Role of T and NK cells and IL7/IL7r interactions during neonatal maturation of lymph nodes

Single nucleotide polymorphism (SNP) characterisation of mouse inbred strains bred at MRC-National Institute for Medical Research

Inbred mouse strains have long proved useful as tools for biomedical research. They remove the effects of genetic background as an experimental variable. Within all mouse colonies, genetic drift is a recognised phenomenon and monitoring and documenting changes is important for experimental design and consistency. This communication documents the initial characterisation through SNP analysis of the inbred mouse strains bred and used at the time at the Medical Research Council National Institute for Medical Research (MRC-NIMR), Mill Hill, now The Crick Institute. These inbred strains were part of the foundation colonies for the many genetically modified mouse strains made at Mill Hill. We found small genetic changes in four of the nine inbred strains. Although phenotypic differences have not yet been found between the NIMR and the correspondent parental strains, I cannot discard that these may arise or have already arisen. This work has also authenticated the 129/SvJEvNimr-Gpi1c strain that was widely used at MRC-NIMR for gene targeting. All these inbred strains have been renamed according to The International Committee on Standardized Genetic Nomenclature for Mice.

Identifying physiological tissue niches that support the HIV reservoir in T cells

Successful antiretroviral therapy (ART) can efficiently suppress Human Immunodeficiency Virus-1 (HIV-1) replication to undetectable levels, but rare populations of infected memory CD4+ T cells continue to persist, complicating viral eradication efforts. Memory T cells utilize distinct homing and adhesion molecules to enter, exit, or establish residence at diverse tissue sites, integrating cellular and environmental cues that maintain homeostasis and life-long protection against pathogens. Critical roles for T cell receptor and cytokine signals driving clonal expansion and memory generation during immunity generation are well established, but whether HIV-infected T cells can utilize similar mechanisms for their own long-term survival is unclear. How infected, but transcriptionally silent T cells maintain their recirculation potential through blood and peripheral tissues, or whether they acquire new capabilities to establish unique peripheral tissue niches, is also not well understood. In this review, we will discuss the cellular and molecular cues that are important for memory T cell homeostasis and highlight opportunities for HIV to hijack normal immunological processes to establish long-term viral persistence.

When the Damage Is Done: Injury and Repair in Thymus Function

Even though the thymus is exquisitely sensitive to acute insults like infection, shock, or common cancer therapies such as cytoreductive chemo- or radiation-therapy, it also has a remarkable capacity for repair. This phenomenon of endogenous thymic regeneration has been known for longer even than its primary function to generate T cells, however, the underlying mechanisms controlling the process have been largely unstudied. Although there is likely continual thymic involution and regeneration in response to stress and infection in otherwise healthy people, acute and profound thymic damage such as that caused by common cancer cytoreductive therapies or the conditioning regimes as part of hematopoietic cell transplantation (HCT), leads to prolonged T cell deficiency; precipitating high morbidity and mortality from opportunistic infections and may even facilitate cancer relapse. Furthermore, this capacity for regeneration declines with age as a function of thymic involution; which even at steady state leads to reduced capacity to respond to new pathogens, vaccines, and immunotherapy. Consequently, there is a real clinical need for strategies that can boost thymic function and enhance T cell immunity. One approach to the development of such therapies is to exploit the processes of endogenous thymic regeneration into novel pharmacologic strategies to boost T cell reconstitution in clinical settings of immune depletion such as HCT. In this review, we will highlight recent work that has revealed the mechanisms by which the thymus is capable of repairing itself and how this knowledge is being used to develop novel therapies to boost immune function.

Chemokines and other mediators in the development and functional organization of lymph nodes

Secondary lymphoid organs like lymph nodes (LNs) are the main inductive sites for adaptive immune responses. Lymphocytes are constantly entering LNs, scanning the environment for their cognate antigen and get replenished by incoming cells after a certain period of time. As only a minor percentage of lymphocytes recognizes cognate antigen, this mechanism of permanent recirculation ensures fast and effective immune responses when necessary. Thus, homing, positioning, and activation as well as egress require precise regulation within LNs. In this review we discuss the mediators, including chemokines, cytokines, growth factors, and others that are involved in the formation of the LN anlage and subsequent functional organization of LNs. We highlight very recent findings in the fields of LN development, steady-state migration in LNs, and the intranodal processes during an adaptive immune response.

Flip the coin: IL-7 and IL-7R in health and disease

The cytokine IL-7 and its receptor, IL-7R, are critical for T cell and, in the mouse, B cell development, as well as differentiation and survival of naive T cells, and generation and maintenance of memory T cells. They are also required for innate lymphoid cell (ILC) development and maintenance, and consequently for generation of lymphoid structures and barrier defense. Here we discuss the central role of IL-7 and IL-7R in the lymphoid system and highlight the impact of their deregulation, placing a particular emphasis on their 'dark side' as promoters of cancer development. We also explore therapeutic implications and opportunities associated with either positive or negative modulation of the IL-7-IL-7R signaling axis.

Cytomegalovirus (CMV) Pneumonitis: Cell Tropism, Inflammation, and Immunity

Human cytomegalovirus (HCMV) is an opportunistic pathogen causing disease mainly in immunocompromised patients or after congenital infection. HCMV infection of the respiratory tract leads to pneumonitis in the immunocompromised host, which is often associated with a bad clinical course. The related mouse cytomegalovirus (MCMV) likewise exhibits a distinct tropism for the lung and thus provides an elegant model to study host-pathogen interaction. Accordingly, fundamental features of cytomegalovirus (CMV) pneumonitis have been discovered in mice that correlate with clinical data obtained from humans. Recent studies have provided insight into MCMV cell tropism and localized inflammation after infection of the respiratory tract. Accordingly, the nodular inflammatory focus (NIF) has been identified as the anatomical correlate of immune control in lungs. Several hematopoietic cells involved in antiviral immunity reside in NIFs and their key effector molecules have been deciphered. Here, we review what has been learned from the mouse model with focus on the microanatomy of infection sites and antiviral immunity in MCMV pneumonitis.

