GC Formation Research Articles

Failed supernova explosions increase the duration of star formation in globular clusters

The duration of SF in GC is an essential aspect for understanding their formation. Contrary to previous presumptions that all stars above 8 $M_ explode as CCSN recent evidence suggests a more complex scenario. We analyse iron spread observations from 55 GC to estimate the number of CCSN explosions before SF termination, thereby determining the SF duration. This work for the first time takes the possibility of failed CCSN into account, when estimating the SF duration. Two scenarios are considered: one where all stars explode as CCSN and another where only stars below 20 $M_ lead to CCSN as most CCSN models predict that no failed CCSN happen below 20 $M_ odot$. This establishes a lower ($ Myr$) and an upper ($ Myr$) limit for the duration of SF . Extending the findings of our previous paper, this study indicates a significant difference in SF duration based on CCSN outcomes, with failed CCSN extending SF by up to a factor of three. Additionally, a new code is introduced to compute the SF duration for a given CCSN model. The extended SF has important implications on GC formation, including enhanced pollution from stellar winds and increased binary star encounters. These results underscore the need for a refined understanding of CCSN in estimating SF durations and the formation of multiple stellar populations in GC

Galaxy archaeology for wet mergers: Globular cluster age distributions in the Milky Way and nearby galaxies

Context. Identifying past wet merger activity in galaxies has been a longstanding issue in extragalactic formation history studies. Gaia’s 6D kinematic measurements in our Milky Way (MW) have vastly extended the possibilities for Galactic archaeology, leading to the discovery of a multitude of early mergers in the MW’s past. As recent work has established a link between younger globular clusters (GCs; less than about 10–11 Gyr old) and wet galaxy merger events, the MW provides an ideal laboratory for testing which GC properties can be used to trace extragalactic galaxy formation histories. Aims. To test the hypothesis that GCs trace wet mergers, we relate the measured GC age distributions of the MW and three nearby galaxies, M 31, NGC 1407, and NGC 3115, to their merger histories and interpret the connection with wet mergers through an empirical model for GC formation. Methods. The GC ages of observed galaxies are taken from a variety of studies to analyze their age distributions side-by-side with the model. For the MW, we additionally cross-match the GCs with their associated progenitor host galaxies to disentangle the connection to the GC age distribution. For the modeled GCs, we take galaxies with similar GC age distributions as observed to compare their accretion histories with those inferred through observations. Results. We find that the MW GC age distribution is bimodal, mainly caused by younger GCs (10–11 Gyr old associated with Gaia-Sausage/Enceladus (GSE) and in part by unassociated high-energy GCs. The GSE GC age distribution also appears to be bimodal. We propose that the older GSE GCs (12–13 Gyr old) were accreted together with GSE, while the younger ones formed as a result of the merger. For the nearby galaxies, we find that clear peaks in the GC age distributions coincide with active early gas-rich merger phases. Even small signatures in the GC age distributions agree well with the expected wet formation histories of the galaxies inferred through other observed tracers. From the models, we predict that the involved cold gas mass can be estimated from the number of GCs found in the formation burst. Conclusions. Multimodal GC age distributions can trace massive wet mergers as a result of GCs being formed through them. From the laboratory of our own MW and nearby galaxies we conclude that the ages of younger GC populations of galaxies can be used to infer the wet merger history of a galaxy.

The formation of globular clusters with top-heavy initial mass functions

ABSTRACT We study the formation of globular clusters (GCs) in massive compact clouds with the low metallicity of Z = 10−3 Z⊙ by performing three-dimensional radiative-hydrodynamic simulations. Considering the uncertainty of the initial mass function (IMF) of stars formed in low-metallicity and high-density clouds, we investigate the impacts of the IMF on the cloud condition for the GC formation with the range of the power-law index of IMF as γ = 1−2.35. We find that the threshold surface density (Σthr) for the GC formation increases from 800 M⊙ pc−2 at γ = 2.35 to 1600 M⊙ pc−2 at γ = 1.5 in the cases of clouds with Mcl = 106 M⊙ because the emissivity of ionizing photons per stellar mass increases as γ decreases. For γ < 1.5, Σthr saturates with ∼2000 M⊙ pc−2 that is quite rare and observed only in local starburst galaxies due to e.g. merger processes. Thus, we suggest that formation sites of low-metallicity GCs could be limited only in the very high-surface density regions. We also find that Σthr can be modelled by a power-law function with the cloud mass (Mcl) and the emissivity of ionizing photons (s*) as $\propto M_{\rm cl}^{-1/5} s_{*}^{2/5}$. Based on the relation between the power-law slope of IMF and Σthr, future observations with e.g. the JWST can allow us to constrain the IMF of GCs.

