B cells are required for follicular helper T (Tfh) cell development, as is the ligand for ICOS (ICOS-L); however, the separable contributions of Ag and ICOS-L delivery by cognate B cells to Tfh-cell development and function are unknown. We find that Tfh-cell and germinal center differentiation are dependent upon cognate B-cell display of ICOS-L, but only when Ag presentation by the latter is limiting, with the requirement for B-cell expression of ICOS-L overcome by robust Ag delivery. These findings demonstrate that Ag-specific B cells provide different, yet compensatory signals for Tfh-cell differentiation, while reconciling conflicting data indicating a requirement for ICOS-L expression on cognate B cells for Tfh-cell development with those demonstrating this requirement could be bypassed in lieu of that tendered by non-cognate B cells. Our findings clarify the separable roles of delivery of Ag and ICOS-L by cognate B cells for Tfh-cell maturation and function, and have implications for using therapeutic ICOS blockade in settings of abundantly available Ag, such as in systemic autoimmunity.
T cells; Costimulation; Autoimmunity
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 non-redundant, essential drivers of TFH cells. This suggests a model in which multiple factors can contribute to the phenotypic characteristics of TFH cells. As type I interferons (IFNs) are often generated in immune responses, we set out to investigate if 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 Bcl6 expression, the master regulator transcription factor for TFH cells, and CXCR5 and PD-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 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, 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 will 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.
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.
CD4 helper T cells are critical for proper immune cell homeostasis and host defense, but are also major contributes to immune and inflammatory disease. Arising from a simple, biphasic model of differentiation, Th1 and Th2 cells, a bewildering number of fates seem to possible for helper T cells. To what extent different helper cell subsets maintain their characteristic gene expression profiles or exhibit functional plasticity is a hotly debated topic. In this review, we will discuss how the expression of “signature cytokines” and “master regulator” transcription factors do not neatly conform to a simple T helper paradigm. While this may seem confusing, the good news is that the newly recognized complexity fits better with our understanding of immunopathogenesis. Finally, we will discuss factors include epigenetic regulation and metabolic alterations that contribute to helper cell specific and plasticity.
T cell plasticity; asthma; allergic disease; epigenetics; histone modification; therapy
CD4+ T cells are critical for the elimination of an immense array of microbial pathogens. Among the ways they accomplish this task is to generate progeny with specialized, characteristic patterns of gene expression. From this perspective, helper cells can be viewed as pluripotent precursors that adopt distinct cell fates. Although there are aspects of helper cell differentiation that can be modeled as a classic cell fate commitment, CD4+ T cells also maintain considerable flexibility in their transcriptional program. This makes sense in terms of host defense but raises the question of how these remarkable cells balance both these requirements, a high degree of specific gene expression and the capacity for plasticity. In this review, we discuss recent advances in our understanding of CD4+ T-cell specification, focusing on how genomic perspectives have influenced our views of these processes. The relative contributions of sensors of the cytokine milieu, especially the signal transducer and activator of transcription (STAT) family transcription factors, ‘master regulators’, and other transcription factors are considered as they relate to the helper cell transcriptome and epigenome.
T-cell plasticity; STAT; epigenetics; histone modification; T-cell differentiation; enhancers; master regulator
The transcription factor T-bet drives the differentiation of NKp46-expressing IL-22–producing innate lymphoid cells
Interleukin (IL)-22–producing innate lymphoid cells (ILCs; ILC22) comprise a heterogeneous population of cells that are dependent on the transcription factor retinoid-related orphan γt (RORγt) and are critical for barrier function of the intestinal mucosa. A distinct ILC22 subset expresses the natural cytotoxicity receptor NKp46 (NKp46+ ILC22); however, the factors that contribute to the generation of this population versus other subsets are largely unknown. Herein, we show that T-bet (encoded by Tbx21) was highly expressed in NKp46+ ILC22, a feature shared by all NKp46+ cells present in the intestine but not by other IL-22–producing populations. Accordingly, the absence of T-bet resulted in loss of NKp46+ ILC22 in the intestinal lamina propria. The residual NKp46+ ILC22 present in Tbx21−/− mice showed a marked reduction of Rorγt expression and impairment in IL-22 production. Generation and functions of gut NK1.1+ cells were also altered. Bone marrow chimera experiments revealed a cell-intrinsic requirement for T-bet in these subsets and competitive reconstitution experiments revealed roles for T-bet in multiple ILC subsets. Thus, T-bet has a general importance for ILC in the gut and plays a selective and critical role in the generation of NKp46+ ILC22.