IL-7 is a Key Driver Cytokine in Spondyloarthritis?

The rationale for a type 17 signature in the pathogenesis of spondyloarthritis (SpA) has been increasing and being ratified in studies recently. IL-7 is a cytokine whose ability to stimulate IL-17 production in both innate and adaptive immunity cells has made it a promising target not only for a better understanding of the disease as well as an important potential therapeutic target in patients with SpA.

Tissue-resident Eomes+ NK cells are the major innate lymphoid cell population in human infant intestine

Innate lymphoid cells (ILC), including natural killer (NK) cells, are implicated in host-defense and tissue-growth. However, the composition and kinetics of NK cells in the intestine during the first year of life, when infants are first broadly exposed to exogenous antigens, are still unclear. Here we show that CD103⁺ NK cells are the major ILC population in the small intestines of infants. When compared to adult intestinal NK cells, infant intestinal NK cells exhibit a robust effector phenotype, characterized by Eomes, perforin and granzyme B expression, and superior degranulation capacity. Absolute intestinal NK cell numbers decrease gradually during the first year of life, coinciding with an influx of intestinal Eomes⁺ T cells; by contrast, epithelial NKp44⁺CD69⁺ NK cells with less cytotoxic capacity persist in adults. In conclusion, NK cells are abundant in infant intestines, where they can provide effector functions while Eomes⁺ T cell responses mature.

Innate lymphoid cells (ILC), including natural killer (NK) cells, are important innate immune regulators. Here the authors show that, in human infant intestines, CD103⁺Eomes⁺ NK cells are the predominant ILC population, but are replaced gradually by Eomes⁺ T cells, while NKp44⁺ NK cells persist in adult intestines.

IL-7-dependent maintenance of ILC3s is required for normal entry of lymphocytes into lymph nodes

Yang et al. identify a new function for IL-7 in permitting efficient entry of T and B lymphocytes into lymph nodes. Maintenance of ILC3s by IL-7 is implicated because ILC3-deficient chimeras exhibit similar trafficking defects.

IL-7 is essential for the development and homeostasis of T and B lymphocytes and is critical for neonatal lymph node organogenesis because Il7^−/− mice lack normal lymph nodes. Whether IL-7 is a continued requirement for normal lymph node structure and function is unknown. To address this, we ablated IL-7 function in normal adult hosts. Either inducible Il7 gene deletion or IL-7R blockade in adults resulted in a rapid loss of lymph node cellularity and a corresponding defect in lymphocyte entry into lymph nodes. Although stromal and dendritic cell components of lymph nodes were present in normal numbers and representation, innate lymphoid cell (ILC) subpopulations were substantially decreased after IL-7 ablation. Testing lymphocyte homing in bone marrow chimeras reconstituted with Rorc^−/− bone marrow confirmed that ILC3s in lymph nodes are required for normal lymphocyte homing. Collectively, our data suggest that maintenance of intact lymph nodes relies on IL-7–dependent maintenance of ILC3 cells.

Tertiary Lymphoid Structures in Cancer: Drivers of Antitumor Immunity, Immunosuppression, or Bystander Sentinels in Disease?

Secondary lymphoid organs are integral to initiation and execution of adaptive immune responses. These organs provide a setting for interactions between antigen-specific lymphocytes and antigen-presenting cells recruited from local infected or inflamed tissues. Secondary lymphoid organs develop as a part of a genetically preprogrammed process during embryogenesis. However, organogenesis of secondary lymphoid tissues can also be recapitulated in adulthood during de novo lymphoid neogenesis of tertiary lymphoid structures (TLSs). These ectopic lymphoid-like structures form in the inflamed tissues afflicted by various pathological conditions, including cancer, autoimmunity, infection, or allograft rejection. Studies are beginning to shed light on the function of such structures in different disease settings, raising important questions regarding their contribution to progression or resolution of disease. Data show an association between the tumor-associated TLSs and a favorable prognosis in various types of human cancer, attracting the speculation that TLSs support effective local antitumor immune responses. However, definitive evidence for the role for TLSs in fostering immune responses in vivo are lacking, with current data remaining largely correlative by nature. In fact, some more recent studies have even demonstrated an immunosuppressive, tumor-promoting role for cancer-associated TLSs. In this review, we will discuss what is known about the development of cancer-associated TLSs and the current understanding of their potential role in the antitumor immune response.

IL-7δ5 protein is expressed in human tissues and induces expression of the oxidized low density lipoprotein receptor 1 (OLR1) in CD14+ monocytes

OBJECTIVES

The 6-exon-spanning 'canonical' Interleukin-7 (IL-7c) is a non-redundant cytokine in human T-cell homeostasis that undergoes extensive alternative pre-mRNA splicing. The IL-7 gene variant lacking, exon 5 (IL-7δ5), exhibits agonistic effects as compared to IL-7c. We studied in this report for the first time the protein expression of IL-7δ5 variant in tissues and its role in monocyte activation.

METHODS

We visualized the expression of IL-7δ5 protein by immunohistochemistry in both healthy and malignant (human) tissues and investigated the impact of IL-7δ5 stimulation on CD14+ monocytes using gene expression analysis and flow cytometry.

RESULTS

IL-7δ5 is largely expressed by human epithelial cells, yet also by stromal cells in malignant lesions. Gene expression analysis in CD14+ monocytes, induced by the 6-exon spanning IL-7 or IL-7δ5 showed similar changes resulting in a pro-inflammatory phenotype and increased expression of genes involved in lipid metabolism. IL7δ5 was superior in inducing upregulation of the oxidised low density lipoprotein receptor (OLR), measured by flow cytometry, in CD14+ cells.

CONCLUSION

IL-7δ5, produced from non-transformed and transformed cells, may contribute to chronic inflammatory responses and development of 'foamy' cells by increased OLR1 expression that mediates increased oxLDL uptake.

