IL-12 receptor β1 deficiency alters in vivo T follicular helper cell response in humans

Functional Diversity of Human Dendritic Cells

At the crossroad between innate and adaptive immunity are the dendritic Cells (DCs), a "novel cell type." discovered in 1973 by Ralph Steinman. Although not entirely appreciated at first, it is clear that they play a critical role as specialized antigen-presenting cells and essential mediators in shaping immune reactivity and tolerance. Dendritic cells are now recognized as a heterogeneous group of cells in terms of cell-surface markers, anatomic location, and function adapted to protect against an array of pathogens and conditions. Importantly, these subsets are also unique to each species. While significant progress has been made on the identification and function of mouse DC subsets, much less is known about human cells. Here we review the fascinating biology of human skin DCs and describe tolerogenic principles that are critical in maintaining immune homeostasis and for controlling inflammation, as well as mechanisms that are fundamental to confer immunity. We surmise that these principles could be applied to DCs across organs, and could be harnessed for the treatment of various human autoimmune, inflammatory diseases, as well as cancer. Importantly, to leverage the relevance of basic research to the clinical setting, it is first necessary to determine the functional homology between mouse and human DCs. We discuss practical steps towards this aim.

Human T follicular helper cells in primary immunodeficiencies

PURPOSE OF REVIEW

To summarize our understanding of the biology of T follicular helper (Tfh) cells and how insights into this are being provided by the study of human monogenic immunological diseases.

RECENT FINDINGS

Antibody production is a key feature of the vertebrate immune system. Antibodies neutralize and clear pathogens, thereby protecting against infectious diseases. Long-lived humoral immunity depends on help provided by Tfh cells, which support the differentiation of antigen-specific B cells into memory and plasma cells. Tfh cells are generated from naïve CD4 T cells following the receipt of inputs from various cell surface receptors. Although genetically modified mice have provided a great understanding of the requirements for generating Tfh cells, it is critical that the requirements for human Tfh cells are also established. This is being achieved by the systematic analysis of humans with monogenic mutations that cause primary immunodeficiencies characterized by impaired humoral immunity following infection or vaccination.

SUMMARY

The elucidation of the mechanisms that regulate Tfh cell generation, differentiation and function should reveal targets for novel therapeutics that may offer opportunities to manipulate these cells to not only improve humoral immunity in the setting of primary immunodeficiencies but also temper their dysregulation in conditions of antibody-mediated autoimmunity.

Mendelian susceptibility to mycobacterial disease: genetic, immunological, and clinical features of inborn errors of IFN-γ immunity

Mendelian susceptibility to mycobacterial disease (MSMD) is a rare condition characterized by predisposition to clinical disease caused by weakly virulent mycobacteria, such as BCG vaccines and environmental mycobacteria, in otherwise healthy individuals with no overt abnormalities in routine hematological and immunological tests. MSMD designation does not recapitulate all the clinical features, as patients are also prone to salmonellosis, candidiasis and tuberculosis, and more rarely to infections with other intramacrophagic bacteria, fungi, or parasites, and even, perhaps, a few viruses. Since 1996, nine MSMD-causing genes, including seven autosomal (IFNGR1, IFNGR2, STAT1, IL12B, IL12RB1, ISG15, and IRF8) and two X-linked (NEMO, and CYBB) genes have been discovered. The high level of allelic heterogeneity has already led to the definition of 18 different disorders. The nine gene products are physiologically related, as all are involved in IFN-γ-dependent immunity. These disorders impair the production of (IL12B, IL12RB1, IRF8, ISG15, NEMO) or the response to (IFNGR1, IFNGR2, STAT1, IRF8, CYBB) IFN-γ. These defects account for only about half the known MSMD cases. Patients with MSMD-causing genetic defects may display other infectious diseases, or even remain asymptomatic. Most of these inborn errors do not show complete clinical penetrance for the case-definition phenotype of MSMD. We review here the genetic, immunological, and clinical features of patients with inborn errors of IFN-γ-dependent immunity.

