PRMT5 Promotes T follicular helper Cell Differentiation and Germinal Center Responses during Influenza Virus Infection

Protein arginine methyltransferases (PRMTs) modify diverse protein targets and regulate numerous cellular processes; yet, their contributions to individual effector T cell responses during infections are incompletely understood. In this study, we identify PRMT5 as a critical regulator of CD4+ T follicular helper cell (Tfh) responses during influenza virus infection in mice. Conditional PRMT5 deletion in murine T cells results in an almost complete ablation of both Tfh and T follicular regulatory populations and, consequently, reduced B cell activation and influenza-specific Ab production. Supporting a potential mechanism, we observe elevated surface expression of IL-2Rα on non-T regulatory effector PRMT5-deficient T cells. Notably, IL-2 signaling is known to negatively impact Tfh differentiation. Collectively, our findings identify PRMT5 as a prominent regulator of Tfh programming, with potential causal links to IL-2 signaling.

Cytotoxic Programming of CD4+ T Cells Is Regulated by Opposing Actions of the Related Transcription Factors Eos and Aiolos

In contrast to the "helper" activities of most CD4+ T effector subsets, CD4+ cytotoxic T lymphocytes (CD4-CTLs) perform functions normally associated with CD8+ T and NK cells. Specifically, CD4-CTLs secrete cytotoxic molecules and directly target and kill compromised cells in an MHC class II-restricted fashion. The functions of these cells have been described in diverse immunological contexts, including their ability to provide protection during antiviral and antitumor responses, as well as being implicated in autoimmunity. Despite their significance to human health, the complete mechanisms that govern their programming remain unclear. In this article, we identify the Ikaros zinc finger transcription factor Eos (Ikzf4) as a positive regulator of CD4-CTL differentiation during murine immune responses against influenza virus infection. We find that the frequency of Eos+ cells is elevated in lung CD4-CTL populations and that the cytotoxic gene program is compromised in Eos-deficient CD4+ T cells. Consequently, we observe a reduced frequency and number of lung-residing, influenza virus-responsive CD4-CTLs in the absence of Eos. Mechanistically, we determine that this is due, at least in part, to reduced expression of IL-2 and IL-15 cytokine receptor subunits on the surface of Eos-deficient CD4+ T cells, both of which support the CD4-CTL program. Finally, we find that Aiolos, a related Ikaros family member and known CD4-CTL antagonist, represses Eos expression by antagonizing STAT5-dependent activation of the Ikzf4 promoter. Collectively, our findings reveal a mechanism wherein Eos and Aiolos act in opposition to regulate cytotoxic programming of CD4+ T cells.

TCDD and CH223191 Alter T Cell Balance but Fail to Induce Anti-Inflammatory Response in Adult Lupus Mice

Aryl hydrocarbon receptor (AhR) responds to endogenous and exogenous ligands as a cytosolic receptor, transcription factor, and E3 ubiquitin ligase. Several studies support an anti-inflammatory effect of AhR activation. However, exposure to the AhR agonist 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) during early stages of development results in an autoimmune phenotype and exacerbates lupus. The effects of TCDD on lupus in adults with pre-existing autoimmunity have not been described. We present novel evidence that AhR stimulation by TCDD alters T cell responses but fails to impact lupus-like disease using an adult mouse model. Interestingly, AhR antagonist CH223191 also changed T cell balance in our model. We next developed a conceptual framework for identifying cellular and molecular factors that contribute to physiological outcomes in lupus and created models that describe cytokine dynamics that were fed into a system of differential equations to predict the kinetics of T follicular helper (Tfh) and regulatory T (Treg) cell populations. The model predicted that Tfh cells expanded to larger values following TCDD exposure compared with vehicle and CH223191. Following the initial elevation, both Tfh and Treg cell populations continuously decayed over time. A function based on the ratio of predicted Treg/Tfh cells showed that Treg cells exceed Tfh cells in all groups, with TCDD and CH223191 showing lower Treg/Tfh cell ratios than the vehicle and that the ratio is relatively constant over time. We conclude that AhR ligands did not induce an anti-inflammatory response to attenuate autoimmunity in adult lupus mice. This study challenges the dogma that TCDD supports an immunosuppressive phenotype.

