Effective CD4 T cell priming requires repertoire scanning by CD301b+ migratory cDC2 cells upon lymph node entry

Innate cells and STAT1-dependent signals orchestrate vaccine-induced protection against invasive Cryptococcus infection

Fungal pathogens are underappreciated causes of significant morbidity and mortality worldwide. In previous studies, we determined that a heat-killed, Cryptococcus neoformans fbp1-deficient strain (HK-fbp1) is a potent vaccine candidate. We determined that vaccination with HK-fbp1 confers protective immunity against lethal Cryptococcosis in an interferon γ (IFNγ)-dependent manner. In this study, we set out to uncover cellular sources and relevant targets of the protective effects of IFNγ in response to the HK-fbp1 vaccine. We found that early IFNγ production peaks at day 3 and that monocytes and neutrophils are important sources of this cytokine after vaccination. Neutralization of IFNγ at day 3 results in impaired CCR2+ monocyte recruitment and reduced differentiation into monocyte-derived dendritic cells (Mo-DC). In turn, depletion of CCR2+ cells prior to immunization results in impaired activation of IFNγ-producing CD4 and CD8 T cells. Thus, monocytes are important targets of innate IFNγ and help promote further IFNγ production by lymphocytes. We employed monocyte-fate mapper and conditional STAT1 knockout mice to uncover that STAT1 activation in CD11c+ cells, including alveolar macrophages, Mo-DCs, and monocyte-derived macrophages (Mo-Mac) is essential for HK-fbp1 vaccine-induced protection. Altogether, our aggregate findings suggest critical roles for innate cells as orchestrators of vaccine-induced protection against Cryptococcus infection.IMPORTANCEThe number of patients susceptible to invasive fungal infections across the world continues to rise at an alarming pace yet current antifungal drugs are often inadequate. Immune-based interventions and novel antifungal vaccines hold the promise of significantly improving patient outcomes. In previous studies, we identified a Cryptococcus neoformans mutant strain (Fbp1-deficient) as a potent, heat-inactivated vaccine candidate capable of inducing homologous and heterologous antifungal protection. In this study, we used a combination of methods together with a cohort of conditional knockout mouse strains to interrogate the roles of innate cells in the orchestration of vaccine-induced antifungal protection. We uncovered novel roles for neutrophils and monocytes as coordinators of a STAT1-dependent cascade of responses that mediate vaccine-induced protection against invasive cryptococcosis. This new knowledge will help guide the future development of much-needed antifungal vaccines.

How type-2 dendritic cells induce Th2 differentiation: Instruction, repression, or fostering T cell-T cell communication?

Allergic disease is caused by the activation of allergen-specific CD4+ type-2 T follicular helper cells (Tfh2) and T helper 2 (Th2) effector cells that secrete the cytokines IL-4, IL-5, IL-9, and IL-13 upon allergen encounter, thereby inducing IgE production by B cells and tissue inflammation. While it is accepted that the priming and differentiation of naïve CD4+ T cells into Th2 requires allergen presentation by type 2 dendritic cells (DC2s), the underlying signals remain unidentified. In this review we focus on the interaction between allergen-presenting DC2s and naïve CD4+ T cells in lymph node (LN), and the potential mechanisms by which DC2s might instruct Th2 differentiation. We outline recent advances in characterizing DC2 development and heterogeneity. We review mechanisms of allergen sensing and current proposed mechanisms of Th2 differentiation, with specific consideration of the role of DC2s and how they might contribute to each mechanism. Finally, we assess recent publications reporting a detailed analysis of DC-T cell interactions in LNs and how they support Th2 differentiation. Together, these studies are starting to shape our understanding of this key initial step of the allergic immune response.

Dendritic cells steering antigen and leukocyte traffic in lymph nodes

Dendritic cells (DCs) play a central role in initiating and shaping the adaptive immune response, thanks to their ability to uptake antigens and present them to T cells. Once in the lymph node (LN), DCs can spread the antigen to other DCs, expanding the pool of cells capable of activating specific T-cell clones. Additionally, DCs can modulate the dynamics of other immune cells, by increasing naïve T-cell dwell time, thereby facilitating the scanning for cognate antigens, and by selectively recruiting other leukocytes. Here we discuss the role of DCs in orchestrating antigen and leukocyte trafficking within the LN, together with the implications of this trafficking on T-cell activation and commitment to effector function.

