Kohei Kometani

SMAD4-deficient intestinal tumors recruit CCR1+ myeloid cells that promote invasion.

Inactivation of TGF-β family signaling is implicated in colorectal tumor progression. Using cis-Apc+/Δ716 Smad4+/− mutant mice (referred to as cis-Apc/Smad4), a model of invasive colorectal cancer in which TGF-β family signaling is blocked, we show here that a new type of immature myeloid cell (iMC) is recruited from the bone marrow to the tumor invasion front. These CD34+ iMCs express the matrix metalloproteinases MMP9 and MMP2 and the CC-chemokine receptor 1 (CCR1) and migrate toward the CCR1 ligand CCL9. In adenocarcinomas, expression of CCL9 is increased in the tumor epithelium. By deleting Ccr1 in the background of the cis-Apc/Smad4 mutant, we further show that lack of CCR1 prevents accumulation of CD34+ iMCs at the invasion front and suppresses tumor invasion. These results indicate that loss of transforming growth factor-β family signaling in tumor epithelium causes accumulation of iMCs that promote tumor invasion.

Memory B cells

The immune system can remember a previously experienced pathogen and can evoke an enhanced response to reinfection that depends on memory lymphocyte populations. Recent advances in tracking antigen-experienced memory B cells have revealed the existence of distinct classes of cells that have considerable functional differences. Some of these differences seem to be determined by the stimulation history during memory cell formation. To induce rapid recall antibody responses, the contributions of other types of cells, such as memory T follicular helper cells, have also now begun to be appreciated. In this Review, we discuss these and other recent advances in our understanding of memory B cells, focusing on the underlying mechanisms that are required for rapid and effective recall antibody responses.

Myeloproliferative stem cell disorders by deregulated Rap1 activation in SPA-1-deficient mice

SPA-1 (signal-induced proliferation-associated gene-1) is a principal Rap1 GTPase-activating protein in hematopoietic progenitors. SPA-1-deficient mice developed a spectrum of myeloid disorders that resembled human chronic myelogenous leukemia (CML) in chronic phase, CML in blast crisis, and myelodysplastic syndrome as well as anemia. Preleukemic SPA-1-deficient mice revealed selective expansion of marrow pluripotential hematopoietic progenitors, which showed abnormal Rap1GTP accumulation. Overexpression of an active form of Rap1 promoted the proliferation of normal hematopoietic progenitors, while SPA-1 overexpression markedly suppressed it. Furthermore, restoring SPA-1 gene in a SPA-1-deficient leukemic blast cell line resulted in the dissolution of Rap1GTP accumulation and concomitant loss of the leukemogenicity in vivo. These results unveiled a role of Rap1 in myeloproliferative stem cell disorders and a tumor suppressor function of SPA-1.

Repression of the Transcription Factor Bach2 Contributes to Predisposition of IgG1 Memory B Cells toward Plasma Cell Differentiation

Memory B cells are essential for generating rapid and robust secondary antibody responses. It has been thought that the unique cytoplasmic domain of IgG causes the prompt activation of antigen-experienced IgG memory B cells. To assess this model, we have generated a mouse containing IgG1 B cells that have never encountered antigen. We found that, upon challenge, antigen-experienced IgG1 memory B cells rapidly differentiated into plasma cells, whereas nonexperienced IgG1 B cells did not, suggesting the importance of the stimulation history. In addition, our results suggest that repression of the Bach2 transcription factor, which results from antigen experience, contributes to predisposition of IgG1 memory B cells to differentiate into plasma cells.

Preferential localization of IgG memory B cells adjacent to contracted germinal centers

It has long been presumed that after leaving the germinal centers (GCs), memory B cells colonize the marginal zone or join the recirculating pool. Here we demonstrate the preferential localization of nitrophenol-chicken γ-globulin-induced CD38+IgG1+ memory B cells adjacent to contracted GCs in the spleen. The memory B cells in this region proliferated after secondary immunization, a response that was abolished by depletion of CD4+ T cells. We also found that these IgG1+ memory B cells could present antigen on their surface, and that this activity was required for their activation. These results implicate this peri-GC region as an important site for survival and reactivation of memory B cells.

Defective immune responses in mice lacking LUBAC‐mediated linear ubiquitination in B cells

The linear ubiquitin chain assembly complex (LUBAC) plays a crucial role in activating the canonical NF‐κB pathway, which is important for B‐cell development and function. Here, we describe a mouse model (B‐HOIPΔlinear) in which the linear polyubiquitination activity of LUBAC is specifically ablated in B cells. Canonical NF‐κB and ERK activation, mediated by the tumour necrosis factor (TNF) receptor superfamily receptors CD40 and TACI, was impaired in B cells from B‐HOIPΔlinear mice due to defective activation of the IKK complex; however, B‐cell receptor (BCR)‐mediated activation of the NF‐κB and ERK pathways was unaffected. B‐HOIPΔlinear mice show impaired B1‐cell development and defective antibody responses to thymus‐dependent and thymus‐independent II antigens. Taken together, these data suggest that LUBAC‐mediated linear polyubiquitination is essential for B‐cell development and activation, possibly via canonical NF‐κB and ERK activation induced by the TNF receptor superfamily, but not by the BCR.

