The miR-142 miRNAs: Shaping the naïve immune system

Immunity in a naïve organism is tightly controlled. Adequate proportions of the many immune cell subsets must be produced to mount efficient responses to eventual challenges. In addition, a functioning immune system is highly dynamic at steady state. Mature immune cells must be positioned properly and/or circulate to facilitate the detection of dangers. They must also be poised to promptly react to unusual encounters, while ignoring innocuous germs and self. Numerous regulatory mechanisms act at the molecular level to generate such an exquisite structure, including miRNA-mediated repression of protein synthesis. Notably, the miRNAs from the miR-142 locus are preferentially expressed in hematopoietic cells. Their importance is underscored by the deeply disturbed immune system seen upon inactivation of the locus in mice. In this review, we explore reported roles for the miR-142 miRNAs in the shaping of immunity in vertebrates, discussing in particular their contributions to the generation, migration and survival of hematopoietic cells.

miR-142 favors naïve B cell residence in peripheral lymph nodes

B lymphocyte development proceeds through a well-ordered sequence of steps, leading to the formation of a sizeable mature B population recognizing a diversity of antigens. These latter cells are ultimately responsible for the production of antibodies upon immune challenges. The detection of threats to the organism is facilitated by the ability of naïve follicular B cells, the main subset of mature B cells in mice, to circulate between lymphoid tissues in search of their cognate antigens. miRNA-mediated fine-tuning of mRNA stability and translation participates in the optimal expression of genetic programs. This regulatory mechanism has been shown to contribute to B cell biology, although the role of individual miRNAs remains understudied. Here, we selectively inactivated the miR-142 locus in B cells. As a consequence, the mature B compartment was visibly perturbed, in agreement with work in miR-142 knockout mice. However, our strategy allowed us to identify roles for the miR-142 locus in B cell physiology obscured by the complexity of the immune phenotype in the null mutant mice. Thus, these miRNAs are necessary for the proper formation of the pre-B cell compartment during development. More remarkably, naïve follicular B cells demonstrated altered migratory properties upon conditional inactivation of the miR-142 locus. The latter mutant cells expressed reduced levels of the homing molecule CD62L. They also migrated more efficiently towards sphingosine-1-phosphate in vitro and displayed an increased abundance of the sphingosine-1-phosphate receptor 1, compatible with improved lymphocyte egress in vivo. In line with these observations, the ablation of the miR-142 locus in B cells caused a paucity of B cells in the lymph nodes. Mutant B cell accumulation in the latter tissues was also compromised upon transfer into a wild-type environment. These changes coincided with suboptimal levels of FOXO1, a positive regulator of CD62L transcription, in mutant B cells. Overall, our findings indicate contributions for the miR-142 locus in various aspects of the B cell life cycle. Notably, this locus appears to favor the establishment of the migratory behavior required for naïve follicular B cell patrolling activity.

A role for the nuclear receptor NR2F6 in peritoneal B cell homeostasis

B cells are key mediators of humoral immunity. Mature B cells fall into various sub-classes that can be separated by their ontogeny, expression of cell surface markers, anatomical location, and function. B1 subsets play important roles in natural immunity and constitute the majority of B cells in newborns. In the adult, B1 cells predominate in the pleural and peritoneal cavities, while the mature B2 follicular subset makes up the major fraction of B cells in lymphoid tissue, although important subsets of antibody-secreting B1 cells are also present at these sites. B1 cells are the main producers of natural IgM but can also contribute to elimination of some pathogens, while B2 cells primarily mediate response to foreign antigens. The differential molecular underpinning of the B1 and B2 subsets remains incompletely understood. Here we demonstrate that germline-deficiency of the orphan nuclear receptor NR2F6 causes a partial loss of B1b and B2 B cells in the peritoneum while leaving peritoneal B1a cells unaltered. A competitive bone marrow chimera in Nr2f6^+/+ host mice produced similar numbers of Nr2f6^+/+ and Nr2f6^-/- peritoneal B1b and B2 cells. The proliferation of Nr2f6^-/- peritoneal B cells was not altered, while the migration marker CXCR5 was reduced on all subsets but Beta7-integrin was reduced only on peritoneal B1b and B2 cells. Similarly, B1b and B2 but not B1a cells, exhibited significantly reduced survival.

