Fibroblastic reticular cells at the nexus of innate and adaptive immune responses

Lymphoid organs guarantee productive immune cell interactions through the establishment of distinct microenvironmental niches that are built by fibroblastic reticular cells (FRC). These specialized immune‐interacting fibroblasts coordinate the migration and positioning of lymphoid and myeloid cells in lymphoid organs and provide essential survival and differentiation factors during homeostasis and immune activation. In this review, we will outline the current knowledge on FRC functions in secondary lymphoid organs such as lymph nodes, spleen and Peyer's patches and will discuss how FRCs contribute to the regulation of immune processes in fat‐associated lymphoid clusters. Moreover, recent evidence indicates that FRC critically impact immune regulatory processes, for example, through cytokine deprivation during immune activation or through fostering the induction of regulatory T cells. Finally, we highlight how different FRC subsets integrate innate immunological signals and molecular cues from immune cells to fulfill their function as nexus between innate and adaptive immune responses.     

Homeostatic chemokines guide lymphoma cells to tumor growth-promoting niches within secondary lymphoid organs

The interaction between lymphoid tumor cells and their tissue microenvironment is thought to promote dissemination and progression of lymphoma. Those type of interactions consists of at least three cornerstones, among them mesenchymal- or bone marrow-derived stromal cells, cells of the innate or adaptive immune response, and the lymphoma cells themselves. The molecular pathways of crosstalk between the lymphoma cells and their nursing stroma are not well understood and their dissection is challenging because of (1) the complexity of stroma cell subpopulations, (2) kinetic and developmental transitions/switches of stroma composition, and (3) inherent technical difficulties to isolate and analyze defined stroma cell subsets. However, recent studies of bone marrow stroma interaction with leukemia or lymphoma cells have revealed therapeutic targets involved in regulating tumor cell mobilization. Release of tumor cells from their supportive niches could be effectuated by inhibition of homing and retention signals. The present review focuses on the effects of homing receptors and cytokines attributed to lymphoid tissue formation in tumor–stroma interactions within secondary lymphoid tissues. We discuss possible cellular and molecular mechanisms of lymphoma–stroma crosstalk and highlight novel therapeutic strategies based on the disruption of tumor–stroma interaction in secondary lymphoid organs.     

The microstructure of secondary lymphoid organs that support immune cell trafficking

Immune cell trafficking in the secondary lymphoid organs is crucial for an effective immune response. Recirculating T cells constantly patrol not only secondary lymphoid organs but also the whole peripheral organs. Thoracic duct lymphocytes represent an ideal cell source for analyzing T cell trafficking: high endothelial venules (HEVs) allow recirculating lymphocytes to transmigrate from the blood directly, and recirculating T cells form a cluster with dendritic cells (DCs) to survey antigen invasions even in a steady state. This cluster becomes an actual site for the antigen presentation when DCs have captured antigens. On activation, effector and memory T cells differentiate into several subsets that have different trafficking molecules and patterns. DCs also migrate actively in a manner depending upon their maturational stages. Danger signals induce the recruitment of several DC precursor subsets with different trafficking patterns and functions. In this review, we describe general and specialized structures of the secondary lymphoid organs for the trafficking of T cells and DCs by a multicolor immunoenzyme staining technique. The lymph nodes, spleen, and Peyer's patches of rats were selected as the major representatives. In vivo trafficking of subsets of T cells and DCs within these organs under steady or emergency states are shown and discussed, and unsolved questions and future prospects are also considered.     