Lymph Nodes and Cancer Metastasis: New Perspectives on the Role of Intranodal Lymphatic Sinuses

The lymphatic system is essential for transporting interstitial fluid, soluble antigen, and immune cells from peripheral tissues to lymph nodes (LNs). Functional integrity of LNs is dependent on intact lymphatics and effective lymph drainage. Molecular mechanisms that facilitate interactions between tumor cells and lymphatic endothelial cells (LECs) during tumor progression still remain to be identified. The cellular and molecular structures of LNs are optimized to trigger a rapid and efficient immune response, and to participate in the process of tumor metastasis by stimulating lymphangiogenesis and establishing a premetastatic niche in LNs. Several molecules, e.g., S1P, CCR7-CCL19/CCL21, CXCL12/CXCR4, IL-7, IFN-γ, TGF-β, and integrin α4β1 play an important role in controlling the activity of LN stromal cells including LECs, fibroblastic reticular cells (FRCs) and follicular dendritic cells (DCs). The functional stromal cells are critical for reconstruction and remodeling of the LN that creates a unique microenvironment of tumor cells and LECs for cancer metastasis. LN metastasis is a major determinant for the prognosis of most human cancers and clinical management. Ongoing work to elucidate the function and molecular regulation of LN lymphatic sinuses will provide insight into cancer development mechanisms and improve therapeutic approaches for human malignancy.

NCR1+ cells appear early in GALT development of the ovine foetus and acquire a c-kit+ phenotype towards the end of gestation

The amount, distribution and phenotype of ovine NCR1+ cells were investigated during developing GALT from day 70 of gestation. Antibodies against CD3 and CD79 were used to identify the compartments of GALT, and the localization of NCR1+ cells were correlated within these structures. Markers CD34 and c-kit, in addition to Ki67, were used to investigate possible origin and the stage of development of the NCR1+ cells. NCR1+ cells were present as single cells in the subepithelial tissue as early as 70 days of gestation, and were predominantly present in the T cell rich IFAs and domes as these intestinal wall compartments developed. While NCR1+ cells proliferated more intensively at mid-gestation (70-104 days), the number of NCR1+ cells also expressing c-kit, increased at the end of gestation. In conclusion, NCR1+ cells appeared early in T cell areas of the gut and displayed a phenotype consistent with intermediate stages of cNK cells and/or a subpopulation of ILC22.

Long-Lasting Impact of Neonatal Exposure to Total Body Gamma Radiation on Secondary Lymphoid Organ Structure and Function

The acute period after total body irradiation (TBI) is associated with an increased risk of infection, principally resulting from the loss of hematopoietic stem cells, as well as disruption of mucosal epithelial barriers. Although there is a return to baseline infection control coinciding with the apparent progressive recovery of hematopoietic cell populations, late susceptibility to infection in radiation-sensitive organs such as lung and kidney is known to occur. Indeed, pulmonary infections are particularly prevalent in hematopoietic cell transplant (HCT) survivors, in both adult and pediatric patient populations. Preclinical studies investigating late outcomes from localized thoracic irradiation have indicated that the mechanisms underlying pulmonary delayed effects are multifactorial, including exacerbated and persistent production of pro-inflammatory molecules and abnormal cross-talk among parenchymal and infiltrating immune and inflammatory cell populations. However, in the context of low-dose TBI, it is not clear whether the observed exacerbated response to infection remains contingent on these same mechanisms. It is possible instead, that after systemic radiation-induced injury, the susceptibility to infection may be independently related to defects in alternative organs that are revealed only through the challenge itself; indeed, we have hypothesized that this defect may be due to radiation-induced chronic effects in the structure and function of secondary lymphoid organs (SLO). In this study, we investigated the molecular and cellular alterations in SLO (i.e., spleen, mediastinal, inguinal and mesenteric lymph nodes) after TBI, and the time points when there appears to be immune competence. Furthermore, due to the high incidence of pulmonary infections in the late post-transplantation period of bone marrow transplant survivors, particularly in children, we focused on outcomes in mice irradiated as neonates, which served as a model for a pediatric population, and used the induction of adaptive immunity against influenza virus as a functional end point. We demonstrated that, in adult animals irradiated as neonates, high endothelial venule (HEV) expansion, generation of follicular helper T cells (TFH) and formation of splenic germinal centers (GC) were rapidly and, more importantly, persistently impaired in SLO, suggesting that the early-life exposure to sublethal radiation had long-lasting effects on the induction of humoral immunity. Although the neonatal TBI did not affect the overall outcome from influenza infection in the adults at the earlier time points assessed, we believe that they nonetheless contribute significantly to the increased mortality observed at subsequent late time points. Furthermore, we speculate that the detrimental and persistent impact on the induction of CD4 T- and B-cell responses in the mediastinal lymph nodes will decrease the animals' ability to respond to other aerial pathogens. Since many of these pathogens are normally cleared by antibodies, our findings provide an explanation for the susceptibility of survivors of childhood HCT to life-threatening respiratory tract infections. These findings have implications regarding the need for increased monitoring in pediatric hematopoietic cell transplant patients, since they indicate that there are ongoing and cumulative defects in SLO, which, importantly, develop during the immediate and early postirradiation period when patients may appear immunologically competent. The identification of changes in immune-related signals may offer bioindicators of progressive dysfunction, and of potential mechanisms that could be targeted so as to reduce the risk of infection from extracellular pathogens. Furthermore, these results support the potential susceptibility of the pediatric population to infection after sublethal irradiation in the context of a nuclear or radiological event.

Insight into lymphoid tissue morphogenesis

Secondary lymphoid organs (SLO) are crucial structures for immune-surveillance and rapid immune responses allowing resident lymphocytes to encounter antigen-presenting cells that carry antigens from peripheral tissues. These structures develop during embryonic life through a tightly regulated process that involves interactions between haematopoietic and mesenchymal cells. Importantly, this morphogenesis potential is maintained throughout life since in chronic inflammatory conditions novel "tertiary lymphoid organs" can be generated by processes that are reminiscent of embryonic SLO development. In this review we will discuss early events in SLO morphogenesis, focusing on haematopoietic and mesenchymal cell subsets implicated on the development of lymphoid organs.

