Separation of splenic red and white pulp occurs before birth in a LTalphabeta-independent manner

High-dimensional intravital microscopy reveals major changes in splenic immune system during postnatal development

Spleen is a key organ for immunologic surveillance, acting as a firewall for antigens and parasites that spread through the blood. However, how spleen leukocytes evolve across the developmental phase, and how they spatially organize and interact in vivo is still poorly understood. Using a novel combination of selected antibodies and fluorophores to image in vivo the spleen immune environment, we described for the first time the dynamics of immune development across postnatal period. We found that spleens from adults and infants had similar numbers and arrangement of lymphoid cells. In contrast, splenic immune environment in newborns is sharply different from adults in almost all parameters analysed. Using this in vivo approach, B cells were the most frequent subtype throughout the development. Also, we revealed how infections - using a model of malaria - can change the spleen immune profile in adults and infants, which could become the key to understanding different severity grades of infection. Our new imaging solutions can be extremely useful for different groups in all areas of biological investigation, paving a way for new intravital approaches and advances.

A diverse fibroblastic stromal cell landscape in the spleen directs tissue homeostasis and immunity

[Figure: see text].

Alternative pathways for the development of lymphoid structures in humans

Lymphoid tissue inducer (LTi) cells are critical for inducing the differentiation of most secondary lymphoid organs (SLOs) in mice. In humans, JAK3 and γc deficiencies result in severe combined immunodeficiency (SCIDs) characterized by an absence of T cells, natural killer cells, innate lymphoid cells (ILCs), and presumably LTi cells. Some of these patients have undergone allogeneic stem cell transplantation (HSCT) in the absence of myeloablation, which leads to donor T cell engraftment, while other leukocyte subsets are of host origin. By using MRI to look for SLOs in nine of these patients 16 to 44 y after HSCT, we discovered that SLOs were exclusively found in the three areas of the abdomen that drain the intestinal tract. A postmortem examination of a child with γc-SCID who had died 3.5 mo after HSCT showed corticomedullary differentiation in the thymus, T cell zones in the spleen, and the appendix, but in neither lymph nodes nor Peyer patches. Tertiary lymphoid organs were observed in the lung. No RAR-related orphan receptor-positive LTi cells could be detected in the existing lymphoid structures. These results suggest that while LTi cells are required for the genesis of most SLOs in humans, SLO in the appendix and in gut-draining areas, as well as tertiary lymphoid organs, can be generated likely by LTi cell-independent mechanisms.

Transcription factor Tlx1 marks a subset of lymphoid tissue organizer-like mesenchymal progenitor cells in the neonatal spleen

The spleen is comprised of spatially distinct compartments whose functions, such as immune responses and removal of aged red blood cells, are tightly controlled by the non-hematopoietic stromal cells that provide regionally-restricted signals to properly activate hematopoietic cells residing in each area. However, information regarding the ontogeny and relationships of the different stromal cell types remains limited. Here we have used in vivo lineage tracing analysis and in vitro mesenchymal stromal cell assays and found that Tlx1, a transcription factor essential for embryonic spleen organogenesis, marks neonatal stromal cells that are selectively localized in the spleen and retain mesenchymal progenitor potential to differentiate into mature follicular dendritic cells, fibroblastic reticular cells and marginal reticular cells. Furthermore, by establishing a novel three-dimensional cell culture system that enables maintenance of Tlx1-expressing cells in vitro, we discovered that signals from the lymphotoxin β receptor and TNF receptor promote differentiation of these cells to express MAdCAM-1, CCL19 and CXCL13, representative functional molecules expressed by different subsets of mature stromal cells in the spleen. Taken together, these findings indicate that mesenchymal progenitor cells expressing Tlx1 are a subset of lymphoid tissue organizer-like cells selectively found in the neonatal spleen.

Perivascular Fibroblasts of the Developing Spleen Act as LTα1β2-Dependent Precursors of Both T and B Zone Organizer Cells

T and B cell compartmentalization is a hallmark of secondary lymphoid organs and is maintained by chemokine-expressing stromal cells. How this stromal cell network initially develops and differentiates into two distinct subsets is poorly known, especially for the splenic white pulp (WP). Here, we show that perivascular fibroblast precursors are triggered by LTα1β2 signals to expand, express CCL19/21, and then differentiate into two functionally distinct fibroblast subsets responsible for B and T cell clustering and WP compartmentalization. Failure to express or sense CCL19 leads to impaired T zone development, while lack of B cells or LTα1β2 leads to an earlier and stronger impairment in WP development. We therefore propose that WP development proceeds in multiple steps, with LTα1β2⁺ B cells acting as major inducer cells driving the expansion and gradual differentiation of perivascular fibroblasts into T and B zone organizer cells.

