Pamela S Ohashi

University Health Network, Toronto, Ontario, Canada

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Publications (279)3067.35 Total impact

  • Douglas G Millar · Pamela S Ohashi
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    ABSTRACT: Thymic selection shapes the repertoire of potentially autoreactive thymocytes that are allowed to mature. The expression pattern of self antigen seen by thymocytes determines the number and functional ability of autoreactive T cells.
    No preview · Article · Jan 2016 · Nature Immunology
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    ABSTRACT: Type I IFNs (IFN-I) are cytokines that can mediate both immune suppression and activation. Dendritic cells (DC) are significant producers of IFN-I, and depending on the context (nature of Ag, duration of exposure to Ag), DC-derived IFN-I can have varying effects on CD8(+) T cell responses. In this study, we report that in the context of a CD8(+) T cell response to a self-Ag, DC-intrinsic expression of IFN regulatory factor 3 is required to induce optimal proliferation and migration of autoreactive CD8(+) T cells, ultimately determining their ability to infiltrate a target tissue (pancreas), and the development of glucose intolerance in rat insulin promoter-glycoprotein (RIP-GP) mice. Moreover, we show that signals through the lymphotoxin-β receptor (LTβR) in DC are also required for the proliferation of autoreactive CD8(+) T cells, the upregulation of VLA4/LFA1 on activated CD8(+) T cells, and their subsequent infiltration into the pancreas both in vitro and in vivo. Importantly, the defects in autoreactive CD8(+) T cell proliferation, accumulation of CD8(+) T cells in the pancreas, and consequent glucose intolerance observed in the context of priming by LTβR(-/-) DC could be rescued by exogenous addition of IFN-I. Collectively, our data demonstrate that the LTβR/IFN-I axis is essential for programming of CD8(+) T cells to mediate immunopathology in a self-tissue. A further understanding of the IFN-I/LTβR axis will provide valuable therapeutic insights for treatment of CD8(+) T cell-mediated autoimmune diseases.
    No preview · Article · Oct 2015 · The Journal of Immunology
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    ABSTRACT: TLR-induced maturation of dendritic cells (DCs) leads to the production of proinflammatory cytokines as well as the upregulation of various molecules involved in T cell activation. These are believed to be the critical events that account for the induction of the adaptive immune response. In this study, we have examined the role of miR-155 in DC function and the induction of immunity. Using a model in which the transfer of self-Ag-pulsed, TLR-matured DCs can induce a functional CD8 T cell response and autoimmunity, we find that DCs lacking miR-155 have an impaired ability to break immune tolerance. Importantly, transfer of self- Ag-pulsed DCs overexpressing miR-155 was sufficient to break tolerance in the absence of TLR stimuli. Although these unstimulated DCs induced T cell function in vivo, there was no evidence for the upregulation of costimulatory ligands or cytokine secretion. Further analysis showed that miR-155 influenced the level of the phosphatase SHIP1 in DCs and that the lack of SHIP1 in DCs was sufficient to break T cell tolerance in vivo, again in the absence of TLR-induced DC maturation. Our study demonstrates that the overexpression of miR-155 in DCs is a critical event that is alone sufficient to break self-tolerance and promote a CD8-mediated autoimmune response in vivo. This process is independent of the induction of conventional DC maturation markers, indicating that miR-155 regulation of SHIP represents a unique axis that regulates DC function in vivo.
    Full-text · Article · Oct 2015 · The Journal of Immunology
  • Ramtin Rahbar · Pamela S. Ohashi
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    ABSTRACT: B7-H4 (B7x, B7S1, VTCN1, DD-0110) is a member of the B7 superfamily that negatively regulates T cell responses1. In addition, B7-H4 expression is increased on tumors and has been shown to be a negative prognostic marker for many cancers2. Unexpectedly1, 2 our recent study demonstrated that B7-H4 inhibited tumor growth and was required to promote effective anti-tumor responses3.