Regulation of adaptive immune responses by innate cells expressing NK markers and antigen-transporting macrophages

A continuing theme of work done in our laboratory involves regulation of adaptive immune response by innate cells, in general, and immuneregulation by natural killer (NK) and NKT cells, in particular. Studies include work with the lung and the eye. In addition to immune surveillance of tumor cells, the NK cell is often associated with secreting cytokines that contribute to the creation of microenvironments conducive to Th1 responses and with defense mechanisms that lessen the initial infecting viral load. Reported studies show that the NKT cells support both T helper cell responses (type 1 and 2), as well as their being absolutely central to the development of antigen-specific T-regulatory cells involved in peripheral tolerance. Because of the multifunctional capabilities of the NKT cell, we propose that yet another cell, such as the antigen-presenting cell (APC), may influence the effector pathway of the NKT cell. We postulate that the APC that transports the antigen from the entry environment provides both trafficking and activation signals for innate cells in the secondary lymphoid organs. Evidence is presented that macrophage-derived signals selectively recruit NKT cells and bias their cytokine synthesis. Data imply that, just as occurs in immune inflammation, a collection of innate and adaptive immune cells interact within the secondary lymphoid tissue to generate antigen-specific tolerance in the periphery.     

Phenotypic and Functional Plasticity of Gamma–Delta (γδ) T Cells in Inflammation and Tolerance

Gamma–delta T cells (γδ T cells) are an unique group of lymphocytes and play an important role in bridging the gap between innate and adaptive immune systems under homeostatic condition as well as during infection and inflammation. They are predominantly localized into the mucosal and epithelial sites, but also exist in other peripheral tissues and secondary lymphoid organs. γδ T cells can produce cytokines and chemokines to regulate the migration of other immune cells, can bring about lysis of infected or stressed cells by secreting granzymes, provide help to B cells and induce IgE production, can present antigen to conventional T cells, activate antigen presenting cells (APC) maturation, and are also known to produce growth factors that regulate the stromal cell function. γδ T cells spontaneously produce IFN-γ and IL-17 cytokines compared to delayed differentiation of Th1 and Th17 cells. In this review, we discussed the current knowledge about the mechanism of γδ T cell function including its mode of antigen recognition, and differentiation into various subsets of γδ T cells. We also explored how γδ T cells interact with different types of innate and adaptive immune cells, and how these interactions shape the immune response highlighting the plasticity and role of these cells—protective or pathogenic under inflammatory and tolerogenic conditions.     

The trafficking of natural killer cells

Summary: Natural killer (NK) cells are large granular lymphocytes of the innate immune system that participate in the early control of microbial infections and cancer. NK cells can induce the death of autologous cells undergoing various forms of stress, recognizing and providing non-microbial 'danger' signals to the immune system. NK cells are widely distributed in lymphoid and non-lymphoid organs. NK cell precursors originate from the bone marrow and go through a complex maturation process that leads to the acquisition of their effector functions, to changes in their expression of integrins and chemotactic receptors, and to their redistribution from the bone marrow and lymph nodes to blood, spleen, liver, and lung. Here, we describe the tissue localization of NK cells, using NKp46 as an NK cell marker, and review the current knowledge on the mechanisms that govern their trafficking in humans and in mice.     

The lymph node neutrophil

Secondary lymphoid organs provide a specialized microenvironment tailored to foster communication between cells of the innate and adaptive immune systems. These interactions allow immune cells to coordinate multilayered defense against pathogens. Until recently dendritic cells and macrophages were thought to comprise the main innate immune cell subsets responsible for delivering signals that drive the adaptive immune response, while the function of neutrophils was largely confined to the innate immune system. However, the discovery of neutrophils in lymph nodes has raised the question of whether neutrophils might play a more extensive role not only in innate immunity per se, but also in coordinating the interactions between innate and adaptive immune responses. In this review we discuss the mechanisms and consequences of neutrophil recruitment to lymph nodes and how this recruitment influences subsequent immune responses both in situ and at distant sites.     

