Setting the stage: an anatomist's view of the immune system

Epicutaneous Administration of 17β-Estradiol Induces Langerhans Cells Depletion

BACKGROUND

Langerhans cells (LC) number and function in mouse vaginal mucosa are affected by 17β-estradiol (E₂) application; nonetheless, its effect on epidermal LC has not been studied. The purpose of this study was to evaluate the effect of topical administration of E₂ on the number, phenotype, and migratory ability of LC in mouse skin.

METHODS

Ears of adult CD1 male mice were topically treated once with several doses. Immunohistochemical staining for CD207 and TUNEL staining were performed. LC migration to lymph nodes and the effect on the expression of costimulatory molecules on cultured dendritic cells (DC) were also evaluated.

RESULTS

E₂ decreased the number of CD207+ LC in a dose-dependent manner. One hour after treatment, 1 and 10 µg/mL E₂ significantly reduced the LC number by 21% and 26%, respectively, after two hours, the reduction was 23% and 41%, respectively. After 48 hours, LC recovered, and after 96 hours of treatment, the CD207+/MHCII+ DC numbers were increased in regional lymph nodes. However, CD86 and CD40 molecules were expressed at lower levels than in positive control. The TUNEL assay did not show apoptotic cells. Furthermore, in cultured DC, E₂ promoted a decrease in CD40 and CD86 expression and an increase in CD273, CD274, MHCII, and CCR7.

CONCLUSIONS

The topical administration of E₂ induced a transitory local diminution of LC population and a tolerogenic phenotype. This decrease in epidermal LC suggests that E₂ may affect skin immune responses, inducing an inhibitory response, which should be considered when prescribing topical E₂ medications.

Lymph Nodes-On-Chip: Promising Immune Platforms for Pharmacological and Toxicological Applications

Organs-on-chip are gaining increasing attention as promising platforms for drug screening and testing applications. However, lymph nodes-on-chip options remain limited although the lymph node is one of the main determinants of the immunotoxicity of newly developed pharmacological drugs. In this review, we describe existing biomimetic lymph nodes-on-chip, their design, and their physiological relevance to pharmacology and shed the light on future directions associated with lymph node-on-chip design and implementation in drug discovery and development.

A comprehensive strategy to clarify the pharmacodynamic constituents and mechanism of Wu-tou decoction based on the constituents migrating to blood and their in vivo process under pathological state

ETHNOPHARMACOLOGICAL RELEVANCE

As a traditional Chinese medicine (TCM) formula, Wu-tou decoction has been used for treating rheumatoid arthritis (RA) for more than a thousand years. Identifying pharmacodynamic constituents (PCs) of WTD and exploring their in vivo process are very meaningful for promoting the modernization of TCM. However, the pathological state might change this process.

AIM OF THE STUDY

Hence, it is necessary and significant to compare the process in vivo of drugs both in normal and disease state and clarify their action mechanism.

MATERIALS AND METHODS

Taking Wu-tou decoction (WTD) as the research object, a comprehensive strategy based on liquid chromatography coupled with mass spectrometry (LC-MS) was developed to identify PCs, clarify and compare their absorption and distribution in normal and model rats, and then explore the potential mechanism of TCM. Firstly, the PCs in WTD were identified. Then, the pharmacokinetics (PK) and tissue distribution of these ingredients were studied. Finally, the constituents with the difference between normal and model rats were selected for target network pharmacological analysis to clarify the mechanism.

RESULTS

A total of 27 PCs of WTD were identified. The absorption and distribution of 20 PCs were successfully analyzed. In the disease state, the absorption and distribution of all these components were improved to have better treatment effects. The results of target network pharmacological analysis indicated that PTGS1, PTGS2, ABCB1, SLC6A4, CHRM2, ESR1, ESR2, CDK2, TNF and IL-6 are 10 key targets for WTD against RA. The regulatory effects of WTD on the expression of PTGS2 and TNF were further verified. Pathway enrichment analysis showed that the key mechanism of WTD against RA is to reduce inflammation and regulate the immune response.