    No preview · Article · Jun 2015 · OncoImmunology
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    ABSTRACT: Isocitrate dehydrogenase-1 (Idh1) is an important metabolic enzyme that produces NADPH by converting isocitrate to α-ketoglutarate. Idh1 is known to reduce reactive oxygen species (ROS) induced in cells by treatment with lipopolysaccharide (LPS) in vitro. Here, we used Idh1-deficient knockout (Idh1 KO) mice to investigate the role of Idh1 in antioxidant defense in vivo. Idh1 KO mice showed heightened susceptibility to death induced by LPS and exhibited increased serum levels of inflammatory cytokines such as tumor necrosis factor-α and interleukin-6. The serum of LPS-injected Idh1 KO mice also contained elevated levels of AST, a marker of inflammatory liver damage. Furthermore, after LPS injection, livers of Idh1 KO mice showed histological evidence of elevated oxidative DNA damage compared with livers of wild-type (WT) mice. Idh1 KO livers showed a faster and more pronounced oxidative stress than WT livers. In line with that, Idh1 KO hepatocytes showed higher ROS levels and an increase in the NADP(+)/NADPH ratio when compared with hepatocytes isolated from WT mice. These results suggest that Idh1 has a physiological function in protecting cells from oxidative stress by regulating the intracellular NADP(+)/NADPH ratio. Our findings suggest that stimulation of Idh1 activity may be an effective therapeutic strategy for reducing oxidative stress during inflammatory responses, including the early stages of septic shock.Cell Death and Differentiation advance online publication, 17 April 2015; doi:10.1038/cdd.2015.38.
    Full-text · Article · Apr 2015 · Cell Death and Differentiation
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    ABSTRACT: The paracaspase MALT1 plays an important role in immune receptor-driven signaling pathways leading to NF-κB activation. MALT1 promotes signaling by acting as a scaffold, recruiting downstream signaling proteins, as well as by proteolytic cleavage of multiple substrates. However, the relative contributions of these two different activities to T and B cell function are not well understood. To investigate how MALT1 proteolytic activity contributes to overall immune cell regulation, we generated MALT1 protease-deficient mice (Malt1(PD/PD)) and compared their phenotype with that of MALT1 knockout animals (Malt1(-/-)). Malt1(PD/PD) mice displayed defects in multiple cell types including marginal zone B cells, B1 B cells, IL-10-producing B cells, regulatory T cells, and mature T and B cells. In general, immune defects were more pronounced in Malt1(-/-) animals. Both mouse lines showed abrogated B cell responses upon immunization with T-dependent and T-independent Ags. In vitro, inactivation of MALT1 protease activity caused reduced stimulation-induced T cell proliferation, impaired IL-2 and TNF-α production, as well as defective Th17 differentiation. Consequently, Malt1(PD/PD) mice were protected in a Th17-dependent experimental autoimmune encephalomyelitis model. Surprisingly, Malt1(PD/PD) animals developed a multiorgan inflammatory pathology, characterized by Th1 and Th2/0 responses and enhanced IgG1 and IgE levels, which was delayed by wild-type regulatory T cell reconstitution. We therefore propose that the pathology characterizing Malt1(PD/PD) animals arises from an immune imbalance featuring pathogenic Th1- and Th2/0-skewed effector responses and reduced immunosuppressive compartments. These data uncover a previously unappreciated key function of MALT1 protease activity in immune homeostasis and underline its relevance in human health and disease. Copyright © 2015 by The American Association of Immunologists, Inc.
    Full-text · Article · Mar 2015 · The Journal of Immunology
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    ABSTRACT: Unlabelled: The B cell-activating factor (BAFF) is critical for B cell development and humoral immunity in mice and humans. While the role of BAFF in B cells has been widely described, its role in innate immunity remains unknown. Using BAFF receptor (BAFFR)-deficient mice, we characterized BAFFR-related innate and adaptive immune functions following infection with vesicular stomatitis virus (VSV) and lymphocytic choriomeningitis virus (LCMV). We identified a critical role for BAFFR signaling in the generation and maintenance of the CD169(+) macrophage compartment. Consequently, Baffr(-) (/) (-) mice exhibited limited induction of innate type I interferon production after viral infection. Lack of BAFFR signaling reduced virus amplification and presentation following viral infection, resulting in highly reduced antiviral adaptive immune responses. As a consequence, BAFFR-deficient mice showed exacerbated and fatal disease after viral infection. Mechanistically, transient lack of B cells in Baffr(-) (/) (-) animals resulted in limited lymphotoxin expression, which is critical for maintenance of CD169(+) cells. In conclusion, BAFFR signaling affects both innate and adaptive immune activation during viral infections. Importance: Viruses cause acute and chronic infections in humans resulting in millions of deaths every year. Innate immunity is critical for the outcome of a viral infection. Innate type I interferon production can limit viral replication, while adaptive immune priming by innate immune cells induces pathogen-specific immunity with long-term protection. Here, we show that BAFFR deficiency not only perturbed B cells, but also resulted in limited CD169(+) macrophages. These macrophages are critical in amplifying viral particles to trigger type I interferon production and initiate adaptive immune priming. Consequently, BAFFR deficiency resulted in reduced enforced viral replication, limited type I interferon production, and reduced adaptive immunity compared to BAFFR-competent controls. As a result, BAFFR-deficient mice were predisposed to fatal viral infections. Thus, BAFFR expression is critical for innate immune activation and antiviral immunity.
