Quantitative interactomics in primary T cells unveils TCR signal diversification extent and dynamics.

The activation of T cells by the T cell antigen receptor (TCR) results in the formation of signaling protein complexes (signalosomes), the composition of which has not been analyzed at a systems level. Here, we isolated primary CD4+ T cells from 15 gene-targeted mice, each expressing one tagged form of a canonical protein of the TCR-signaling pathway. Using affinity purification coupled with mass spectrometry, we analyzed the composition and dynamics of the signalosomes assembling around each of the tagged proteins over 600 s of TCR engagement. We showed that the TCR signal-transduction network comprises at least 277 unique proteins involved in 366 high-confidence interactions, and that TCR signals diversify extensively at the level of the plasma membrane. Integrating the cellular abundance of the interacting proteins and their interaction stoichiometry provided a quantitative and contextual view of each documented interaction, permitting anticipation of whether ablation of a single interacting protein can impinge on the whole TCR signal-transduction network.

Single-cell transcriptomics uncovers an instructive T-cell receptor role in adult γδ T-cell lineage commitment.

After entering the adult thymus, bipotent T-cell progenitors give rise to αß or γδ T cells. To determine whether the γδ T-cell receptor (TCR) has an instructive role in γδ T-cell lineage commitment or only "confirms" a pre-established γδ Τ-cell lineage state, we exploited mice lacking expression of LAT, an adaptor required for γδ TCR signaling. Although these mice showed a T-cell development block at the CD4- CD8- double-negative third (DN3) stage, 0.3% of their DN3 cells expressed intermediate levels of γδ TCR (further referred to as γδint ) at their surface. Single-cell transcriptomics of LAT-deficient DN3 γδint cells demonstrated no sign of commitment to the γδ T-cell lineage, apart from γδ TCR expression. Although the lack of LAT is thought to tightly block DN3 cell development, we unexpectedly found that 25% of LAT-deficient DN3 γδint cells were actively proliferating and progressed up to the DN4 stage. However, even those cells failed to turn on the transcriptional program associated with the γδ T-cell lineage. Therefore, the γδ TCR-LAT signaling axis builds upon a γδ T-cell uncommitted lineage state to fully instruct adult γδ T-cell lineage specification.

Broad and Largely Concordant Molecular Changes Characterize Tolerogenic and Immunogenic Dendritic Cell Maturation in Thymus and Periphery.

Dendritic cells (DCs) are instrumental in the initiation of T cell responses, but how thymic and peripheral tolerogenic DCs differ globally from Toll-like receptor (TLR)-induced immunogenic DCs remains unclear. Here, we show that thymic XCR1(+) DCs undergo a high rate of maturation, accompanied by profound gene-expression changes that are essential for central tolerance and also happen in germ-free mice. Those changes largely overlap those occurring during tolerogenic and, more unexpectedly, TLR-induced maturation of peripheral XCR1(+) DCs, arguing against the commonly held view that tolerogenic DCs undergo incomplete maturation. Interferon-stimulated gene (ISG) expression was among the few discriminators of immunogenic and tolerogenic XCR1(+) DCs. Tolerogenic XCR1(+) thymic DCs were, however, unique in expressing ISGs known to restrain virus replication. Therefore, a broad functional convergence characterizes tolerogenic and immunogenic XCR1(+) DC maturation in the thymus and periphery, maximizing antigen presentation and signal delivery to developing and to conventional and regulatory mature T cells.

Unsupervised High-Dimensional Analysis Aligns Dendritic Cells across Tissues and Species.