Eos Promotes TH2 Differentiation by Interacting with and Propagating the Activity of STAT5

The Ikaros zinc-finger transcription factor Eos has largely been associated with sustaining the immunosuppressive functions of regulatory T cells. Paradoxically, Eos has more recently been implicated in promoting proinflammatory responses in the dysregulated setting of autoimmunity. However, the precise role of Eos in regulating the differentiation and function of effector CD4+ T cell subsets remains unclear. In this study, we find that Eos is a positive regulator of the differentiation of murine CD4+ TH2 cells, an effector population that has been implicated in both immunity against helminthic parasites and the induction of allergic asthma. Using murine in vitro TH2 polarization and an in vivo house dust mite asthma model, we find that EosKO T cells exhibit reduced expression of key TH2 transcription factors, effector cytokines, and cytokine receptors. Mechanistically, we find that the IL-2/STAT5 axis and its downstream TH2 gene targets are one of the most significantly downregulated pathways in Eos-deficient cells. Consistent with these observations, we find that Eos forms, to our knowledge, a novel complex with and supports the tyrosine phosphorylation of STAT5. Collectively, these data define a regulatory mechanism whereby Eos propagates STAT5 activity to facilitate TH2 cell differentiation.

Prime-Pull Immunization of Mice with a BcfA-Adjuvanted Vaccine Elicits Sustained Mucosal Immunity That Prevents SARS-CoV-2 Infection and Pathology

Vaccines against SARS-CoV-2 that induce mucosal immunity capable of preventing infection and disease remain urgently needed. In this study, we demonstrate the efficacy of Bordetella colonization factor A (BcfA), a novel bacteria-derived protein adjuvant, in SARS-CoV-2 spike-based prime-pull immunizations. We show that i.m. priming of mice with an aluminum hydroxide- and BcfA-adjuvanted spike subunit vaccine, followed by a BcfA-adjuvanted mucosal booster, generated Th17-polarized CD4+ tissue-resident memory T cells and neutralizing Abs. Immunization with this heterologous vaccine prevented weight loss following challenge with mouse-adapted SARS-CoV-2 (MA10) and reduced viral replication in the respiratory tract. Histopathology showed a strong leukocyte and polymorphonuclear cell infiltrate without epithelial damage in mice immunized with BcfA-containing vaccines. Importantly, neutralizing Abs and tissue-resident memory T cells were maintained until 3 mo postbooster. Viral load in the nose of mice challenged with the MA10 virus at this time point was significantly reduced compared with naive challenged mice and mice immunized with an aluminum hydroxide-adjuvanted vaccine. We show that vaccines adjuvanted with alum and BcfA, delivered through a heterologous prime-pull regimen, provide sustained protection against SARS-CoV-2 infection.

Aiolos represses CD4+ T cell cytotoxic programming via reciprocal regulation of TFH transcription factors and IL-2 sensitivity

During intracellular infection, T follicular helper (TFH) and T helper 1 (TH1) cells promote humoral and cell-mediated responses, respectively. Another subset, CD4-cytotoxic T lymphocytes (CD4-CTLs), eliminate infected cells via functions typically associated with CD8+ T cells. The mechanisms underlying differentiation of these populations are incompletely understood. Here, we identify the transcription factor Aiolos as a reciprocal regulator of TFH and CD4-CTL programming. We find that Aiolos deficiency results in downregulation of key TFH transcription factors, and consequently reduced TFH differentiation and antibody production, during influenza virus infection. Conversely, CD4-CTL programming is elevated, including enhanced Eomes and cytolytic molecule expression. We further demonstrate that Aiolos deficiency allows for enhanced IL-2 sensitivity and increased STAT5 association with CD4-CTL gene targets, including Eomes, effector molecules, and IL2Ra. Thus, our collective findings identify Aiolos as a pivotal regulator of CD4-CTL and TFH programming and highlight its potential as a target for manipulating CD4+ T cell responses.

Eos regulates IL-7 signaling and central memory T cell differentiation

CD4+ central memory T cells (TCM) are key players in recall immune responses and thus are critical to long-term immunity. Despite their importance, much remains to be defined regarding the mechanisms that promote their differentiation and survival. Ikaros zinc finger (IkZF) transcription factors are known regulators of T and B cell differentiation programs. To date, the IkZF factor Eos has been mainly linked to regulatory T cell suppressive functions. Now, we have found that Eos is expressed at elevated levels in CD4+ TCM populations. Comparison between WT and Eos-deficient T cells in a murine model of influenza infection revealed a decrease in the percentage of CD4+ TCM cells in the absence of Eos, suggesting that Eos promotes TCM differentiation and/or survival. Using an established in vitro model of CD4+ TCM-like cell differentiation, we found that loss of Eos correlated with reduced expression of genes encoding key TCM transcription factors and cell surface markers. Further, Eos deficiency resulted in decreased STAT5 activation downstream of IL-7 signaling, a known positive regulator of TCM populations. In WT cells, IL-7 treatment increased Eos expression, suggesting a positive feed-forward relationship exists between Eos expression and IL-7/STAT5 signaling. Overall, our findings demonstrate that Eos is an important regulator of TCM populations. Understanding the mechanisms by which Eos regulates TCM function and survival may therefore be of interest in exploiting TCM protection against re-infection for therapeutic benefit.