CONCLUSION

These results indicated that this strategy could better understand the in vivo process and mechanism of WTD under the pathological state. Furthermore, this strategy is also appropriate for other TCM.

Multi-Compartment 3D-Cultured Organ-on-a-Chip: Towards a Biomimetic Lymph Node for Drug Development

The interaction of immune cells with drugs and/or with other cell types should be mechanistically investigated in order to reduce attrition of new drug development. However, they are currently only limited technologies that address this need. In our work, we developed initial but significant building blocks that enable such immune-drug studies. We developed a novel microfluidic platform replicating the Lymph Node (LN) microenvironment called LN-on-a-chip, starting from design all the way to microfabrication, characterization and validation in terms of architectural features, fluidics, cytocompatibility, and usability. To prove the biomimetics of this microenvironment, we inserted different immune cell types in a microfluidic device, which showed an in-vivo-like spatial distribution. We demonstrated that the developed LN-on-a-chip incorporates key features of the native human LN, namely, (i) similarity in extracellular matrix composition, morphology, porosity, stiffness, and permeability, (ii) compartmentalization of immune cells within distinct structural domains, (iii) replication of the lymphatic fluid flow pattern, (iv) viability of encapsulated cells in collagen over the typical timeframe of immunotoxicity experiments, and (v) interaction among different cell types across chamber boundaries. Further studies with this platform may assess the immune cell function as a step forward to disclose the effects of pharmaceutics to downstream immunology in more physiologically relevant microenvironments.

Immunology-Guided Biomaterial Design for Mucosal Cancer Vaccines

Cancer of mucosal tissues is a major cause of worldwide mortality for which only palliative treatments are available for patients with late-stage disease. Engineered cancer vaccines offer a promising approach for inducing antitumor immunity. The route of vaccination plays a major role in dictating the migratory pattern of lymphocytes, and thus vaccine efficacy in mucosal tissues. Parenteral immunization, specifically subcutaneous and intramuscular, is the most common vaccination route. However, this induces marginal mucosal protection in the absence of tissue-specific imprinting signals. To circumvent this, the mucosal route can be utilized, however degradative mucosal barriers must be overcome. Hence, vaccine administration route and selection of materials able to surmount transport barriers are important considerations in mucosal cancer vaccine design. Here, an overview of mucosal immunity in the context of cancer and mucosal cancer clinical trials is provided. Key considerations are described regarding the design of biomaterial-based vaccines that will afford antitumor immune protection at mucosal surfaces, despite limited knowledge surrounding mucosal vaccination, particularly aided by biomaterials and mechanistic immune-material interactions. Finally, an outlook is given of how future biomaterial-based mucosal cancer vaccines will be shaped by new discoveries in mucosal vaccinology, tumor immunology, immuno-therapeutic screens, and material-immune system interplay.

T-cell lymphoproliferative processes in the spleen

T-cell lymphoproliferative processes in the spleen are rare and it is important to study normal T cell subsets in the spleen to understand the splenic milieu in which they arise. True malignant T-cell processes including hepatosplenic T-cell lymphoma and T-cell large granular lymphocytic leukemia occur in the spleen, but other atypical reactive T-cell proliferations and those of uncertain significance also have been described. Proper distinction of florid T cell responses from malignant T-cell neoplasms has important therapeutic implications for the patient.