Cell signaling and the aging of B cells

The uniqueness of each B cell lies in the structural diversity of the B-cell antigen receptor allowing the virtually limitless recognition of antigens, a necessity to protect individuals against a range of challenges. B-cell development and response to stimulation are exquisitely regulated by a group of cell surface receptors modulating various signaling cascades and their associated genetic programs. The effects of these signaling pathways in optimal antibody-mediated immunity or the aberrant promotion of immune pathologies have been intensely researched in the past in young individuals. In contrast, we are only beginning to understand the contribution of these pathways to the changes in B cells of old organisms. Thus, critical transcription factors such as E2A and STAT5 show differential expression or activity between young and old B cells. As a result, B-cell physiology appears altered, and antibody production is impaired. Here, we discuss selected phenotypic changes during B-cell aging and attempt to relate them to alterations of molecular mechanisms.

Context-specific regulation of cell survival by a miRNA-controlled BIM rheostat

Knockout of the ubiquitously expressed miRNA-17∼92 cluster in mice produces a lethal developmental lung defect, skeletal abnormalities, and blocked B lymphopoiesis. A shared target of miR-17∼92 miRNAs is the pro-apoptotic protein BIM, central to life-death decisions in mammalian cells. To clarify the contribution of miR-17∼92:Bim interactions to the complex miR-17∼92 knockout phenotype, we used a system of conditional mutagenesis of the nine Bim 3' UTR miR-17∼92 seed matches. Blocking miR-17∼92:Bim interactions early in development phenocopied the lethal lung phenotype of miR-17∼92 ablation and generated a skeletal kinky tail. In the hematopoietic system, instead of causing the predicted B cell developmental block, it produced a selective inability of B cells to resist cellular stress; and prevented B and T cell hyperplasia caused by Bim haploinsufficiency. Thus, the interaction of miR-17∼92 with a single target is essential for life, and BIM regulation by miRNAs serves as a rheostat controlling cell survival in specific physiological contexts.

Inflammation and the Alteration of B-Cell Physiology in Aging

Aging results for the immune system in a departure from the optimal homeostatic state seen in young organisms. This divergence regrettably contributes to a higher frequency of compromised responses to infections and inefficient classical vaccination in aged populations. In B cells, the cornerstone of humoral immunity, the development and distribution of the various mature B cell subsets are impacted by aging in both humans and mice. In addition, aged mature B cells demonstrate limited capacity to mount efficient antibody responses. An expected culprit for the decline in effective immunity is the rise of the systemic levels of pro-inflammatory molecules during aging, establishing a chronic low-grade inflammation. Indeed, numerous alterations affecting directly or indirectly B cells in old people and mice are reminiscent of various effects of acute inflammation on this cell type in young adults. The present mini-review will highlight the possible adverse contributions of the persistent low-level inflammation observed in susceptible older organisms to the inadequate B-cell physiology.

TET enzymes control antibody production and shape the mutational landscape in germinal centre B cells

Upon activation by antigen, B cells form germinal centres where they clonally expand and introduce affinity-enhancing mutations into their B-cell receptor genes. Somatic mutagenesis and class switch recombination (CSR) in germinal centre B cells are initiated by the activation-induced cytidine deaminase (AID). Upon germinal centre exit, B cells differentiate into antibody-secreting plasma cells. Germinal centre maintenance and terminal fate choice require transcriptional reprogramming that associates with a substantial reconfiguration of DNA methylation patterns. Here we examine the role of ten-eleven-translocation (TET) proteins, enzymes that facilitate DNA demethylation and promote a permissive chromatin state by oxidizing 5-methylcytosine, in antibody-mediated immunity. Using a conditional gene ablation strategy, we show that TET2 and TET3 guide the transition of germinal centre B cells to antibody-secreting plasma cells. Optimal AID expression requires TET function, and TET2 and TET3 double-deficient germinal centre B cells show defects in CSR. However, TET2/TET3 double-deficiency does not prevent the generation and selection of high-affinity germinal centre B cells. Rather, combined TET2 and TET3 loss-of-function in germinal centre B cells favours C-to-T and G-to-A transition mutagenesis, a finding that may be of significance for understanding the aetiology of B-cell lymphomas evolving in conditions of reduced TET function.

