 has an inherent plasticity for signaling that can be exploited therapeutically with recombinant ligands for this receptor.

A complex network of regulatory elements in Ikaros and their activity during hemo‐lymphopoiesis

Regulated expression of Ikaros is critical for normal hemopoiesis and lymphocyte development. To elucidate the mechanisms underlying transcription of Ikaros, tissue‐specific DNase I‐hypersensitive sites (DHS) were mapped throughout the Ikaros locus, and several promoters were identified. The activity of these regulatory regions was elucidated using an enhanced green fluorescent protein (EGFP) reporter in transgenic mice. Two genomic fragments, each containing a distinct promoter and its associated DHS cluster, were found to be active in the myeloid (DHS‐C2 and DHS‐C3) and B‐cell (DHS‐C3) lineages. Although neither of these regulatory regions was active within the majority of differentiating thymocytes and mature T cells, the DHS‐C3 region was active at the earliest stages (DN1–DN3) of T‐cell differentiation. However, when the DHS‐C3 region was combined with the downstream intronic DHS‐C6 cluster, its activity was maintained and raised to higher levels at subsequent stages of T‐cell differentiation. This combination of regulatory elements provided reporter expression that closely resembles that of endogenous Ikaros during hemo‐lymphopoiesis, and it decreased (but did not alleviate) position effect variegation within the expressing cell types.