Dirk Baumjohann

Persistent Antigen and Germinal Center B Cells Sustain T Follicular Helper Cell Responses and Phenotype

T follicular helper (Tfh) cells provide help to B cells and are crucial for establishment of germinal center (GC) reactions, including production of high-affinity antibodies and generation of memory B cells and long-lived plasma cells. Here we report that the magnitude of the Tfh cell response was dictated by the amount of antigen and directly correlated with the magnitude of the GC B cell response. In addition, maintenance of the Tfh cell phenotype required sustained antigenic stimulation by GC B cells. In lymphopenic conditions, a strong and prolonged Tfh cell response led to bystander B cell activation, hypergammaglobulinemia, and production of poly- and self-reactive antibodies. These data demonstrate that antigen dose determines the size and duration of the Tfh cell response and GC reaction, highlight the transient nature of the Tfh cell phenotype, and suggest a link between overstimulation of Tfh cells and the development of dysregulated humoral immune responses.

MicroRNA-mediated regulation of T helper cell differentiation and plasticity

CD4+ T helper (TH) cells regulate appropriate cellular and humoral immune responses to a wide range of pathogens and are central to the success of vaccines. However, their dysregulation can cause allergies and autoimmune diseases. The CD4+ T cell population is characterized not only by a range of distinct cell subsets, such as TH1, TH2 and TH17 cells, regulatory T cells and T follicular helper cells — each with specific functions and gene expression programmes — but also by plasticity between the different TH cell subsets. In this Review, we discuss recent advances and emerging ideas about how microRNAs — small endogenously expressed oligonucleotides that modulate gene expression — are involved in the regulatory networks that determine TH cell fate decisions and that regulate their effector functions.

The concerted action of GM-CSF and Flt3-ligand on in vivo dendritic cell homeostasis

Dendritic cell (DC) development is efficiently supported by Flt3-ligand or GM-CSF in vitro, and lymphoid-organ DC maintenance in vivo is critically dependent on Flt3-ligand. However, the relevance of GM-CSF for lymphoid-tissue DC maintenance and the importance of both cytokines for nonlymphoid organ DC homeostasis are not defined. Here, we show that, although Gm-csfr and Flt3 are both expressed in DC progenitors, Gm-csfr is expressed predominantly in monocytes, classical DCs (cDCs), and skin DCs, whereas Flt3 is expressed in both cDCs and plasmacytoid DCs (pDCs). In accordance with the respective cytokine receptor expression, DC progenitor and pDC numbers are primarily affected by Flt3-ligand deficiency, whereas both splenic and lymph node cDCs and dermal DCs are reduced in the absence of either GM-CSF or Flt3-ligand. Combined lack of GM-CSF and Flt3-ligand in newly generated double-deficient mice leads to further significant reductions of DC progenitors and dermal DCs. In line with the decrease of respective DC subsets, T-cell and antigen-specific IgG responses decline progressively, from wild-type to GM-CSF- to Flt3-ligand- to double-deficient mice, upon subcutaneous antigen delivery. These data thus show the concerted action of GM-CSF and Flt3-ligand on DC homeostasis in vivo.

Cutting Edge: Distinct Waves of BCL6 Expression during T Follicular Helper Cell Development

T follicular helper (TFH) cells are central to the development and regulation of T cell-dependent humoral immune responses. The transcriptional repressor BCL6 is required for TFH responses, but the kinetics of BCL6 protein expression in activated CD4+ T cells have not been established. We measured BCL6 expression during TFH cell development at the single-cell level using intracellular staining and YFP-BCL6 fusion protein reporter mice. BCL6 was immediately upregulated in all dividing T cells during dendritic cell–T cell interactions. A second wave of early BCL6 expression coincided with the induction of CXCR5, resulting in a distinct and stable TFH cell population. Cognate B cells were not required for the induction of BCL6, but supported the expansion of TFH cells. These data suggest that BCL6 participates in very early events in TFH cell development, and that repeated encounters with APCs reinforce BCL6 expression, thereby establishing the TFH cell phenotype.