Dendritic cells (DCs) are professional antigen-presenting cells that hold great therapeutic potential. Multiple DC subsets have been described, and it remains challenging to align them across tissues and species to analyze their function in the absence of macrophage contamination. Here, we provide and validate a universal toolbox for the automated identification of DCs through unsupervised analysis of conventional flow cytometry and mass cytometry data obtained from multiple mouse, macaque, and human tissues. The use of a minimal set of lineage-imprinted markers was sufficient to subdivide DCs into conventional type 1 (cDC1s), conventional type 2 (cDC2s), and plasmacytoid DCs (pDCs) across tissues and species. This way, a large number of additional markers can still be used to further characterize the heterogeneity of DCs across tissues and during inflammation. This framework represents the way forward to a universal, high-throughput, and standardized analysis of DC populations from mutant mice and human patients.

High-dimensional mass cytometry analysis of NK cell alterations in AML identifies a subgroup with adverse clinical outcome.

Natural killer (NK) cells are major antileukemic immune effectors. Leukemic blasts have a negative impact on NK cell function and promote the emergence of phenotypically and functionally impaired NK cells. In the current work, we highlight an accumulation of CD56-CD16+ unconventional NK cells in acute myeloid leukemia (AML), an aberrant subset initially described as being elevated in patients chronically infected with HIV-1. Deep phenotyping of NK cells was performed using peripheral blood from patients with newly diagnosed AML (n = 48, HEMATOBIO cohort, NCT02320656) and healthy subjects (n = 18) by mass cytometry. We showed evidence of a moderate to drastic accumulation of CD56-CD16+ unconventional NK cells in 27% of patients. These NK cells displayed decreased expression of NKG2A as well as the triggering receptors NKp30 and NKp46, in line with previous observations in HIV-infected patients. High-dimensional characterization of these NK cells highlighted a decreased expression of three additional major triggering receptors required for NK cell activation, NKG2D, DNAM-1, and CD96. A high proportion of CD56-CD16+ NK cells at diagnosis was associated with an adverse clinical outcome and decreased overall survival (HR = 0.13; P = 0.0002) and event-free survival (HR = 0.33; P = 0.018) and retained statistical significance in multivariate analysis. Pseudotime analysis of the NK cell compartment highlighted a disruption of the maturation process, with a bifurcation from conventional NK cells toward CD56-CD16+ NK cells. Overall, our data suggest that the accumulation of CD56-CD16+ NK cells may be the consequence of immune escape from innate immunity during AML progression.

Self-antigen-driven activation induces instability of regulatory T cells during an inflammatory autoimmune response.

Stable Foxp3 expression is crucial for regulatory T (Treg) cell function. We observed that antigen-driven activation and inflammation in the CNS promoted Foxp3 instability selectively in the autoreactive Treg cells that expressed high amounts of Foxp3 before experimental autoimmune encephalitis induction. Treg cells with a demethylated Treg-cell-specific demethylated region in the Foxp3 locus downregulated Foxp3 transcription in the inflamed CNS during the induction phase of the response. Stable Foxp3 expression returned at the population level with the resolution of inflammation or was rescued by IL-2-anti-IL-2 complex treatment during the antigen priming phase. Thus, a subset of fully committed self-antigen-specific Treg cells lost Foxp3 expression during an inflammatory autoimmune response and might be involved in inadequate control of autoimmunity. These results have important implications for Treg cell therapies and give insights into the dynamics of the Treg cell network during autoreactive CD4(+) T cell effector responses in vivo.

Origins and functional specialization of macrophages and of conventional and monocyte-derived dendritic cells in mouse skin.

In the skin, the lack of markers permitting the unambiguous identification of macrophages and of conventional and monocyte-derived dendritic cells (DCs) complicates understanding of their contribution to skin integrity and to immune responses. By combining CD64 and CCR2 staining, we successfully identified each of these cell types and studied their origin, transcriptomic signatures, and migratory and T cell stimulatory properties. We also analyzed the impact of microbiota on their development and their contribution to skin inflammation during contact hypersensitivity. Dermal macrophages had a unique scavenging role and were unable to migrate and activate T cells. Conventional dermal DCs excelled both at migrating and activating T cells. In the steady-state dermis, monocyte-derived DCs are continuously generated by extravasated Ly-6C(hi) monocytes. Their T cell stimulatory capacity combined with their poor migratory ability made them particularly suited to activate skin-tropic T cells. Therefore, a high degree of functional specialization occurs among the mononuclear phagocytes of the skin.