Expression and function of interleukin-7 in secondary and tertiary lymphoid organs

Interleukin-7 (IL-7) is known since many years as stromal-cell derived cytokine that plays a key role for the adaptive immune system. It promotes lymphocyte development in the bone marrow and thymus as well as naive and memory T cell homeostasis in the periphery. More recently, IL-7 reporter mice and other approaches have led to the further characterization of the various stromal cell sources of IL-7 in secondary lymphoid organs (SLO) and other tissues. We will review these advances along with a discussion of the regulation of IL-7 and its receptor, and compare the biological effects IL-7 has on adaptive as well as innate immune cells in SLO. Finally, we will review the role of IL-7 in development of SLO and tertiary lymphoid tissues that frequently are associated with sites of chronic inflammation.     

Interaction of the selectin ligand PSGL-1 with chemokines CCL21 and CCL19 facilitates efficient homing of T cells to secondary lymphoid organs

P-selectin glycoprotein ligand 1 (PSGL-1) is central to the trafficking of immune effector cells to areas of inflammation through direct interactions with P-selectin, E-selectin and L-selectin. Here we show that PSGL-1 was also required for efficient homing of resting T cells to secondary lymphoid organs but functioned independently of selectin binding. PSGL-1 mediated an enhanced chemotactic T cell response to the secondary lymphoid organ chemokines CCL21 and CCL19 but not to CXCL12 or to inflammatory chemokines. Our data show involvement of PSGL-1 in facilitating the entry of T cells into secondary lymphoid organs, thereby demonstrating the bifunctional nature of this molecule.     

Dendritic cells: immune regulators in health and disease.

Dendritic cells (DCs) are bone marrow-derived cells of both lymphoid and myeloid stem cell origin that populate all lymphoid organs including the thymus, spleen, and lymph nodes, as well as nearly all nonlymphoid tissues and organs. Although DCs are a moderately diverse set of cells, they all have potent antigen-presenting capacity for stimulating naive, memory, and effector T cells. DCs are members of the innate immune system in that they can respond to dangers in the host environment by immediately generating protective cytokines. Most important, immature DCs respond to danger signals in the microenvironment by maturing, i.e., differentiating, and acquiring the capacity to direct the development of primary immune responses appropriate to the type of danger perceived. The powerful adjuvant activity that DCs possess in stimulating specific CD4 and CD8 T cell responses has made them targets in vaccine development strategies for the prevention and treatment of infections, allograft reactions, allergic and autoimmune diseases, and cancer. This review addresses the origins and migration of DCs to their sites of activity, their basic biology as antigen-presenting cells, their roles in important human diseases and, finally, selected strategies being pursued to harness their potent antigen-stimulating activity.     

树突状细胞的生物力学与力学生物学研究进展

树突状细胞(dendritic cells, DCs)是目前已知机体内功能最为强大的抗原提呈细胞,具有高效的抗原摄取、加工和处理能力,能够在二级淋巴组织向幼稚T细胞提呈抗原,从而诱导免疫应答或耐受,在启动和放大先天性及适应性免疫中发挥关键作用。DCs在发挥其生理学功能的过程中经历复杂的化学和力学微环境变化,并表现出不同力学表型和免疫表型,深入理解调控DCs力学表型和免疫表型的化学和力学因素是利用其治疗免疫相关疾病的先决条件。本文主要介绍DCs生物力学与力学生物学研究的进展,并探讨其在免疫相关疾病治疗中的潜在应用和未来发展方向。     

Crucial functions of the Rap1 effector molecule RAPL in lymphocyte and dendritic cell trafficking

Immunosurveillance requires the coordinated regulation of chemokines and adhesion molecules to guide immune cell migration. However, the critical molecule for governing the high trafficking capability of immune cells is not clear. Here we show that the effector molecule RAPL is indispensable in the integrin-mediated adhesion and migration of lymphocytes and dendritic cells. RAPL deficiency caused defective chemokine-triggered lymphocyte adhesion and migration to secondary lymphoid organs, resulting in atrophic lymphoid follicles and deficient marginal zone B cells, concomitant with increased immature B cells in the blood. Furthermore, splenic dendritic cells were diminished and defective in adhesion. After being activated with inflammatory stimuli, skin and splenic dendritic cells failed to migrate into either the draining lymph nodes or the white pulp of the spleen. Thus, RAPL is a crucial immune cell trafficking regulator essential for immunosurveillance.     