    Full-text · Article · Feb 2015 · Journal of Virology
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    ABSTRACT: P.A.L; A.M.; A.A.P. contributed equally During virus infection and autoimmune disease, inflammatory dendritic cells (iDCs) differentiate from blood monocytes and infiltrate infected tissue. Following acute infection with hepatotropic viruses, iDCs are essential for re-stimulating virus-specific CD8+ T cells and therefore contribute to virus control. Here we used the lymphocytic choriomeningitis virus (LCMV) model system to identify novel signals, which influence the recruitment and activation of iDCs in the liver. We observed that intrinsic expression of Toso (Faim3, FcμR) influenced the differentiation and activation of iDCs in vivo and DCs in vitro. Lack of iDCs in Toso-deficient (Toso-/-) mice reduced CD8+ T-cell function in the liver and resulted in virus persistence. Furthermore, Toso-/- DCs failed to induce autoimmune diabetes in the rat insulin promoter-glycoprotein (RIP-GP) autoimmune diabetes model. In conclusion, we found that Toso has an essential role in the differentiation and maturation of iDCs, a process that is required for the control of persistence-prone virus infection.
    Full-text · Article · Jan 2015 · Cell Death and Differentiation
  • Linh T Nguyen · Pamela S Ohashi
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    ABSTRACT: Dysfunctional T cells can render the immune system unable to eliminate infections and cancer. Therapeutic targeting of the surface receptors that inhibit T cell function has begun to show remarkable success in clinical trials. In this Review, we discuss the potential mechanisms of action of the clinical agents that target two of these receptors, programmed cell death protein 1 (PD1) and lymphocyte activation gene 3 protein (LAG3). We also suggest correlative studies that may define the predominant mechanisms of action and identify predictive biomarkers.
    No preview · Article · Dec 2014 · Nature reviews. Immunology
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    ABSTRACT: The B7 family plays a critical role in both positive and negative regulation of immune responses by engaging a variety of receptors on lymphocytes. Importantly, blocking co-inhibitory molecules using antibodies specific for CTLA-4 and PD-1 enhances tumor immunity in a subset of patients. Therefore, it is critical to understand the role of different B7 family members since they may be suitable therapeutic targets. B7-H4 is another member that inhibits T-cell function, and it is also up-regulated on a variety of tumors and has been proposed to promote tumor growth. Here we investigate the role of B7-H4 in tumor development and show that B7-H4 expression inhibits tumor growth in two mouse models. Furthermore, we show that B7-H4 expression is required for antitumor immune responses in a mouse model of mammary tumorigenesis. We found the expression levels of B7-H4 correlate with MHC class I expression in both mouse and human samples. We show that IFNγ up-regulates B7-H4 expression on mouse embryo fibroblasts and that the up-regulation of B7-H4 on tumors is dependent on T cells. Notably, breast cancer patients with increased B7-H4 expression show a prolonged time to recurrence. These studies demonstrate a positive role for B7-H4 in promoting antitumor immunity. Copyright © 2014, American Association for Cancer Research.
    Preview · Article · Dec 2014
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    ABSTRACT: During the course of many chronic viral infections, the antiviral T cell response becomes attenuated through a process that is regulated in part by the host. While elevated expression of the immunosuppressive cytokine IL-10 is involved in the suppression of viral-specific T cell responses, the relevant cellular sources of IL-10, as well as the pathways responsible for IL-10 induction, remain unclear. In this study, we traced IL-10 production over the course of chronic lymphocytic choriomeningitis virus (LCMV) infection in an IL-10 reporter mouse line. Using this model, we demonstrated that virus-specific T cells with reduced inflammatory function, particularly Th1 cells, display elevated and sustained IL-10 expression during chronic LCMV infection. Furthermore, ablation of IL-10 from the T cell compartment partially restored T cell function and reduced viral loads in LCMV-infected animals. We found that viral persistence is needed for sustained IL-10 production by Th1 cells and that the transcription factor BLIMP-1 is required for IL-10 expression by Th1 cells. Restimulation of Th1 cells from LCMV-infected mice promoted BLIMP-1 and subsequent IL-10 expression, suggesting that constant antigen exposure likely induces the BLIMP-1/IL-10 pathway during chronic viral infection. Together, these data indicate that effector T cells self-limit their responsiveness during persistent viral infection via an IL-10-dependent negative feedback loop.