New insights into IL-12-mediated tumor suppression

During the past two decades, interleukin-12 (IL-12) has emerged as one of the most potent cytokines in mediating antitumor activity in a variety of preclinical models. Through pleiotropic effects on different immune cells that form the tumor microenvironment, IL-12 establishes a link between innate and adaptive immunity that involves different immune effector cells and cytokines depending on the type of tumor or the affected tissue. The robust antitumor response exerted by IL-12, however, has not yet been successfully translated into the clinics. The majority of clinical trials involving treatment with IL-12 failed to show sustained antitumor responses and were associated to toxic side effects. Here we discuss the therapeutic effects of IL-12 from preclinical to clinical studies, and will highlight promising strategies to take advantage of the antitumor activity of IL-12 while limiting adverse effects.

Transcriptional and epigenetic networks of helper T and innate lymphoid cells

The discovery of the specification of CD4(+) helper T cells to discrete effector 'lineages' represented a watershed event in conceptualizing mechanisms of host defense and immunoregulation. However, our appreciation for the actual complexity of helper T-cell subsets continues unabated. Just as the Sami language of Scandinavia has 1000 different words for reindeer, immunologists recognize the range of fates available for a CD4(+) T cell is numerous and may be underestimated. Added to the crowded scene for helper T-cell subsets is the continuously growing family of innate lymphoid cells (ILCs), endowed with common effector responses and the previously defined 'master regulators' for CD4(+) helper T-cell subsets are also shared by ILC subsets. Within the context of this extraordinary complexity are concomitant advances in the understanding of transcriptomes and epigenomes. So what do terms like 'lineage commitment' and helper T-cell 'specification' mean in the early 21st century? How do we put all of this together in a coherent conceptual framework? It would be arrogant to assume that we have a sophisticated enough understanding to seriously answer these questions. Instead, we review the current status of the flexibility of helper T-cell responses in relation to their genetic regulatory networks and epigenetic landscapes. Recent data have provided major surprises as to what master regulators can or cannot do, how they interact with other transcription factors and impact global genome-wide changes, and how all these factors come together to influence helper cell function.

The cytokine TGF-β co-opts signaling via STAT3-STAT4 to promote the differentiation of human TFH cells

Understanding the developmental mechanisms of T follicular helper (T_FH) cells in humans is a highly relevant topic to clinic. However, factors that drive human CD4⁺ helper T (T_H) cell differentiation program towards T_FH cells remain largely undefined. Here we show that TGF-β provides critical additional signals for the transcription factors STAT3 and STAT4 to promote the initial T_FH differentiation programs in humans. This mechanism does not appear to be shared with mouse T_H cells. The developing human Bcl-6⁺ T_FH cells also expressed RORγt, a transcription factor typically expressed by T_H17 cells. Our study documents a mechanism by which T_FH and T_H17 cells co-emerge in inflammatory environments in humans, as often observed in many human autoimmune diseases.

Targeting human dendritic cells in situ to improve vaccines

Dendritic cells (DCs) provide a critical link between innate and adaptive immunity. The potent antigen presenting properties of DCs makes them a valuable target for the delivery of immunogenic cargo. Recent clinical studies describing in situ DC targeting with antibody-mediated targeting of DC receptor through DEC-205 provide new opportunities for the clinical application of DC-targeted vaccines. Further advances with nanoparticle vectors which can encapsulate antigens and adjuvants within the same compartment and be targeted against diverse DC subsets also represent an attractive strategy for targeting DCs. This review provides a brief summary of the rationale behind targeting dendritic cells in situ, the existing pre-clinical and clinical data on these vaccines and challenges faced by the next generation DC-targeted vaccines.

Phenotype and functions of memory Tfh cells in human blood

Our understanding of the origin and functions of human blood CXCR5(+) CD4(+) T cells found in human blood has changed dramatically in the past years. These cells are currently considered to represent a circulating memory compartment of T follicular helper (Tfh) lineage cells. Recent studies have shown that blood memory Tfh cells are composed of phenotypically and functionally distinct subsets. Here, we review the current understanding of human blood memory Tfh cells and the subsets within this compartment. We present a strategy to define these subsets based on cell surface profiles. Finally, we discuss how increased understanding of the biology of blood memory Tfh cells may contribute insight into the pathogenesis of autoimmune diseases and the mode of action of vaccines.

