Summary
G‐protein–coupled receptor (GPCR) signaling is essential for the spatiotemporal control of leukocyte dynamics during immune responses. For efficient navigation through mammalian tissues, most ...leukocyte types express more than one GPCR on their surface and sense a wide range of chemokines and chemoattractants, leading to basic forms of leukocyte movement (chemokinesis, haptokinesis, chemotaxis, haptotaxis, and chemorepulsion). How leukocytes integrate multiple GPCR signals and make directional decisions in lymphoid and inflamed tissues is still subject of intense research. Many of our concepts on GPCR‐controlled leukocyte navigation in the presence of multiple GPCR signals derive from in vitro chemotaxis studies and lower vertebrates. In this review, we refer to these concepts and critically contemplate their relevance for the directional movement of several leukocyte subsets (neutrophils, T cells, and dendritic cells) in the complexity of mouse tissues. We discuss how leukocyte navigation can be regulated at the level of only a single GPCR (surface expression, competitive antagonism, oligomerization, homologous desensitization, and receptor internalization) or multiple GPCRs (synergy, hierarchical and non‐hierarchical competition, sequential signaling, heterologous desensitization, and agonist scavenging). In particular, we will highlight recent advances in understanding GPCR‐controlled leukocyte navigation by intravital microscopy of immune cells in mice.
Secondary lymphoid tissues are the sites of both innate and adaptive host defense. Aside from the relatively static nonhematopoietic stromal elements and some macrophages and dendritic cells, most of ...the cells in these tissues are in constant movement, but the organs maintain a defined microanatomy with preferred locations for the bulk of T cells, B cells, and other lymphocytes and subsets of myeloid cells. Here we describe both the cell dynamics and spatial organization of lymph nodes and review how both physical features and molecular cues guide cell movement to optimize host defense. We emphasize the role of locality in improving the efficiency of a system requiring rare cells to find each other and interact productively through membrane-bound or short-range secreted mediators and highlight how changes in steady-state cell positioning during an infectious challenge contribute to rapid generation of productive responses.
Neutrophil recruitment from blood to extravascular sites of sterile or infectious tissue damage is a hallmark of early innate immune responses, and the molecular events leading to cell exit from the ...bloodstream have been well defined. Once outside the vessel, individual neutrophils often show extremely coordinated chemotaxis and cluster formation reminiscent of the swarming behaviour of insects. The molecular players that direct this response at the single-cell and population levels within the complexity of an inflamed tissue are unknown. Using two-photon intravital microscopy in mouse models of sterile injury and infection, we show a critical role for intercellular signal relay among neutrophils mediated by the lipid leukotriene B4, which acutely amplifies local cell death signals to enhance the radius of highly directed interstitial neutrophil recruitment. Integrin receptors are dispensable for long-distance migration, but have a previously unappreciated role in maintaining dense cellular clusters when congregating neutrophils rearrange the collagenous fibre network of the dermis to form a collagen-free zone at the wound centre. In this newly formed environment, integrins, in concert with neutrophil-derived leukotriene B4 and other chemoattractants, promote local neutrophil interaction while forming a tight wound seal. This wound seal has borders that cease to grow in kinetic concert with late recruitment of monocytes and macrophages at the edge of the displaced collagen fibres. Together, these data provide an initial molecular map of the factors that contribute to neutrophil swarming in the extravascular space of a damaged tissue. They reveal how local events are propagated over large-range distances, and how auto-signalling produces coordinated, self-organized neutrophil-swarming behaviour that isolates the wound or infectious site from surrounding viable tissue.
Innate lymphoid cells (ILCs) participate in tissue homeostasis, inflammation, and early immunity against infection. It is unclear how ILCs acquire effector function and whether these mechanisms ...differ between organs. Through multiplexed single-cell mRNA sequencing, we identified cKit
CD127
TCF-1
early differentiation stages of T-bet
ILC1s. These cells were present across different organs and had the potential to mature toward CD127
TCF-1
and CD127
TCF-1
ILC1s. Paralleling a gradual loss of TCF-1, differentiating ILC1s forfeited their expansion potential while increasing expression of effector molecules, reminiscent of T cell differentiation in secondary lymphoid organs. The transcription factor Hobit was induced in TCF-1
ILC1s and was required for their effector differentiation. These findings reveal sequential mechanisms of ILC1 lineage commitment and effector differentiation that are conserved across tissues. Our analyses suggest that ILC1s emerge as TCF-1
cells in the periphery and acquire a spectrum of organ-specific effector phenotypes through a uniform Hobit-dependent differentiation pathway driven by local cues.
The lymphatic network that transports interstitial fluid and antigens to lymph nodes constitutes a conduit system that can be hijacked by invading pathogens to achieve systemic spread unless ...dissemination is blocked in the lymph node itself. Here, we show that a network of diverse lymphoid cells (natural killer cells, γδ T cells, natural killer T cells, and innate-like CD8+ T cells) are spatially prepositioned close to lymphatic sinus-lining sentinel macrophages where they can rapidly and efficiently receive inflammasome-generated IL-18 and additional cytokine signals from the pathogen-sensing phagocytes. This leads to rapid IFNγ secretion by the strategically positioned innate lymphocytes, fostering antimicrobial resistance in the macrophage population. Interference with this innate immune response loop allows systemic spread of lymph-borne bacteria. These findings extend our understanding of the functional significance of cellular positioning and local intercellular communication within lymph nodes while emphasizing the role of these organs as highly active locations of innate host defense.