NK cells protect secondary lymphoid tissue from cytomegalovirus via a CD30‐dependent mechanism

The pathogenic outcomes of viral infection are often reminiscent of a dysfunctional immune system. Thus, cytomegalovirus (CMV) causes disruption of the lymphoid architecture and the functionality of lymphocytes, both of which are features of CD30 deficiency. It was therefore plausible that CD30 might interfere with CMV infection. The present study identifies CD30 as an inducible NK‐cell receptor critical for innate immunity against CMV. Expression of CD30 integrates survival signals to NK cells that allow them to prevent viral spread and subsequent disintegration of secondary lymphoid tissue. Deficiency in CD30 results in exaggerated NK cell death and complete abrogation of the lymphoid architecture. Our data define the necessity of NK cells for protection of secondary lymphoid organs and describe a mechanism by which this protection is conferred.     

Making friends in out-of-the-way places: how cells of the immune system get together and how they conduct their business as revealed by intravital imaging

Summary: A central characteristic of the immune system is the constantly changing location of most of its constituent cells. Lymphoid and myeloid cells circulate in the blood, and subsets of these cells enter, move, and interact within, then leave organized lymphoid tissues. When inflammation is present, various hematopoietic cells also exit the vasculature and migrate within non-lymphoid tissues, where they carry out effector functions that support host defense or result in autoimmune pathology. Effective innate and adaptive immune responses involve not only the action of these individual cells but also productive communication among them, often requiring direct membrane contact between rare antigen-specific or antigen-bearing cells. Here, we describe our ongoing studies using two-photon intravital microscopy to probe the in situ behavior of the cells of the immune system and their interactions with non-hematopoietic stromal elements. We emphasize the importance of non-random cell migration within lymphoid tissues and detail newly established mechanisms of traffic control that operate at multiple organizational scales to facilitate critical cell contacts. We also describe how the methods we have developed for imaging within lymphoid sites are being applied to other tissues and organs, revealing dynamic details of host-pathogen interactions previously inaccessible to direct observation.     

Perisinusoidal B cells in the bone marrow participate in T-independent responses to blood-borne microbes

Mature recirculating B cells are generally assumed to exist in follicular niches in secondary lymphoid organs, and these cells mediate T-dependent humoral immune responses. We show here that a large proportion of mature B lymphocytes occupy an anatomically and functionally distinct perisinusoidal niche in the bone marrow. Perisinusoidal B cells circulate freely, as revealed by parabiosis studies. However, unlike their counterparts in the follicular niche, these cells are capable of being activated in situ by blood-borne microbes in a T-independent manner to generate specific IgM antibodies. The bone marrow represents a unique type of secondary lymphoid organ in which mature B cells are strategically positioned in the path of circulating microbes.     

Sphingosine-1-Phosphate: a Master Regulator of Lymphocyte Egress and Immunity

Sphingosine-1-phosphate (S1P) is a central factor responsible for lymphocyte distribution in the body. S1P is able to control the integrity of various effector cell populations within many lymphoid tissues by directing lymphocyte egress. In this review, we give an overview of the generation and degradation of S1P in specific lymphoid microenvironments. Furthermore, we discuss, sometimes contradictory, the functions of the five S1P receptors on different cells in diverse tissues and give an idea of additional counteracting chemotactic signals for lymphocyte immigration and emigration. We focus special attention to recent discoveries of S1P-specific transporters, like spinster-homolog-2 and the active secretion of S1P by endothelial cells, erythrocytes and platelets. In addition, we describe the microanatomical structures as well as entry and egress routes into lymphoid organs which lymphocytes use for efficient trafficking. Finally, we give an overview of open questions regarding the regulation of lymphocyte homing from primary lymphoid organs to secondary lymphoid organs and back again.     