Colonic patch and colonic SILT development are independent and differentially regulated events

Intestinal lymphoid tissues have to simultaneously ensure protection against pathogens and tolerance toward commensals. Despite such vital functions, their development in the colon is poorly understood. Here, we show that the two distinct lymphoid tissues of the colon-colonic patches and colonic solitary intestinal lymphoid tissues (SILTs)-can easily be distinguished based on anatomical location, developmental timeframe, and cellular organization. Furthermore, whereas colonic patch development depended on CXCL13-mediated lymphoid tissue inducer (LTi) cell clustering followed by LTα-mediated consolidation, early LTi clustering at SILT anlagen did not require CXCL13, CCR6, or CXCR3. Subsequent dendritic cell recruitment to and gp38(+)VCAM-1(+) lymphoid stromal cell differentiation within SILTs required LTα; B-cell recruitment and follicular dendritic cell differentiation depended on MyD88-mediated signaling, but not the microflora. In conclusion, our data demonstrate that different mechanisms, mediated mainly by programmed stimuli, induce the formation of distinct colonic lymphoid tissues, therefore suggesting that these tissues may have different functions.

Diversity, function, and transcriptional regulation of gut innate lymphocytes

The innate immune system plays a critical early role in host defense against viruses, bacteria, and tumor cells. Until recently, natural killer (NK) cells and lymphoid tissue inducer (LTi) cells were the primary members of the innate lymphocyte family: NK cells form the front-line interface between the external environment and the adaptive immune system, while LTi cells are essential for secondary lymphoid tissue formation. More recently, it has become apparent that the composition of this family is much more diverse than previously appreciated and newly recognized populations play distinct and essential functions in tissue protection. Despite the importance of these cells, the developmental relationships between different innate lymphocyte populations remain unclear. Here we review recent advances in our understanding of the development of different innate immune cell subsets, the transcriptional programs that might be involved in driving fate decisions during development, and their relationship to NK cells.

Human hemato-lymphoid system mice: current use and future potential for medicine

To directly study complex human hemato-lymphoid system physiology and respective system-associated diseases in vivo, human-to-mouse xenotransplantation models for human blood and blood-forming cells and organs have been developed over the past three decades. We here review the fundamental requirements and the remarkable progress made over the past few years in improving these systems, the current major achievements reached by use of these models, and the future challenges to more closely model and study human health and disease and to achieve predictive preclinical testing of both prevention measures and potential new therapies.

Mesenchymal cell differentiation during lymph node organogenesis

Secondary lymphoid tissues such as lymph nodes are essential for the interactions between antigen presenting cells and lymphocytes that result in adaptive immune responses that protect the host against invading pathogens. The specialized architecture of these organs facilitates the cognate interactions between antigen-loaded dendritic cells and lymphocytes expressing their specific receptor as well as B-T cell interactions that are at the core of long lasting adaptive immune responses. Lymph nodes develop during embryogenesis as a result of a series of cross-talk interactions between a hematopoietically derived cell lineage called lymphoid tissue inducer cells and stromal cells of mesenchymal origin to form the anlagen of these organs. This review will present an overview of the different signaling pathways and maturation steps that mesenchymal cells undergo during the process of lymph node formation such as cell specification, priming, and maturation to become lymphoid tissue stromal organizer cells.

Lymphotoxin-β receptor signaling through NF-κB2-RelB pathway reprograms adipocyte precursors as lymph node stromal cells

Lymph node development during embryogenesis involves lymphotoxin-β receptor engagement and subsequent differentiation of a poorly defined population of mesenchymal cells into lymphoid tissue organizer cells. Here, we showed that embryonic mesenchymal cells with characteristics of adipocyte precursors present in the microenvironment of lymph nodes gave rise to lymph node organizer cells. Signaling through the lymphotoxin-β receptor controlled the fate of adipocyte precursor cells by blocking adipogenesis and instead promoting lymphoid tissue stromal cell differentiation. This effect involved activation of the NF-κB2-RelB signaling pathway and inhibition of the expression of the key adipogenic factors Pparγ and Cebpα. In vivo organogenesis assays show that embryonic and adult adipocyte precursor cells can migrate into newborn lymph nodes and differentiate into a variety of lymph node stromal cells. Thus, we propose that adipose tissues act as a source of lymphoid stroma for lymph nodes and other lymphoid structures associated with fat.

IL-7-producing stromal cells are critical for lymph node remodeling

Nonhematopoietic stromal cells of secondary lymphoid organs form important scaffold and fluid transport structures, such as lymph node (LN) trabeculae, lymph vessels, and conduits. Furthermore, through the production of chemokines and cytokines, these cells generate a particular microenvironment that determines lymphocyte positioning and supports lymphocyte homeostasis. IL-7 is an important stromal cell-derived cytokine that has been considered to be derived mainly from T-cell zone fibroblastic reticular cells. We show here that lymphatic endothelial cells (LECs) are a prominent source of IL-7 both in human and murine LNs. Using bacterial artificial chromosome transgenic IL-7-Cre mice, we found that fibroblastic reticular cells and LECs strongly up-regulated IL-7 expression during LN remodeling after viral infection and LN reconstruction after avascular transplantation. Furthermore, IL-7-producing stromal cells contributed to de novo formation of LyveI-positive lymphatic structures connecting reconstructed LNs with the surrounding tissue. Importantly, diphtheria toxin-mediated depletion of IL-7-producing stromal cells completely abolished LN reconstruction. Taken together, this study identifies LN LECs as a major source of IL-7 and shows that IL-7-producing stromal cells are critical for reconstruction and remodeling of the distinct LN microenvironment.

