Herbert C. Morse

Immunodeficiency and Chronic Myelogenous Leukemia-like Syndrome in Mice with a Targeted Mutation of the ICSBP Gene

Interferon consensus sequence binding protein (ICSBP) is a transcription factor of the interferon (IFN) regulatory factor (IRF) family. Mice with a null mutation of ICSBP exhibit two prominent phenotypes related to previously described activities of the IRF family. The first is enhanced susceptibility to virus infections associated with impaired production of IFN(gamma). The second is deregulated hematopoiesis in both ICSBP-/- and ICSBP+/- mice that manifests as a syndrome similar to human chronic myelogenous leukemia. The chronic period of the disease progresses to a fatal blast crisis characterized by a clonal expansion of undifferentiated cells. Normal mice injected with cells from mice in blast crisis developed acute leukemia within 6 weeks of transfer. These results suggest a novel role for ICSBP in regulating the proliferation and differentiation of hematopoietic progenitor cells.

Regulation of B cell differentiation and plasma cell generation by IL-21, a novel inducer of Blimp-1 and Bcl-6.

IL-21 is a type I cytokine whose receptor is expressed on T, B, and NK cells. Within the B cell lineage, IL-21 regulates IgG1 production and cooperates with IL-4 for the production of multiple Ab classes in vivo. Using IL-21-transgenic mice and hydrodynamics-based gene delivery of IL-21 plasmid DNA into wild-type mice as well as in vitro studies, we demonstrate that although IL-21 induces death of resting B cells, it promotes differentiation of B cells into postswitch and plasma cells. Thus, IL-21 differentially influences B cell fate depending on the signaling context, explaining how IL-21 can be proapoptotic for B cells in vitro yet critical for Ag-specific Ig production in vivo. Moreover, we demonstrate that IL-21 unexpectedly induces expression of both Blimp-1 and Bcl-6, indicating mechanisms as to how IL-21 can serve as a complex regulator of B cell maturation and terminal differentiation. Finally, BXSB-Yaa mice, which develop a systemic lupus erythematosus-like disease, have greatly elevated IL-21, suggesting a role for IL-21 in the development of autoimmune disease.

TRIM family proteins and their emerging roles in innate immunity

The superfamily of tripartite motif-containing (TRIM) proteins is conserved throughout the metazoan kingdom and has expanded rapidly during vertebrate evolution; there are now more than 60 TRIM proteins known in humans and mice. Many TRIM proteins are induced by type I and type II interferons, which are crucial for many aspects of resistance to pathogens, and several are known to be required for the restriction of infection by lentiviruses. In this Review, we describe recent data that reveal broader antiviral and antimicrobial activities of TRIM proteins and discuss their involvement in the regulation of pathogen-recognition and transcriptional pathways in host defence.

New genes involved in cancer identified by retroviral tagging

Retroviral insertional mutagenesis in BXH2 and AKXD mice induces a high incidence of myeloid leukemia and B- and T-cell lymphoma, respectively. The retroviral integration sites (RISs) in these tumors thus provide powerful genetic tags for the discovery of genes involved in cancer. Here we report the first large-scale use of retroviral tagging for cancer gene discovery in the post-genome era. Using high throughput inverse PCR, we cloned and analyzed the sequences of 884 RISs from a tumor panel composed primarily of B-cell lymphomas. We then compared these sequences, and another 415 RIS sequences previously cloned from BXH2 myeloid leukemias and from a few AKXD lymphomas, against the recently assembled mouse genome sequence. These studies identified 152 loci that are targets of retroviral integration in more than one tumor (common retroviral integration sites, CISs) and therefore likely to encode a cancer gene. Thirty-six CISs encode genes that are known or predicted to be genes involved in human cancer or their homologs, whereas others encode candidate genes that have not yet been examined for a role in human cancer. Our studies demonstrate the power of retroviral tagging for cancer gene discovery in the post-genome era and indicate a largely unrecognized complexity in mouse and presumably human cancer.