Origin and specialization of splenic macrophages

Macrophage heterogeneity in the spleen has been long documented, with four subsets populating the different splenic compartments. The diverse environments on the splenic compartments determine their varied phenotype and functions. In the white pulp, highly phagocytic macrophages contribute to the generation of the immune response. The marginal zone contains two populations of macrophages, which also contribute to the immune response. Their strategic position in the bloodstream and their unique phenotype confer them a crucial role in the defense against blood borne pathogens, placing them at the crossroad between innate and adaptive immune responses. Macrophages in the red pulp are classically linked to homeostatic and metabolic functions in erythrocyte phagocytosis and iron recycling. We review here recent advances demonstrating the importance of macrophage ontogeny and organ development in determining the phenotype, transcriptional profile and, ultimately, the functions of the populations of splenic macrophages.

Spleen development is modulated by neonatal gut microbiota

The full development of the mammalian immune system occurs after birth upon exposure to non self-antigens. The gut is the first site of bacterial colonization where it is crucial to create the appropriate microenvironment able to balance effector or tolerogenic responses to external stimuli. It is a well-established fact that at mucosal sites bacteria play a key role in developing the immune system but we ignore how colonising bacteria impact the maturation of the spleen. Here we addressed this issue. Taking advantage of the fact that milk SIgA regulates bacterial colonization of the newborn intestine, we generated immunocompetent mice born either from IgA pro-efficient or IgA deficient females. Having demonstrated that SIgA in maternal milk modulates neonatal gut microbiota by promoting an increased diversity of the colonizing species we also found that immunocompetent pups, not exposed to milk SIgA, fail to properly develop the FDC network and primary follicles in the spleen compromising the response to T-dependent antigens. The presence of a less diverse microbiota with a higher representation of pathogenic species leads to a fast replenishment of the marginal zone and the IgM plasma cell compartment of the spleen as well as IgA plasma cells in the gut.

Origin and Immunological Functions of Spleen Stromal Cells

The mammalian spleen is a peripheral lymphoid organ that plays a central role in host defense. Consequently, the lack of spleen is often associated with immunodeficiency and increased risk of overwhelming infections. Growing evidence suggests that non-hematopoietic stromal cells are central players in spleen development, organization, and immune functions. In addition to its immunological role, the spleen also provides a site for extramedullary hematopoiesis (EMH) in response to injuries. A deeper understanding of the biology of stromal cells is therefore essential to fully comprehend how these cells modulate the immune system during normal and pathological conditions. Here, we review the specificities of the different mouse spleen stromal cell subsets and complement the murine studies with human data when available.

"Double-duty" conventional dendritic cells in the amphibian Xenopus as the prototype for antigen presentation to B cells

Two populations of dendritic cells (DCs) are found in mammals, one derived from hematopoietic precursors (conventional/cDC), and another derived from mesenchymal precursors, the follicular DC (FDC); the latter is specialized for antigen presentation to B cells, and has only been definitively demonstrated in mammals. Both cDC and FDC are necessary for induction of germinal centers (GC) and GC-dependent class switch recombination (CSR) and somatic hypermutation (SHM). We demonstrate that in Xenopus, an amphibian in which immunoglobulin CSR and SHM occur without GC formation, a single type of DC has properties of both cDC and FDC, including high expression of MHC class II for the former and display of native antigen at the cell surface for the latter. Our data confirm that the advent of FDC functionality preceded emergence of bona fide FDC, which was in turn crucial for the development of GC formation and efficient affinity maturation in mammals.