CD4+ T cells differentiate into multiple effector types, but it is unclear how they form memory T cells during infection in vivo. Profiling virus-specific CD4+ T cells revealed that effector cells with T helper 1 (Th1) or T follicular helper (Tfh) cell characteristics differentiated into memory cells, although expression of Tfh cell markers declined over time. In contrast to virus-specific effector CD8+ T cells, increased IL-7R expression was not a reliable marker of CD4+ memory precursor cells. However, decreased Ly6C and T-bet (Tbx21) expression distinguished a subset of Th1 cells that displayed greater longevity and proliferative responses to secondary infection. Moreover, the gene expression profile of Ly6CloT-betint Th1 effector cells was virtually identical to mature memory CD4+ T cells, indicating early maturation of memory CD4+ T cell features in this subset during acute viral infection. This study provides a framework for memory CD4+ T cell development after acute viral infection.
CD4 T follicular helper (TFH) cells interact with and stimulate the generation of antigen-specific B cells. TFH cell interaction with B cells correlates with production of SIV-specific immunoglobulins. However, the fate of TFH cells and their participation in SIV-induced antibody production is not well understood. We investigated the phenotype, function, location, and molecular signature of TFH cells in rhesus macaques. Similar to their human counterparts, TFH cells in rhesus macaques represented a heterogeneous population with respect to cytokine function. In a highly differentiated subpopulation of TFH cells, characterized by CD150lo expression, production of Th1 cytokines was compromised while IL-4 production was augmented, and cells exhibited decreased survival, cycling, and trafficking capacity. TFH cells exhibited a distinct gene profile that was markedly altered by SIV infection. TFH cells were infected by SIV; yet, in some animals, these cells actually accumulated during chronic SIV infection. Generalized immune activation and increased IL-6 production helped drive TFH differentiation during SIV infection. Accumulation of TFH cells was associated with increased frequency of activated germinal center B cells and SIV-specific antibodies. Therefore, chronic SIV does not disturb the ability of TFH cells to help B cell maturation and production of SIV-specific immunoglobulins.
Follicular helper T (TFH) cells, defined by expression of the surface markers CXCR5 and PD-1 and synthesis of IL-21, require upregulation of the transcriptional repressor Bcl6 for their development and function in B cell maturation in germinal centers. We have explored the role of B cells, and the cytokines IL-6 and IL-21, in the in vivo regulation of Bcl6 expression and TFH cell development. We found that TFH cells are characterized by a Bcl6-dependent downregulation of P-selectin glycoprotein ligand-1 (PSGL1, a CCL19- and CCL21-binding protein), indicating that, like CXCR5 and PD-1 upregulation, modulation of PSGL1 expression is part of the TFH cell program of differentiation. B cells were neither required for initial upregulation of Bcl6 nor PSGL1 downregulation, suggesting these events preceded T-B cell interactions, although they were required for full development of the TFH cell phenotype, including CXCR5 and PD-1 upregulation, and IL-21 synthesis. Bcl6 upregulation and TFH cell differentiation were independent of IL-6 and IL-21, revealing that either cytokine is not absolutely required for development of Bcl6+ TFH cells in vivo. These data increase our understanding of Bcl6 regulation in TFH cells and their differentiation in vivo, and identifies a new surface marker that may be functionally relevant in this subset.