    Preview · Article · Jul 2014 · Journal of Clinical Investigation
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    ABSTRACT: Despite development of new antiviral drugs, viral infections are still a major health problem. The most potent antiviral defense mechanism is the innate production of type I interferon (IFN-I), which not only limits virus replication but also promotes antiviral T cell immunity through mechanisms, which remain insufficiently studied. Using the murine lymphocytic choriomeningitis virus model system, we show here that IFN-I signaling on T cells prevented their rapid elimination in vivo. Microarray analyses uncovered that IFN-I triggered the expression of selected inhibitory NK-cell-receptor ligands. Consequently, T cell immunity of IFN-I receptor (IFNAR)-deficient T cells could be restored by NK cell depletion or in NK-cell-deficient hosts (Nfil3(-/-)). The elimination of Ifnar1(-/-) T cells was dependent on NK-cell-mediated perforin expression. In summary, we identified IFN-I as a key player regulating the protection of T cells against regulatory NK cell function.
    Full-text · Article · Jun 2014 · Immunity
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    ABSTRACT: Vaccines for cancer immunotherapy are of interest but in general have not yet achieved the desired therapeutic efficacy in clinical trials. We present here a novel model to evaluate vaccine strategies by following tissue destruction in a transgenic model, where a defined antigen is expressed on pancreatic islets. We found that the transfer of syngeneic antigen-pulsed dendritic cells (DCs) resulted in autoimmune cytotoxic T-lymphocyte activation that was not observed following vaccinations that were based on peptides and adjuvants. Importantly, the induction of diabetes by DC transfer is dependent upon the maturation of DCs prior to transfer. Furthermore, diabetes induction only occurred if DCs were pulsed with the immunodominant epitope in addition to at least one other peptide, suggesting greater cytolytic activity upon engagement of multiple T-cell specificities. While the tumor environment undoubtedly will be more complex than healthy tissue, the insights gained through this model provide useful information on variables that can affect CD8-mediated tissue cytolysis in vivo.
    Full-text · Article · Mar 2014 · PLoS ONE
  • Evan F Lind · Pamela S Ohashi
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    ABSTRACT: The activation of T cells is a tightly regulated process that has evolved to maximize protective immune responses to pathogens while minimizing damage to self-tissues. A delicate balance of cell-intrinsic, costimulatory, and transcriptional pathways as well as micro-environmental cues such as local cytokines controls the magnitude and nature of T-cell responses in vivo. The discovery of functional small noncoding RNAs called micro-RNAs (miRNAs) has introduced new mechanisms that contribute to the regulation of protein translation and cellular responses to stimuli. miRNAs are short (approximately 22 bp) RNA species, which bind to mRNAs and suppress translation. Due to their short length and imperfect base pairing requirements, each miRNA has the potential to regulate various pathways through the translational inhibition of multiple mRNAs. The human and mouse genomes each encode hundreds of miRNAs, and studying the function of miRNAs has led to the realization that they play important roles in diverse biological processes from development and cancer to immunity. This review focuses on the function of mir-155 in T cells and the impact of this miRNA on autoimmunity, tumor immunity, and pathogen-induced immunity.
    No preview · Article · Feb 2014 · European Journal of Immunology
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    ABSTRACT: CD8(+) T-cell functions are critical for preventing chronic viral infections by eliminating infected cells. For healthy immune responses, beneficial destruction of infected cells must be balanced against immunopathology resulting from collateral damage to tissues. These processes are regulated by factors controlling CD8(+) T-cell function, which are still incompletely understood. Here, we show that the interferon regulatory factor 4 (IRF4) and its cooperating binding partner B-cell-activating transcription factor (BATF) are necessary for sustained CD8(+) T-cell effector function. Although Irf4(-/-) CD8(+) T cells were initially capable of proliferation, IRF4 deficiency resulted in limited CD8(+) T-cell responses after infection with the lymphocytic choriomeningitis virus. Consequently, Irf4(-/-) mice established chronic infections, but were protected from fatal immunopathology. Absence of BATF also resulted in reduced CD8(+) T-cell function, limited immunopathology, and promotion of viral persistence. These data identify the transcription factors IRF4 and BATF as major regulators of antiviral cytotoxic T-cell immunity.Cell Death and Differentiation advance online publication, 14 February 2014; doi:10.1038/cdd.2014.19.