STAT4 deficiency fails to induce lung Th2 or Th17 immunity following primary or secondary respiratory syncytial virus (RSV) challenge but enhances the lung RSV-specific CD8+ T cell immune response to secondary challenge

Immune-mediated lung injury is a hallmark of lower respiratory tract illness caused by respiratory syncytial virus (RSV). STAT4 plays a critical role in CD4+ Th1 lineage differentiation and gamma interferon (IFN-γ) protein expression by CD4+ T cells. As CD4+ Th1 differentiation is associated with negative regulation of CD4+ Th2 and Th17 differentiation, we hypothesized that RSV infection of STAT4-/- mice would result in enhanced lung Th2 and Th17 inflammation and impaired lung Th1 inflammation compared to wild-type (WT) mice. We performed primary and secondary RSV challenges in WT and STAT4-/- mice and used STAT1-/- mice as a positive control for the development of RSV-specific lung Th2 and Th17 inflammation during primary challenge. Primary RSV challenge of STAT4-/- mice resulted in decreased T-bet and IFN-γ expression levels in CD4+ T cells compared to those of WT mice. Lung Th2 and Th17 inflammation did not develop in primary RSV-challenged STAT4-/- mice. Decreased IFN-γ expression by NK cells, CD4+ T cells, and CD8+ T cells was associated with attenuated weight loss and enhanced viral clearance with primary challenge in STAT4-/- mice compared to WT mice. Following secondary challenge, WT and STAT4-/- mice also did not develop lung Th2 or Th17 inflammation. In contrast to primary challenge, secondary RSV challenge of STAT4-/- mice resulted in enhanced weight loss, an increased lung IFN-γ expression level, and an increased lung RSV-specific CD8+ T cell response compared to those of WT mice. These data demonstrate that STAT4 regulates the RSV-specific CD8+ T cell response to secondary infection but does not independently regulate lung Th2 or Th17 immune responses to RSV challenge.

IMPORTANCE

STAT4 is a protein critical for both innate and adaptive immune responses to viral infection. Our results show that STAT4 regulates the immune response to primary and secondary challenge with RSV but does not restrain RSV-induced lung Th2 or Th17 immune responses. These findings suggest that STAT4 expression may influence lung immunity and severity of illness following primary and secondary RSV infections.

Therapeutic potential of STAT4 in autoimmunity

INTRODUCTION

STAT4, which acts as the major signaling transducing STATs in response to IL-12, is a central mediator in generating inflammation during protective immune responses and immune-mediated diseases.

AREAS COVERED

This review summarizes that STAT4 is essential for the differentiation and function of a wide variety of immune cells, including natural killer cells, mast cells, dendritic cells and T helper cells. In addition, STAT4-mediated signaling promoted the production of autoimmune-associated components, which are implicated in the pathogenesis of autoimmune diseases, such as rheumatoid arthritis, systemic lupus erythematosus, systemic sclerosis and psoriasis.

EXPERT OPINION

Due to its crucial roles in inflammation and autoimmunity, STAT4 may have promise as an effective therapeutic target for autoimmune diseases. Understanding the molecular mechanisms driving STAT4, together with knowledge on the ability of current immunosuppressive treatment to target this process, may open an avenue to novel therapeutic options.

Effects of microRNA-21 on the interleukin 12/signal transducer and activator of transcription 4 signaling pathway in asthmatic mice

Objective

To study the effect of microRNA-21 (miRNA-21) on the regulation of the interleukin 12 (IL-12)/signal transducer and activator of transcription 4 (STAT4) pathway in the lung tissue of asthmatic mice.

Material and methods

Forty five male C57BL/6 mice were randomly divided into three groups of 15 mice each: normal control, asthmatic model, and dexamethasone. Our mouse model of allergic asathma was established using OVA sensitization and challenge. Hematoxylin and eosin staining was performed to observe the pathological changes in lung tissue morphology. Both the total cell number and the amount of eosinophils (EOS) in the bronchoalveolar lavage fluid (BALF) were manually counted. The expression of miRNA-21 was detected by real time quantitative PCR. The expression levels of IL-12 and STAT4 in lung tissue were assayed via western blot, and immunohistochemistry was used to observe the distribution of their expression.