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► Spatial and functional organization in lymph nodes prevents systemic pathogen spread ► A macrophage-lymphocyte communication loop promotes innate immune host defense ► Innate effectors are prepositioned close to macrophages to rapidly produce IFNγ ► Inflammasome-generated IL-1 and IL-18 activate and orchestrate the innate defense
A network of diverse lymphoid cells are spatially prepositioned close to sentinel macrophages lining the lymph nodes where they can rapidly and efficiently receive cytokine signals from the pathogen-sensing phagocytes, thus blocking the dissemination of bacteria and viruses that attempt to hijack the lymphatic network.
Differentiation of CD4
T cells into either follicular helper T (T
) or type 1 helper T (T
1) cells influences the balance between humoral and cellular adaptive immunity, but the mechanisms whereby ...pathogens elicit distinct effector cells are incompletely understood. Here we analyzed the spatiotemporal dynamics of CD4
T cells during infection with recombinant vesicular stomatitis virus (VSV), which induces early, potent neutralizing antibodies, or recombinant lymphocytic choriomeningitis virus (LCMV), which induces a vigorous cellular response but inefficient neutralizing antibodies, expressing the same T cell epitope. Early exposure of dendritic cells to type I interferon (IFN), which occurred during infection with VSV, induced production of the cytokine IL-6 and drove T
cell polarization, whereas late exposure to type I IFN, which occurred during infection with LCMV, did not induce IL-6 and allowed differentiation into T
1 cells. Thus, tight spatiotemporal regulation of type I IFN shapes antiviral CD4
T cell differentiation and might instruct vaccine design strategies.
Microbes or danger signals trigger inflammasome sensors, which induce polymerization of the adaptor ASC and the assembly of ASC specks. ASC specks recruit and activate caspase-1, which induces ...maturation of the cytokine interleukin 1β (IL-1β) and pyroptotic cell death. Here we found that after pyroptosis, ASC specks accumulated in the extracellular space, where they promoted further maturation of IL-1β. In addition, phagocytosis of ASC specks by macrophages induced lysosomal damage and nucleation of soluble ASC, as well as activation of IL-1β in recipient cells. ASC specks appeared in bodily fluids from inflamed tissues, and autoantibodies to ASC specks developed in patients and mice with autoimmune pathologies. Together these findings reveal extracellular functions of ASC specks and a previously unknown form of cell-to-cell communication.
Abstract
T cell exhaustion is a hallmark of cancer and persistent infections, marked by inhibitory receptor upregulation, diminished cytokine secretion, and impaired cytolytic activity. Terminally ...exhausted T cells are steadily replenished by a precursor population (Tpex), but the metabolic principles governing Tpex maintenance and the regulatory circuits that control their exhaustion remain incompletely understood. Using a combination of gene-deficient mice, single-cell transcriptomics, and metabolomic analyses, we show that mitochondrial insufficiency is a cell-intrinsic trigger that initiates the functional exhaustion of T cells. At the molecular level, we find that mitochondrial dysfunction causes redox stress, which inhibits the proteasomal degradation of hypoxia-inducible factor 1α (HIF-1α) and promotes the transcriptional and metabolic reprogramming of Tpex cells into terminally exhausted T cells. Our findings also bear clinical significance, as metabolic engineering of chimeric antigen receptor (CAR) T cells is a promising strategy to enhance the stemness and functionality of Tpex cells for cancer immunotherapy.
Tissue-resident memory CD8
T cells (T
) constitute a major component of the immune-surveillance system in nonlymphoid organs. Local, noncognate factors are both necessary and sufficient to support ...the programming of T
cell fate in tissue-infiltrating T cells. Recent evidence suggests that TCR signals received in infected nonlymphoid tissues additionally contribute to T
cell formation. Here, we asked how antigen-dependent pathways influence the generation of skin-resident memory T cells that arise from a polyclonal repertoire of cells induced by infection with an antigenically complex virus and recombinant vaccine vector. We found that CD8
T cells of different specificities underwent antigen-dependent competition in the infected tissue, which shaped the composition of the local pool of T
cells. This local cross-competition was active for T cells recognizing antigens that are coexpressed by infected cells. In contrast, T
cell development remained largely undisturbed by the presence of potential competitors when antigens expressed in the same tissue were segregated through infection with antigenically distinct viral quasispecies. Functionally, local cross-competition might serve as a gatekeeping mechanism to regulate access to the resident memory niche and to fine-tune the local repertoire of antiviral T
cells.
Malaria infection begins when a female Anopheles mosquito injects Plasmodium sporozoites into the skin of its host during blood feeding. Skin-deposited sporozoites may enter the bloodstream and ...infect the liver, reside and develop in the skin, or migrate to the draining lymph nodes (DLNs). Importantly, the DLN is where protective CD8(+) T cell responses against malaria liver stages are induced after a dermal route of infection. However, the significance of parasites in the skin and DLN to CD8(+) T cell activation is largely unknown. In this study, we used genetically modified parasites, as well as antibody-mediated immobilization of sporozoites, to determine that active sporozoite migration to the DLNs is required for robust CD8(+) T cell responses. Through dynamic in vivo and static imaging, we show the direct uptake of parasites by lymph-node resident DCs followed by CD8(+) T cell-DC cluster formation, a surrogate for antigen presentation, in the DLNs. A few hours after sporozoite arrival to the DLNs, CD8(+) T cells are primed by resident CD8α(+) DCs with no apparent role for skin-derived DCs. Together, these results establish a critical role for lymph node resident CD8α(+) DCs in CD8(+) T cell priming to sporozoite antigens while emphasizing a requirement for motile sporozoites in the induction of CD8(+) T cell-mediated immunity.
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