Hiroyuki Gonda

Lymph Node Fibroblastic Reticular Cells Construct the Stromal Reticulum via Contact with Lymphocytes

The sophisticated microarchitecture of the lymph node, which is largely supported by a reticular network of fibroblastic reticular cells (FRCs) and extracellular matrix, is essential for immune function. How FRCs form the elaborate network and remodel it in response to lymphocyte activation is not understood. In this work, we established ERTR7+gp38+VCAM-1+ FRC lines and examined the production of the ER-TR7 antigen. Multiple chemokines produced by FRCs induced T cell and dendritic cell chemotaxis and adhesion to the FRC surface. FRCs can secrete the ER-TR7 antigen as an extracellular matrix component to make a reticular meshwork in response to contact with lymphocytes. The formation of the meshwork is induced by stimulation with tumor necrosis factor-α or lymphotoxin-α in combination with agonistic antibody to lymphotoxin-β receptor in a nuclear factor-κB (RelA)–dependent manner. These findings suggest that signals from lymphocytes induce FRCs to form the network that supports the movement and interactions of immune effectors within the lymph node.

The Balance Between Pax5 and Id2 Activities Is the Key to AID Gene Expression

Pax5 activity is enhanced in activated B cells and is essential for class switch recombination (CSR). We show that inhibitor of differentiation (Id)2 suppresses CSR by repressing the gene expression of activation-induced cytidine deaminase (AID), which has been shown to be indispensable for CSR. Furthermore, a putative regulatory region of AID contains E2A- and Pax5-binding sites, and the latter site is indispensable for AID gene expression. Moreover, the DNA-binding activity of Pax5 is decreased in Id2-overexpressing B cells and enhanced in Id2−/− B cells. The kinetics of Pax5, but not E2A, occupancy to AID locus is the same as AID expression in primary B cells. Finally, enforced expression of Pax5 induces AID transcription in pro–B cell lines. Our results provide evidence that the balance between Pax5 and Id2 activities has a key role in AID gene expression.

Organizer-Like Reticular Stromal Cell Layer Common to Adult Secondary Lymphoid Organs

Mesenchymal stromal cells are crucial components of secondary lymphoid organs (SLOs). Organogenesis of SLOs involves specialized stromal cells, designated lymphoid tissue organizer (LTo) in the embryonic anlagen; in the adult, several distinct stromal lineages construct elaborate tissue architecture and regulate lymphocyte compartmentalization. The relationship between the LTo and adult stromal cells, however, remains unclear, as does the precise number of stromal cell types that constitute mature SLOs are unclear. From mouse lymph nodes, we established a VCAM-1+ICAM-1+MAdCAM-1+ reticular cell line that can produce CXCL13 upon LTβR stimulation and support primary B cell adhesion and migration in vitro. A similar stromal population sharing many characteristics with the LTo, designated marginal reticular cells (MRCs), was found in the outer follicular region immediately underneath the subcapsular sinus of lymph nodes. Moreover, MRCs were commonly observed at particular sites in various SLOs even in Rag2−/− mice, but were not found in ectopic lymphoid tissues, suggesting that MRCs are a developmentally determined element. These findings lead to a comprehensive view of the stromal composition and architecture of SLOs.

Roles of p-ERM and Rho–ROCK signaling in lymphocyte polarity and uropod formation

Front–rear asymmetry in motile cells is crucial for efficient directional movement. The uropod in migrating lymphocytes is a posterior protrusion in which several proteins, including CD44 and ezrin/radixin/moesin (ERM), are concentrated. In EL4.G8 T-lymphoma cells, Thr567 phosphorylation in the COOH-terminal domain of ezrin regulates the selective localization of ezrin in the uropod. Overexpression of the phosphorylation-mimetic T567D ezrin enhances uropod size and cell migration. T567D ezrin also induces construction of the CD44-associated polar cap, which covers the posterior cytoplasm in staurosporine-treated, uropod-disrupted EL4.G8 cells or in naturally unpolarized X63.653 myeloma cells in an actin cytoskeleton–dependent manner. Rho-associated coiled coil–containing protein kinase (ROCK) inhibitor Y-27632 disrupts the uropod but not the polar cap, indicating that Rho–ROCK signaling is required for posterior protrusion but not for ERM phosphorylation. Phosphorylated ezrin associates with Dbl through its NH2-terminal domain and causes Rho activation. Moreover, constitutively active Q63L RhoA is selectively localized in the rear part of the cells. Thus, phosphorylated ERM has a potential function in establishing plasma membrane “posteriority” in the induction of the uropod in T lymphocytes.

