Largen: a molecular regulator of mammalian cell size control.

Little is known about how mammalian cells maintain cell size homeostasis. We conducted a novel genetic screen to identify cell-size-controlling genes and isolated Largen, the product of a gene (PRR16) that increased cell size upon overexpression in human cells. In vitro evidence indicated that Largen preferentially stimulates the translation of specific subsets of mRNAs, including those encoding proteins affecting mitochondrial functions. The involvement of Largen in mitochondrial respiration was consistent with the increased mitochondrial mass and greater ATP production in Largen-overexpressing cells. Furthermore, Largen overexpression led to increased cell size in vivo, as revealed by analyses of conditional Largen transgenic mice. Our results establish Largen as an important link between mRNA translation, mitochondrial functions, and the control of mammalian cell size.

ARIH2 is essential for embryogenesis, and its hematopoietic deficiency causes lethal activation of the immune system.

The E3 ligase ARIH2 has an unusual structure and mechanism of elongating ubiquitin chains. To understand its physiological role, we generated gene-targeted mice deficient in ARIH2. ARIH2 deficiency resulted in the embryonic death of C57BL/6 mice. On a mixed genetic background, the lethality was attenuated, with some mice surviving beyond weaning and then succumbing to an aggressive multiorgan inflammatory response. We found that in dendritic cells (DCs), ARIH2 caused degradation of the inhibitor IκBß in the nucleus, which abrogated its ability to sequester, protect and transcriptionally coactivate the transcription factor subunit p65 in the nucleus. Loss of ARIH2 caused dysregulated activation of the transcription factor NF-κB in DCs, which led to lethal activation of the immune system in ARIH2-sufficent mice reconstituted with ARIH2-deficient hematopoietic stem cells. Our data have therapeutic implications for targeting ARIH2 function.

IL-7 engages multiple mechanisms to overcome chronic viral infection and limit organ pathology.

Understanding the factors that impede immune responses to persistent viruses is essential in designing therapies for HIV infection. Mice infected with LCMV clone-13 have persistent high-level viremia and a dysfunctional immune response. Interleukin-7, a cytokine that is critical for immune development and homeostasis, was used here to promote immunity toward clone-13, enabling elucidation of the inhibitory pathways underlying impaired antiviral immune response. Mechanistically, IL-7 downregulated a critical repressor of cytokine signaling, Socs3, resulting in amplified cytokine production, increased T cell effector function and numbers, and viral clearance. IL-7 enhanced thymic output to expand the naive T cell pool, including T cells that were not LCMV specific. Additionally, IL-7 promoted production of cytoprotective IL-22 that abrogated liver pathology. The IL-7-mediated effects were dependent on endogenous IL-6. These attributes of IL-7 have profound implications for its use as a therapeutic in the treatment of chronic viral diseases.

Adjuvant IL-7 antagonizes multiple cellular and molecular inhibitory networks to enhance immunotherapies.

Identifying key factors that enhance immune responses is crucial for manipulating immunity to tumors. We show that after a vaccine-induced immune response, adjuvant interleukin-7 (IL-7) improves antitumor responses and survival in an animal model. The improved immune response is associated with increased IL-6 production and augmented T helper type 17 cell differentiation. Furthermore, IL-7 modulates the expression of two ubiquitin ligases: Casitas B-lineage lymphoma b (Cbl-b), a negative regulator of T cell activation, is repressed, and SMAD-specific E3 ubiquitin protein ligase-2 (Smurf2) is enhanced, which antagonizes transforming growth factor-beta signaling. Notably, we show that although short term IL-7 therapy potently enhances vaccine-mediated immunity, in the absence of vaccination it is inefficient in promoting antitumor immune responses, despite inducing homeostatic proliferation of T cells. The ability of adjuvant IL-7 to antagonize inhibitory networks at the cellular and molecular level has major implications for immunotherapy in the treatment of tumors.

CD4 T cells, lymphopenia, and IL-7 in a multistep pathway to autoimmunity.

There are many inhibitory mechanisms that function at the cellular and molecular levels to maintain tolerance. Despite this, self-reactive clones escape regulatory mechanisms and cause autoimmunity in certain circumstances. We hypothesized that the same mechanisms that permit T cells to expand during homeostatic proliferation may inadvertently promote autoimmunity under certain conditions. One major homeostatic cytokine is IL-7, and studies have linked it or its receptor to the development of multiple sclerosis and other autoimmune diseases. We show in a model of beta-islet cell self-reactivity that the transfer of activated autoreactive CD4 T cells can prime and expand endogenous autoreactive CD8 T cells in a CD28- and CD40-dependent manner through the licensing of dendritic cells. Despite this, mice do not develop diabetes. However, the provision of exogenous IL-7 or the physiological production of IL-7 associated with lymphopenia was able to profoundly promote the expansion of self-reactive clones even in the presence of regulatory T cells. Autoimmune diabetes rapidly ensued with CD4 help and the subsequent activation of CD8 T cells, which contributed to disease progression. With the advent of many biologicals targeting TNFalpha, IL-6, and IL-1 and their effective use in the treatment of autoimmune diseases, we propose that IL-7 and its receptor may be promising targets for biological agents in the treatment of autoimmunity.

B7RP-1 is not required for the generation of Th2 responses in a model of allergic airway inflammation but is essential for the induction of inhalation tolerance.