Supported by a grant from NIAID (NIH- R01 AI134972) and funds through The Ohio State University College of Medicine

Aiolos restrains the acquisition of cytotoxic features by CD4+ T cells

Classically, CD4+ T cells have been defined as “helper” type cells, providing aid to other immune cell populations via the secretion of cytokines and direct cell-cell interactions. More recently, a subset of CD4+ T cells with cytotoxic capabilities, termed CD4+ cytotoxic T lymphocytes (CD4-CTLs), have been observed in both mice and humans, performing protective functions in the settings of infection and cancer while conversely contributing to the pathogenesis of autoimmunity. Despite their well-documented importance in several disease contexts, the complete mechanisms that underlie their differentiation and function remain unknown. Here, we identify the Ikaros family member, Aiolos, as a novel regulator of CD4+ CTL differentiation and function. We find that Aiolos deficiency results in increased expression of key CD4+ CTL transcription factors and effector molecules both in vitro and in an in vivo murine model of influenza infection. Mechanistically, we find that Aiolos deficiency results in increased IL-2/STAT5 signaling, supporting a repressive role for Aiolos in CD4+ CTL differentiation via negative regulation of the IL-2/STAT5 pathway. Collectively, this work identifies Aiolos as a novel negative regulator of CD4+ cytotoxic gene programming, and thus may represent a therapeutic target for the treatment of autoimmune diseases and enhanced anti-tumor immunity.

Sponsored by a grant from NIAID (NIH-RO1 AI134972) and funds through The Ohio State University College of Medicine

Making memories with Bcl-6

For over a decade, mutual antagonism between the transcriptional repressors Bcl-6 and Blimp-1 has been appreciated as a key mechanistic determinant of lymphoid differentiation programs. Now, in this issue of JEM, Ciucci et al. (2021. J. Exp. Med.https://doi.org/10.1084/jem.20202343) demonstrate that this relationship is "central" to the generation of T cell memory.

Dynamic Roles for IL-2-STAT5 Signaling in Effector and Regulatory CD4+ T Cell Populations

CD4⁺ Th cells are responsible for orchestrating diverse, pathogen-specific immune responses through their differentiation into a number of subsets, including T_H1, T_H2, T_H9, T follicular helper, T follicular regulatory, and regulatory T cells. The differentiation of each subset is guided by distinct regulatory requirements, including those derived from extracellular cytokine signals. IL-2 has emerged as a critical immunomodulatory cytokine that both positively and negatively affects the differentiation of individual Th cell subsets. IL-2 signals are propagated, in part, via activation of STAT5, which functions as a key regulator of CD4⁺ T cell gene programs. In this review, we discuss current understanding of the mechanisms that allow IL-2-STAT5 signaling to exert divergent effects across CD4⁺ T cell subsets and highlight specific roles for this pathway in the regulation of individual Th cell differentiation programs.

Established and emergent roles for Ikaros transcription factors in lymphoid cell development and function

Ikaros zinc finger transcription factors are important regulators of the gene programs underlying the development of hematopoietic cell lineages. The family consists of five members: Ikaros, Helios, Aiolos, Eos, and Pegasus, which engage in both homo- and heterotypic intrafamilial interactions to exert diverse functional effects. Pioneering studies focused on the role of these factors in early lymphoid development, as their absence resulted in severe defects in lymphocyte populations. More recent work has now begun to define nuanced, stage-specific roles for Ikaros family members in the differentiation and function of mature T, B, and innate lymphoid cell populations including natural killer (NK) cells. The precise transcriptional mechanisms by which these factors function, both independently and collaboratively, is an area of active investigation. However, several key themes appear to be emerging regarding the pathways influenced by Ikaros family members, including the end-to-end regulation of cytokine signaling. Here, we review roles for Ikaros factors in lymphoid cell development, differentiation, and function, including a discussion of the current understanding of the transcriptional mechanisms they employ and considerations for the future study of this important transcription factor family.