ICSBP is essential for the development of mouse type I interferon-producing cells and for the generation and activation of CD8α+ dendritic cells

Interferon (IFN) consensus sequence-binding protein (ICSBP) is a transcription factor playing a critical role in the regulation of lineage commitment, especially in myeloid cell differentiation. In this study, we have characterized the phenotype and activation pattern of subsets of dendritic cells (DCs) in ICSBP−/− mice. Remarkably, the recently identified mouse IFN-producing cells (mIPCs) were absent in all lymphoid organs from ICSBP−/− mice, as revealed by lack of CD11clowB220+Ly6C+CD11b− cells. In parallel, CD11c+ cells isolated from ICSBP−/− spleens were unable to produce type I IFNs in response to viral stimulation. ICSBP−/− mice also displayed a marked reduction of the DC subset expressing the CD8α marker (CD8α+ DCs) in spleen, lymph nodes, and thymus. Moreover, ICSBP−/− CD8α+ DCs exhibited a markedly impaired phenotype when compared with WT DCs. They expressed very low levels of costimulatory molecules (intercellular adhesion molecule [ICAM]-1, CD40, CD80, CD86) and of the T cell area-homing chemokine receptor CCR7, whereas they showed higher levels of CCR2 and CCR6, as revealed by reverse transcription PCR. In addition, these cells were unable to undergo full phenotypic activation upon in vitro culture in presence of maturation stimuli such as lipopolysaccharide or poly (I:C), which paralleled with lack of Toll-like receptor (TLR)3 mRNA expression. Finally, cytokine expression pattern was also altered in ICSBP−/− DCs, as they did not express interleukin (IL)-12p40 or IL-15, but they displayed detectable IL-4 mRNA levels. On the whole, these results indicate that ICSBP is a crucial factor in the regulation of two possibly linked processes: (a) the development and activity of mIPCs, whose lack in ICSBP−/− mice may explain their high susceptibility to virus infections; (b) the generation and activation of CD8α+ DCs, whose impairment in ICSBP−/− mice can be responsible for the defective generation of a Th1 type of immune response.

BORIS, a novel male germ-line-specific protein associated with epigenetic reprogramming events, shares the same 11-zinc-finger domain with CTCF, the insulator protein involved in reading imprinting marks in the soma

CTCF, a conserved, ubiquitous, and highly versatile 11-zinc-finger factor involved in various aspects of gene regulation, forms methylation-sensitive insulators that regulate X chromosome inactivation and expression of imprinted genes. We document here the existence of a paralogous gene with the same exons encoding the 11-zinc-finger domain as mammalian CTCF genes and thus the same DNA-binding potential, but with distinct amino and carboxy termini. We named this gene BORIS for Brother of the Regulator of Imprinted Sites. BORIS is present only in the testis, and expressed in a mutually exclusive manner with CTCF during male germ cell development. We show here that erasure of methylation marks during male germ-line development is associated with dramatic up-regulation of BORIS and down-regulation of CTCF expression. Because BORIS bears the same DNA-binding domain that CTCF employs for recognition of methylation marks in soma, BORIS is a candidate protein for the elusive epigenetic reprogramming factor acting in the male germ line.

AID is required for germinal center–derived lymphomagenesis

Most human B cell non-Hodgkins lymphomas (B-NHLs) derive from germinal centers (GCs), the structure in which B cells undergo somatic hypermutation (SHM) and class switch recombination (CSR) before being selected for high-affinity antibody production. The pathogenesis of B-NHL is associated with distinct genetic lesions, including chromosomal translocations and aberrant SHM, which arise from mistakes occurring during CSR and SHM. A direct link between these DNA remodeling events and GC lymphoma development, however, has not been demonstrated. Here we have crossed three mouse models of B cell lymphoma driven by oncogenes (Myc, Bcl6 and Myc/Bcl6; refs. 5,6) with mice lacking activation-induced cytidine deaminase (AID), the enzyme required for both CSR and SHM. We show that AID deficiency prevents Bcl6-dependent, GC-derived B-NHL, but has no impact on Myc-driven, pre-GC lymphomas. Accordingly, abrogation of AID is associated with the disappearance of CSR- and SHM-mediated structural alterations. These results show that AID is required for GC-derived lymphomagenesis, supporting the notion that errors in AID-mediated antigen-receptor gene modification processes are principal contributors to the pathogenesis of human B-NHL.