LymphoAtlas: a dynamic and integrated phosphoproteomic resource of TCR signaling in primary T cells reveals ITSN2 as a regulator of effector functions.

T-cell receptor (TCR) ligation-mediated protein phosphorylation regulates the activation, cellular responses, and fates of T cells. Here, we used time-resolved high-resolution phosphoproteomics to identify, quantify, and characterize the phosphorylation dynamics of thousands of phosphorylation sites in primary T cells during the first 10 min after TCR stimulation. Bioinformatic analysis of the data revealed a coherent orchestration of biological processes underlying T-cell activation. In particular, functional modules associated with cytoskeletal remodeling, transcription, translation, and metabolic processes were mobilized within seconds after TCR engagement. Among proteins whose phosphorylation was regulated by TCR stimulation, we demonstrated, using a fast-track gene inactivation approach in primary lymphocytes, that the ITSN2 adaptor protein regulated T-cell effector functions. This resource, called LymphoAtlas, represents an integrated pipeline to further decipher the organization of the signaling network encoding T-cell activation. LymphoAtlas is accessible to the community at: https://bmm-lab.github.io/LymphoAtlas.

Plasticity of Foxp3(+) T cells reflects promiscuous Foxp3 expression in conventional T cells but not reprogramming of regulatory T cells.

The emerging notion of environment-induced reprogramming of Foxp3(+) regulatory T (Treg) cells into helper T (Th) cells remains controversial. By genetic fate mapping or adoptive transfers, we have identified a minor population of nonregulatory Foxp3(+) T cells exhibiting promiscuous and transient Foxp3 expression, which gave rise to Foxp3(-) ("exFoxp3") Th cells and selectively accumulated in inflammatory cytokine milieus or in lymphopenic environments including those in early ontogeny. In contrast, Treg cells did not undergo reprogramming under those conditions irrespective of their thymic or peripheral origins. Moreover, although a few Treg cells transiently lose Foxp3 expression, such "latent" Treg cells retained their memory and robustly re-expressed Foxp3 and suppressive function upon activation. This study establishes that Treg cells constitute a stable cell lineage, whose committed state in a changing environment is ensured by DNA demethylation of the Foxp3 locus irrespectively of ongoing Foxp3 expression.

In vivo imaging of partially reversible th17 cell-induced neuronal dysfunction in the course of encephalomyelitis.

Neuronal damage in autoimmune neuroinflammation is the correlate for long-term disability in multiple sclerosis (MS) patients. Here, we investigated the role of immune cells in neuronal damage processes in animal models of MS by monitoring experimental autoimmune encephalomyelitis (EAE) by using two-photon microscopy of living anaesthetized mice. In the brainstem, we detected sustained interaction between immune and neuronal cells, particularly during disease peak. Direct interaction of myelin oligodendrocyte glycoprotein (MOG)-specific Th17 and neuronal cells in demyelinating lesions was associated with extensive axonal damage. By combining confocal, electron, and intravital microscopy, we showed that these contacts remarkably resembled immune synapses or kinapses, albeit with the absence of potential T cell receptor engagement. Th17 cells induced severe, localized, and partially reversible fluctuation in neuronal intracellular Ca(2+) concentration as an early sign of neuronal damage. These results highlight the central role of the Th17 cell effector phenotype for neuronal dysfunction in chronic neuroinflammation.

Deletion of Notch1 converts pro-T cells to dendritic cells and promotes thymic B cells by cell-extrinsic and cell-intrinsic mechanisms.