IkZF and STAT factor regulatory modules as drivers of TH1 and TFH cell differentiation decisions

CD4+ T helper cells are capable of differentiating into subsets that regulate distinct aspects of the immune response. The differentiation of these populations is driven by subset-specific cytokine signals and downstream transcription factor networks that are responsible for promoting the unique gene expression programs of each cell type. It is well-established that cytokine signals are propagated by Signal Transducer and Activator of Transcription (STAT) factors. Recently, we demonstrated that STAT3 and Aiolos, a member of the Ikaros Zinc Finger (IkZF) family, form a novel transcriptional complex that directly induces expression of the T follicular helper (TFH) cell lineage defining factor, Bcl-6. Interestingly, we have also found that a second IkZF factor Eos interacts with STAT5 and forms an opposing complex in T helper 1 (TH1) cells that promotes expression of the Bcl-6 antagonist, Blimp-1. However, whether additional IkZF/STAT complexes function downstream of other cytokine signals in these cell populations is unclear. Here, we report that in vitro-generated TH1 and TFH1-like cells similarly express Ikaros concurrent with increased STAT4 activation downstream of IL-12 signaling. Using co-immunoprecipitation analyses, we were also able to detect interactions between Ikaros and STAT4. Furthermore, as with the other IkZF/STAT complexes, we find that interactions between Ikaros and STAT4 were dependent upon the IkZF C-terminal ZF domain, suggesting that this is a conserved regulatory feature of IkZF/STAT complex formation. Current efforts are now aimed at identifying whether mechanistic interplay between the Ikaros/STAT4 complex and the other IkZF/STAT factors may ultimately dictate TH1 or TFH cell fate and function.

Selective Histone Deacetylase 6 Inhibition Normalizes B Cell Activation and Germinal Center Formation in a Model of Systemic Lupus Erythematosus

Autoantibody production by plasma cells (PCs) plays a pivotal role in the pathogenesis of systemic lupus erythematosus (SLE). The molecular pathways by which B cells become pathogenic PC secreting autoantibodies in SLE are incompletely characterized. Histone deactylase 6 (HDAC6) is a unique cytoplasmic HDAC that modifies the interaction of a number of tubulin- associated proteins; inhibition of HDAC6 has been shown to be beneficial in murine models of SLE, but the downstream pathways accounting for the therapeutic benefit have not been clearly delineated. In the current study, we sought to determine whether selective HDAC6 inhibition would abrogate abnormal B cell activation in SLE. We treated NZB/W lupus mice with the selective HDAC6 inhibitor, ACY-738, for 4 weeks beginning at 20 weeks-of age. After only 4 weeks of treatment, manifestation of lupus nephritis (LN) were greatly reduced in these animals. We then used RNAseq to determine the genomic signatures of splenocytes from treated and untreated mice and applied computational cellular and pathway analysis to reveal multiple signaling events associated with B cell activation and differentiation in SLE that were modulated by HDAC6 inhibition. PC development was abrogated and germinal center (GC) formation was greatly reduced. When the HDAC6 inhibitor-treated lupus mouse gene signatures were compared to human lupus patient gene signatures, the results showed numerous immune, and inflammatory pathways increased in active human lupus were significantly decreased in the HDAC6 inhibitor treated animals. Pathway analysis suggested alterations in cellular metabolism might contribute to the normalization of lupus mouse spleen genomic signatures, and this was confirmed by direct measurement of the impact of the HDAC6 inhibitor on metabolic activities of murine spleen cells. Taken together, these studies show HDAC6 inhibition decreases B cell activation signaling pathways and reduces PC differentiation in SLE and suggest that a critical event might be modulation of cellular metabolism.

IL-12 signaling drives the differentiation and function of a TH1-derived TFH1-like cell population

CD4⁺ T follicular helper (T_FH) cells provide help to B cells and promote antibody-mediated immune responses. Increasing evidence supports the existence of T_FH populations that secrete cytokines typically associated with the effector functions of other CD4⁺ T cell subsets. These include T helper 1 (T_H1)-biased T_FH (T_FH1) cells that have recognized roles in both immune responses to pathogens and also the pathogenesis of autoimmune disease. Given their apparent importance to human health, there is interest in understanding the mechanisms that regulate T_FH1 cell formation and function. However, their origin and the molecular requirements for their differentiation are unclear. Here, we describe a population of murine T_H1-derived, T_FH1-like cells that express the chemokine receptor Cxcr3 and produce both the T_H1 cytokine interferon-γ and the T_FH-associated cytokine interleukin-21 (IL-21). Furthermore, these T_FH1-like cells promote B cell activation and antibody production at levels indistinguishable from conventional IL-6-derived T_FH-like cells. Regarding their regulatory requirements, we find that IL-12 signaling is necessary for the differentiation and function of this T_FH1-like cell population. Specifically, IL-12-dependent activation of STAT4, and unexpectedly STAT3, promotes increased expression of IL-21 and the T_FH lineage-defining transcription factor Bcl-6 in T_FH1-like cells. Taken together, these findings provide insight into the potential origin and differentiation requirements of T_FH1 cells.