Globular cluster formation histories, masses, and radii inferred from gravitational waves

ABSTRACT Globular clusters (GCs) are found in all types of galaxies and harbour some of the most extreme stellar systems, including black holes that may dynamically assemble into merging binary black holes (BBHs). Uncertain GC properties, including when they formed, their initial masses and sizes, affect their production rate of BBH mergers. Using the gravitational-wave transient catalogue (GWTC-3), we measure that dynamically assembled BBHs – those that are consistent with isotropic spin directions – make up ${61^{+29}_{-44}\%}$ of the total merger rate, with a local merger rate of ${10.9^{+16.8}_{-9.3}}$ Gpc−3 yr−1 rising to ${58.9^{+149.4}_{-46.0}}$ Gpc−3 yr−1 at z = 1. We assume that this inferred rate describes the contribution from GCs and compare it against the Cluster Monte Carlo (cmc) simulation catalogue to directly fit for the GC initial mass function, virial radius distribution, and formation history. We find that GC initial masses are consistent with a Schechter function with slope ${\beta _m = -1.9^{+0.8}_{-0.8}}$ . Assuming a mass function slope of βm = −2 and a mass range between 104–$10^8\, \mathrm{ M}_\odot$ , we infer a GC formation rate at z = 2 of ${5.0^{+9.4}_{-4.0}}$ Gpc−3 yr−1, or ${2.1^{+3.9}_{-1.7}}\times 10^6\, \mathrm{ M}_\odot$ Gpc−3 yr−1 in terms of mass density. We find that the GC formation rate probably rises more steeply than the global star formation rate between z = 0 and z = 3 (82 per cent credibility) and implies a local number density that is ${f_\mathrm{ev} = 22.6^{+29.9}_{-16.2}}$ times higher than the observed density of survived GCs. This is consistent with expectations for cluster evaporation, but may suggest that other environments contribute to the rate of BBH mergers with significantly tilted spins.

Dynamic mitochondrial transcription and translation in B cells control germinal center entry and lymphomagenesis

Germinal center (GC) B cells undergo proliferation at very high rates in a hypoxic microenvironment but the cellular processes driving this are incompletely understood. Here we show that the mitochondria of GC B cells are highly dynamic, with significantly upregulated transcription and translation rates associated with the activity of transcription factor A, mitochondrial (TFAM). TFAM, while also necessary for normal B cell development, is required for entry of activated GC precursor B cells into the germinal center reaction; deletion of Tfam significantly impairs GC formation, function and output. Loss of TFAM in B cells compromises the actin cytoskeleton and impairs cellular motility of GC B cells in response to chemokine signaling, leading to their spatial disorganization. We show that B cell lymphoma substantially increases mitochondrial translation and that deletion of Tfam in B cells is protective against the development of lymphoma in a c-Myc transgenic mouse model. Finally, we show that pharmacological inhibition of mitochondrial transcription and translation inhibits growth of GC-derived human lymphoma cells and induces similar defects in the actin cytoskeleton.

A role for Hes1 in constraining germinal center B cell formation.

Germinal center is a transient lymphoid tissue structure in which B cells undergo affinity maturation and differentiate into memory B cells and plasma cells. GC formation depends on B cell expression of BCL6, a master transcription regulator of the GC state. Bcl6 expression is under elaborate control by external signals. HES1 plays important roles in T-cell lineage commitment, although little is known about its potential roles in GC formation. Here we report that B-cell-specific HES1 deletion causes a significant increase in GC formation, leading to increased production of plasma cells. We further provide evidence that HES1 inhibits BCL6 expression in a bHLH domain-dependent manner. Our study suggests a new layer of regulation of GC initiation mediated by HES1 and, by inference, Notch signals in vivo.