Determinants of postnatal spleen tissue regeneration and organogenesis

The spleen is an organ that filters the blood and is responsible for generating blood-borne immune responses. It is also an organ with a remarkable capacity to regenerate. Techniques for splenic auto-transplantation have emerged to take advantage of this characteristic and rebuild spleen tissue in individuals undergoing splenectomy. While this procedure has been performed for decades, the underlying mechanisms controlling spleen regeneration have remained elusive. Insights into secondary lymphoid organogenesis and the roles of stromal organiser cells and lymphotoxin signalling in lymph node development have helped reveal similar requirements for spleen regeneration. These factors are now considered in the regulation of embryonic and postnatal spleen formation, and in the establishment of mature white pulp and marginal zone compartments which are essential for spleen-mediated immunity. A greater understanding of the cellular and molecular mechanisms which control spleen development will assist in the design of more precise and efficient tissue grafting methods for spleen regeneration on demand. Regeneration of organs which harbour functional white pulp tissue will also offer novel opportunities for effective immunotherapy against cancer as well as infectious diseases.

Impaired lymphoid extracellular matrix impedes antibacterial immunity in epidermolysis bullosa

Genetic loss of collagen VII causes recessive dystrophic epidermolysis bullosa (RDEB), a skin fragility disorder that, unexpectedly, manifests also with elevated colonization of commensal bacteria and frequent wound infections. Here, we describe an unprecedented systemic function of collagen VII as a member of a unique innate immune-supporting multiprotein complex in spleen and lymph nodes. In this complex, collagen VII specifically binds and sequesters the innate immune activator cochlin in the lumen of lymphoid conduits. In genetic mouse models, loss of collagen VII increased bacterial colonization by diminishing levels of circulating cochlin LCCL domain. Intraperitoneal injection of collagen VII, which restored cochlin in the spleen, but not in the skin, reactivated peripheral innate immune cells via cochlin and reduced bacterial skin colonization. Systemic administration of the cochlin LCCL domain was alone sufficient to diminish bacterial supercolonization of RDEB mouse skin. Human validation demonstrated that RDEB patients displayed lower levels of systemic cochlin LCCL domain with subsequently impaired macrophage response in infected wounds. This study identifies an intrinsic innate immune dysfunction in RDEB and uncovers a unique role of the lymphoid extracellular matrix in systemic defense against bacteria.

The evolution of innate lymphoid cells

Innate lymphoid cells (ILCs) are the most recently discovered group of immune cells. Understanding their biology poses many challenges. We discuss here the current knowledge on the appearance of ILC subsets during evolution and propose how the connection between ILCs and T cells contributes to the robustness of immunity and hence to the fitness of the hosts.

Stromal Cell Subsets Directing Neonatal Spleen Regeneration

Development of lymphoid tissue is determined by interactions between stromal lymphoid tissue organiser (LTo) and hematopoietic lymphoid tissue inducer (LTi) cells. A failure for LTo to receive appropriate activating signals during embryogenesis through lymphotoxin engagement leads to a complete cessation of lymph node (LN) and Peyer's patch development, identifying LTo as a key stromal population for lymphoid tissue organogenesis. However, little is known about the equivalent stromal cells that induce spleen development. Here, by dissociating neonatal murine spleen stromal tissue for re-aggregation and transplant into adult mouse recipients, we have identified a MAdCAM-1⁺CD31⁺CD201⁺ spleen stromal organizer cell-type critical for new tissue formation. This finding provides an insight into the regulation of post-natal spleen tissue organogenesis, and could be exploited in the development of spleen regenerative therapies.

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.

Stromal Fibroblasts in Tertiary Lymphoid Structures: A Novel Target in Chronic Inflammation

Tertiary lymphoid structures (TLS) are organized aggregates of lymphocytes, myeloid, and stromal cells that provide ectopic hubs for acquired immune responses. TLS share phenotypical and functional features with secondary lymphoid organs (SLO); however, they require persistent inflammatory signals to arise and are often observed at target sites of autoimmune disease, chronic infection, cancer, and organ transplantation. Over the past 10 years, important progress has been made in our understanding of the role of stromal fibroblasts in SLO development, organization, and function. A complex and stereotyped series of events regulate fibroblast differentiation from embryonic life in SLOs to lymphoid organ architecture observed in adults. In contrast, TLS-associated fibroblasts differentiate from postnatal, locally activated mesenchyme, predominantly in settings of inflammation and persistent antigen presentation. Therefore, there are critical differences in the cellular and molecular requirements that regulate SLO versus TLS development that ultimately impact on stromal and hematopoietic cell function. These differences may contribute to the pathogenic nature of TLS in the context of chronic inflammation and malignant transformation and offer a window of opportunity for therapeutic interventions in TLS associated pathologies.