Ikaros Zinc Finger Transcription Factors: Regulators of Cytokine Signaling Pathways and CD4+ T Helper Cell Differentiation

CD4⁺ T helper cells are capable of differentiating into a number of effector subsets that perform diverse functions during adaptive immune responses. The differentiation of each of these subsets is governed, in large part, by environmental cytokine signals and the subsequent activation of downstream, cell-intrinsic transcription factor networks. Ikaros zinc finger (IkZF) transcription factors are known regulators of immune cell development, including that of CD4⁺ T cell subsets. Over the past decade, members of the IkZF family have also been implicated in the differentiation and function of individual T helper cell subsets, including T helper 1 (T_H1), T_H2, T_H17, T follicular (T_FH), and T regulatory (T_REG) cells. Now, an increasing body of literature suggests that the distinct cell-specific cytokine environments responsible for the development of each subset result in differential expression of IkZF factors across T helper populations. Intriguingly, recent studies suggest that IkZF members influence T helper subset differentiation in a feed-forward fashion through the regulation of these same cytokine-signaling pathways. Here, we review the increasingly prominent role for IkZF transcription factors in the differentiation of effector CD4⁺ T helper cell subsets.

Additive manufacturing of three-dimensional (3D) microfluidic-based microelectromechanical systems (MEMS) for acoustofluidic applications

Three-dimensional (3D) printing now enables the fabrication of 3D structural electronics and microfluidics. Further, conventional subtractive manufacturing processes for microelectromechanical systems (MEMS) relatively limit device structure to two dimensions and require post-processing steps for interface with microfluidics. Thus, the objective of this work is to create an additive manufacturing approach for fabrication of 3D microfluidic-based MEMS devices that enables 3D configurations of electromechanical systems and simultaneous integration of microfluidics. Here, we demonstrate the ability to fabricate microfluidic-based acoustofluidic devices that contain orthogonal out-of-plane piezoelectric sensors and actuators using additive manufacturing. The devices were fabricated using a microextrusion 3D printing system that contained integrated pick-and-place functionality. Additively assembled materials and components included 3D printed epoxy, polydimethylsiloxane (PDMS), silver nanoparticles, and eutectic gallium-indium as well as robotically embedded piezoelectric chips (lead zirconate titanate (PZT)). Electrical impedance spectroscopy and finite element modeling studies showed the embedded PZT chips exhibited multiple resonant modes of varying mode shape over the 0-20 MHz frequency range. Flow visualization studies using neutrally buoyant particles (diameter = 0.8-70 μm) confirmed the 3D printed devices generated bulk acoustic waves (BAWs) capable of size-selective manipulation, trapping, and separation of suspended particles in droplets and microchannels. Flow visualization studies in a continuous flow format showed suspended particles could be moved toward or away from the walls of microfluidic channels based on selective actuation of in-plane or out-of-plane PZT chips. This work suggests additive manufacturing potentially provides new opportunities for the design and fabrication of acoustofluidic and microfluidic devices.

Local and long-range transcriptional regulation of the Bcl6 locus by Ikaros zinc finger factors

T follicular helper (TFH) cells play a pivotal role in the humoral immune response by providing help to B cells, resulting in germinal center formation and the subsequent generation of pathogen-neutralizing antibodies. The development of TFH cells requires the expression of the transcriptional repressor Bcl-6; however, the mechanisms underlying Bcl6 transcriptional regulation are incompletely understood. Here we identify the Ikaros Zinc Finger (IkZF) transcription factors Aiolos and Ikaros as novel regulators of Bcl6 expression. We demonstrate that the expression of Aiolos and Ikaros is upregulated in antigen-specific TFH cells generated in response to both Listeria monocytogenes and influenza infection. Mechanistically, we find that Aiolos and Ikaros directly associate with the Bcl6 promoter. This association corresponds with chromatin remodeling events consistent with gene activation including the recruitment of chromatin remodeling complexes, increased histone acetylation, and RNA polymerase II binding. Additionally, we find that Aiolos and Ikaros are enriched at a conserved noncoding sequence 273 kb (CNS273) upstream of the Bcl6 transcriptional start site. CNS273 exhibits a covalent histone modification profile indicative of an active enhancer and is capable of enhancing Bcl6 promoter activity in an Aiolos-dependent manner. Collectively, these data provide insight into the multifaceted mechanisms by which Aiolos and Ikaros regulate Bcl6 expression.