The transcription factor Mef2d regulates B:T synapse-dependent GC-TFH differentiation and IL-21-mediated humoral immunity.

Communication between CD4 T cells and cognate B cells is key for the former to fully mature into germinal center-T follicular helper (GC-TFH) cells and for the latter to mount a CD4 T cell-dependent humoral immune response. Although this interaction occurs in a B:T synapse-dependent manner, how CD4 T cells transcriptionally regulate B:T synapse formation remains largely unknown. Here, we report that Mef2d, an isoform of the myocyte enhancer factor 2 (Mef2) transcription factor family, is a critical regulator of this process. In CD4 T cells, Mef2d negatively regulates expression of Sh2d1a, which encodes SLAM-associated protein (SAP), a critical regulator of B:T synapses. We found that Mef2d regulates Sh2d1a expression via DNA binding-dependent transcriptional repression, inhibiting SAP-dependent B:T synapse formation and preventing antigen-specific CD4 T cells from differentiating into GC-TFH cells. Mef2d also impeded IL-21 production by CD4 T cells, an important B cell help signaling molecule, via direct repression of the Il21 gene. In contrast, CD4 T cell-specific disruption of Mef2d led to a substantial increase in GC-TFH differentiation in response to protein immunization, concurrent with enhanced SAP expression. MEF2D mRNA expression inversely correlates with human systemic lupus erythematosus (SLE) patient autoimmune parameters, including circulating TFH-like cell frequencies, autoantibodies, and SLEDAI scores. These findings highlight Mef2d as a pivotal rheostat in CD4 T cells for controlling GC formation and antibody production by B cells.

S1PR4 deficiency results in reduced germinal center formation but only marginally affects antibody production.

Splenic B cells exhibit a high expression of the G protein-coupled sphingosine-1-phosphate (S1P) receptor type 4 (S1PR4). Little is known about the functional relevance of S1PR4 expression on those cells. In this study, S1PR4-deficient mice were used to study the role of S1PR4-mediated S1P signaling in B cell motility in vitro and for the maintenance of the splenic architecture under steady state conditions as well as in polymicrobial abdominal sepsis in vivo. Finally, the impact of S1PR4 deficiency on antibody production after immunization with T cell dependent antigens was assessed. Loss of S1PR4 resulted in minor alterations of the splenic architecture concerning the presence of B cell follicles. After sepsis induction, the germinal center response was severely impaired in S1PR4-deficient animals. Splenic B cells showed reduced motility in the absence of S1PR4. However, titres of specific antibodies showed only minor reductions in S1PR4-deficient animals. These observations suggest that S1P signaling mediated by S1PR4 modifies chemokine-induced splenic B cell chemotaxis, thus modulating splenic microarchitecture, GC formation and T-cell dependent antibody production.

First-in-Class Histone Acetyltransferase (HAT) Activator, YF2, Modulates Immune Evasion in DLBCL, Enhancing the Effects of Immune Checkpoint Blockade