Macro-clusters: CD301b+ DCs prime Th2 responses

In this issue of JEM, Lyons-Cohen et al. (https://doi.org/10.1084/jem.20231282) reveal that lymph node macro-clusters provide a spatial niche where CD301b+ cDC2s and CD4+ T cells interact. These integrin-mediated cellular hubs promote enhanced co-stimulation and cytokine signaling to drive Th2 differentiation.

Clec12A, CD301b, and FcγRIIB/III define the heterogeneity of murine DC2s and DC3s

Over the last decade, multiple studies have investigated the heterogeneity of murine conventional dendritic cells type 2 (cDC2s). However, their phenotypic similarity with monocytes and macrophages renders their clear identification challenging. By creating a protein atlas utilizing multiparameter flow cytometry, we show that ESAM+ cDC2s are a specialized feature of the spleen strongly differing in their proteome from other cDC2s. In contrast, all other tissues are populated by Clec12A+ cDC2s or Clec12A- cDC2s (high or low for Fcγ receptors, C-type lectin receptors, and CD11b, respectively), rendering Clec12A+ cDC2s classical sentinels. Further, expression analysis of CD301b, Clec12A, and FcγRIIB/III provides a conserved definition of cDC2 heterogeneity, including the discovery of putative FcγRIIB/III+ DC3s across tissues. Finally, our data reveal that cell identity (ontogeny) dictates the proteome that is further fine-tuned by the tissue environment on macrophages and dendritic cells (DCs), while monocytes and plasmacytoid DCs (pDCs) display subset intrinsic default settings.

Dendritic cells: the yin and yang in disease progression

Dendritic cells (DCs) are antigen presenting cells that link innate and adaptive immunity. DCs have been historically considered as the most effective and potent cell population to capture, process and present antigens to activate naïve T cells and originate favorable immune responses in many diseases, such as cancer. However, in the last decades, it has been observed that DCs not only promote beneficial responses, but also drive the initiation and progression of some pathologies, including inflammatory bowel disease (IBD). In line with those notions, different therapeutic approaches have been tested to enhance or impair the concentration and role of the different DC subsets. The blockade of inhibitory pathways to promote DCs or DC-based vaccines have been successfully assessed in cancer, whereas the targeting of DCs to inhibit their functionality has proved to be favorable in IBD. In this review, we (a) described the general role of DCs, (b) explained the DC subsets and their role in immunogenicity, (c) analyzed the role of DCs in cancer and therapeutic approaches to promote immunogenic DCs and (d) analyzed the role of DCs in IBD and therapeutic approaches to reduced DC-induced inflammation. Therefore, we aimed to highlight the "yin-yang" role of DCs to improve the understand of this type of cells in disease progression.

Rapid activation of IL-2 receptor signaling by CD301b+ DC-derived IL-2 dictates the outcome of helper T cell differentiation

Effector T helper (Th) cell differentiation is fundamental to functional adaptive immunity. Different subsets of dendritic cells (DCs) preferentially induce different types of Th cells, but the fate instruction mechanism for Th type 2 (Th2) differentiation remains enigmatic, as the critical DC-derived cue has not been clearly identified. Here, we show that CD301b+ DCs, a major Th2-inducing DC subset, drive Th2 differentiation through cognate interaction by 'kick-starting' IL-2 receptor signaling in CD4T cells. Mechanistically, CD40 engagement induces IL-2 production selectively from CD301b+ DCs to maximize CD25 expression in CD4 T cells, which is required specifically for the Th2 fate decision. On the other hand, CD25 in CD301b+ DCs facilitates directed action of IL-2 toward cognate CD4T cells. Furthermore, CD301b+ DC-derived IL-2 skews CD4T cells away from the T follicular helper fate. These results highlight the critical role of DC-intrinsic CD40-IL-2 axis in bifurcation of Th cell fate.

Role of mouse dendritic cell subsets in priming naive CD4 T cells

Conventional dendritic cells (cDCs) are potent antigen-presenting cells that consist of developmentally, phenotypically, and functionally distinct subsets. Following immunization, each subset of cDCs acquires the antigen and presents it to CD4T (CD4+ T (cells)) cells with distinct spatiotemporal kinetics in the secondary lymphoid organs, often causing multiple waves of antigen presentation to CD4T cells. Here, we review the current understanding of the kinetics of antigen presentation by each cDC subset and its functional consequences in priming naive CD4T cells, and discuss its implications in the differentiation of CD4T cells.