An IL-2/STAT5/Eos regulatory axis promotes Th1 differentiation

Members of the Ikaros Zinc Finger Transcription Factor (IkZF) family are known regulators of hematopoiesis, including the development of CD4+ T helper cell populations. Our recent work established that the IkZF family member Aiolos cooperates with STAT3 to positively regulate Bcl-6 expression in T follicular helper (Tfh) cells. Given the conservation between members of the STAT and IkZF families, and their roles in T helper cell development, we hypothesized that additional IkZF/STAT interactions may regulate the differentiation of other T helper cell subsets. Here, we find that IL-2 signaling induces expression of the IkZF factor Eos in Th1 cells. Knockdown of Eos results in a significant reduction in the expression of signature Th1 genes including Prdm1, Il2ra, and Ifng. Mechanistically, we find that Eos and STAT5 physically interact and form a transcriptional complex that is capable of inducing Prdm1 expression. In support of these data, we also find that Eos and STAT5 are co-enriched at the regulatory regions of Th1 genes including Prdm1 and Il2ra. Taken together, these findings are suggestive of a novel, cooperative role for the IkZF family member Eos and STAT5 in regulating the Th1 differentiation program.

Integrated STAT3 and Ikaros Zinc Finger Transcription Factor Activities Regulate Bcl-6 Expression in CD4+ Th Cells

B cell lymphoma-6 (Bcl-6) is a transcriptional repressor that is required for the differentiation of T follicular helper (T_FH) cell populations. Currently, the molecular mechanisms underlying the transcriptional regulation of Bcl-6 expression are unclear. In this study, we have identified the Ikaros zinc finger transcription factors Aiolos and Ikaros as novel regulators of Bcl-6. We found that increased expression of Bcl-6 in CD4⁺ Th cell populations correlated with enhanced enrichment of Aiolos and Ikaros at the Bcl6 promoter. Furthermore, overexpression of Aiolos or Ikaros, but not the related family member Eos, was sufficient to induce Bcl6 promoter activity. Intriguingly, STAT3, a known Bcl-6 transcriptional regulator, physically interacted with Aiolos to form a transcription factor complex capable of inducing the expression of Bcl6 and the T_FH-associated cytokine receptor Il6ra Importantly, in vivo studies revealed that the expression of Aiolos was elevated in Ag-specific T_FH cells compared with that observed in non-T_FH effector Th cells generated in response to influenza infection. Collectively, these data describe a novel regulatory mechanism through which STAT3 and the Ikaros zinc finger transcription factors Aiolos and Ikaros cooperate to regulate Bcl-6 expression.

Control of lupus nephritis by changes of gut microbiota

BACKGROUND

Systemic lupus erythematosus, characterized by persistent inflammation, is a complex autoimmune disorder with no known cure. Immunosuppressants used in treatment put patients at a higher risk of infections. New knowledge of disease modulators, such as symbiotic bacteria, can enable fine-tuning of parts of the immune system, rather than suppressing it altogether.

RESULTS

Dysbiosis of gut microbiota promotes autoimmune disorders that damage extraintestinal organs. Here we report a role of gut microbiota in the pathogenesis of renal dysfunction in lupus. Using a classical model of lupus nephritis, MRL/lpr, we found a marked depletion of Lactobacillales in the gut microbiota. Increasing Lactobacillales in the gut improved renal function of these mice and prolonged their survival. We used a mixture of 5 Lactobacillus strains (Lactobacillus oris, Lactobacillus rhamnosus, Lactobacillus reuteri, Lactobacillus johnsonii, and Lactobacillus gasseri), but L. reuteri and an uncultured Lactobacillus sp. accounted for most of the observed effects. Further studies revealed that MRL/lpr mice possessed a "leaky" gut, which was reversed by increased Lactobacillus colonization. Lactobacillus treatment contributed to an anti-inflammatory environment by decreasing IL-6 and increasing IL-10 production in the gut. In the circulation, Lactobacillus treatment increased IL-10 and decreased IgG2a that is considered to be a major immune deposit in the kidney of MRL/lpr mice. Inside the kidney, Lactobacillus treatment also skewed the Treg-Th17 balance towards a Treg phenotype. These beneficial effects were present in female and castrated male mice, but not in intact males, suggesting that the gut microbiota controls lupus nephritis in a sex hormone-dependent manner.

CONCLUSIONS

This work demonstrates essential mechanisms on how changes of the gut microbiota regulate lupus-associated immune responses in mice. Future studies are warranted to determine if these results can be replicated in human subjects.