The recently described ICOS-B7RP-1 costimulatory pathway has been implicated in the generation of effector Th2 responses and, hence, has become an attractive therapeutic target for allergic diseases. In the present study, we used B7RP-1-deficient mice to investigate the role of B7RP-1 in the generation and maintenance of Th2 responses in a model of mucosal allergic airway inflammation. We found that exposure of B7RP-1 knockout mice to aerosolized OVA in the context of GM-CSF leads to airway eosinophilic inflammation. This response was long lasting because rechallenge of mice with the same Ag recapitulated airway eosinophilia. Moreover, significant expression of T1/ST2 on T cells and production of Th2-affiliated cytokines (IL-5, IL-4, and IL-13) and Igs (IgE and IgG1) conclusively demonstrate the generation of a Th2 response in the absence of B7RP-1. In addition, expression of two major Th2-associated costimulatory molecules-CD28 and ICOS-indicates T cell activation in the absence of B7RP-1 signaling. Finally, B7RP-1 knockout mice are resistant to the induction of inhalation tolerance as indicated by the sustained eosinophilia in the lung and IL-5 production. In summary, our results demonstrate that in a model of mucosal allergic sensitization, the ICOS-B7RP-1 pathway is redundant for the generation of Th2 responses but essential for the induction of inhalation tolerance.

The inducible costimulator plays the major costimulatory role in humoral immune responses in the absence of CD28.

CD28 plays crucial costimulatory roles in T cell proliferation, cytokine production, and germinal center response. Mice that are deficient in the inducible costimulator (ICOS) also have defects in cytokine production and germinal center response. Because the full induction of ICOS in activated T cells depends on CD28 signal, the T cell costimulatory capacity of ICOS in the absence of CD28 has remained unclear. We have clarified this issue by comparing humoral immune responses in wild-type, CD28 knockout (CD28 KO), and CD28-ICOS double-knockout (DKO) mice. DKO mice had profound defects in Ab responses against environmental Ags, T-dependent protein Ags, and vesicular stomatitis virus that extended far beyond those observed in CD28 KO mice. However, DKO mice mounted normal Ab responses against a T-independent Ag, indicating that B cell function itself was normal. Restimulated CD4(+) DKO T cells that had been primed in vivo showed decreased proliferation and reduced IL-4 and IL-10 production compared with restimulated CD4(+) T cells from CD28 KO mice. Thus, in the absence of CD28, ICOS assumes the major T cell costimulatory role for humoral immune responses. Importantly, CD28-mediated ICOS up-regulation is not essential for ICOS function in vivo.

Survivin loss in thymocytes triggers p53-mediated growth arrest and p53-independent cell death.

Because survivin-null embryos die at an early embryonic stage, the role of survivin in thymocyte development is unknown. We have investigated the role by deleting the survivin gene only in the T lineage and show here that loss of survivin blocks the transition from CD4- CD8- double negative (DN) thymocytes to CD4+ CD8+ double positive cells. Although the pre-T cell receptor signaling pathway is intact in survivin-deficient thymocytes, the cells cannot respond to its signals. In response to proliferative stimuli, cycling survivin-deficient DN cells exhibit cell cycle arrest, a spindle formation defect, and increased cell death. Strikingly, loss of survivin activates the tumor suppressor p53. However, the developmental defects caused by survivin deficiency cannot be rescued by p53 inactivation or introduction of Bcl-2. These lines of evidence indicate that developing thymocytes depend on the cytoprotective function of survivin and that this function is tightly coupled to cell proliferation but independent of p53 and Bcl-2. Thus, survivin plays a critical role in early thymocyte development.

Costimulation through the inducible costimulator ligand is essential for both T helper and B cell functions in T cell-dependent B cell responses.

Costimulation through the inducible costimulator (ICOS) and its ligand (ICOSL) is essential for T cell-dependent B cell responses, but the cellular and temporal dynamics underlying its in vivo effects are poorly defined. Here we have shown that Icosl(-/-) and Icos(-/-) mice had similar phenotypes and that ICOS-ICOSL costimulation modulated the early but not late phases of IgG1 affinity maturation. Exploiting the adoptive transfer of T or B cells from primed Icosl(-/-) mice, we provided genetic evidence that costimulation through ICOSL was essential for primary but not secondary helper T cell responses and for the control of both T and B cell activities, resulting in T cell-dependent IgG1 production.

Role of ICOS versus CD28 in antiviral immunity.

The costimulatory protein ICOS is inducibly expressed on activated T cells. Previous results have shown that ICOS(-/-) mice are defective in germinal center formation, antibody (Ab) production and class switch as well as Th1 and Th2 cytokine production in response to protein or parasite antigens. However, ICOS-Ig failed to block antiviral Ab responses. To date the immune response to viruses has not been examined in ICOS(-/-) mice. In this report we compared antiviral Ab responses to LCMV, VSV and influenza virus in ICOS(-/-) versus wild-type mice. Our results show that ICOS is important in the Ab response to all three viruses, with greater effects on primary as compared to secondary responses. Although ICOS(-/-) mice are impaired in some immune responses following influenza infection, the effects were less severe than for CD28(-/-) mice. There was no defect in initial influenza-specific CD8 T cell expansion in ICOS(-/-) mice or in cytotoxic effector function. However, ICOS was important in maintaining CD4 cytokine production and CD8 T cell numbers late in the primary response. Upon secondary infection, ICOS(-/-) mice show wild-type levels of influenza-specific CD8 T cells, whereas CD28(-/-) mice show greatly impaired secondary CD8 T cell expansion. Overall, our results show that ICOS plays a clear role in the primary response to viruses at the level of Ab production, germinal center formation and Th cytokine production, but has diminished effects following secondary viral challenge.