Notch1 signaling is required for T cell development and has been implicated in fate decisions in the thymus. We showed that Notch1 deletion in progenitor T cells (pro-T cells) revealed their latent developmental potential toward becoming conventional and plasmacytoid dendritic cells. In addition, Notch1 deletion in pro-T cells resulted in large numbers of thymic B cells, previously explained by T-to-B cell fate conversion. Single-cell genotyping showed, however, that the majority of these thymic B cells arose from Notch1-sufficient cells by a cell-extrinsic pathway. Fate switching nevertheless exists for a subset of thymic B cells originating from Notch1-deleted pro-T cells. Chimeric mice lacking the Notch ligand delta-like 4 (Dll4) in thymus epithelium revealed an essential role for Dll4 in T cell development. Thus, Notch1-Dll4 signaling fortifies T cell commitment by suppressing non-T cell lineage potential in pro-T cells, and normal Notch1-driven T cell development repels excessive B cells in the thymus.

Intestinal lamina propria dendritic cell subsets have different origin and functions.

The intestinal immune system discriminates between tolerance toward the commensal microflora and robust responses to pathogens. Maintenance of this critical balance is attributed to mucosal dendritic cells (DCs) residing in organized lymphoid tissue and dispersed in the subepithelial lamina propria. In situ parameters of lamina propria DCs (lpDCs) remain poorly understood. Here, we combined conditional cell ablation and precursor-mediated in vivo reconstitution to establish that lpDC subsets have distinct origins and functions. CD103(+) CX(3)CR1(-) lpDCs arose from macrophage-DC precursors (MDPs) via DC-committed intermediates (pre-cDCs) through a Flt3L growth-factor-mediated pathway. CD11b(+) CD14(+) CX(3)CR1(+) lpDCs were derived from grafted Ly6C(hi) but not Ly6C(lo) monocytes under the control of GM-CSF. Mice reconstituted exclusively with CX(3)CR1(+) lpDCs when challenged in an innate colitis model developed severe intestinal inflammation that was driven by graft-derived TNF-alpha-secreting CX(3)CR1(+) lpDCs. Our results highlight the critical importance of the lpDC subset balance for robust gut homeostasis.

Identification of a new stromal cell type involved in the regulation of inflamed B cell follicles.

Lymph node (LN) stromal cells provide survival signals and adhesive substrata to lymphocytes. During an immune response, B cell follicles enlarge, questioning how LN stromal cells manage these cellular demands. Herein, we used a murine fate mapping system to describe a new stromal cell type that resides in the T cell zone of resting LNs. We demonstrated that upon inflammation, B cell follicles progressively trespassed into the adjacent T cell zone and surrounded and converted these stromal cells into CXCL13 secreting cells that in return delineated the new boundaries of the growing follicle. Acute B cell ablation in inflamed LNs abolished CXCL13 secretion in these cells, while LT-ß deficiency in B cells drastically affected this conversion. Altogether, we reveal the existence of a dormant stromal cell subset that can be functionally awakened by B cells to delineate the transient boundaries of their expanding territories upon inflammation.

Temporal lineage tracing of Aire-expressing cells reveals a requirement for Aire in their maturation program.

Understanding the cellular dynamics of Aire-expressing lineage(s) among medullary thymic epithelial cells (AEL-mTECs) is essential for gaining insight into the roles of Aire in establishment of self-tolerance. In this study, we monitored the maturation program of AEL-mTECs by temporal lineage tracing, in which bacterial artificial chromosome transgenic mice expressing tamoxifen-inducible Cre recombinase under control of the Aire regulatory element were crossed with reporter strains. We estimated that the half-life of AEL-mTECs subsequent to Aire expression was ∼7-8 d, which was much longer than that reported previously, owing to the existence of a post-Aire stage. We found that loss of Aire did not alter the overall lifespan of AEL-mTECs, inconsistent with the previous notion that Aire expression in medullary thymic epithelial cells (mTECs) might result in their apoptosis for efficient cross-presentation of self-antigens expressed by AEL-mTECs. In contrast, Aire was required for the full maturation program of AEL-mTECs, as exemplified by the lack of physiological downregulation of CD80 during the post-Aire stage in Aire-deficient mice, thus accounting for the abnormally increased CD80(high) mTECs seen in such mice. Of interest, increased CD80(high) mTECs in Aire-deficient mice were not mTEC autonomous and were dependent on cross-talk with thymocytes. These results further support the roles of Aire in the differentiation program of AEL-mTECs.