We reported the discovery of YF2, a EP300 (p300) and CREBBP (CBP) HAT activator which demonstrates selective cytotoxicity in HAT-mutated DLBCL cell lines and induces histone acetylation in vitro and in vivo. Recently, we have focused on the role of HAT induction in regulating immune response in DLBCLs. CBP has been shown to function as an enhancer that regulates GC B-cell fate by controlling antigen presentation and terminal differentiation. HAT mutations are linked to reduced MHC-II HLA transcripts due to decreased acetylation at the promoter regions which leads to decreased GC B:T cell entanglement and derangements in the GC reaction. Numerous studies have reported the effect of epigenetic drugs on antigen presentation. This response is caused by disruption of the BLC6/SMRT/HDAC3 repressor complex which co-occupies enhancers in the MHC Class II loci. Previously, we demonstrated YF2 induced p300 mediated acetylation of BCL6; abrogating its activity. More recently, we demonstrated through RNA-seq that prolonged treatment of YF2 upregulated the expression of several MHC Class I-II genes resulting in the upregulation of the immune regulation signaling pathway. Further, treatment with YF2 up-regulated MHC-I and -II expression in a dose dependent manner. In addition to regulating immune response, BCL6 is a master regulator of GC formation. We hypothesize that induction of HATs will modulate immune response in splenic and circulating B and T-cells. Immunized BALB/c mice were treated with saline or 50mg/kg YF2 for 10 days. Spleen and blood samples were analyzed by flow cytometry. We observed a significant reduction in splenic weight, B-cell frequency, and GCs formation in YF2 treated mice. YF2 treatment increased the frequency of mature and transitional B-cells in the spleen. Further, YF2 treatment reduced light zone and increased dark zone cells in the spleen. YF2 treatment also increased the frequency of plasma cells and plasmablast. MHC-I and -II expression was significantly upregulated in YF2 treated mice. YF2 exposure resulted in an increase of PD-L1hi B-cells. Lastly, circulating B-cells showed significant increase in MHC-II expression following YF2 treatment. In splenic T-cells, we observed a reduction in CD4+ helper T-cells and an increase in CD8+ cytotoxic T-cells in YF2 treated mice. YF2 caused a reduction in naïve CD4+ cells and an increase in the frequency of central and effector memory cells Lastly, we examined the frequency of inhibitory molecules PD1, TIGIT, CD226, Lag3, and VISTA. PD-1, TIGIT, and CD226 were upregulated with YF2 treatment in CD4+ cells and CD226 was upregulated in CD8+ cells. Together, these results demonstrate YF2 is able to modulate immune response in B and T-cells. DLBCLs are known to have limited sensitivity to checkpoint blockade with PD-1 and PD-L1 inhibitors due to low PD-L1 and PD-1 expression which is in part driven by HAT mutations and overexpression of BCL6; which regulates PD-L1. We hypothesize that combination treatment of YF2 and PD-1 inhibitor in the immunocompetent syngeneic A20 mouse tumor xenograft model will result in decreased tumorigenicity and improved survival in mice. Xenografted and immunized BALBc mice were treated with saline, YF2, PD-1 inhibitor, or a combination of YF2 and PD-1 inhibitor for 28 days. We observed, a significant reduction in tumor volume in both the PD-1 and combination treated groups. Kaplan-Meier analysis showed an increase in median survival for all treatment with the combination treatment resulting in the greatest increase in median survival vs. control (Fig A). Spatial analysis was performed using the 10x Visium platform and sc-RNA-seq to examine the tumor microenvironment in control and YF2 treated mouse tumor samples (Fig B). We observed a more aggressive phenotype in the untreated tumors as well as an increase in the number of distinct clusters indicating a more heterogenous tumor population. Lastly, PBMCs from patient samples were isolated and treated with YF2. Compared to healthy donor patients, YF2 induced cytotoxicity across various lymphoma subtype. Normal PBMCs were not affected by treatment. Based on the success of YF2 in the preclinical setting, the use of this first-in-class drug for treatment of lymphoma was approved to proceed in clinical studies in patients with relapsed/refractory lymphoma. Patient samples will be collected and analyzed for immune response. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

Globular cluster formation with multiple stellar populations: a single-binary composite scenario

ABSTRACT We discuss a GC formation scenario in which the first generation (1G) of single asymptotic giant branch (AGB) stars and intermediate-mass close binaries (IMCBs) eject gas, from which the second generation (2G) of stars can be formed. The two key parameters in the scenario are the fractions of binary stars (fb) and the slopes (α) of the stellar initial mass functions (IMFs) for 1G stars. Principle results derived by analytic and one-zone models of GC formation are as follows. The mass fraction of 2G stars (f2g) can be higher than ≈0.4 for α < 1.8 and is not so dependent on fb. The ratio of the initial mass of a GC to the present-day mass (Mgc) ranges from 2 to 7 depending on α for 0.5 ≤ fb ≤ 0.9. The differences in [Na/Fe] between 1G and 2G stars can be as large as 0.7 for a wide range of model parameters. The Li abundances of 2G stars can be as high as those of 1G even if the pristine gas from IMCBs is assumed to be Li-free. Formation histories of 2G stars show at least two peaks owing to two peaks in the total ejection rate of gas from IMCB populations. The observed correlation between f2g and Mgc can be due to α depending on Mgc. The hypothetical long duration of 2G formation (≈108 yr) is possible, because massive star formation can be suppressed through frequent dynamical interaction between 1G stars and gas clouds.