Emergence and Evolution of Secondary Lymphoid Organs

For effective adaptive immunity to foreign antigens (Ag), secondary lymphoid organs (SLO) provide the confined environment in which Ag-restricted lymphocytes, with very low precursor frequencies, interact with Ag on Ag-presenting cells (APC). The spleen is the primordial SLO, arising in conjunction with adaptive immunity in early jawed vertebrates. The spleen, especially the spleen's lymphoid compartment, the white pulp (WP), has undergone numerous modifications over evolutionary time. We describe the progressive advancement of splenic WP complexity, which evolved in parallel with the increasing functionality of adaptive immunity. The Ag-presenting function of follicular dendritic cells (FDC) also likely emerged at the inception of adaptive immunity, and we propose that a single type of hematopoietically derived APC displayed Ag to both T and B cells. A dedicated FDC, derived from a vascular precursor, is a recent evolutionary innovation that likely permitted the robust affinity maturation found in mammals.

Divergence of Vascular Specification in Visceral Lymphoid Organs-Genetic Determinants and Differentiation Checkpoints

Despite their functional similarities, peripheral lymphoid tissues are remarkably different according to their developmental properties and structural characteristics, including their specified vasculature. Access of leukocytes to these organs critically depends on their interactions with the local endothelium, where endothelial cells are patterned to display a restricted set of adhesion molecules and other regulatory compounds necessary for extravasation. Recent advances in high throughput analyses of highly purified endothelial subsets in various lymphoid tissues as well as the expansion of various transgenic animal models have shed new light on the transcriptional complexities of lymphoid tissue vascular endothelium. This review is aimed at providing a comprehensive analysis linking the functional competence of spleen and intestinal lymphoid tissues with the developmental programming and functional divergence of their vascular specification, with particular emphasis on the transcriptional control of endothelial cells exerted by Nkx2.3 homeodomain transcription factor.

CXCL13 responsiveness but not CXCR5 expression by late transitional B cells initiates splenic white pulp formation

Secondary lymphoid organs (SLO) provide the structural framework for coconcentration of Ag and Ag-specific lymphocytes required for an efficient adaptive immune system. The spleen is the primordial SLO, and evolved concurrently with Ig/TCR:pMHC-based adaptive immunity. The earliest cellular/histological event in the ontogeny of the spleen's lymphoid architecture, the white pulp (WP), is the accumulation of B cells around splenic vasculature, an evolutionarily conserved feature since the spleen's emergence in early jawed vertebrates such as sharks. In mammals, B cells are indispensable for both formation and maintenance of SLO microarchitecture; their expression of lymphotoxin α1β2 (LTα1β2) is required for the LTα1β2:CXCL13 positive feedback loop without which SLO cannot properly form. Despite the spleen's central role in the evolution of adaptive immunity, neither the initiating event nor the B cell subset necessary for WP formation has been identified. We therefore sought to identify both in mouse. We detected CXCL13 protein in late embryonic splenic vasculature, and its expression was TNF-α and RAG-2 independent. A substantial influx of CXCR5(+) transitional B cells into the spleen occurred 18 h before birth. However, these late embryonic B cells were unresponsive to CXCL13 (although responsive to CXCL12) and phenotypically indistinguishable from blood-derived B cells. Only after birth did B cells acquire CXCL13 responsiveness, accumulate around splenic vasculature, and establish the uniquely splenic B cell compartment, enriched for CXCL13-responsive late transitional cells. Thus, CXCL13 is the initiating component of the CXCL13:LTα1β2 positive feedback loop required for WP ontogeny, and CXCL13-responsive late transitional B cells are the initiating subset.