Cooperative activities between STAT3 and Ikaros Zinc Finger transcription factors regulate Bcl-6 expression in CD4+ T helper cells

CD4+ T cell subtype specialization depends upon unique lineage-defining transcription factors that direct T helper cell development by both activating cell-specific gene expression programs and repressing alternative T helper cell fates. One such example is the transcriptional repressor Bcl-6, required for the development of T follicular helper (TFH) cell populations. Here, we identify the Ikaros zinc finger (IkZF) transcription factors Ikaros and Aiolos as novel regulators of Bcl-6 expression. We find that increased expression of Bcl-6 in T helper cells is associated with enhanced enrichment of Aiolos and Ikaros at the Bcl6 promoter. Additionally, we demonstrate that signal transducer and activator of transcription 3 (STAT3) physically interacts with Aiolos, suggesting a cooperative role for these factors in the activation of Bcl-6 expression. Mechanistically, the association of Ikaros, Aiolos, and STAT3 with the Bcl6 promoter was accompanied by chromatin remodeling events consistent with gene activation including increased histone acetylation. Importantly, expression of Aiolos and Ikaros was elevated in in vivo generated TFH cell populations compared to other IkZF family members. Collectively, these data describe a novel regulatory mechanism wherein STAT3 and the IkZF transcription factors Ikaros and Aiolos cooperate to regulate Bcl-6 expression. Elucidating the complex network of signals and factors that regulate Bcl-6 expression may enhance the potential to design more efficacious vaccines and develop novel immunotherapeutic approaches due to the wide-ranging importance of this transcriptional regulator.

T follicular helper cell programming by cytokine-mediated events

CD4⁺ T cells, or T helper cells, are critical mediators and coordinators of adaptive immunity. Unique effector T helper cell populations have been identified that perform distinct functions in response to pathogenic infection. The T follicular helper (Tfh) cells are one such subset, which has been identified as the primary T-cell population responsible for interacting with B cells to promote effective antibody-mediated immune responses. Since their initial description at the turn of the century, and subsequent classification as a distinct T helper cell subset, there has been substantial interest in elucidating the regulatory mechanisms that govern Tfh cell formation. The collective insight from this body of work has demonstrated that Tfh cell differentiation is a complex and multistage process regulated by a litany of cell-intrinsic and cell-extrinsic factors. As with the development of the other recognized T helper cell subsets, specific cytokines exercise prominent roles in both the positive and negative regulation of Tfh cell development. However, the exact composition of, and stage-specific requirements for, these environmental factors in the governance of Tfh cell differentiation remain incompletely understood. In this review, we summarize what is known regarding the role of cytokines in both the promotion and inhibition of Tfh cell differentiation and function.

IL-2, IL-7, and IL-15: Multistage regulators of CD4(+) T helper cell differentiation

Cytokines represent a class of environmental factors that are critical drivers of immune cell development. Cytokines of the common gamma-chain family, including interleukin (IL)-2, IL-7, and IL-15, have been the subject of intense experimental scrutiny and have well-defined roles as regulators of diverse immune cell types including CD4(+) T helper cells. Because of their pleiotropic effects on T-cell development and function, researchers and clinicians have attempted to harness the capabilities of these cytokines for therapeutic benefit. In this review, we summarize the recent progress in our understanding of the molecular mechanisms underlying the effects of these cytokines on CD4(+) T cell development and briefly discuss how these immunomodulatory cytokines are being used in efforts to treat human disease.

Differential regulation of TH1, TFH, and TCM gene programs by cytokine-mediated events

During the course of an immune response, multiple effector T helper cell types emerge to combat the initial infection, with subsequent emergence of long-term CD4+ T memory cells to ensure that a second exposure to the same pathogen will not cause disease. CD4+ T helper cell populations that are critical for this process include T helper 1 (TH1) and T follicular Helper (TFH) effector cells and long-lasting central memory T (TCM) cells. Previous work has demonstrated a level of flexibility between effector TH1 and TFH-like cell populations, with the respective gene programs regulated by alterations in interleukin 2 (IL-2) signaling. Here, we demonstrate that in response to decreased IL-2 signaling, effector TH1 cells are capable of initiating the expression of both TFH and TCM-like gene programs, including cell surface expression of IL-6Ra and IL-7R. Importantly, exposure of these cells to either IL-6 or IL-7 augments or inhibits the TFH gene program, respectively, suggesting a potential mechanism whereby cytokine availability may influence TFH or TCM cell function. Consistent with this model, we have identified a novel population of IL-6Ra+IL-7R+ cells that emerge at late time points post-infection in an in vivo mouse model of influenza infection. Further research into the temporal and spatial expression of IL-2, IL-6, and IL-7 in the modulation of these cells represents an exciting avenue of research with wide-ranging therapeutic implications.