Participation of the spleen in the IgA immune response in the gut

The role of the spleen in the induction of an immune response to orally administered antigens is still under discussion. Although it is well known that after oral antigen administration specific germinal centres are not only formed in the Peyers patches (PP) and the mesenteric lymph nodes (mLN) but also in the spleen, there is still a lack of functional data showing a direct involvement of splenic B cells in an IgA immune response in the gut. In addition, after removal of mLN a high level of IgA+ B cells was observed in the gut. Therefore, in this study we analysed the role of the spleen in the induction of IgA+ B cells in the gut after mice were orally challenged with antigens. Here we have shown that antigen specific splenic IgM+ B cells after in vitro antigen stimulation as well as oral immunisation of donor mice were able to migrate into the gut of recipient mice, where they predominantly switch to IgA+ plasma cells. Furthermore, stimulation of recipient mice by orally administered antigens enhanced the migration of the splenic B cells into the gut as well as their switch to IgA+ plasma cells. Removal of the mLN led to a higher activation level of the splenic B cells. Altogether, our results imply that splenic IgM+ B cells migrate in the intestinal lamina propria, where they differentiate into IgA+ plasma cells and subsequently proliferate. In conclusion, we demonstrated that the spleen plays a major role in the gut immune response serving as a reservoir of immune cells that migrate to the site of antigen entrance.

3D bioprinting of tissues and organs for regenerative medicine

3D bioprinting is a pioneering technology that enables fabrication of biomimetic, multiscale, multi-cellular tissues with highly complex tissue microenvironment, intricate cytoarchitecture, structure-function hierarchy, and tissue-specific compositional and mechanical heterogeneity. Given the huge demand for organ transplantation, coupled with limited organ donors, bioprinting is a potential technology that could solve this crisis of organ shortage by fabrication of fully-functional whole organs. Though organ bioprinting is a far-fetched goal, there has been a considerable and commendable progress in the field of bioprinting that could be used as transplantable tissues in regenerative medicine. This paper presents a first-time review of 3D bioprinting in regenerative medicine, where the current status and contemporary issues of 3D bioprinting pertaining to the eleven organ systems of the human body including skeletal, muscular, nervous, lymphatic, endocrine, reproductive, integumentary, respiratory, digestive, urinary, and circulatory systems were critically reviewed. The implications of 3D bioprinting in drug discovery, development, and delivery systems are also briefly discussed, in terms of in vitro drug testing models, and personalized medicine. While there is a substantial progress in the field of bioprinting in the recent past, there is still a long way to go to fully realize the translational potential of this technology. Computational studies for study of tissue growth or tissue fusion post-printing, improving the scalability of this technology to fabricate human-scale tissues, development of hybrid systems with integration of different bioprinting modalities, formulation of new bioinks with tuneable mechanical and rheological properties, mechanobiological studies on cell-bioink interaction, 4D bioprinting with smart (stimuli-responsive) hydrogels, and addressing the ethical, social, and regulatory issues concerning bioprinting are potential futuristic focus areas that would aid in successful clinical translation of this technology.

Designed Surface Topographies Control ICAM-1 Expression in Tonsil-Derived Human Stromal Cells

Fibroblastic reticular cells (FRCs), the T-cell zone stromal cell subtype in the lymph nodes, create a scaffold for adhesion and migration of immune cells, thus allowing them to communicate. Although known to be important for the initiation of immune responses, studies about FRCs and their interactions have been impeded because FRCs are limited in availability and lose their function upon culture expansion. To circumvent these limitations, stromal cell precursors can be mechanotranduced to form mature FRCs. Here, we used a library of designed surface topographies to trigger FRC differentiation from tonsil-derived stromal cells (TSCs). Undifferentiated TSCs were seeded on a TopoChip containing 2176 different topographies in culture medium without differentiation factors, then monitored cell morphology and the levels of ICAM-1, a marker of FRC differentiation. We identified 112 and 72 surfaces that upregulated and downregulated, respectively, ICAM-1 expression. By monitoring cell morphology, and expression of the FRC differentiation marker ICAM-1 via image analysis and machine learning, we discovered correlations between ICAM-1 expression, cell shape and design of surface topographies and confirmed our findings by using flow cytometry. Our findings confirmed that TSCs are mechano-responsive cells and identified particular topographies that can be used to improve FRC differentiation protocols.