T cell-derived IL-17 mediates epithelial changes in the airway and drives pulmonary neutrophilia

Th17 cells are a proinflammatory subset of effector T cells that have been implicated in the pathogenesis of asthma. Their production of the cytokine IL-17 is known to induce local recruitment of neutrophils, but the direct impact of IL-17 on the lung epithelium is poorly understood. In this study, we describe a novel mouse model of spontaneous IL-17-driven lung inflammation that exhibits many similarities to asthma in humans. We have found that STAT3 hyperactivity in T lymphocytes causes an expansion of Th17 cells, which home preferentially to the lungs. IL-17 secretion then leads to neutrophil infiltration and lung epithelial changes, in turn leading to a chronic inflammatory state with increased mucus production and decreased lung function. We used this model to investigate the effects of IL-17 activity on airway epithelium and identified CXCL5 and MIP-2 as important factors in neutrophil recruitment. The neutralization of IL-17 greatly reduces pulmonary neutrophilia, underscoring a key role for IL-17 in promoting chronic airway inflammation. These findings emphasize the role of IL-17 in mediating neutrophil-driven pulmonary inflammation and highlight a new mouse model that may be used for the development of novel therapies targeting Th17 cells in asthma and other chronic pulmonary diseases.

Immune surveillance and therapy of lymphomas driven by Epstein-Barr virus protein LMP1 in a mouse model

B cells infected by Epstein-Barr virus (EBV), a transforming virus endemic in humans, are rapidly cleared by the immune system, but some cells harboring the virus persist for life. Under conditions of immunosuppression, EBV can spread from these cells and cause life-threatening pathologies. We have generated mice expressing the transforming EBV latent membrane protein 1 (LMP1), mimicking a constitutively active CD40 coreceptor, specifically in B cells. Like human EBV-infected cells, LMP1+ B cells were efficiently eliminated by T cells, and breaking immune surveillance resulted in rapid, fatal lymphoproliferation and lymphomagenesis. The lymphoma cells expressed ligands for a natural killer (NK) cell receptor, NKG2D, and could be targeted by an NKG2D-Fc fusion protein. These experiments indicate a central role for LMP1 in the surveillance and transformation of EBV-infected B cells in vivo, establish a preclinical model for B cell lymphomagenesis in immunosuppressed patients, and validate a new therapeutic approach.

Development of immunoglobulin lambda-chain-positive B cells, but not editing of immunoglobulin kappa-chain, depends on NF-kappaB signals

By genetically ablating IκB kinase (IKK)-mediated NF-κB activation in the B cell lineage, and by analyzing a mouse mutant in which Igλ⁺ B cells are generated in the absence of rearrangements in Igk, we define two distinct, consecutive phases of early B cell development that differ in their dependence on IKK-mediated NF-κB signaling. During the first phase, in which NF-κB signaling is dispensable, predominantly Igκ⁺ B cells are generated and undergo efficient receptor editing. In the second phase, predominantly Igλ⁺ B cells are generated, whose development is ontogenetically timed to occur after Igk rearrangements. This second phase of development is dependent on NF-κB signals, which can be substituted by transgenic expression of the pro-survival factor Bcl2.