IL-7 signalling represses Bcl-6 and the TFH gene program

The transcriptional repressor Bcl-6 is linked to the development of both CD4⁺ T follicular helper (T_FH) and central memory T (T_CM) cells. Here, we demonstrate that in response to decreased IL-2 signalling, T helper 1 (T_H1) cells upregulate Bcl-6 and co-initiate T_FH- and T_CM-like gene programs, including expression of the cytokine receptors IL-6Rα and IL-7R. Exposure of this potentially bi-potent cell population to IL-6 favours the T_FH gene program, whereas IL-7 signalling represses T_FH-associated genes including Bcl6 and Cxcr5, but not the T_CM-related genes Klf2 and Sell. Mechanistically, IL-7-dependent activation of STAT5 contributes to Bcl-6 repression. Importantly, antigen-specific IL-6Rα⁺IL-7R⁺ CD4⁺ T cells emerge from the effector population at late time points post influenza infection. These data support a novel role for IL-7 in the repression of the T_FH gene program and evoke a divergent regulatory mechanism by which post-effector T_H1 cells may contribute to long-term cell-mediated and humoral immunity.

It remains incompletely understood how cytokines shape T_H1 cell differentiation to central memory T (T_CM) and follicular T helper (T_FH) cells. Here the authors show that T_H1 cells can co-initiate the expression of both T_FH and T_CM gene programs and that IL-7 signalling represses T_FH-associated but not T_CM-associated genes.

Bcl-6 directly represses the gene program of the glycolysis pathway

Despite our increasing knowledge of the molecular events that induce the glycolysis pathway in effector T cells, very little is known about the transcriptional mechanisms that dampen the glycolysis program in quiescent cell populations such as memory T cells. Here, we show that the transcription factor Bcl-6 directly repressed genes involved in the glycolysis pathway, including Slc2a1, Slc2a3, Pkm2 and Hk2, in T_H1 cells exposed to low amounts of interleukin 2 (IL-2). Thus, Bcl-6 plays an opposing role to the IL-2-sensitive glycolytic transcriptional program that c-Myc and HIF-1α promote in effector T cells. Additionally, the Th1-lineage-specifying factor T-bet functionally antagonized the Bcl-6-dependent repression of genes in the glycolysis pathway, implicating the molecular balance between these two factors in metabolic gene program regulation.

Multiannual observations of acetone, methanol, and acetaldehyde in remote tropical atlantic air: implications for atmospheric OVOC budgets and oxidative capacity

Oxygenated volatile organic compounds (OVOCs) in the atmosphere are precursors to peroxy acetyl nitrate (PAN), affect the tropospheric ozone budget, and in the remote marine environment represent a significant sink of the hydroxyl radical (OH). The sparse observational database for these compounds, particularly in the tropics, contributes to a high uncertainty in their emissions and atmospheric significance. Here, we show measurements of acetone, methanol, and acetaldehyde in the tropical remote marine boundary layer made between October 2006 and September 2011 at the Cape Verde Atmospheric Observatory (CVAO) (16.85° N, 24.87° W). Mean mixing ratios of acetone, methanol, and acetaldehyde were 546 ± 295 pptv, 742 ± 419 pptv, and 428 ± 190 pptv, respectively, averaged from approximately hourly values over this five-year period. The CAM-Chem global chemical transport model reproduced annual average acetone concentrations well (21% overestimation) but underestimated levels by a factor of 2 in autumn and overestimated concentrations in winter. Annual average concentrations of acetaldehyde were underestimated by a factor of 10, rising to a factor of 40 in summer, and methanol was underestimated on average by a factor of 2, peaking to over a factor of 4 in spring. The model predicted summer minima in acetaldehyde and acetone, which were not apparent in the observations. CAM-Chem was adapted to include a two-way sea-air flux parametrization based on seawater measurements made in the Atlantic Ocean, and the resultant fluxes suggest that the tropical Atlantic region is a net sink for acetone but a net source for methanol and acetaldehyde. Inclusion of the ocean fluxes resulted in good model simulations of monthly averaged methanol levels although still with a 3-fold underestimation in acetaldehyde. Wintertime acetone levels were better simulated, but the observed autumn levels were more severely underestimated than in the standard model. We suggest that the latter may be caused by underestimated terrestrial biogenic African primary and/or secondary OVOC sources by the model. The model underestimation of acetaldehyde concentrations all year round implies a consistent significant missing source, potentially from secondary chemistry of higher alkanes produced biogenically from plants or from the ocean. We estimate that low model bias in OVOC abundances in the remote tropical marine atmosphere may result in up to 8% underestimation of the global methane lifetime due to missing model OH reactivity. Underestimation of acetaldehyde concentrations is responsible for the bulk (∼70%) of this missing reactivity.