Modeling the kinematics of globular cluster systems

ABSTRACT Globular clusters (GCs) are old massive star clusters that serve as ‘fossils’ of galaxy formation. The advent of Gaia observatory has enabled detailed kinematics studies of the Galactic GCs and revolutionized our understanding of the connections between GC properties and galaxy assembly. However, lack of kinematic measurements of extragalactic GCs limits the sample size of GC systems that we can fully study. In this work, we present a model for GC formation and evolution, which includes positional and kinematic information of individual GCs by assigning them to particles in the Illustris TNG50-1 simulation based on age and location. We calibrate the three adjustable model parameters using observed properties of the Galactic and extragalactic GC systems, including the distributions of position, systemic velocity, velocity dispersion, anisotropy parameter, orbital actions, and metallicities. We also analyse the properties of GCs from different origins. In outer galaxy, ex situ clusters are more dominant than the clusters formed in situ. This leads to the GC metallicities decreasing outwards due to the increasing abundance of accreted, metal-poor clusters. We also find the ex-situ GCs to have greater velocity dispersions and orbital actions, in agreement with their accretion origin.

Cell-specific regulation of antigen-specific germinal center formation and antibody responses by thymic stromal lymphopoietin

Abstract Germinal centers (GCs) are dynamic immune structures that expand upon T-dependent immune challenge to generate high-affinity antibody-producing plasma cells and long-lived memory. GCs require stringent regulation to optimize GC-derived antibody responses. Here, we identified thymic stromal lymphopoietin (TSLP), as a novel regulator of GCs. After immunization with NP-OVA in Alum, TSLP receptor-deficient (TSLPR−/−) mice fail to form antigen-specific GCs and antibodies. TSLPR expression is elevated on T follicular helper cells (Tfh) and GC B cells during inflammation, so we asked how cell-specific loss of TSLPR signaling could disrupt overall GC activity. When TSLPR was deleted in CD4 T cells with a tamoxifen-inducible Cre (CD4CreERT2/+TSLPRFLOX/FLOX), we observed a similar reduction in T follicular helper cells (Tfh), GC formation, and antibody responses after immunization when compared to CD4CreERT2/+ and TSLPRFLOX/FLOX controls. Conversely, when TSLPR signaling was lost on mature B cells (CD21Cre/+TSLPRFLOX/FLOX) and GC B cells (AIDCre/+TSLPRFLOX/FLOX), we observed an increase in antigen-specific GC B cells and antigen-specific memory B cells. TSLPR signaling regulates IRF4 expression in dendritic cells, and we observed a similar reduction in IRF4 expression in Tfh and GC B cells from TSLPR−/− mice compared to TSLPR+/+ controls. Collectively, these findings suggest that TSLPR signaling promotes IRF4 expression in GC B cells and Tfh cells, resulting in distinct roles in the GC. TSLPR and IRF4 signaling in T cells is required for the establishment and maintenance of antigen-specific Tfh cells, whereas TSLPR/IRF4 signaling in B cells inhibits antigen-specific GC B cell retention and the establishment of B cell memory. Supported by a grant from the NIH (F32 AI154787)

Th1/Tfh hybrid (Ifng+Il21+) T cells contribute to delay of germinal center formation in Plasmodium chabaudi infection