Comparative transcriptional and functional profiling defines conserved programs of intestinal DC differentiation in humans and mice

Dendritic cells (DCs) that orchestrate mucosal immunity have been studied in mice. Here we characterized human gut DC populations and defined their relationship to previously studied human and mouse DCs. CD103+Sirpα− DCs were related to human blood CD141+ DCs and to mouse intestinal CD103+CD11b− DCs and expressed markers of cross-presenting DCs. CD103+Sirpα+ DCs aligned with human blood CD1c+ DCs and mouse intestinal CD103+CD11b+ DCs and supported the induction of regulatory T cells. Both CD103+ DC subsets induced the TH17 subset of helper T cells, while CD103−Sirpα+ DCs induced the TH1 subset of helper T cells. Comparative analysis of transcriptomes revealed conserved transcriptional programs among CD103+ DC subsets and identified a selective role for the transcriptional repressors Bcl-6 and Blimp-1 in the specification of CD103+CD11b− DCs and intestinal CD103+CD11b+ DCs, respectively. Our results highlight evolutionarily conserved and divergent programming of intestinal DCs.Dendritic cells (DCs) that orchestrate mucosal immunity have been studied in mice. Here we characterize human gut DC populations, and define their relationship to previously studied human and mouse DCs. CD103+Sirpα− DCs were related to human blood CD141+ and to mouse intestinal CD103+CD11b− DCs and expressed markers of cross-presenting DCs. CD103+Sirpα+ DCs aligned with human blood CD1c+ DCs and mouse intestinal CD103+CD11b+ DCs and supported regulatory T cell induction. Both CD103+ DC subsets induced TH17 cells, while CD103−Sirpα+ DCs induced TH1 cells. Comparative transcriptomics revealed conserved transcriptional programs among CD103+ DC subsets and uncovered a selective role for Bcl-6 and Blimp-1 in CD103+Sirpα− and intestinal CD103+CD11b+ DC specification, respectively. These results highlight evolutionarily conserved and divergent programming of intestinal DCs.

A microRNA upregulated in asthma airway T cells promotes TH2 cytokine production

MicroRNAs (miRNAs) exert powerful effects on immunological function by tuning networks of target genes that orchestrate cell activity. We sought to identify miRNAs and miRNA-regulated pathways that control the type 2 helper T cell (TH2 cell) responses that drive pathogenic inflammation in asthma. Profiling miRNA expression in human airway-infiltrating T cells revealed elevated expression of the miRNA miR-19a in asthma. Modulating miR-19 activity altered TH2 cytokine production in both human and mouse T cells, and TH2 cell responses were markedly impaired in cells lacking the entire miR-17∼92 cluster. miR-19 promoted TH2 cytokine production and amplified inflammatory signaling by direct targeting of the inositol phosphatase PTEN, the signaling inhibitor SOCS1 and the deubiquitinase A20. Thus, upregulation of miR-19a in asthma may be an indicator and a cause of increased TH2 cytokine production in the airways.

In vivo magnetic resonance imaging of dendritic cell migration into the draining lymph nodes of mice

Dendritic cell (DC) migration into the draining lymph nodes is critical for T cell priming. Here, we show that magnetic resonance imaging (MRI) can be used to visualize DC migration in vivo. We combined clinically approved small particles of iron oxide (SPIO) with protamine sulfate to achieve efficient uptake by murine bone marrow‐derived DC. SPIO‐DC were largely unaltered and after injection into the footpads of mice, they migrated into the T cell areas of the draining lymph nodes, which could be visualized by MRI. Distinct MRI signal reduction patterns correlated with the detection of SPIO‐DC mainly within Thy‐1.2+ B220– T cell areas, as confirmed by iron staining and immunohistology. Clear signal reduction patterns could still be observed with 1 × 106 injected SPIO‐DC at high resolution, resulting in the detection of about 2000 DC. Control injections of homing‐incompetent SPIO‐DC derived from CCR7–/– mice or SPIO alone did not reach the T cell areas. Taken together, the results demonstrate that clinically approved contrast agents allow the non‐invasive visualization of DC migration into the draining lymph node by MRI in vivo at high resolution. This protocol therefore also allows dynamic imaging of immune responses and MRI‐based tracking of human DC in patients.