Results

The expression levels of miRNA-21 as well as the total number of BALF cells and EOS were significantly higher in the asthmatic model group than in the control or dexamethasone groups, with significantly higher amounts found in the dexamethasone group than in the control group. The expression levels of IL-12 and STAT4 proteins were lower in the asthmatic model group than in the control and dexamethasone groups, with a significantly lower expression of IL-12 and STAT4 in the dexamethasone group than in the control group.

Conclusions

The expression level of miRNA-21 was significantly increased and the expression level of IL-12 and STAT4 proteins was significantly decreased in allergic asthmatic mice compared with normal control mice. These findings suggest a role for miRNA-21 and the IL-12/STAT4 pathway in the development of allergic asthma.

Bcl-6 controlled TFH polarization and memory: the known unknowns

Upon antigenic activation in vivo, naïve CD4 T cells can differentiate into one of several helper (Th) subsets under the control of lineage-specifying transcription factors to tailor immune responses against different types of pathogens. Follicular T-helper (TFH) cells are a recently defined subset that is controlled by Bcl-6 and specializes in promoting B cell-mediated humoral immunity. TFH cells exhibit unique spatiotemporal and functional features, but it is not settled as to how Bcl-6 promotes the TFH development, how TFH cells relate to other Th subsets, and how TFH cells relate to memory. Here we review recent advances and crucial gaps in our understanding of Bcl-6-controlled TFH development and function.

Tailored immune responses: novel effector helper T cell subsets in protective immunity

Differentiation of naïve CD4⁺ cells into functionally distinct effector helper T cell subsets, characterised by distinct "cytokine signatures," is a cardinal strategy employed by the mammalian immune system to efficiently deal with the rapidly evolving array of pathogenic microorganisms encountered by the host. Since the T(H)1/T(H)2 paradigm was first described by Mosmann and Coffman, research in the field of helper T cell biology has grown exponentially with seven functionally unique subsets having now been described. In this review, recent insights into the molecular mechanisms that govern differentiation and function of effector helper T cell subsets will be discussed in the context of microbial infections, with a focus on how these different helper T cell subsets orchestrate immune responses tailored to combat the nature of the pathogenic threat encountered.

Type I IFN induces binding of STAT1 to Bcl6: divergent roles of STAT family transcription factors in the T follicular helper cell genetic program

CD4(+) T follicular helper cells (TFH) are critical for the formation and function of B cell responses to infection or immunization, but also play an important role in autoimmunity. The factors that contribute to the differentiation of this helper cell subset are incompletely understood, although several cytokines including IL-6, IL-21, and IL-12 can promote TFH cell formation. Yet, none of these factors, nor their downstream cognate STATs, have emerged as nonredundant, essential drivers of TFH cells. This suggests a model in which multiple factors can contribute to the phenotypic characteristics of TFH cells. Because type I IFNs are often generated in immune responses, we set out to investigate whether these factors are relevant to TFH cell differentiation. Type I IFNs promote Th1 responses, thus one possibility was these factors antagonized TFH-expressed genes. However, we show that type I IFNs (IFN-α/β) induced B cell lymphoma 6 (Bcl6) expression, the master regulator transcription factor for TFH cells, and CXCR5 and programmed cell death-1 (encoded by Pdcd1), key surface molecules expressed by TFH cells. In contrast, type I IFNs failed to induce IL-21, the signature cytokine for TFH cells. The induction of Bcl6 was regulated directly by STAT1, which bound to the Bcl6, Cxcr5, and Pdcd1 loci. These data suggest that type I IFNs (IFN-α/β) and STAT1 can contribute to some features of TFH cells but are inadequate in inducing complete programming of this subset.

The antigen presenting cells instruct plasma cell differentiation

The professional antigen presenting cells (APCs), including many subsets of dendritic cells and macrophages, not only mediate prompt but non-specific response against microbes, but also bridge the antigen-specific adaptive immune response through antigen presentation. In the latter, typically activated B cells acquire cognate signals from T helper cells in the germinal center of lymphoid follicles to differentiate into plasma cells (PCs), which generate protective antibodies. Recent advances have revealed that many APC subsets provide not only “signal 1” (the antigen), but also “signal 2” to directly instruct the differentiation process of PCs in a T-cell-independent manner. Herein, the different signals provided by these APC subsets to direct B cell proliferation, survival, class switching, and terminal differentiation are discussed. We furthermore propose that the next generation of vaccines for boosting antibody response could be designed by targeting APCs.