Abstract Germinal center (GC) formation in P. chabaudi rodent malaria is delayed for three weeks, which can prolong the infection. T follicular helper (Tfh) cells express CXCR5 for homing to the B cell area, and IL-21 which promotes antibody production. T helper type 1 (Th1) cells express CXCR3 and IFN-γ to promote phagocytosis. Previous data suggests that Th1 responses control peak parasitemia while systemic Th1 cytokines inhibit GC formation. In P. chabaudi, majority of CD4+ effector T cells (Teff) express markers of both Th1 and Tfh (e.g., CXCR5, CXCR3, Th1/Tfh hybrid), including IFN-γ and IL-21. The contribution of Th1/Tfh hybrid T cells in protection and antibody production remains unclear. We tested the impact of Il21+Tfh-like, Ifng+Th1-like, and Ifng+Il21+Th1/Tfh in protection by transferring these subsets from infected triple cytokine reporters (Il21-rfp Il4-gfp Ifng-Thy1.1) into TCR KO mice that are then infected. We are currently assessing changes in parasitemia and pathology. We found that Th1/Tfh cells do not help in vivo like recent data suggest they do in vitro. Th1/Tfh significantly generated few GC B cells, compared to Il21+ Tfh-like cells. Recipients of Th1-like cells had significantly lower parasite-specific IgG and GCs by immunohistology than Tfh-like at 21 dpi, hybrid Th1/Tfh were intermediate. A cytokine kinetics study of Th subsets in infected cytokine reporter mouse distinguished Il4+Il21+ GC Tfh from Th1 cells. The inflammation-driven chemokine receptors CXCR3 and CXCR6 are highly expressed on GC Tfh and Th1/Tfh hybrid during the first week of infection, but decline as GCs develop, suggesting a role for T cell migration which could explain the delay in GC formation and parasite-specific antibody production in malaria. Supported by R01AI 135061

Brg1 Supports B Cell Proliferation and Germinal Center Formation Through Enhancer Activation.

Activation and differentiation of B cells depend on extensive rewiring of gene expression networks through changes in chromatin structure and accessibility. The chromatin remodeling complex BAF with its catalytic subunit Brg1 was previously identified as an essential regulator of early B cell development, however, how Brg1 orchestrates gene expression during mature B cell activation is less clear. Here, we find that Brg1 is required for B cell proliferation and germinal center formation through selective interactions with enhancers. Brg1 recruitment to enhancers following B cell activation was associated with increased chromatin accessibility and transcriptional activation of their coupled promoters, thereby regulating the expression of cell cycle-associated genes. Accordingly, Brg1-deficient B cells were unable to mount germinal center reactions and support the formation of class-switched plasma cells. Our findings show that changes in B cell transcriptomes that support B cell proliferation and GC formation depend on enhancer activation by Brg1. Thus, the BAF complex plays a critical role during the onset of the humoral immune response.

Expression of Foxp3 by T follicular helper cells in end-stage germinal centers.

Germinal centers (GCs) are the site of immunoglobulin somatic hypermutation and affinity maturation, processes essential to an effective antibody response. The formation of GCs has been studied in detail, but less is known about what leads to their regression and eventual termination, factors that ultimately limit the extent to which antibodies mature within a single reaction. We show that contraction of immunization-induced GCs is immediately preceded by an acute surge in GC-resident Foxp3+ T cells, attributed at least partly to up-regulation of the transcription factor Foxp3 by T follicular helper (TFH) cells. Ectopic expression of Foxp3 in TFH cells is sufficient to decrease GC size, implicating the natural up-regulation of Foxp3 by TFH cells as a potential regulator of GC lifetimes.

Regulating germinal center contraction

Immunology Germinal centers (GCs) in secondary lymphoid organs are where mature B cells expand and differentiate. Although GC formation is well studied, the control of GC duration and contraction is less well understood. Using intravital imaging of mouse GCs and single-cell RNA sequencing, Jacobsen et al. report that T follicular helper (TFH) cells are a critical player in this process. They found that some late-GC TFH cells upregulate the transcription factor FOXP3 and acquire a regulatory T cell–like phenotype. These cells are distinct from T follicular regulatory (TFR) cells and, unlike TFR cells, are needed to shut down the GC reaction. Tweaking this process may be key to extending GC lifetimes and enhancing antibody responses in the context of vaccination. Science , abe5146, this issue p. [eabe5146][1] [1]: /lookup/doi/10.1126/science.abe5146

Rap1 Is Essential for B-Cell Locomotion, Germinal Center Formation and Normal B-1a Cell Population.