Heterogeneity of stromal cells in the human splenic white pulp. Fibroblastic reticulum cells, follicular dendritic cells and a third superficial stromal cell type

At least three phenotypically and morphologically distinguishable types of branched stromal cells are revealed in the human splenic white pulp by subtractive immunohistological double-staining. CD271 is expressed in fibroblastic reticulum cells of T-cell zones and in follicular dendritic cells of follicles. In addition, there is a third CD2711- and CD271+/) stromal cell population surrounding T-cell zones and follicles. At the surface of follicles the third population consists of individually variable partially overlapping shells of stromal cells exhibiting CD90 (Thy-1), MAdCAM-1, CD105 (endoglin), CD141 (thrombomodulin) and smooth muscle α-actin (SMA) with expression of CD90 characterizing the broadest shell and SMA the smallest. In addition, CXCL12, CXCL13 and CCL21 are also present in third-population stromal cells and/or along fibres. Not only CD27+ and switched B lymphocytes, but also scattered IgD++ B lymphocytes and variable numbers of CD4+ T lymphocytes often occur close to the third stromal cell population or one of its subpopulations at the surface of the follicles. In contrast to human lymph nodes, neither podoplanin nor RANKL (CD254) were detected in adult human splenic white pulp stromal cells. The superficial stromal cells of the human splenic white pulp belong to a widespread cell type, which is also found at the surface of red pulp arterioles surrounded by a mixed T-cell/B-cell population. Superficial white pulp stromal cells differ from fibroblastic reticulum cells and follicular dendritic cells not only in humans, but apparently also in mice and perhaps in rats. However, the phenotype of white pulp stromal cells is species-specific and more heterogeneous than described so far.

Follicular dendritic cells: dynamic antigen libraries

Follicular dendritic cells (FDCs) are essential for high-affinity antibody production and for the development of B cell memory. Historically, FDCs have been characterized as 'accessory' cells that passively support germinal centre (GC) responses. However, recent observations suggest that FDCs actively shape humoral immunity. In this Review, we discuss recent findings concerning the antigen acquisition and retention functions of FDCs, and relevant implications for protective immunity. Furthermore, we describe the roles of FDCs within GCs in secondary lymphoid organs and discuss FDC development within this dynamic environment. Finally, we discuss how a better understanding of FDCs could facilitate the design of next-generation vaccines.

Murine spleen tissue regeneration from neonatal spleen capsule requires lymphotoxin priming of stromal cells

Spleen is a tissue with regenerative capacity, which allows autotransplantation of human spleen fragments to counteract the effects of splenectomy. We now reveal in a murine model that transplant of neonatal spleen capsule alone leads to the regeneration of full spleen tissue. This finding indicates that graft-derived spleen stromal cells, but not lymphocytes, are essential components of tissue neogenesis, a finding verified by transplant and regeneration of Rag1KO spleen capsules. We further demonstrate that lymphotoxin and lymphoid tissue inducer cells participate in two key elements of spleen neogenesis, bulk tissue regeneration and white pulp organization, identifying a lymphotoxin-dependent pathway for neonatal spleen regeneration that contrasts with previously defined lymphotoxin-independent embryonic spleen organogenesis.

Nkx2-5(+)islet1(+) mesenchymal precursors generate distinct spleen stromal cell subsets and participate in restoring stromal network integrity

Secondary lymphoid organ stromal cells comprise different subsets whose origins remain unknown. Herein, we exploit a genetic lineage-tracing approach to show that splenic fibroblastic reticular cells (FRCs), follicular dendritic cells (FDCs), marginal reticular cells (MRCs), and mural cells, but not endothelial cells, originate from embryonic mesenchymal progenitors of the Nkx2-5(+)Islet1(+) lineage. This lineage include embryonic mesenchymal cells with lymphoid tissue organizer (LTo) activity capable also of supporting ectopic lymphoid-like structures and a subset of resident spleen stromal cells that proliferate and regenerate the splenic stromal microenvironment following resolution of a viral infection. These findings identify progenitor cells that generate stromal diversity in spleen development and repair and suggest the existence of multipotent stromal progenitors in the adult spleen with regenerative capacity.

Stromal cells of the mouse spleen

The composition and function of stromal cells in the white pulp of the spleen resemble to a large extent the situation in other secondary lymphoid organs such as lymph nodes. The stromal cells play an important role in the support and guidance of lymphocytes and myeloid cells in the T and B cell zones of the spleen. Major differences of the spleen are found in the way cells enter the white pulp and the composition of stromal cells in the red pulp. In this review, the features of stromal cells of both white and red pulp will be described in light of the function of the spleen.