Long-term tolerance to skin commensals is established neonatally through a specialized dendritic cell subgroup

Early-life establishment of tolerance to commensal bacteria at barrier surfaces carries enduring implications for immune health but remains poorly understood. Here, we showed that tolerance in skin was controlled by microbial interaction with a specialized subset of antigen-presenting cells. More particularly, CD301b+ type 2 conventional dendritic cells (DCs) in neonatal skin were specifically capable of uptake and presentation of commensal antigens for the generation of regulatory T (Treg) cells. CD301b+ DC2 were enriched for phagocytosis and maturation programs, while also expressing tolerogenic markers. In both human and murine skin, these signatures were reinforced by microbial uptake. In contrast to their adult counterparts or other early-life DC subsets, neonatal CD301b+ DC2 highly expressed the retinoic-acid-producing enzyme, RALDH2, the deletion of which limited commensal-specific Treg cell generation. Thus, synergistic interactions between bacteria and a specialized DC subset critically support early-life tolerance at the cutaneous interface.

Pulmonary inflammation promoted by type-2 dendritic cells is a feature of human and murine schistosomiasis

Schistosomiasis is a parasitic disease affecting over 200 million people in multiple organs, including the lungs. Despite this, there is little understanding of pulmonary immune responses during schistosomiasis. Here, we show type-2 dominated lung immune responses in both patent (egg producing) and pre-patent (larval lung migration) murine Schistosoma mansoni (S. mansoni) infection. Human pre-patent S. mansoni infection pulmonary (sputum) samples revealed a mixed type-1/type-2 inflammatory cytokine profile, whilst a case-control study showed no significant pulmonary cytokine changes in endemic patent infection. However, schistosomiasis induced expansion of pulmonary type-2 conventional dendritic cells (cDC2s) in human and murine hosts, at both infection stages. Further, cDC2s were required for type-2 pulmonary inflammation in murine pre-patent or patent infection. These data elevate our fundamental understanding of pulmonary immune responses during schistosomiasis, which may be important for future vaccine design, as well as for understanding links between schistosomiasis and other lung diseases.

Protocol to quantify and characterize contact between T cells and antigen-presenting cells in the antigen-draining lymph nodes of mice using flow cytometry

Physical contact between T cells and antigen-presenting cells (APCs) is essential for priming antigen-specific T cells, but quantitating the antigen-dependent T cell-APC contact can be laborious. Here, we present a simple flow-cytometry-based protocol for quantitating T cell-APC contacts in the antigen-draining lymph node in mice immunized with ovalbumin (OVA). This protocol quantifies the contact between adoptively transferred OVA-specific TCR transgenic CD4T (OT-II) cells and dendritic cell (DC) subsets. This approach can be applied to other types of intercellular interactions between T cells and APCs. For complete details on the use and execution of this protocol, please refer to Tatsumi et al. (2021).1.

Type 2 cytokines in the thymus activate Sirpα+ dendritic cells to promote clonal deletion

The thymus contains a diversity of dendritic cells (DCs) that exist in defined locations and have different antigen-processing and -presenting features. This suggests that they play nonredundant roles in mediating thymocyte selection. In an effort to eliminate SIRPα+ classic DC2 subsets, we discovered that a substantial proportion expresses the surface lectin, CD301b, in the thymus. These cells resemble the CD301b+ type 2 immune response promoting DCs that are present in the skin-draining lymph nodes. Transcriptional and phenotypic comparison to other DC subsets in the thymus revealed that thymic CD301b+ cDCs represent an activated state that exhibits enhanced antigen processing and presentation. Furthermore, a CD301b+ cDC2 subset demonstrated a type 2 cytokine signature and required steady-state interleukin-4 receptor signaling. Selective ablation of CD301b+ cDC2 subsets impaired clonal deletion without affecting regulatory T cells (Treg cells). The T cell receptor α repertoire sequencing confirmed that a cDC2 subset promotes deletion of conventional T cells with minimal effect on Treg cell selection. Together, these findings suggest that cytokine-induced activation of DCs in the thymus substantially enforces central tolerance.