Differential expression of regulator of G-protein signalling transcripts and in vivo migration of CD4+ naïve and regulatory T cells

The immune response of T lymphocytes to pathogens is initiated in draining secondary lymphoid organs, and activated cells then migrate to the site of infection. Thus, control of naive and regulatory CD4+ T-cell migration is crucial; however, it is poorly understood in physiological and pathological conditions. We found that CD4+ subpopulations displayed characteristic regulator of G-protein signalling (RGS) gene expression profiles. Regulatory T cells express higher levels of RGS1, RGS9 and RGS16 than naive cells. These genes are up-regulated upon cell activation and their level of expression correlates with in vivo cell migration. Using parabiosis, we showed that regulatory T lymphocytes migrate less than naive T cells and that migrant naive T cells express even lower RGS levels than their static counterparts. Our results show an inverse correlation between the capacity to migrate and the levels of RGS1, RGS9 and RGS16 for both naive and regulatory T cells. Taken together, these results suggest a role for RGS molecules in chemokine-induced lymphocyte migration and demonstrate the peculiarity of regulatory T cells in terms of phenotype and migration ability, providing new insights into their function.     

Innate lymphoid cells in secondary lymphoid organs

The family of innate lymphoid cells (ILCs) has attracted attention in recent years as its members are important regulators of immunity, while they can also cause pathology. In both mouse and man, ILCs were initially discovered in developing lymph nodes as lymphoid tissue inducer (LTi) cells. These cells form the prototypic members of the ILC family and play a central role in the formation of secondary lymphoid organs (SLOs). In the absence of LTi cells, lymph nodes (LN) and Peyer's Patches (PP) fail to form in mice, although the splenic white pulp can develop normally. Besides LTi cells, the ILC family encompasses helper-like ILCs with functional distinctions as seen by T-helper cells, as well as cytotoxic natural killer (NK) cells. ILCs are still present in adult SLOs where they have been shown to play a role in lymphoid tissue regeneration. Furthermore, ILCs were implicated to interact with adaptive lymphocytes and influence the adaptive immune response. Here, we review the recent literature on the role of ILCs in secondary lymphoid tissue from the formation of SLOs to mature SLOs in adults, during homeostasis and pathology.     

Innate immune mediators in cancer: between defense and resistance

Chronic inflammation in the tumor microenvironment and evasion of the antitumor effector immune response are two of the emerging hallmarks required for oncogenesis and cancer progression. The innate immune system not only plays a critical role in perpetuating these tumor-promoting hallmarks but also in developing antitumor adaptive immune responses. Thus, understanding the dual role of the innate system in cancer immunology is required for the design of combined immunotherapy strategies able to tackle established tumors. Here, we review recent advances in the understanding of the role of cell populations and soluble components of the innate immune system in cancer, with a focus on complement, the adapter molecule Stimulator of Interferon Genes, natural killer cells, myeloid cells, and B cells.     

T follicular helper cells in cancer,tertiary lymphoid structures,and beyond

With the emergence and success of checkpoint blockade immunotherapy, immuno-oncology has primarily focused on CD8 T cells, whose cytotoxic programs directly target tumor cells. However, the limited response rate of current immunotherapy regimens has prompted investigation into other types of tumor-infiltrating immune cells, such as CD4 T cells and B cells, and how they interact with CD8 T cells in a coordinated network. Recent studies have demonstrated the potential therapeutic benefits of CD4 T follicular helper (TFH) cells and B cells in cancer, highlighting the important role of their crosstalk and interactions with other immune cell components in the tumor microenvironment. These interactions also occur in tumor-associated tertiary lymphoid structures (TLS), which resemble secondary lymphoid organs (SLOs) with orchestrated vascular, chemokine, and cellular infrastructures that support the developmental pathways of functional immune cells. In this review, we discuss recent breakthroughs on TFH biology and T cell-B cell interactions in tumor immunology, and their potential as novel therapeutic targets to advance cancer treatment.