Essential role of Id2 in negative regulation of IgE class switching.

Serum concentrations of immunoglobulin E (IgE) in normal circumstances are kept much lower than those of other Ig isotypes to avoid allergic reactions. B cells lacking Id2 have increased E2A activity, which leads to specific enhancement of germline transcription of the immunoglobulin ε locus. As a consequence, Id2-deficient B cells undergo class switch recombination (CSR) to IgE at a much higher frequency than wild-type B cells. In contrast, Id2 is induced in wild-type B cells by transforming growth factor-β1 (TGF-β1) and suppresses IgE CSR. Our results provide evidence for the inhibitory and selective role of Id2 in IgE CSR in response to TGF-β1. Id2 might act as molecular safeguard to suppress IgE CSR to prevent serious complications such as allergic hypersensitivity during the normal course of immune responses.

Targeting of Krüppel-associated Box-containing Zinc-finger Proteins to Centromeric Heterochromatin : Implication for the Gene Silencing Mechanisms

Krüppel-associated box-containing zinc finger proteins (KRAB-ZFPs) repress transcription via functional interaction with the corepressor KRAB-associated protein-1 (KAP-1). KAP-1 directly interacts with heterochromatin protein 1 (HP1), a dose-dependent regulator of heterochromatin-mediated silencing. Here we show that two KRAB-ZFPs that we previously identified, KRAZ1 and KRAZ2, are targeted to foci of centromeric heterochromatin containing HP1α through the interaction with KAP-1. Centromeric targeting potential of KRAZ1 and KAP-1 is strictly correlated with their silencing activities; a KRAB mutant of KRAZ1 that is unable to bind KAP-1 and KAP-1 deletions unable to bind HP1 cannot localize to centromeric foci nor repress transcription. We provide evidence that this correlation is likely to be functionally relevant. First, overexpression of the VP16 transactivation domain fused with the KAP-1 deletion that binds to KRAB but not to HP1 leads to dramatic redistribution of KRAZ1 from centromeric foci and simultaneously converts KRAZ1-mediated silencing into strong transcriptional activation. Second, a specific inhibitor of histone deacetylases, trichostatin A, effectively redistributes KRAZ1 and KAP-1 from centromeric foci and partially relieves their silencing activities. These data strongly suggest that KRAB-ZFPs/KAP-1 silence transcription by dynamic recruitment of the target locus to the specific gene silencing compartment, centromeric heterochromatin, in a histone deacetylase-dependent manner.

Th1-Biased Tertiary Lymphoid Tissue Supported by CXC Chemokine Ligand 13-Producing Stromal Network in Chronic Lesions of Autoimmune Gastritis

Secondary lymphoid tissue is developmentally programmed and characterized by well-ordered compartmentalization of lymphocyte subsets and specialized stromal cells supporting the tissue architecture. By contrast, tertiary lymphoid tissue is defined as that induced in ectopic sites by inflammation, although its immunological role is largely unknown. In this study, we characterize the lymphoid tissue induced in the chronic lesion of murine autoimmune gastritis (AIG). Within the lymphoid cluster in the gastric mucosa, there is a clear segregation of T and B cells. Follicle-like B cell areas are always located on the luminal side of the mucosa, while T cells are located in the basal part. A typical lymphoid reticular network and follicular dendritic cells support the structure. Importantly, complement receptor 1+ follicular dendritic cells within the follicle express a B cell homing chemokine, CXC chemokine ligand 13. The number and size of the clusters correlate with the age of the mice and the serum autoantibody titer, suggesting the functional importance of the clusters in local Ab production, although involvement of the autoantibody in the disease progression is still unclear. AIG gastric lesions are known to constitute a Th1-biased, memory T cell-dependent immunomicroenvironment. The expression pattern of cytokines, including lymphotoxin-β, and chemokines in the AIG stomach is consistent with this observation. Taken together, these facts suggest that, during the chronic phase of autoimmunity, long-lasting lymphocyte infiltration probably induces a unique tertiary lymphoid tissue that has a function distinct from that of regional lymph nodes. These neolymphoid tissues may maintain the local self reactivity supporting the vicious cycle of Th1-type reaction as well as autoantibody production.