Conditional ablation of NKp46+ cells using a novel Ncr1(greenCre) mouse strain: NK cells are essential for protection against pulmonary B16 metastases.

To study gene functions specifically in NKp46+ cells we developed novel Cre mice allowing for conditional gene targeting in cells expressing Ncr1 (encoding NKp46). We generated transgenic Ncr1(greenCre) mice carrying an EGFPcre fusion under the control of a proximal Ncr1 promoter that faithfully directed EGFPcre expression to NKp46+ cells from lymphoid and nonlymphoid tissues. This approach allowed for direct detection of Cre-expressing NKp46+ cells via their GFP signature by flow cytometry and histology. Cre was functional as evidenced by the NKp46+ cell-specific expression of RFP in Ncr1(greenCre) Rosa-dtRFP reporter mice. We generated Ncr1(greenCre) Il2rg(fl/fl) mice that lack NKp46+ cells in an otherwise intact hematopoietic environment. Il2rg encodes the common gamma chain (γc ), which is an essential receptor subunit for cytokines (IL-2, -4, -7, -9, -15, and -21) that stimulate lymphocyte development and function. In Ncr1(greenCre) Il2rg(fl/fl) mice, NK cells are severely reduced and the few remaining NKp46+ cells escaping γc deletion failed to express GFP. Using this new NK-cell-deficient model, we demonstrate that the homeostasis of NKp46+ cells from all tissues (including the recently described intraepithelial ILC1 subset) requires Il2rg. Finally, Ncr1(greenCre) Il2rg(fl/fl) mice are unable to reject B16 lung metastases demonstrating the essential role of NKp46+ cells in antimelanoma immune responses.

Promiscuous Foxp3-cre activity reveals a differential requirement for CD28 in Foxp3⁺ and Foxp3⁻ T cells.

Costimulatory signals by CD28 are critical for thymic regulatory T-cell (Treg) development. To determine the functional relevance of CD28 for peripheral Treg post thymic selection, we crossed the widely used Forkhead box protein 3 (Foxp3)-CreYFP mice to mice bearing a conditional Cd28 allele. Treg-specific CD28 deficiency provoked a severe autoimmune syndrome as a result of a strong disadvantage in competitive fitness and proliferation of CD28-deficient Tregs. By contrast, Treg survival and lineage integrity were not affected by the lack of CD28. This data demonstrate that, even after the initial induction requirement, Treg maintain a higher dependency on CD28 signalling than conventional T cells for homeostasis. In addition, we found the Foxp3-CreYFP allele to be a hypomorph, with reduced Foxp3 protein levels. Furthermore, we report here the stochastic activity of the Foxp3-CreYFP allele in non-Tregs, sufficient to recombine some conditional alleles (including Cd28) but not others (including R26-RFP). This hypomorphism and 'leaky' expression of the Foxp3-CreYFP allele should be considered when analysing the conditionally mutated Treg.

Evaluating the efficiency of isotope transmission for improved panel design and a comparison of the detection sensitivities of mass cytometer instruments.

The recent introduction of mass cytometry, a technique coupling a cell introduction system generating a stream of single cells with mass spectrometry, has greatly increased the number of parameters that can be measured per single cell. As with all new technology there is a need for dissemination of standardization and quality control procedures. Here, we characterize variations in sensitivity observed across the mass range of a mass cytometer, using different lanthanide tags. We observed a five-fold difference in lanthanide detection over the mass range and demonstrated that each instrument has its own sensitivity pattern. Therefore, the selection of lanthanide combinations is a key step in the establishment of a staining panel for mass cytometry-based experiments, particularly for multicenter studies. We propose the sensitivity pattern as the basis for panel design, instrument standardization and future implementation of normalization algorithms.