NIK overexpression amplifies, whereas ablation of its TRAF3-binding domain replaces BAFF:BAFF-R-mediated survival signals in B cells

BAFF-R-dependent activation of the alternative NF-kappaB pathway plays an essential role in mature B cell survival. Mutations leading to overexpression of NIK and deletion of the TRAF3 gene are implicated in human multiple myeloma. We show that overexpression of NIK in mouse B lymphocytes amplifies alternative NF-kappaB activation and peripheral B cell numbers in a BAFF-R-dependent manner, whereas uncoupling NIK from TRAF3-mediated control causes maximal p100 processing and dramatic hyperplasia of BAFF-R-independent B cells. NIK controls alternative NF-kappaB signaling by increasing the protein levels of its negative regulator TRAF3 in a dose-dependent fashion. This mechanism keeps NIK protein levels below detection even when they cause B cell hyperplasia, so that contributions of NIK to B cell pathologies can easily be overlooked.

Canonical NF-kappaB activity, dispensable for B cell development, replaces BAFF-receptor signals and promotes B cell proliferation upon activation

The maintenance of mature B cells hinges on signals emitted from the BAFF-R cell-surface receptor, but the nature of these signals is incompletely understood. Inhibition of canonical NF-kappaB transcription factor activity through ablation of the essential scaffold protein NEMO arrests B cell development at the same stage as BAFF-R deficiency. Correspondingly, activation of this pathway by constitutively active IkappaB Kinase2 renders B cell survival independent of BAFF-R:BAFF interactions and prevents proapoptotic PKCdelta nuclear translocation. In addition, canonical NF-kappaB activity mediates differentiation and proper localization of follicular and marginal zone B cells in the absence of BAFF-R, but not CD19. By replacing BAFF-R signals, constitutive canonical NF-kappaB signaling, a hallmark of various B cell lymphomas, causes accumulation of resting B cells and promotes their proliferation and survival upon activation, but does not per se induce lymphomagenesis. Therefore, canonical NF-kappaB activity can substitute for BAFF-R signals in B cell development and pathogenesis.

Identification and characterization of p100HB, a new mutant form of p100/NF-kappa B2

P100, which is encoded by NF-kappa B2, inhibits Rel dimers. It can also be processed into p52, one of the DNA binding sub-units of NF-kappa B/Rel factors. Several p100 C-terminal truncations that result from gene rearrangements are associated with lymphomagenesis. Here, we characterized a new p100 mutant that we termed p100HB. It originates from a point-mutation that generates a premature stop-codon, and thus the protein lacks the last 125 amino acids. We have detected p100HB in several human tumor cell lines. The truncated protein is mainly unprocessed, and although it still binds Rel dimers, it has reduced inhibitory potency compared to p100 and translocates into the nucleus. Thus, p100HB may be associated with deregulated NF-kappa B/Rel functions.

RelB/p50 dimers are differentially regulated by tumor necrosis factor-alpha and lymphotoxin-beta receptor activation: critical roles for p100

Tumor necrosis factor-alpha (TNF-alpha) and lymphotoxin-beta receptor (LTbetaR) signaling both play important roles in inflammatory and immune responses through activation of NF-kappaB. Using various deficient mouse embryonic fibroblast cells, we have compared the signaling pathways leading to NF-kappaB induction in response to TNF-alpha and LTbetaR activation. We demonstrate that LTbetaR ligation induces not only RelA/p50 dimers but also RelB/p50 dimers, whereas TNF-alpha induces only RelA/p50 dimers. LTbetaR-induced binding of RelB/p50 requires processing of p100 that is mediated by IKKalpha but is independent of IKKbeta, NEMO/IKKgamma, and RelA. Moreover, we show that RelB, p50, and p100 can associate in the same complex and that TNF-alpha but not LTbeta signaling increases the association of p100 with RelB/p50 dimers in the nucleus, leading to the specific inhibition of RelB DNA binding. These results suggest that the alternative NF-kappaB pathway based on p100 processing may account not only for the activation of RelB/p52 dimers but also for that of RelB/p50 dimers and that p100 regulates the binding activity of RelB/p50 dimers via at least two distinct mechanisms depending on the signaling pathway involved.