Stroma cell priming in enteric lymphoid organ morphogenesis

The lymphoid system is equipped with a network of specialized platforms located at strategic sites, which grant strict immune-surveillance and efficient immune responses. The development of these peripheral secondary lymphoid organs (SLO) occurs mainly in utero, while tertiary lymphoid structures can form in adulthood generally in response to persistent infection and inflammation. Regardless of the lymphoid tissue and intrinsic cellular and molecular differences, it is now well established that the recruitment of fully functional lymphoid tissue inducer (LTi) cells to presumptive lymphoid organ sites, and their consequent close and reciprocal interaction with resident stroma cells, are central to SLO formation. In contrast, the nature of events that initially prime resident sessile stroma cells to recruit and retain LTi cells remains poorly understood. Recently, new findings revealed early phases of SLO development putting emphasis on mesenchymal and lymphoid tissue initiator cells. Herein we discuss the main tenets of enteric lymphoid organs genesis and focus in the most recent findings that open new perspectives to the understanding of the early phases of lymphoid morphogenesis.

Multifaceted roles of interleukin-7 signaling for the development and function of innate lymphoid cells

Recently, additional innate lymphocyte subsets have been identified that express germline encoded immunoreceptors and respond to cytokine cues. Among these, innate lymphoid cells (ILC) at mucosal surfaces are of significant interest because they were found to play important roles for lymphoid organogenesis, tissue homeostasis and repair, for immunity to various infections but also have been involved as disease-promoting cells in models of chronic inflammatory diseases and of autoimmunity. Their functional and transcriptional programs strikingly resemble that of the various T helper cell subsets suggesting that these programs are already pre-formed in the innate immune system and that these may be more conserved than previously appreciated. Interestingly, all ILC subsets express the interleukin 7 receptor α chain and IL-7 signaling has been involved in various aspects of their developmental and functional programs. Here, we will review the role of IL-7 signaling for the differentiation, maintenance and function of two important ILC subsets, lymphoid tissue inducer cells (i.e., RORγt(+) ILC) and natural helper cells (i.e., type 2 ILC). We will also put emphasis on the recently discovered role of IL-7 in controlling plasticity of RORγt(+) ILC.

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.

IL-7: the global builder of the innate lymphoid network and beyond, one niche at a time

The development and homeostasis of adaptive and innate lymphocytes is dependent on the stromal cytokine IL-7. The initial priming of immune responses to pathogenic challenges is executed by innate lymphoid cells (ILCs) with programmed capacity to rapidly secrete effector cytokines. How ILCs are controlled by IL-7 in distinct anatomical locale has evolved into a more complex problem as IL-7 receptor is not only expressed on ILCs, but also on surrounding neighbors, including vascular endothelium and mesenchymal cells that compete for limiting IL-7. For the generation of γδ T and B cells IL-7 is required for the production of antigen receptors, and it is likely that IL-7 performs critical function in facilitating ILC effector programming in addition to its regulatory actions on cell survival and proliferation. Most of our current understanding of the highly calibrated regulatory circuits of IL-7 function and IL-7 receptor signaling has derived from studies of adaptive, conventional lymphocytes. Here we highlight recent advances in mapping the gene circuits and cellular interactions that regulate temporospatial activities of IL-7 in diverse macro and micro niches that have direct relevance to deciphering the sphere of impact of IL-7 on ILC differentiation.

RANKL induces organized lymph node growth by stromal cell proliferation

RANK and its ligand RANKL play important roles in the development and regulation of the immune system. We show that mice transgenic for Rank in hair follicles display massive postnatal growth of skin-draining lymph nodes. The proportions of hematopoietic and nonhematopoietic stromal cells and their organization are maintained, with the exception of an increase in B cell follicles. The hematopoietic cells are not activated and respond to immunization by foreign Ag and adjuvant. We demonstrate that soluble RANKL is overproduced from the transgenic hair follicles and that its neutralization normalizes lymph node size, inclusive area, and numbers of B cell follicles. Reticular fibroblastic and vascular stromal cells, important for secondary lymphoid organ formation and organization, express RANK and undergo hyperproliferation, which is abrogated by RANKL neutralization. In addition, they express higher levels of CXCL13 and CCL19 chemokines, as well as MAdCAM-1 and VCAM-1 cell-adhesion molecules. These findings highlight the importance of tissue-derived cues for secondary lymphoid organ homeostasis and identify RANKL as a key molecule for controlling the plasticity of the immune system.

Interdependence of stromal and immune cells for lymph node function

Lymph nodes are strategically located throughout the body to allow lymphocytes to efficiently encounter their cognate antigen and become activated. The structure of the lymph nodes is such that B and T lymphocytes each have their own microdomain. This structure is provided by lymph node stromal cells, which also provide the lymphocytes with a scaffold upon which to migrate. Here, we discuss how stromal cells differentiate from mesenchymal precursor cells in response to the interaction with lymphocytes, while these stromal cells in turn provide necessary survival factors for the lymphocytes. We propose that during immune reactions, the interactions of stromal and immune cells are similarly important for controlling the expanding lymphocyte pool.

Functional CD47/signal regulatory protein alpha (SIRP(alpha)) interaction is required for optimal human T- and natural killer- (NK) cell homeostasis in vivo

The homeostatic control mechanisms regulating human leukocyte numbers are poorly understood. Here, we assessed the role of phagocytes in this process using human immune system (HIS) BALB/c Rag2(-/-)IL-2Rγc(-/-) mice in which human leukocytes are generated from transplanted hematopoietic progenitor cells. Interactions between signal regulatory protein alpha (SIRPα; expressed on phagocytes) and CD47 (expressed on hematopoietic cells) negatively regulate phagocyte activity of macrophages and other phagocytic cells. We previously showed that B cells develop and survive robustly in HIS mice, whereas T and natural killer (NK) cells survive poorly. Because human CD47 does not interact with BALB/c mouse SIRPα, we introduced functional CD47/SIRPα interactions in HIS mice by transducing mouse CD47 into human progenitor cells. Here, we show that this procedure resulted in a dramatic and selective improvement of progenitor cell engraftment and human T- and NK-cell homeostasis in HIS mouse peripheral lymphoid organs. The amount of engrafted human B cells also increased but much less than that of T and NK cells, and total plasma IgM and IgG concentrations increased 68- and 35-fold, respectively. Whereas T cells exhibit an activated/memory phenotype in the absence of functional CD47/SIRPα interactions, human T cells accumulated as CD4(+) or CD8(+) single-positive, naive, resting T cells in the presence of functional CD47/SIRPα interactions. Thus, in addition to signals mediated by T cell receptor (TCR)/MHC and/or IL/IL receptor interactions, sensing of cell surface CD47 expression by phagocyte SIRPα is a critical determinant of T- and NK-cell homeostasis under steady-state conditions in vivo.