Integrin regulation by Rap1 is indispensable for lymphocyte recirculation. In mice having B-cell-specific Rap1a/b double knockouts (DKO), the number of B cells in lymph nodes decreased to approximately 4% of that of control mice, and B cells were present in the spleen and blood. Upon the immunization with NP-CGG, DKO mice demonstrated the defective GC formation in the spleen, and the reduced NP-specific antibody production. In vitro, Rap1 deficiency impaired the movement of activated B cells along the gradients of chemoattractants known to be critical for their localization in the follicles. Furthermore, B-1a cells were almost completely absent in the peritoneal cavity, spleen and blood of adult DKO mice, and the number of B-cell progenitor/precursor (B-p) were reduced in neonatal and fetal livers. However, DKO B-ps normally proliferated, and differentiated into IgM+ cells in the presence of IL-7. CXCL12-dependent migration of B-ps on the VCAM-1 was severely impaired by Rap1 deficiency. Immunostaining study of fetal livers revealed defects in the co-localization of DKO B-ps and IL-7-producing stromal cells. This study proposes that the profound effects of Rap1-deficiency on humoral responses and B-1a cell generation may be due to or in part caused by impairments of the chemoattractant-dependent positioning and the contact with stromal cells.

The massive M31 cluster G1: detailed chemical abundances from integrated light spectroscopy★

ABSTRACT G1, also known as Mayall II, is one of the most massive star clusters in M31. Its mass, ellipticity, and location in the outer halo make it a compelling candidate for a former nuclear star cluster. This paper presents an integrated light abundance analysis of G1, based on a moderately high-resolution (R = 15 000) spectrum obtained with the high-resolution spectrograph on the Hobby–Eberly Telescope in 2007 and 2008. To independently determine the metallicity, a moderate-resolution (R ∼ 4000) spectrum of the Ca ii triplet lines in the near-infrared was also obtained with the Astrophysical Research Consortium’s 3.5-m telescope at Apache Point Observatory. From the high-resolution spectrum, G1 is found to be a moderately metal-poor cluster, with $[\rm {Fe/H}]~=~-0.98\pm 0.05$. G1 also shows signs of α-enhancement (based on Mg, Ca, and Ti) and lacks the s-process enhancements seen in dwarf galaxies (based on comparisons of Y, Ba, and Eu), indicating that it originated in a fairly massive galaxy. Intriguingly, G1 also exhibits signs of Na and Al enhancement, a unique signature of GCs – which suggests that G1’s formation is intimately connected with GC formation. G1’s high [Na/Fe] also extends previous trends with cluster velocity dispersion to an even higher mass regime, implying that higher mass clusters are more able to retain Na-enhanced ejecta. The effects of intracluster abundance spreads are discussed in a subsequent paper. Ultimately, G1’s chemical properties are found to resemble other M31 GCs, though it also shares some similarities with extragalactic nuclear star clusters.

A relic star cluster under the floor

Stellar Astrophysics Globular clusters (GCs) are gravitationally bound assemblies of thousands to millions of stars that orbit in the outskirts of large galaxies. GCs consist of old stars with low metallicity containing low proportions of chemical elements heavier than hydrogen and helium. However, GCs appear to have a minimum metallicity, known as the floor, implying that at least some of those elements were required for their formation. Larsen et al. have found a GC in the nearby Andromeda Galaxy with a metallicity beneath the floor. This unexpected discovery will inform models of GC formation and incorporation into galaxies. Science , this issue p. [970][1] [1]: /lookup/doi/10.1126/science.abb1970

Exploring POU2AF1 (BOB-1) Dependency and Transcription Addiction in Multiple Myeloma

