Tomonori Kaifu

The B7 family member B7-H6 is a tumor cell ligand for the activating natural killer cell receptor NKp30 in humans

Cancer development is often associated with the lack of specific and efficient recognition of tumor cells by the immune system. Natural killer (NK) cells are lymphocytes of the innate immune system that participate in the elimination of tumors. We report the identification of a tumor cell surface molecule that binds NKp30, a human receptor which triggers antitumor NK cell cytotoxicity and cytokine secretion. This previously unannotated gene belongs to the B7 family and, hence, was designated B7-H6. B7-H6 triggers NKp30-mediated activation of human NK cells. B7-H6 was not detected in normal human tissues but was expressed on human tumor cells, emphasizing that the expression of stress-induced self-molecules associated with cell transformation serves as a mode of cell recognition in innate immunity.

B7-H6/NKp30 interaction: a mechanism of alerting NK cells against tumors.

Natural killer (NK) cells are lymphocytes of the innate immune system that sense target cells through a panel of activating and inhibitory receptors. Together with NKG2D, the natural cytotoxicity receptors (NCRs) are major activating receptors involved in tumor cell detection. Although numerous NKG2D ligands have been identified, characterization of the molecules interacting with the NCRs is still incomplete. The identification of B7-H6 as a counter structure of the NCR NKp30 shed light on the molecular basis of NK cell immunosurveillance. We review here the current knowledge on NKp30 and B7-H6, and we discuss their potential role in anti-tumor immunity.

DCIR Maintains Bone Homeostasis by Regulating IFN-γ Production in T Cells

Dendritic cell immunoreceptor (DCIR) is a C-type lectin receptor mainly expressed in DCs. Dcir−/− mice spontaneously develop autoimmune enthesitis and ankylosis accompanied by fibrocartilage proliferation and ectopic ossification. However, the mechanisms of new bone/cartilage formation in Dcir−/− mice remain to be elucidated. In this study, we show that DCIR maintains bone homeostasis by regulating IFN-γ production under pathophysiological conditions. DCIR deficiency increased bone volume in femurs and caused aberrant ossification in joints, whereas these symptoms were abolished in Rag2−/−Dcir−/− mice. IFN-γ–producing T cells accumulated in lymph nodes and joints of Dcir−/− mice, and purified Dcir−/− DCs enhanced IFN-γ+ T cell differentiation. The ankylotic changes and bone volume increase were suppressed in the absence of IFN-γ. Thus, IFN-γ is a positive chondrogenic and osteoblastogenic factor, and DCIR is a crucial regulator of bone metabolism; consequently, both factors are potential targets for therapies directed against bone metabolic diseases.

Featured Article: Accelerated decline of physical strength in peroxiredoxin-3 knockout mice

As a member of peroxiredoxin family, peroxiredoxin-3 plays a major role in the control of mitochondrial level of reactive oxygen species. During the breeding of experimental mice, we noticed that the peroxiredoxin-3 knockout mice were listless with aging. In the present study, we compared the swimming exercise performance and oxidative status between peroxiredoxin-3 knockout mice (n = 15) and wild-type littermates (n = 15). At the age of 10 months, the physical strength of peroxiredoxin-3 knockout mice was much lower than the wild-type littermates. Increased oxidative damage and decreased mitochondrial DNA copy number of the animal skeletal muscles were observed in peroxiredoxin-3 knockout mice as compared to that in the wild-type littermates. In addition, we found increased apoptotic cells in the brains of peroxiredoxin-3 knockout mice. Our results suggest that the deficiency of peroxiredoxin-3 induces accelerated oxidative stress and mitochondrial impairment, resulting in the decrease of energy supply and cellular activities. Peroxiredoxin-3 might be involved in the inhibition of aging process.

Generation of a Tlx1CreER‐Venus knock‐in mouse strain for the study of spleen development

The spleen is a lymphoid organ that serves as a unique niche for immune reactions, extramedullary hematopoiesis, and the removal of aged erythrocytes from the circulation. While much is known about the immunological functions of the spleen, the mechanisms governing the development and organization of its stromal microenvironment remain poorly understood. Here we report the generation and analysis of a Tlx1CreER‐Venus knock‐in mouse strain engineered to simultaneously express tamoxifen‐inducible CreERT2 and Venus fluorescent protein under the control of regulatory elements of the Tlx1 gene, which encodes a transcription factor essential for spleen development. We demonstrated that Venus as well as CreER expression recapitulates endogenous Tlx1 transcription within the spleen microenvironment. When Tlx1CreER‐Venus mice were crossed with the Cre‐inducible reporter strain, Tlx1‐expressing cells as well as their descendants were specifically labeled following tamoxifen administration. We also showed by cell lineage tracing that asplenia caused by Tlx1 deficiency is attributable to altered contribution of mesenchymal cells in the spleen anlage to the pancreatic mesenchyme. Thus, Tlx1CreER‐Venus mice represent a new tool for lineage tracing and conditional gene manipulation of spleen mesenchymal cells, essential approaches for understanding the molecular mechanisms of spleen development. genesis 52:916–923, 2014.