Role of Id proteins in B lymphocyte activation: new insights from knockout mouse studies

Id (inhibitor of differentiation) proteins play important roles in cell differentiation, cell cycle control, and apoptosis. They act as negative regulators of basic helix-loop-helix-type transcription factors, which positively regulate differentiation of various cell types. Id proteins work to block B lymphocyte (B cell) maturation at an early differentiation step, as demonstrated by gain-of-function studies. In recent years a series of gene-targeted mice lacking different Ids have been generated. Analyses of these gene-targeted mice provide information useful for understanding the physiological roles of Ids in B cell biology. Id3 is required for proper B cell functions and acts by controlling the cell cycle. Upon B cell activation, Id2 acts as a negative regulator to prevent potentially harmful effects brought about by excessive immunological reactions; one of its special roles is to maintain low serum concentrations of immunoglobulin E (IgE). The Id2 protein does this by antagonizing E2A and Pax5 activities, both of which are required for proper B cell activation. This review presents several new insights into B cell differentiation and activation programs and the physiological role of Id proteins in B cell activation.

Spontaneous Large-Scale Lymphoid Neogenesis and Balanced Autoimmunity versus Tolerance in the Stomach of H+/K+-ATPase-Reactive TCR Transgenic Mouse

Autoimmunity is often accompanied by the development of ectopic lymphoid tissues in the target organ, and these tissues have been believed to have close relevance to the severity of the disease. However, the true relationship between the extent of such lymphoid structures and the intensity or type of immune responses mediated by self-reactive T cells has remained unclear. In the present study, we generated transgenic mice expressing TCR from an autoimmune gastritis (AIG)-inducing Th1 cell clone specific for one of the major stomach self-Ags, H+/K+-ATPase α subunit. The transgenic mice spontaneously develop massive lymphoid neogenesis with a highly organized tissue structure in the gastric mucosa, demonstrating Ag-specific, T cell-mediated induction of the lymphoid tissues. Nevertheless, the damage of surrounding tissue and autoantibody production were considerably limited compared with those in typical AIG induced by neonatal thymectomy. Such a moderate pathology is likely due to the locally restricted activation and Th2 skewing of self-reactive T cells, as well as the accumulation of naturally occurring regulatory T cells in the target organ. Altogether, the findings suggest that lymphoid neogenesis in chronic autoimmunity does not simply correlate with the destructive response; rather, the overall activation status of the T cell network, i.e., the balance of self-reactivity and tolerance, in the local environment has an impact.

DNA polymerase β is not essential for the formation of palindromic (P) region of T cell receptor gene

Formation of palindromic (P) region at the variable (V)-diversity (D)-joining (J) junction in DNA polymerase beta (pol-beta) deficient mice were investigated by sequencing of reverse transcriptase-polymerase chain reaction (RT-PCR) products of mRNAs encoding the beta chain of T cell receptor (TCR). Total 42 and 43 cDNA clones encoding V(beta8)-D(beta)-J(beta)-C(beta) from E18.5 embryonic thymocytes of pol-beta gene knocked-out and wild type control mouse, respectively, were sequenced. Among them five and six clones from pol-beta knocked-out and wild type, respectively, have P insertions of two nucleotides. This result unequivocally indicates that pol-beta, which is one of the repair-type DNA polymerases most abundantly expressed in thymus and spleen, is not essential for the formation of P region.