Multiple Myeloma (MM) is a complex disease driven by numerous genetic and epigenetic alterations. Recurrent IgH translocations, copy number abnormalities and somatic mutations all contribute to myelomagenesis, yet true drivers of the disease have yet to be identified. In order to investigate the enhancer landscape and identify new dependencies and actionable therapeutic targets in MM, we have generated high-quality active enhancer landscape in MM cell lines as well as in large cohort of primary patient myeloma cells (n=70) and normal plasma cell. We integrated this data with an in-house curated atlas of 600+ active enhancer profile across a wide range of tumor types and normal tissues. Combining this data with gene expression and genetic dependency (CRISPR KO) enables a multidimensional integration of how transcriptional regulation intersects with tumor specific dependencies. We identified genes that are super-enhancer associated, expressed, and required specifically in MM versus other tumor types or normal tissues. Many of the specific and potent dependencies in MM are transcription factors, and especially those establishing plasma cell identity. Among others, the POU2AF1 gene, which encodes the OCA-B/BOB-1, a B cell transcriptional coactivator protein, represented the most striking dependency in MM. BOB-1 is a gene regulatory factor that regulates B-cell development, maturation and GC formation. Although BOB-1 is expressed throughout B-cell development, we found it to be highly expressed in CD138+ plasma cells from patients with precursor conditions (MGUS and SMM) as well and established MM compared to normal plasma cells, giving us the rationale to study the transcriptional program associated with BOB-1 in MM cells. To confirm CRISPR data described above, we have performed loss-of-function (LOF) studies using shRNA, siRNAs as well as antisense GapMers specific for BOB-1. Downregulation of BOB-1 in a panel of MM cell lines caused MM cell growth inhibition in a time-dependent manner which was associated with G2/M cell cycle arrest and induction of MM cell apoptosis. Transcriptomic analysis by RNA-sequencing revealed a set of 72 genes (adj p-value=0.2) commonly modulated in AMO-1 and NCI-929 MM cell lines upon BOB-1 depletion, as compared with scrambled cells. Among the top 5 downregulated genes in BOB-1 knockdown samples, we found AMPD1, KCNN3, BHLHA15, HID1 and XBP1. The modulation of BOB1 target genes was confirmed at the protein level by Western blot analysis in BOB-1 LOF and gain-of-function (GOF) cell systems. Analysis of patient expression datasets revealed that these genes are positively correlated with BOB-1 expression in primary MM cells and overexpressed in MM cells compared to normal plasma cells. One of these 5 genes positively controlled by BOB-1, BHLHA15, is the gene coding for the transcription factor Mist1, which functions as a coregulator of the unfolded protein response (UPR) master transcription factor XBP1. Target genes of the XBP1- BHLHA15 axis are involved in lipid synthesis, protein folding and secretion, and ER-associated protein degradation. As a result, BOB-1 knockdown increased de novo protein synthesis in MM cells compared to control cells. Interestingly, Mist1 gene expression is induced during ER stress by XBP1, but as ER stress subsides, Mist1 serves as a feedback inhibitor. Moreover, we identified that LOF studies in two MM cell lines confirmed increased expression of BOB-1 and XBP1 upon KD of Mist1. However, XBP1 and/or Mist1 are dispensable for MM cell survival, as gene depletion did not affect MM cell viability. Among the genes most significantly upregulated by BOB-1 depletion was heme oxygenase 1 HMOX1 gene, whose expression is significantly lower in-patient MM cells compared to normal plasma cells. Moreover, its lower expression significantly correlated with poor clinical outcome; and siRNA depletion of HMOX1 increased MM cell growth in MM cell lines, suggesting important role in MM. Mechanistic studies aimed at investigating the functions of HMOX1 in MM and its relation with BOB-1 are ongoing and will be presented. In conclusion, we here report BOB1 as an important transcriptional regulator in MM and its two down-stream novel target genes (BHLHA15 and HMOX1) with potential significant biological role in MM. Disclosures Hajek: BMS: Consultancy, Honoraria, Research Funding; Oncopeptides: Consultancy; PharmaMar: Consultancy, Honoraria; Abbvie: Consultancy, Honoraria; Takeda: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Consultancy, Research Funding; Celgene: Consultancy, Honoraria, Research Funding; Janssen: Consultancy, Honoraria, Research Funding; Amgen: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding. Fulciniti:NIH: Research Funding. Munshi:Janssen: Consultancy; C4: Current equity holder in private company; Karyopharm: Consultancy; Amgen: Consultancy; OncoPep: Consultancy, Current equity holder in private company, Membership on an entity's Board of Directors or advisory committees, Patents & Royalties; BMS: Consultancy; Legend: Consultancy; Adaptive: Consultancy; AbbVie: Consultancy; Takeda: Consultancy.