Humanized mouse models of HIV infection

Because of the limited tropism of HIV, in vivo modeling of this virus has been almost exclusively limited to other lentiviruses, such as simian immunodeficiency virus, that reproduce many important characteristics of HIV infection. However, there are significant genetic and biological differences among lentiviruses and some HIV-specific interventions are not effective against other lentiviruses in nonhuman hosts. For these reasons, much emphasis has recently been placed on developing alternative animal models that support HIV replication and recapitulate key aspects of HIV infection and pathogenesis in humans. Humanized mice, CD34+ hematopoietic progenitor cell transplanted immunodeficient mice, and in particular mice also implanted with human thymus/liver tissue (bone marrow liver thymus mice) that develop a functional human immune system, have been the focus of a great deal of attention as possible models to study virtually all aspects of HIV biology and pathogenesis. Humanized mice are systemically reconstituted with human lymphoid cells, offering rapid, reliable, and reproducible experimental systems for HIV research. Peripheral blood of humanized mice can be readily sampled longitudinally to assess reconstitution with human cells and to monitor HIV replication, permitting the evaluation of multiple parameters of HIV infection such as viral load levels, CD4+ T-cell depletion, immune activation, as well as the effects of therapeutic interventions. Of high relevance to HIV transmission is the extensive characterization and validation of the reconstitution with human lymphoid cells of the female reproductive tract and of the gastrointestinal tract of humanized bone marrow liver thymus mice that renders them susceptible to both vaginal and rectal HIV infection. Other important attributes of all types of humanized mice include: (i) their small size and cost that make them widely accessible; (ii) multiple cohorts of humanized mice can be made from multiple human donors and each cohort has identical human cells, permitting control of intragenetic variables; (iii) continuous de novo production of human immune cells from the transplanted CD34+ cells within each humanized mouse facilitates long-term experiments; (iv) both primary and laboratory HIV isolates can be used for experiments; and (v) in addition to therapeutic interventions, rectal and vaginal HIV prevention approaches can be studied. In summary, humanized mice can have an important role in virtually all aspects of HIV research, including the analysis of HIV replication, the evaluation of HIV restriction factors, the characterization of successful biomedical HIV prevention strategies, the evaluation of new treatment regimens, and the evaluation of novel HIV eradication strategies.

Programmed death-1 is required for systemic self-tolerance in newly generated T cells during the establishment of immune homeostasis

Lymphopenia driven T cell activation is associated with autoimmunity. That lymphopenia does not always lead to autoimmunity suggests that control mechanisms may exist. We assessed the importance of the co-inhibitory receptor programmed death-1 (PD-1) in the control of lymphopenia-driven autoimmunity in newly generated T cells vs. established peripheral T cells and in thymic selection. PD-1 was not required for negative selection in the thymus or for maintenance of self tolerance following transfer of established PD-1⁻/⁻ peripheral T cells to a lymphopenic host. In contrast, PD-1 was essential for systemic self tolerance in newly generated T cells under lymphopenic conditions, as PD-1⁻/⁻ recent thymic emigrants (RTE), generated after transfer of PD-1⁻/⁻ hematopoietic stem cell (HSC) precursors or thymocytes into lymphopenic adult Rag⁻/⁻ recipients, induced a rapidly lethal multi-organ inflammatory disease. Disease could be blocked by using lymph node deficient recipients, indicating that lymphopenia driven PD-1⁻/⁻ T cell activation required access to sufficient lymph node stroma. These data suggested that PD-1⁻/⁻ mice themselves might be substantially protected from autoimmunity because their T cell repertoire is first generated early in life, a period naturally deficient in lymph node stroma. Consistent with this idea, neonatal Rag⁻/⁻ recipients of PD-1⁻/⁻ HSC were resistant to disease. Thus, a critical role of PD-1 resides in the control of RTE in lymphopenia. The data suggest that PD-1 and a paucity of lymphoid stroma cooperate to control autoimmunity in newly generated T cells. Clinical therapies for autoimmune disease employing lymphoablation and hematopoietic stem cell transplantation will need to take into account functional polymorphisms in the PD-1 pathway, if the treatment is to ameliorate rather than exacerbate autoimmunity.

Shared dependence on the DNA-binding factor TOX for the development of lymphoid tissue-inducer cell and NK cell lineages

TOX is a DNA-binding factor required for development of CD4(+) T cells, natural killer T cells and regulatory T cells. Here we document that both natural killer (NK) cell development and lymphoid tissue organogenesis were also inhibited in the absence of TOX. We found that the development of lymphoid tissue-inducer cells, a rare subset of specialized cells that has an integral role in lymphoid tissue organogenesis, required TOX. Tox was upregulated considerably in immature NK cells in the bone marrow, consistent with the loss of mature NK cells in the absence of this nuclear protein. Thus, many cell lineages of the immune system share a TOX-dependent step for development.

New insights into the development of lymphoid tissues

Secondary lymphoid organs are important locations for the initiation of adaptive immune responses. They develop before birth, and their formation requires interaction between lymphotoxin-α₁ß₂-expressing lymphoid-tissue inducer cells and lymphotoxin-ß receptor-expressing stromal organizer cells. Here, we discuss new insights into the earliest phases of peripheral lymph node and Peyer's patch formation that occur before lymphotoxin-ß receptor signalling and suggest a role for the developing nervous system. In addition, we discuss the differing requirements for the postnatal formation of mucosa-associated lymphoid tissues and tertiary lymphoid structures that develop at sites of chronic inflammation.