The novel BORIS + CTCF gene family is uniquely involved in the epigenetics of normal biology and cancer

CTCF is a ubiquitous 11 zinc finger (ZF) protein with highly versatile functions: in addition to transcriptional silencing or activating in a context-dependent fashion, it organizes epigenetically controlled chromatin insulators that regulate imprinted genes in soma. Recently, we have identified a CTCF paralogue, termed BORIS for Brother of the Regulator of Imprinted Sites, that is expressed only in the testis. BORIS has the same exons encoding the 11 ZF domain as mammalian CTCF genes, and hence interacts with similar cis elements, but encodes amino and carboxy termini distinct from those in CTCF. Normally, CTCF and BORIS are expressed in a mutually exclusive pattern that correlates with re-setting of methylation marks during male germ cell differentiation. The antagonistic features of these two gene siblings are underscored by showing that while CTCF overexpression blocks cell proliferation, expression of BORIS in normally BORIS-negative cells promotes cell growth which can lead to transformation. The suggestion that BORIS directs epigenetic reprogramming at CTCF target sites impinges on the observations that human BORIS is not only abnormally activated in a wide range of human cancers, but also maps to the cancer-associated amplification region at 20q13. The sibling rivalry occasioned by aberrant expression of BORIS in cancer may interfere with normal functions of CTCF including growth suppression, and contribute to epigenetic dysregulation which is a common feature in human cancer.

A critical role for IL-21 receptor signaling in the pathogenesis of systemic lupus erythematosus in BXSB-Yaa mice

Interleukin 21 (IL-21) is a pleiotropic cytokine produced by CD4 T cells that affects the differentiation and function of T, B, and NK cells by binding to a receptor consisting of the common cytokine receptor γ chain and the IL-21 receptor (IL-21R). IL-21, a product associated with IL-17-producing CD4 T cells (TH17) and follicular CD4 T helper cells (TFH), has been implicated in autoimmune disorders including the severe systemic lupus erythematosus (SLE)-like disease characteristic of BXSB-Yaa mice. To determine whether IL-21 plays a significant role in this disease, we compared IL-21R-deficient and -competent BXSB-Yaa mice for multiple parameters of SLE. The deficient mice showed none of the abnormalities characteristic of SLE in IL-21R-competent Yaa mice, including hypergammaglobulinemia, autoantibody production, reduced frequencies of marginal zone B cells and monocytosis, renal disease, and premature morbidity. IL-21 production associated with this autoimmune disease was not a product of TH17 cells and was not limited to conventional CXCR5+ TFH but instead was produced broadly by ICOS+ CD4+ splenic T cells. IL-21 arising from an abnormal population of CD4 T cells is thus central to the development of this lethal disease, and, more generally, could play an important role in human SLE and related autoimmune disorders.

Identification of a common ecotropic viral integration site, Evi-1, in the DNA of AKXD murine myeloid tumors.

AKXD-23 recombinant inbred mice develop myeloid tumors at a high frequency, unlike other AKXD recombinant inbred strains which develop B-cell lymphomas, T-cell lymphomas, or both. AKXD-23 myeloid tumors are monoclonal, and their DNA contains somatically acquired proviruses, suggesting that they are retrovirally induced. We identified a common site of ecotropic proviral integration that is present in the DNA of all AKXD-23 myeloid tumors that were analyzed and in the DNA of all myeloid tumors that occur in AKXD strains other than AKXD-23. We designated this locus Evi-1 (ecotropic viral integration site 1). Rearrangements in the Evi-1 locus were also detected in the DNA of a number of myeloid tumors and myeloid cell lines isolated from strains other than AKXD. In contrast, few Evi-1 rearrangements were detected in the DNA of T- or B-cell tumors. Evi-1 may thus identify a new proto-oncogene locus that is involved in myeloid disease.