Absence of Nkx2-3 homeodomain transcription factor induces the formation of LYVE-1-positive endothelial cysts without lymphatic commitment in the spleen

In contrast to peripheral lymph nodes possessing lymphatic and blood vasculature, the spleen in both humans and rodents is largely devoid of functioning lymphatic capillaries. Here it is reported that in mice lacking homeodomain transcription factor Nkx2-3, the spleen contains an extensive network of lymphocyte-filled sacs lined by cells expressing LYVE-1 antigen, a marker associated with lymphatic endothelium cells (LECs). Real-time quantitative PCR analyses of Nkx2-3 mutant spleen revealed a substantial increase of LYVE-1 and podoplanin mRNA levels, without the parallel increase of mRNA for VEGFR-3 (vascular endothelial growth factor receptor Type 3) and Prox1 (Prospero homeobobox protein 1), two markers specific for LECs. Although these structures express VEGFR-2/flk-1, they lack Prox1 protein, indicating their non-LEC endothelial origin. The LYVE-1(+) structures are bordered with ER-TR7(+) fibroblastic reticular cells with small clusters of macrophages expressing MARCO and sialoadhesin. Short-term cell-tracing studies using labeled lymphocytes indicate that these LYVE-1(+) cysts are largely excluded from the systemic circulation. Cells expressing LYVE-1 glycoprotein as putative precursors for such structures are detectable in the spleen of late-stage embryos, and the formation of LYVE-1(+) structures is independent from the activity of lymphotoxin β-receptor. Thus the splenic vascular defects in Nkx2-3 deficiency include the generation of LYVE-1(+) cysts, comprised of endothelial cells without being committed along the LEC lineage.

Stromal cell-immune cell interactions

Interaction between different types of hematopoietic cells is essential for proper functioning of the immune system. For instance, the cytokines produced by antigen-presenting dendritic cells will determine the type of T cell response that is induced. However, hematopoietic cells are also strongly influenced by the surrounding nonhematopoietic cells. The cells that form these microenvironments are collectively called stromal cells. Here, we focus on the stromal cells present within secondary lymphoid organs and discuss their importance for various aspects of the immune system.

Transcription factor Nkx2-3 controls the vascular identity and lymphocyte homing in the spleen

The vasculature in the spleen and peripheral lymph nodes (pLNs) is considerably different, which affects both homing of lymphocytes and antigenic access to these peripheral lymphoid organs. In this paper, we demonstrate that in mice lacking the homeodomain transcription factor Nkx2-3, the spleen develops a pLN-like mRNA expression signature, coupled with the appearance of high endothelial venules (HEVs) that mediate L-selectin-dependent homing of lymphocytes into the mutant spleen. These ectopic HEV-like vessels undergo postnatal maturation and progressively replace MAdCAM-1 by pLN addressin together with the display of CCL21 arrest chemokine in a process that is reminiscent of HEV formation in pLNs. Similarly to pLNs, development of HEV-like vessels in the Nkx2-3-deficient spleen depends on lymphotoxin-β receptor-mediated signaling. The replacement of splenic vessels with a pLN-patterned vasculature impairs the recirculation of adoptively transferred lymphocytes and reduces the uptake of blood-borne pathogens. The Nkx2-3 mutation in BALB/c background causes a particularly disturbed splenic architecture, characterized by the near complete lack of the red pulp, without affecting lymph nodes. Thus, our observations reveal that the organ-specific patterning of splenic vasculature is critically regulated by Nkx2-3, thereby profoundly affecting the lymphocyte homing mechanism and blood filtering capacity of the spleen in a tissue-specific manner.

Lymphoid tissue inducer cells and the evolution of CD4 dependent high-affinity antibody responses

Phylogeny indicates that in mammals memory CD4-dependent antibody responses evolved after monotremes split from the common ancestor of marsupial and eutherian mammals. This was strongly associated with the development of segregated B and T cell areas and the development of a linked lymph node network. The evolution of the lymphotoxin beta receptor in these higher mammals was key to the development of these new functions. Here, we argue that lymphoid tissue inducer cells played a pivotal role not only in the development of organized lymphoid structures but also in the subsequent genesis of the CD4-dependent class-switched memory antibody responses that depend on an organized infrastructure to work. In this review, we concentrate on the role of this cell type in the making of a tolerant CD4 T cell repertoire and in the sustenance of CD4 T cell responses for protective immunity.