Visualisation of lymphoid organ development

This chapter provides information on imaging tools that can be employed to visualise and study lymphoid organ development. We focus on the use of genetically modified mouse models that take advantage of fluorescent protein expression in discrete cell populations, thus allowing live cell imaging during lymphoid organogenesis. We discuss approaches that allow characterisation of the cell types involved in the formation of lymphoid organs, including (i) functional assays in explant organ cultures and (ii) high-resolution whole-mount immunostaining methods, which are useful for the characterisation of specific cell populations in the context of the whole developing organ.

Effects of splenectomy on spontaneously chronic pancreatitis in aly/aly mice

Background and Aim. Mice with alymphoplasia (aly/aly) mutation characterized by a lack of lymph nodes, Peyer's patches, and well-defined lymphoid follicles in the spleen were found. In this study, we used splenectomized aly/aly mice to elucidate the effects of secondary lymphoid organs in the development of aly/aly autoimmune pancreatitis. Methods. Forty-eight 10-week-old aly/aly mice were divided into two groups for splenectomy and sham operation. Histological and immunohistochemical analyses of the pancreas were performed at the ages of 20, 30, and 40 weeks old after operation, respectively. Results. Our results showed that mononuclear cell infiltration was restricted to the interlobular connective tissues at the age of 20 weeks, and not increase obviously at the age of 30 and 40 weeks in splenectomized aly/aly mice. Furthermore, an apparent decrease in the expressions of CD4⁺ T, CD8⁺ T, and B cells was detected in the pancreatic tissues compared with sham aly/aly mice, however, no significant difference in macrophage expression between mice with and without a splenectomy. Conclusions. Inflammation infiltration and development of the pancreatitis in aly/aly mice were suppressed effectively after splenectomy, which was, at least partly, correlated to inhibition of the infiltration of T and B cells in pancreatic tissues but not to macrophages.

Ontogeny of stromal organizer cells during lymph node development

The development of secondary lymphoid organs, such as lymph nodes (LNs), in the embryo results from the reciprocal action between lymphoid tissue inducer (LTi) cells and stromal cells. However, the initial events inducing LN anlagen formation before the LTi stromal cells cross-talk interactions take place are not fully elucidated. In this study, we show that the inguinal LN anlagen in mouse embryos developed from mesenchymal cells surrounding the lymph sacs, spherical structures of endothelial cells that bud from veins. Using inguinal and mesenteric LNs (mLNs), we provide evidence supporting a two-step maturation model for stromal cells: first, ICAM-1(-)VCAM-1(-) mesenchymal precursor cells become ICAM-1(int)VCAM-1(int) cells, in a process independent of LTi cells and lymphotoxin beta receptor (LTbetaR) signaling. The second step involves the maturation of ICAM-1(int)VCAM-1(int) cells to ICAM-1(high)VCAM-1(high) mucosal addressin cell adhesion molecule-1(+) organizer cells and depends on both LTi cells and LTbetaR. Addition of alphaLTbetaR agonist to LN organ cultures was sufficient to induce ICAM-1(int)VCAM-1(int) cells to mature. In LtbetaR(-/-) embryos, both inguinal and mLN stromal cells showed a block at the ICAM-1(int)VCAM-1(int) stage, and, contrary to inguinal LNs, mLNs persist longer and contained LTi cells, which correlated with the sustained gene expression of Il-7, Cxcl13, and, to a lesser degree, Ccl21. Taken together, these results highlight the importance of the signals and cellular interactions that induce the maturation of stromal cells and ultimately lead to the formation of lymphoid tissues.

The IL-7 signaling pathway regulates lymph node development independent of peripheral lymphocytes

Lymph node (LN) organogenesis is initiated by the interaction between hematopoietic lymphoid tissue inducer (LTi) cells and the mesenchymal organizer cells. Mice in which the IL-7 signaling pathway has been disrupted have a severe defect in LN development; however, the reasons underlying this defect are as yet unknown. In this study, we show that the overexpression of thymic stromal lymphopoietin (TSLP) increased LTi cell numbers and restored LN development in IL-7(-/-) and RAG2(-/-) gamma(c)(-/-) mice. The TSLP-mediated LN restoration was strictly dependent on LTi cells and independent of lymphocyte colonization. Increased LTi cell numbers in the LN anlagen of RAG2(-/-) gamma(c)(-/-) TSLP transgenic mice were associated with the restoration of organizer cells, suggesting that LTi cell number is a critical parameter for LN organogenesis. Our results shed light on the minimal cellular requirement for LN development during ontogeny. We show that the presence of LTi and organizer cells, but not of peripheral lymphocytes, is critical for LN development and persistence and further suggest that the IL-7 signaling pathway regulates LN organogenesis by controlling the size of the LTi cell pool.

Natural killer receptors: the burden of a name

A population of cells that expresses the NK cell receptor NKp46 and produces interleukin (IL)-22 have recently attracted considerable attention. The identity of these cells is still the subject of speculation, being variably defined as a novel NK cell subset or as a population containing conventional NK (cNK) cell precursors. In this issue, two studies shed light on this conundrum, demonstrating that NKp46(+) IL-22(+) cells and cNK cells belong to distinct lineages.

IL7-hCD25 and IL7-Cre BAC transgenic mouse lines: new tools for analysis of IL-7 expressing cells

IL-7 is a cytokine that is required for T-cell development and homeostasis as well as for lymph node organogenesis. Despite the importance of IL-7 in the immune system and its potential therapeutic relevance, questions remain regarding the sites of IL-7 synthesis, specific cell types involved and molecular mechanisms regulating IL-7 expression. To address these issues, we generated two bacterial artificial chromosome (BAC) transgenic mouse lines in which IL-7 regulatory elements drive expression of either Cre recombinase or a human CD25 (hCD25) cell surface reporter molecule. Expression of the IL-7.hCD25 BAC transgene, detected by reactivity with anti-hCD25 antibody, mimicked endogenous IL-7 expression. Fetal and adult tissues from crosses between IL-7.Cre transgenic mice and Rosa26R or R26-EYFP reporters demonstrated X-gal or YFP staining in tissues known to express endogenous IL-7 at some stage during development. These transgenic lines provide novel genetic tools to identify IL-7 producing cells in various tissues and to manipulate gene expression selectively in IL-7 expressing cells.