Roles for helper T cell lineage-specifying transcription factors in cellular specialization

The development of specialized helper T cells has garnered much attention because of their critical role in coordinating the immune response to invading pathogens. Recent research emphasizing novel functions for specialized helper T cells in a variety of infectious disease settings, as well as autoimmune states, has reshaped our view on the capabilities of helper T cells. Notably, one previously underappreciated aspect of the lifespan of helper T cells is that they often retain the capacity to respond to changes in the environment by altering the composition of helper T cell lineage-specifying transcription factors they express, which, in turn, changes their phenotype. This emerging realization is changing our views on the stability versus flexibility of specialized helper T cell subtypes. Now, there is a new concerted effort to define the mechanistic events that contribute to the potential for flexibility in specialized helper T cell gene expression programs in the different environmental circumstances that allow for the re-expression of helper T cell lineage-specifying transcription factors. In addition, we are also now beginning to appreciate that "helper T cell" lineage-specifying transcription factors are expressed in diverse types of innate and adaptive immune cells and this may allow them to play roles in coordinating aspects of the immune response. Our current challenges include defining the conserved mechanisms that are utilized by these lineage-specifying transcription factors to coordinate gene expression programs in different settings as well as the mechanistic events that contribute to the differential downstream consequences that these factors mediate in unique cellular environments. In this review, we will explore our evolving views on these topics, often times using the Th1-lineage-specifying transcription factor T-bet as an example.

Helper T cell plasticity: impact of extrinsic and intrinsic signals on transcriptomes and epigenomes

CD4(+) helper T cells are crucial for autoimmune and infectious diseases; however, the recognition of the many, diverse fates available continues unabated. Precisely what controls specification of helper T cells and preserves phenotypic commitment is currently intensively investigated. In this review, we will discuss the major factors that impact helper T cell fate choice, ranging from cytokines and the microbiome to metabolic control and epigenetic regulation. We will also discuss the technological advances along with the attendant challenges presented by "big data," which allow the understanding of these processes on comprehensive scales.

Human dendritic cells - stars in the skin

"A properly functioning adaptive immune system signifies the best features of life. It is diverse beyond compare, tolerant without fail, and capable of behaving appropriately with a myriad of infections and other challenges. Dendritic cells (DCs) are required to explain how this remarkable system is energized and directed." This is a quote by one of the greatest immunologists our community has ever known, and the father of dendritic cells, Ralph Steinman. Steinman's discovery of DCs in 1973 and his subsequent research opened a new field of study within immunology: DC biology and in particular the role of DCs in immune regulation in health and disease. Here, I review themes from our work and others on the complex network of dendritic cells in the skin and discuss the significance of skin DCs in understanding aspects of host defense against infections, the pathology of inflammatory skin diseases, and speculate on the future effective immune-based therapies.

How T follicular helper cells and the germinal centre response change with age

Normal ageing is accompanied by a decline in the function of the immune system that causes an increased susceptibility to infections and an impaired response to vaccination in older individuals. This results in an increased disease burden in the aged population, even with good immunisation programmes in place. The decreased response to vaccination is partly due to the diminution of the germinal centre response with age, caused by impaired T-cell help to B cells. Within the germinal centre, T-cell help is provided by a specialised subset of CD4(+) T cells; T follicular helper (Tfh) cells. Tfh cells provide survival and selection signals to germinal centre B cells, allowing them to egress from the germinal centre and become long-live plasma cells or memory B cells, and provide life-long protection against subsequent infection. This review will discuss the cellular and molecular changes in both Tfh cells and germinal centre B cells that occur with advancing age, which result in a smaller germinal centre response and a less effective response to immunisation.