Effective B cell–mediated immunity and antibody responses often require help from CD4+ T cells. It is thought that a distinct CD4+ effector T cell subset, called T follicular helper cells (TFH), provides this help; however, the molecular requirements for TFH differentiation are unknown. We found that expression of the transcription factor Bcl6 in CD4+ T cells is both necessary and sufficient for in vivo TFH differentiation and T cell help to B cells in mice. In contrast, the transcription factor Blimp-1, an antagonist of Bcl6, inhibits TFH differentiation and help, thereby preventing B cell germinal center and antibody responses. These findings demonstrate that TFH cells are required for proper B cell responses in vivo and that Bcl6 and Blimp-1 play central but opposing roles in TFH differentiation.
Interleukin 17 (IL-17)-producing CD4+ T (TH-17) cells share a developmental relationship with FoxP3+ regulatory T (Treg) cells. Here we show that a TH-17 population differentiates within the thymus in a manner influenced by self-antigen recognition, and by the cytokines IL-6 and transforming growth factor (TGF)-β. Like previously described TH-17 cells, TH-17 cells that develop in the thymus expressed the orphan nuclear receptor RORγt and the IL-23 receptor. These cells also expressed α4β1 integrins and the chemokine receptor CCR6, and were recruited to the lung, gut, and liver. In the liver these cells secreted IL-22 in response to self-antigen and mediated host protection during inflammation. Thus, TH-17 cells, like Treg cells, can be selected by self-antigens in the thymus.
The role of specialized follicular helper T (TFH) cells in the germinal center has become well recognized, but it is less clear how effector T cells govern the extrafollicular response, the dominant pathway of high-affinity, isotype-switched autoantibody production in the MRL/MpJ-Faslpr (MRLlpr) mouse model of lupus. MRLlpr mice lacking the Icos gene have impaired extrafollicular differentiation of immunoglobulin (Ig) G+ plasma cells accompanied by defects in CXC chemokine receptor (CXCR) 4 expression, interleukin (IL) 21 secretion, and B cell helper function in CD4 T cells. These phenotypes reflect the selective loss of a population of T cells marked by down-regulation of P-selectin glycoprotein ligand 1 (PSGL-1; also known as CD162). PSGL-1lo T cells from MRLlpr mice express CXCR4, localize to extrafollicular sites, and uniquely mediate IgG production through IL-21 and CD40L. In other autoimmune strains, PSGL-1lo T cells are also abundant but may exhibit either a follicular or extrafollicular phenotype. Our findings define an anatomically distinct extrafollicular population of cells that regulates plasma cell differentiation in chronic autoimmunity, indicating that specialized humoral effector T cells akin to TFH cells can occur outside the follicle.
Live attenuated vectors based on recombinant vesicular stomatitis viruses (rVSVs) expressing foreign antigens are highly effective vaccines in animal models. In this study, we report that an rVSV (VSV-GMCSF1) expressing high levels of murine granulocyte-macrophage colony-stimulating factor (GM-CSF) from the first position in the viral genome is highly attenuated in terms of viral dissemination and pathogenesis after intranasal delivery to mice. However, this highly attenuated virus generated antibody and T-cell responses equivalent to those induced by a control virus expressing enhanced green fluorescent protein (EGFP) from the first position (VSV-EGFP1). The better containment and clearance of VSV-GMCSF1 may be due to enhanced recruitment of macrophages to the site of infection but is not explained by a greater induction of interferons. The primary CD8 T-cell and neutralizing antibody responses to VSV-GMCSF1 were equivalent to those generated by VSV-EGFP1, while the CD8 T-cell memory and recall responses to the vector were enhanced in mice infected with VSV-GMCSF1. It is likely that the GM-CSF produced by immunization with this virus results in an enhanced recruitment of antigen-presenting cells, leading to better acute and long-term T-cell responses. This recruitment appears to cancel out any negative effect of viral attenuation on immunogenicity.