Secondary lymphoid organs: responding to genetic and environmental cues in ontogeny and the immune response

Secondary lymphoid organs (SLOs) include lymph nodes, spleen, Peyer's patches, and mucosal tissues such as the nasal-associated lymphoid tissue, adenoids, and tonsils. Less discretely anatomically defined cellular accumulations include the bronchus-associated lymphoid tissue, cryptopatches, and isolated lymphoid follicles. All SLOs serve to generate immune responses and tolerance. SLO development depends on the precisely regulated expression of cooperating lymphoid chemokines and cytokines such as LTalpha, LTbeta, RANKL, TNF, IL-7, and perhaps IL-17. The relative importance of these factors varies between the individual lymphoid organs. Participating in the process are lymphoid tissue initiator, lymphoid tissue inducer, and lymphoid tissue organizer cells. These cells and others that produce crucial cytokines maintain SLOs in the adult. Similar signals regulate the transition from inflammation to ectopic or tertiary lymphoid tissues.

Interleukin-22-producing innate immune cells: new players in mucosal immunity and tissue repair?

Mucosal tissues, lying at the interface with the external environment, are constantly challenged by microbial, physical and chemical assaults. To provide the necessary immune defence to such challenges, lymph nodes and Peyer's patches are formed in utero in response to inductive signals from lymphoid-tissue inducer (LTi) cells. As discussed in this Progress article, a series of recent reports has identified a population of interleukin-22-producing mucosal cells in the gut and tonsils that share features with both LTi cells (by expressing RORgammat) and natural killer cells (by expressing NKp46) and that might be involved in immunity and homeostasis in mucosal tissues.

Human fetal lymphoid tissue-inducer cells are interleukin 17-producing precursors to RORC+ CD127+ natural killer-like cells

The human body contains over 500 individual lymph nodes, yet the biology of their formation is poorly understood. Here we identify human lymphoid tissue-inducer cells (LTi cells) as lineage-negative RORC+ CD127+ cells with the functional ability to interact with mesenchymal cells through lymphotoxin and tumor necrosis factor. Human LTi cells were committed natural killer (NK) cell precursors that produced interleukin 17 (IL-17) and IL-22. In vitro, LTi cells gave rise to RORC+ CD127+ NK cells that retained the ability to produce IL-17 and IL-22. Postnatally, similar populations of LTi cell-like cells and RORC+ CD127+ NK cells were present in tonsils, and both secreted IL-17 and IL-22 but no interferon-gamma. Our data indicate that lymph node organogenesis is controlled by an NK cell precursor population with adaptive immune features and demonstrate a previously unappreciated link between the innate and adaptive immune systems.

Development of secondary lymphoid organs

Secondary lymphoid organs develop during embryogenesis or in the first few weeks after birth according to a highly coordinated series of interactions between newly emerging hematopoietic cells and immature mesenchymal or stromal cells. These interactions are orchestrated by homeostatic chemokines, cytokines, and growth factors that attract hematopoietic cells to sites of future lymphoid organ development and promote their survival and differentiation. In turn, lymphotoxin-expressing hematopoietic cells trigger the differentiation of stromal and endothelial cells that make up the scaffolding of secondary lymphoid organs. Lymphotoxin signaling also maintains the expression of adhesion molecules and chemokines that govern the ultimate structure and function of secondary lymphoid organs. Here we describe the current paradigm of secondary lymphoid organ development and discuss the subtle differences in the timing, molecular interactions, and cell types involved in the development of each secondary lymphoid organ.

Modulation of lymphocyte function with inhibitory CD2: loss of NK and NKT cells

Analysis of the NK cell developmental pathway suggests that CD2 expression may be important in regulating NK maturation. To test this hypothesis, we developed mice containing only an inhibitory CD2 molecule by linking the extracellular domain of CD2 to an intracellular immunoreceptor tyrosine-based inhibitory motif (ITIM) motif. Mice containing the CD2 Tg(ITIM) transgene, introduced into a CD2 KO background, have no morphologically detectable lymph nodes, although development of the thymus appears normal. In addition, these mice had major loss of both NK and NKT subsets in peripheral organs, while T and B cell frequencies were intact. Expression of CD2 was low on T cells and lacking on B cells and functional defects were observed in these populations. NKT cells expressing CD4 were absent, while the CD8+ and double negative NKT cells were retained. Small subsets of NK cells were detected but expression of CD2 on these cells was very low or absent, and their maturation was impaired. Based on the phenotype described here, we believe that these mice represent a unique model to study lymphoid organ and lymphocyte development.

From induced to programmed lymphoid tissues: the long road to preempt pathogens

In mammalian fetuses, ontogeny dictates the development of secondary lymphoid tissues, the spleen, the lymph nodes and the Peyer's patches. In adults, commensals, infectious microbes and inflammation induce the formation of small reversible lymphoid tissues resembling lymph nodes or Peyer's patches. Crucial to lymphoid-tissue genesis is lymphotoxin-beta receptor-mediated activation of inflammatory effectors and structural chemokines. This pathway might have existed since the advent of the vertebrate lymphocyte system to organize local adaptive immunity in response to microbes; however, it only operates in the absence of microbes in mammals, resulting in the development of secondary lymphoid tissues and in preempting pathogens.

Human-hemato-lymphoid-system mice: opportunities and challenges

With the recent advances in human-hemato-lymphoid-system mice, this commentary discusses the utility of these mice and further improvements required to generate an accessible system that allows predictive in vivo human hematology and immunology research.