Dendritic-cell-based therapeutic cancer vaccines

The past decade has seen tremendous developments in novel cancer therapies through the targeting of tumor-cell-intrinsic pathways whose activity is linked to genetic alterations and the targeting of tumor-cell-extrinsic factors, such as growth factors. Furthermore, immunotherapies are entering the clinic at an unprecedented speed after the demonstration that T cells can efficiently reject tumors and that their antitumor activity can be enhanced with antibodies against immune-regulatory molecules (checkpoint blockade). Current immunotherapy strategies include monoclonal antibodies against tumor cells or immune-regulatory molecules, cell-based therapies such as adoptive transfer of ex-vivo-activated T cells and natural killer cells, and cancer vaccines. Herein, we discuss the immunological basis for therapeutic cancer vaccines and how the current understanding of dendritic cell and T cell biology might enable the development of next-generation curative therapies for individuals with cancer.

Human T follicular helper (Tfh) cells and disease

The generation of protective antibodies by B cells following natural infection or vaccination requires 'help' from CD4(+) T cells. T follicular helper (Tfh) cells are the specialized CD4(+) T cell subset that has evolved the appropriate mechanisms to induce the activation and differentiation of B cells into immunoglobulin (Ig) secreting cells. As such, appropriate control of Tfh cell generation and function is essential to human health as overactivation is likely to result in autoimmunity, whereas underactivation is often associated with immunodeficiency. Furthermore, an understanding of the regulation of these cells may be invaluable to improved vaccine development strategies. Traditionally Tfh cells have been identified by their anatomical location in secondary lymphoid tissues, which has hindered the study of these cells in humans as access to these tissues is often not feasible. However, recent studies have identified the circulating counterparts to tissue Tfh cells and with this has come a wealth of knowledge gained from the study of these cells in human disease. Here we review some of the recent developments on the role of human Tfh cells in health and disease.

Regulatory mechanisms that control T-follicular helper and T-helper 1 cell flexibility

Following antigenic stimulation, CD4(+) T cells have the potential to differentiate into a number of specialized effector cell subtypes. To date, much progress has been made in defining the basic molecular mechanisms that regulate initial helper T-cell differentiation decisions. Emerging research in the field is now uncovering more complexity in the series of events that control helper T-cell commitment decisions than was previously appreciated. During the commitment process, helper T cells need to integrate both signals derived from the T-cell receptor and from the surrounding microenvironment. These external signals are then translated into internal changes in gene expression potential to ultimately define the functional characteristics of the cell. In this review, this topic will be discussed from the perspective of T-follicular helper (Tfh) and T-helper type 1 (Th1) cell differentiation. The focus will be on examining how the cytokine environment is perceived by signaling through signal transducer and activator of transcription (STAT) family proteins to initiate fate choices. The activities of STAT proteins are then in turn translated into changes in the molecular balance between B-cell lymphoma 6 (Bcl-6) and T-box expressed in T cells (T-bet), the helper T-cell lineage-specifying transcription factors that regulate Tfh and Th1 effector cell differentiation, respectively. Collectively, the knowledge of the molecular pathways that regulate Tfh and Th1 commitment have provided insight into the relationship between these two specialized helper T-cell subtypes and the potential for flexibility in their gene programs.

Human dendritic cells subsets as targets and vectors for therapy

The skin immune system includes a complex network of dendritic cells (DCs). In addition to generating cellular and humoral immunity against pathogens, skin DCs are involved in tolerogenic mechanisms that maintain immune homeostasis and in pathogenic chronic inflammation in which immune responses are unrestrained. Harnessing DC function by directly targeting DC-derived molecules or by selectively modulating DC subsets is a novel strategy for ameliorating inflammatory diseases. In this short review, we discuss recent advances in understanding the functional specialization of skin DCs and the potential implication for future DC-based therapeutic strategies.

Human dendritic cell subsets in vaccination

Owing to their properties, dendritic cells (DCs) are often called 'nature's adjuvants' and thus have become the natural targets for antigen delivery. DCs provide an essential link between the innate and the adaptive immune responses. DCs are at the center of the immune system owing to their ability to control both tolerance and immunity. DCs are thus key targets for both preventive and therapeutic vaccination. Herein, we will discuss recent progresses in our understanding of DC subsets physiology as it applies to vaccination.