Association of Interleukin 10 Gene Polymorphisms with Autoimmune Thyroid Disease: Meta-Analysis

The aim of this study was to perform a meta-analysis of eligible studies and to derive a precise estimate of the association between interleukin 10 (IL10) polymorphisms and susceptibility to autoimmune thyroid disease (AITD). Meta-analyses were conducted on the associations between AITD and the -1082 G/A (rs1800896), -819 C/T (rs1800871) and -592 C/A (rs1800872) polymorphisms in IL10, and the haplotype of these polymorphisms and AITD. A total of 2903 AITD patients and 3060 controls in 10 eligible studies were included in the meta-analysis. This meta-analysis showed significant associations between IL10 at the -1082 G allele and overall AITD (OR: 1.44, 95% CI 1.13-1.82, P = 0.003), but no association between the IL10 -592 C allele and the -819 C allele and AITD. Subgroup studies demonstrated significant associations between the -1082 G allele and susceptibility to Graves' disease. Ethnicity-specific meta-analysis revealed significant associations between the -1082 G allele and AITD susceptibility in Asian populations; however, in Middle Eastern populations, no association was evident. Meta-analysis of the IL10 haplotype revealed an association between the ATA haplotype and AITD (OR: 1.17, 95% CI 1.00-1.36, P = 0.04). Meta-analysis demonstrates that the IL10 polymorphisms are associated with susceptibility to AITD.

A Stromal Cell Niche for Human and Mouse Type 3 Innate Lymphoid Cells

Adaptive immunity critically depends on the functional compartmentalization of secondary lymphoid organs. Mesenchymal stromal cells create and maintain specialized niches that support survival, activation, and expansion of T and B cells, and integrated analysis of lymphocytes and their niche has been instrumental in understanding adaptive immunity. Lymphoid organs are also home to type 3 innate lymphoid cells (ILC3), innate effector cells essential for barrier immunity. However, a specialized stromal niche for ILC3 has not been identified. A novel lineage-tracing approach now identifies a subset of murine fetal lymphoid tissue organizer cells that gives rise exclusively to adult marginal reticular cells. Moreover, both cell types are conserved from mice to humans and colocalize with ILC3 in secondary lymphoid tissues throughout life. In sum, we provide evidence that fetal stromal organizers give rise to adult marginal reticular cells and form a dedicated stromal niche for innate ILC3 in adaptive lymphoid organs.

A Tale from TGF-β Superfamily for Thymus Ontogeny and Function

Multiple signaling pathways control every aspect of cell behavior, organ formation, and tissue homeostasis throughout the lifespan of any individual. This review takes an ontogenetic view focused on the large superfamily of TGF-β/bone morphogenetic protein ligands to address thymus morphogenesis and function in T cell differentiation. Recent findings on a role of GDF11 for reversing aging-related phenotypes are also discussed.

Application of tissue engineering to the immune system: development of artificial lymph nodes

The goal of tissue engineering and regenerative medicine is to develop synthetic versions of human organs for transplantation, in vitro toxicology testing and to understand basic mechanisms of organ function. A variety of different approaches have been utilized to replicate the microenvironments found in lymph nodes including the use of a variety of different bio-materials, culture systems, and the application of different cell types to replicate stromal networks found in vivo. Although no system engineered so far can fully replicate lymph node function, progress has been made in the development of microenvironments that can promote the initiation of protective immune responses. In this review we will explore the different approaches utilized to recreate lymph node microenvironments and the technical challenges required to recreate a fully functional immune system in vitro.

Transitional B cells: how well are the checkpoints for specificity understood?

It is crucial for the immune system to minimise the number of circulating mature self-reactive B cells, in order to reduce the potential for the development of autoantibody-related autoimmune diseases. Studies of animal models have identified two major checkpoints that ensure that such cells do not contribute to the naïve B cell repertoire. The first is in the bone marrow as B cells develop and the second is in the spleen; B cells that are released from the bone marrow as transitional B cells go through more stringent selection in the spleen before they develop into mature naïve B cells. Transitional B cells and their maturation have mostly been studied in mice. However, recent studies characterised human transitional B cells and found considerable differences to current models. In this review, we will consider these differences alongside known differences in mouse and human splenic function and ask whether human transitional B cells might develop along a different pathway.