Impaired lymphoid organ development in mice lacking the heparan sulfate modifying enzyme glucuronyl C5-epimerase

The development of lymphoid organs depends on cross talk between hematopoietic cells and mesenchymal stromal cells and on vascularization of the lymphoid primordia. These processes are orchestrated by cytokines, chemokines, and angiogenic factors that require tight spatiotemporal regulation. Heparan sulfate (HS) proteoglycans are molecules designed to specifically bind and regulate the bioactivity of soluble protein ligands. Their binding capacity and specificity are controlled by modification of the HS side chain by HS-modifying enzymes. Although HS proteoglycans have been implicated in the morphogenesis of several organ systems, their role in controlling lymphoid organ development has thus far remained unexplored. In this study, we report that modification of HS by the HS-modifying enzyme glucuronyl C5-epimerase (Glce), which controls HS chain flexibility, is required for proper lymphoid organ development. Glce(-/-) mice show a strongly reduced size of the fetal spleen as well as a spectrum of defects in thymus and lymph node development, ranging from dislocation to complete absence of the organ anlage. Once established, however, the Glce(-/-) primordia recruited lymphocytes and developed normal architectural features. Furthermore, Glce(-/-) lymph node anlagen transplanted into wild-type recipient mice allowed undisturbed lymphocyte maturation. Our results indicate that modification of HS by Glce is required for controlling the activity of molecules that are instructive for early lymphoid tissue morphogenesis but may be dispensable at later developmental stages and for lymphocyte maturation and differentiation.

Lymphoid tissue inducer cells: bridges between the ancient innate and the modern adaptive immune systems

Phylogeny indicates that adaptive immunity evolved first in diffusely distributed lymphoid tissues found in the lamina propria (LP) of the gut. B follicular structures appeared later, probably initially in isolated lymphoid follicles in the LP and then in organized lymphoid tissues such as lymph nodes and Peyer's patches. The development of these new lymphoid structures was enabled by gene duplication and evolution of new tumor necrosis family members. Here, we argue that lymphoid tissue inducer cells (LTis) had a pivotal role, not only in the development of organized lymphoid structures, but also in the subsequent genesis of the CD4-dependent class-switched memory antibody responses. In this review, we concentrate on the latter function: the sustenance by LTis of CD4 T-cell responses for protective immunity.

Chemokine CXCL13 is essential for lymph node initiation and is induced by retinoic acid and neuronal stimulation

The location of embryonic lymph node development is determined by the initial clustering of lymphoid tissue-inducer (LTi) cells. Here we demonstrate that both the chemokine CXCL13 and the chemokine CCL21 attracted LTi cells at embryonic days 12.5-14.5 and that initial clustering depended exclusively on CXCL13. Retinoic acid (RA) induced early CXCL13 expression in stromal organizer cells independently of lymphotoxin signaling. Notably, neurons adjacent to the lymph node anlagen expressed enzymes essential for RA synthesis. Furthermore, stimulation of parasymphathetic neural output in adults led to RA receptor (RAR)-dependent induction of CXCL13 in the gut. Therefore, our data show that the initiation of lymph node development is controlled by RA-mediated expression of CXCL13 and suggest that RA may be provided by adjacent neurons.

Transplantation of embryonic spleen tissue reveals a role for adult non-lymphoid cells in initiating lymphoid tissue organization

In this report we describe a transplantation system where embryonic spleens are grafted into adult hosts. This model can be used to analyze the cellular and molecular requirements for the development and organization of splenic microenvironments. Whole embryonic day 15 (ED15) spleens, grafted under the kidney capsule of adult mice, were colonized by host-derived lymphocytes and DC and developed normal splenic architecture. Grafts were also able to form germinal centers in response to T-dependent antigen. Using this system we demonstrated that adult host-derived lymphotoxin (LT) α was sufficient for the development of ED15 LTα^−/− grafts. Grafting of ED15 LTα^−/− spleens into RAG^−/− hosts followed by transfer of LT α^−/− splenocytes revealed no requirement for lymphocyte-derived LT α in the induction of CCL21 or the development of T-zone stroma. These data suggest that interactions between adult lymphoid-tissue inducer-like cells and embryonic stromal cells initiated T-zone development. Furthermore, adult lymphoid tissue inducer-like cells were shown to develop from bone marrow-derived progenitors. The model described here demonstrates a method of transferring whole splenic microenvironments and dissecting the stromal and hematopoietic signals involved in spleen development and organization.