    Full-text · Article · Feb 2014 · Cell death and differentiation

  • No preview · Article · Jan 2014 · Zeitschrift für Gastroenterologie

  • No preview · Article · Jan 2014 · Zeitschrift für Gastroenterologie
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    ABSTRACT: D.B. and A.B. contributed equally to this puplication. The ability to mount a strong immune response against pathogens is crucial for mammalian survival. However, excessive and uncontrolled immune reactions can lead to autoimmunity. Unraveling how the reactive versus tolerogenic state is controlled might point toward novel therapeutic strategies to treat autoimmune diseases. The surface receptor Toso/Faim3 has been linked to apoptosis, IgM binding, and innate immune responses. In this study, we used Toso-deficient mice to investigate the importance of Toso in tolerance and autoimmunity. We found that Toso(-/-) mice do not develop severe experimental autoimmune encephalomyelitis (EAE), a mouse model for the human disease multiple sclerosis. Toso(-/-) dendritic cells were less sensitive to Toll-like receptor stimulation and induced significantly lower levels of disease-associated inflammatory T-cell responses. Consistent with this observation, the transfer of Toso(-/-) dendritic cells did not induce autoimmune diabetes, indicating their tolerogenic potential. In Toso(-/-) mice subjected to EAE induction, we found increased numbers of regulatory T cells and decreased encephalitogenic cellular infiltrates in the brain. Finally, inhibition of Toso activity in vivo at either an early or late stage of EAE induction prevented further disease progression. Taken together, our data identify Toso as a unique regulator of inflammatory autoimmune responses and an attractive target for therapeutic intervention.
    Full-text · Article · Jan 2014 · Proceedings of the National Academy of Sciences
  • Sara R. Hamilton · Sarah Q. Crome · Pamela S. Ohashi
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    ABSTRACT: The development of a healthy adaptive immune system requires a T cell repertoire diverse enough to recognize foreign antigen while limiting recognition of self-antigens to avoid autoimmunity. However, this idea gives rise to many questions. If all lymphocytes that recognize and respond to self-antigens are eliminated, will the remaining repertoire be sufficient to respond to the array of unknown pathogens? And if all self-recognizing T cells are tolerized, how does autoimmunity and immunosurveillance occur? The T cell repertoire needs to be capable of the precarious balance between tolerance and immunity to allow for specific recognition and clearance of dangerous pathogens and transformed cells, while remaining tolerant to normal self-tissues, commensal bacteria, and food and environmental antigens. To strike this balance, there are many regulatory checkpoints in the development of the immune system and in the maintenance of immune homeostasis. This chapter will outline T cell selection, mechanisms of tolerance, and how dysregulation of these processes may lead to autoimmunity.
    No preview · Article · Dec 2013
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    Full-text · Article · Nov 2013

Publication Stats

26k Citations
3,067.35 Total Impact Points


  • 2003-2015
    • University Health Network
      • Campbell Family Institute for Breast Cancer Research
      Toronto, Ontario, Canada
  • 1985-2015
    • University of Toronto
      • • Department of Immunology
      • • Department of Medical Biophysics
      • • Ontario Cancer Institute
      Toronto, Ontario, Canada
  • 1994-2013
    • Ontario Institute for Cancer Research
      Toronto, Ontario, Canada
  • 1993-2013
    • The Princess Margaret Hospital
      Toronto, Ontario, Canada
  • 2012
    • University of Ottawa
      Ottawa, Ontario, Canada
  • 2010-2011
    • Université de Sherbrooke
      • • Department of Pediatrics
      • • Centre de recherche clinique Étienne-Le Bel
      Sherbrooke, Quebec, Canada
  • 2004
    • University of Texas at El Paso
      • Department of Biological Sciences
      El Paso, Texas, United States
  • 2000
    • Amgen
      사우전드오크스, California, United States
  • 1999
    • Samuel Lunenfeld Research Institute
      Toronto, Ontario, Canada
    • University of Wuerzburg
      • Institute for Medical Radiation and Cell Research
      Würzburg, Bavaria, Germany
  • 1998
    • University of Lausanne
      Lausanne, Vaud, Switzerland
  • 1997
    • Howard Hughes Medical Institute
      Ashburn, Virginia, United States
  • 1996
    • Amgen Canada
      Mississauga, Ontario, Canada
  • 1989-1996
    • University of Zurich
      • • Institut für Experimentelle Immunologie
      • • Institute of Veterinary Pathology
      Zürich, ZH, Switzerland
  • 1992
    • Ludwig Institute for Cancer Research
      لا هویا, California, United States
  • 1990-1992
    • University Hospital Zürich
      Zürich, Zurich, Switzerland