Improved method to retain cytosolic reporter protein fluorescence while staining for nuclear proteins.

Staining of transcription factors (TFs) together with retention of fluorescent reporter proteins is hindered by loss of fluorescence using current available methods. In this study, it is shown that current TF staining protocols do not destroy fluorescent proteins (FPs) but rather that fixation is not sufficient to retain FPs in the cytosol of the permeabilized cells. In this article, a simple and reliable protocol is elaborated, which allows efficient TF and cytokine staining while retaining FPs inside fixed cells.

Differential requirement for ROCK in dendritic cell migration within lymphatic capillaries in steady-state and inflammation.

Dendritic cell (DC) migration via lymphatic vessels to draining lymph nodes (dLNs) is crucial for the initiation of adaptive immunity. We imaged this process by intravital microscopy (IVM) in the ear skin of transgenic mice bearing red-fluorescent vasculature and yellow-fluorescent DCs. DCs within lymphatic capillaries were rarely transported by flow, but actively migrated within lymphatics and were significantly faster than in the interstitium. Pharmacologic blockade of the Rho-associated protein kinase (ROCK), which mediates nuclear contraction and de-adhesion from integrin ligands, significantly reduced DC migration from skin to dLNs in steady-state. IVM revealed that ROCK blockade strongly reduced the velocity of interstitial DC migration, but only marginally affected intralymphatic DC migration. By contrast, during tissue inflammation, ROCK blockade profoundly decreased both interstitial and intralymphatic DC migration. Inhibition of intralymphatic migration was paralleled by a strong up-regulation of ICAM-1 in lymphatic endothelium, suggesting that during inflammation ROCK mediates de-adhesion of DC-expressed integrins from lymphatic-expressed ICAM-1. Flow chamber assays confirmed an involvement of lymphatic-expressed ICAM-1 and DC-expressed ROCK in DC crawling on lymphatic endothelium. Overall, our findings further define the role of ROCK in DC migration to dLNs and reveal a differential requirement for ROCK in intralymphatic DC crawling during steady-state and inflammation.

CD64 expression distinguishes monocyte-derived and conventional dendritic cells and reveals their distinct role during intramuscular immunization.

Although most vaccines are administered i.m., little is known about the dendritic cells (DCs) that are present within skeletal muscles. In this article, we show that expression of CD64, the high-affinity IgG receptor FcγRI, distinguishes conventional DCs from monocyte-derived DCs (Mo-DCs). By using such a discriminatory marker, we defined the distinct DC subsets that reside in skeletal muscles and identified their migratory counterparts in draining lymph nodes (LNs). We further used this capability to analyze the functional specialization that exists among muscle DCs. After i.m. administration of Ag adsorbed to alum, we showed that alum-injected muscles contained large numbers of conventional DCs that belong to the CD8α(+)- and CD11b(+)-type DCs. Both conventional DC types were capable of capturing Ag and of migrating to draining LNs, where they efficiently activated naive T cells. In alum-injected muscles, Mo-DCs were as numerous as conventional DCs, but only a small fraction migrated to draining LNs. Therefore, alum by itself poorly induces Mo-DCs to migrate to draining LNs. We showed that addition of small amounts of LPS to alum enhanced Mo-DC migration. Considering that migratory Mo-DCs had, on a per cell basis, a higher capacity to induce IFN-γ-producing T cells than conventional DCs, the addition of LPS to alum enhanced the overall immunogenicity of Ags presented by muscle-derived DCs. Therefore, a full understanding of the role of adjuvants during i.m. vaccination needs to take into account the heterogeneous migratory and functional behavior of muscle DCs and Mo-DCs revealed in this study.