CD40L+ CD4+ memory T cells migrate in a CD62P-dependent fashion into reactive lymph nodes and license dendritic cells for T cell priming

There is growing evidence that the maturation state of dendritic cells (DCs) is a critical parameter determining the balance between tolerance and immunity. We report that mouse CD4+ effector memory T (TEM) cells, but not naive or central memory T cells, constitutively expressed CD40L at levels sufficient to induce DC maturation in vitro and in vivo in the absence of antigenic stimulation. CD4+ TEM cells were excluded from resting lymph nodes but migrated in a CD62P-dependent fashion into reactive lymph nodes that were induced to express CD62P, in a transient or sustained fashion, on high endothelial venules. Trafficking of CD4+ TEM cells into chronic reactive lymph nodes maintained resident DCs in a mature state and promoted naive T cell responses and experimental autoimmune encephalomyelitis (EAE) to antigens administered in the absence of adjuvants. Antibodies to CD62P, which blocked CD4+ TEM cell migration into reactive lymph nodes, inhibited DC maturation, T cell priming, and induction of EAE. These results show that TEM cells can behave as endogenous adjuvants and suggest a mechanistic link between lymphocyte traffic in lymph nodes and induction of autoimmunity.

Self-Enforcing Feedback Activation between BCL6 and Pre-B Cell Receptor Signaling Defines a Distinct Subtype of Acute Lymphoblastic Leukemia

Studying 830 pre-B ALL cases from four clinical trials, we found that human ALL can be divided into two fundamentally distinct subtypes based on pre-BCR function. While absent in the majority of ALL cases, tonic pre-BCR signaling was found in 112 cases (13.5%). In these cases, tonic pre-BCR signaling induced activation of BCL6, which in turn increased pre-BCR signaling output at the transcriptional level. Interestingly, inhibition of pre-BCR-related tyrosine kinases reduced constitutive BCL6 expression and selectively killed patient-derived pre-BCR(+) ALL cells. These findings identify a genetically and phenotypically distinct subset of human ALL that critically depends on tonic pre-BCR signaling. In vivo treatment studies suggested that pre-BCR tyrosine kinase inhibitors are useful for the treatment of patients with pre-BCR(+) ALL.

Experimental priming of encephalitogenic Th1/Th17 cells requires pertussis toxin-driven IL-1 beta production by myeloid cells

CD4+ Th17 are heterogeneous in terms of cytokine production and capacity to initiate autoimmune diseases, such as experimental autoimmune encephalomyelitis (EAE). Here we demonstrate that experimental priming of encephalitogenic Th cells expressing RORγt and T-bet and producing IL-17A, IFN-γ and GM-CSF but not IL-10 (Th1/Th17), is dependent on the presence of pertussis toxin (PTX) at the time of immunization. PTX induces early production of IL-1β by CD11b+CCR2+Gr1+ myeloid cells, which are rapidly recruited to antigen-draining lymph nodes. PTX-induced generation of Th1/Th17 cells is impaired in IL-1β- and ASC-deficient mice and in mice in which myeloid cells are depleted or fail to migrate to lymph nodes and requires expression of IL-1R1 and MyD88 on both T cells and non-T cells. Collectively, these data shed light on the enigmatic function of PTX in EAE induction and suggest that inflammatory monocytes and microbial infection can influence differentiation of pathogenic Th1/Th17 cells in autoimmune diseases through production of IL-1β.