Bach2 maintains T cells in a naive state by suppressing effector memory-related genes

The transcriptional repressor BTB and CNC homology 2 (Bach2) is thought to be mainly expressed in B cells with specific functions such as class switch recombination and somatic hypermutation, but its function in T cells is not known. We found equal Bach2 expression in T cells and analyzed its function using Bach2-deficient (−/−) mice. Although T-cell development was normal, numbers of peripheral naive T cells were decreased, which rapidly produced Th2 cytokines after TCR stimulation. Bach2−/− naive T cells highly expressed genes related to effector-memory T cells such as CCR4, ST-2 and Blimp-1. Enhanced expression of these genes induced Bach2−/− naive T cells to migrate toward CCR4-ligand and respond to IL33. Forced expression of Bach2 restored the expression of these genes. Using Chromatin Immunoprecipitation (ChIP)-seq analysis, we identified S100 calcium binding protein a, Heme oxigenase 1, and prolyl hydroxylase 3 as Bach2 direct target genes, which are highly expressed in effector-memory T cells. These findings indicate that Bach2 suppresses effector memory-related genes to maintain the naive T-cell state and regulates generation of effector-memory T cells.

Regulated selection of germinal-center cells into the memory B cell compartment

Despite the importance of memory B cells in protection from reinfection, how such memory cells are selected and generated during germinal-center (GC) reactions remains unclear. We found here that light-zone (LZ) GC B cells with B cell antigen receptors (BCRs) of lower affinity were prone to enter the memory B cell pool. Mechanistically, cells in this memory-prone fraction had higher expression of the transcriptional repressor Bach2 than that of their counterparts with BCRs of higher affinity. Haploinsufficiency of Bach2 resulted in reduced generation of memory B cells, independently of suppression of the gene encoding the transcription factor Blimp-1. Bach2 expression in GC cells was inversely correlated with the strength of help provided by T cells. Thus, we propose an instructive model in which weak help from T cells maintains relatively high expression of Bach2, which predisposes GC cells to enter the memory pool.

ERKs Induce Expression of the Transcriptional Repressor Blimp-1 and Subsequent Plasma Cell Differentiation

The differentiation of B cells into antibody-producing plasma cells requires extracellular signal–regulated kinases. Differential Role for ERKs Given that cellular differentiation is associated with cell cycle arrest, the processes of differentiation and proliferation are thought to be mutually exclusive, and the transcriptional programs underlying these events are frequently stimulated by distinct extracellular cues. The extracellular signal–regulated kinase (ERK) signaling pathway is responsible for driving proliferation in many cell types, including immune cells, and thus is thought to be involved only in dividing cells (see the Perspective by Allman and Cancro). In B cells, the transcriptional program of proliferating cells is suppressed by the transcriptional repressor Blimp-1, which is required for the differentiation of B cells into antibody-presenting plasma cells, a terminally differentiated cell type. Through experiments with mice deficient in both ERK1 and ERK2 in B cells, Yasuda et al. provide evidence of a critical role for the ERKs in driving the differentiation of plasma cells and show that ERK signaling is required for the expression of Blimp-1. Together, these data expand the role of ERK signaling in B cells and should prompt investigation of the potential involvement of ERK proteins in the differentiation of other cell types. In immune cells, the positive role of the extracellular signal–regulated kinase (ERK) signaling pathway in cell cycle progression and survival is well established; however, it is unclear whether ERK signaling plays a role in cell differentiation. Here, we report that ERKs are essential for the differentiation of B cells into antibody-producing plasma cells and that ERKs induce the expression of Prdm1, which encodes Blimp-1, a transcriptional repressor and “master regulator” of plasma cell differentiation. Transgenic mice with conditional deletion of both ERK1 and ERK2 in germinal center (GC) B cells lacked plasma cells differentiated after GC formation, and memory B cells from these mice failed to differentiate into plasma cells. In addition, ERK1- and ERK2-deficient B cells exhibited impaired Prdm1 expression upon stimulation with antibody against CD40 in the presence of interleukin-4; conversely, enforced expression of Prdm1 in ERK1- and ERK2-deficient B cells restored the generation of plasma cells. Thus, our study suggests that cytokines stimulate ERKs to induce the production of Blimp-1 and that ERKs thereby contribute to the process of cellular differentiation.

Unique properties of memory B cells of different isotypes

Summary:  Memory antibody responses are typically seen to T‐cell‐dependent antigens and are characterized by the rapid production of high titers of high‐affinity antigen‐specific antibody. The hallmark of T‐cell‐dependent memory B cells is their expression of a somatically mutated, isotype‐switched B‐cell antigen receptor, features that are mainly generated in germinal centers. Classical studies have focused on isotype‐switched memory B cells (mainly IgG isotype) and demonstrated their unique intrinsic properties in terms of localization and responsiveness to antigen re‐exposure. However, recent advances in monitoring antigen‐experienced B cells have revealed the considerable heterogeneity of memory B cells, which include unswitched IgM+ and/or unmutated memory B cells. The IgM and IgG type memory B cells reside in distinct locations and appear to possess distinct origins and effector functions, together orchestrating humoral memory responses.