Dendritic Cell Immunoreceptor (DCIR): An ITIM-Harboring C-Type Lectin Receptor

C-type lectin receptors (CLRs) have been recognized as one of the pattern recognition receptors that trigger immune responses against pathogens. Dendritic cell immunoreceptor (DCIR) is a type II membrane protein that contains an extracellular carbohydrate recognition domain (CRD) and a long cytoplasmic tail with an immunoreceptor tyrosine-based inhibition motif (ITIM). Only one molecular species is identified in humans (DCIR), while four family molecules (DCIR1-4) are present in mice. Human DCIR and mouse DCIR1 dampen immune responses through ITIM-mediated reaction. DCIR binds mannose and fucose and also pathogenic organisms, but the structure of the ligand carbohydrates still remains to be determined. DCIR1 is important for the homeostasis of the immune system, and the deficiency causes autoimmune diseases. DCIR also acts as an attachment factor for HIV in dendritic cells and HIV-infected T cells. DCIR1 is also implicated in the pathogenesis of mosquito-transmitted virus and protozoan infections. This chapter highlights the roles of human and mouse DCIR in immune responses and immune homeostasis revealed by in vitro cell-based studies as well as in vivo gene-depleted mouse analyses.

DCIR in the "osteo-immune" system.

Dendritic cell immunoreceptor (Clec4a2, DCIR) is one of C-type lectin receptors (CLRs), which is predominantly expressed in dendritic cells (DCs) [1]. DCIR contains a carbohydrate recognition domain in the extracellular part and an immunoreceptor tyrosine-based inhibitory motif (ITIM) in the cytoplasmic region which can negatively regulate immune signaling by recruiting phosphatases, SHP-1 and SHP-2. Previously, Fujikado et al. reported that Dcir−/− mice spontaneously develop autoimmune sialadenitis and enthesitis, that eventually causes ankylotic changes of joints with age [2]. These mice are also highly susceptible to collagen-induced arthritis and experimental autoimmune encephalomyelitis, animal models for rheumatoid arthritis (RA) and multiple sclerosis, respectively. This is because antigen presentation is enhanced in Dcir−/− mice, due to over-expansion of DC population. They showed that Dcir-deficient bone marrow cells differentiate into DCs more efficiently in vitro upon treatment with GM-CSF, because STAT5 activation is augmented in Dcir−/− cells [2]. Thus, DCIR plays an important role in regulating homeostasis of the immune system by regulating DC development and expansion. Recently, Maruhashi and Kaifu et al. investigated the mechanism how joint ankylosis is induced in Dcir−/− mice [3]. They found that not only joint ankylosis but also mild bone volume increase of thigh bones occurs in Dcir−/− mice. Interestingly, these abnormalities are completely abolished in Dcir−/−Rag2−/−mice and Dcir−/−Ifng−/−mice. IFN-γ-producing T cell population is increased in Dcir−/− mice, and co-culture of purified Dcir−/− DCs with wild-type T cells promotes differentiation of IFN-γ-producing T cells more efficiently than wild-type DCs, consistent with the report by Kaneko et al., in which they showed that DCs derived from SHP-1-deficient mice preferentially support IFN-γ-producing Th1 cell differentiation [4]. Furthermore, Maruhashi and Kaifu et al. showed that IFN-γ enhances proliferation and differentiation of chondrocytes and osteoblasts in vitro, suggesting that these osteogenic and chondrogenic activities of IFN-γ directly contribute to the bone abnormalities in Dcir−/− mice [3]. These observations suggest that DCIR is critically involved in the regulation of bone homeostasis through regulation of IFN-γ-producing T cell differentiation via ITIM-SHP-1 activation. Thus, DCIR is important not only for immune homeostasis but also for bone metabolism, indicating that DCIR is one of regulators that coordinate bone formation and immune activation. As such, the immune and bone systems share a large number of regulatory components including cytokines, signaling molecules, transcriptional factors, and receptors [5]. However, the biological meaning of this coordination largely remains to be elucidated. Ankylosing spondylitis (AS), a form of seronegative spondyloarthritis, is an inflammatory joint disorder of the axial skeleton [6]. The primary clinical symptom is axial and peripheral enthesitis, an inflammation at the sites of attachment of ligaments, tendons and joint capsules to bone. Ankylosis and joint immobility subsequently develop due to heterotopic cartilage and bone formation. It is still unclear, however, how entheseal inflammation is coupled to ankylosis in AS. Regarding this, it is worth noting that the pathology observed in the axial and peripheral joints of Dcir−/− mice closely resembles that of AS in humans. However, the etiopathogenesis seems different, because HLA-B27 and endoplasmic reticulum-stress-induced activation of the IL-23/IL-17 axis are involved in AS but not in the ankylotic changes in Dcir−/− mice [6]. Nonetheless, the critical roles of IFN-γ in the development of ankylosis in Dcir−/− mice suggest that IFN-γ may also be involved in the development of ankylosis in AS patients. CLRs are widely recognized as one of pattern recognition receptors that sense pathogen-derived carbohydrate structures and initiate innate and adaptive immune responses against pathogens [1]. Actually, DCIR-Fc fusion protein binds helminthes such as S. mansoni and T. spiralis. DCIR signaling suppresses TLR8- and TLR9-mediated cytokine productions. Furthermore, HIV-1 binds DCs through DCIR, resulting in the promotion of HIV-1 transmission to CD4+ T cells. However, the present report by Maruhashi and Kaifu et al. clearly shows that DCIR is important for the regulation of bone metabolism independently from pathogen infection, suggesting the presence of endogenous ligands [3]. Endogenous ligands are also reported recently for other CLRs such as Mincle, Clec12a, and Clec9a, suggesting that this group of lectins also plays important roles apart from host defense against pathogens [7]. The presence of EPS motif that enables binding to galactose-containing ligands in the carbohydrate recognition domain suggests glycans as the ligands [1]. Identificaiton of DCIR ligans should be important for understanding the molecular mechanisms how DCs differentiation and bone formation are controlled, and may provide us a clue to develop novel therapeutics for bone metabolic disorders.