Self-antigen presentation by mouse B cells results in regulatory T-cell induction rather than anergy or clonal deletion

Multiple mechanisms operate to ensure T-cell tolerance toward self-antigens. Three main processes have been described: clonal deletion, anergy, and deviation to CD4(+) regulatory T cells (Tregs) that suppress autoreactive T cells that have escaped the first 2 mechanisms. Although it is accepted that dendritic cells (DCs) and B cells contribute in maintaining T-cell tolerance to self-antigens, their relative contribution and the processes involved under physiologic conditions remain only partially characterized. In this study, we used different transgenic mouse models to obtain chimeras where a neo self-antigen is expressed by thymic epithelium and/or by DCs or B cells. We found that expression of cognate ligand in the thymus enhances antigen-specific FoxP3(+) cells independently of whether the self-antigen is expressed on thymic epithelium or only on DCs, but not on B cells. On the contrary, self-antigen expression by B cells was very efficient in inducing FoxP3(+) cells in the periphery, whereas self-antigen expression by DC led mainly to deletion and anergy of antigen-specific FoxP3(-) cells. The results presented in this study underline the role of B cells in Treg induction and may have important implications in clinical protocols aimed at the peripheral expansion of Tregs in patients.

Incorporating parasite systematics in comparative analyses of variation in spleen mass and testes sizes of rodents

Parasite diversity is hypothesized to act on host life-history traits through investment in immunity. In order to incorporate the diversity of the parasite community that an individual host or a host species may face, two indices can be used: Taxonomic Species Richness and Taxonomic Entropy, where the taxonomic information is incorporated with the taxonomic weight. We tested whether these indices correlate with several morphological traits potentially implicated in immune defence and in reproduction, using data on gastrointestinal helminths and their rodent hosts sampled in Southeast Asia. We found no relationship between parasite diversity indices and either spleen mass or testes size at the intraspecific level, i.e. at the level of individuals. At the interspecific level, we found no relationship between the parasite diversity indices and testes size. However, we found that female spleen mass is significantly influenced by the specific species richness of parasites, whereas male spleen mass is influenced by individual mean parasite diversity indices. We concluded that female spleen mass may have evolved in response to gastrointestinal helminth pressure acting at species levels, while in males, the individual spleen mass could be constrained by other factors, such as the blood storage function of the spleen.

Stromal cells directly mediate the re-establishment of the lymph node compartments after transplantation by CXCR5 or CCL19/21 signalling

Lymph nodes (LN) are highly organized and have characteristic compartments. Destruction of these compartments leads to an inability to fulfil their immunological function. However, it is not yet clearly understood which mechanisms are involved in the development and maintenance of this organization. After transplantation of LN into the mesentery, the LN regenerate to fully functional LN. In this study, the question was addressed, how stromal cells in the B-cell follicles (follicular dendritic cells), which were identified by CD21/CD35, and stromal cells in the T-cell area (gp38+ cells) are involved via chemokine signalling. The gp38+ cells and CD21/CD35+ cells were detected in the transplanted LN (EGFP, plt/plt and CXCR5(-/-) mice) over a period of 8 weeks to analyse their competence to reconstruct the compartmental organization. The presence of gp38+ cells was stable during regeneration and these cells reconstructed the T-cell area within 4 weeks. After transplantation of plt/plt LN CCL19/CCL21 expression was observed leading to partial restoration of the T-cell area. In contrast, there were changes in the presence and morphology of CD21/CD35+ cells within the B-cell area during reconstruction, which was dependent on the presence of B cells and CXCL13/CXCR5 signalling. Hence, CD21/CD35+ cells and gp38+ cells are involved in the establishment of the compartmental organization of lymph nodes but using different ways to recruit lymphocytes via chemokine signalling.

Screening large numbers of expression patterns of transcription factors in late stages of the mouse thymus

Transcription factor families are well known to be involved in the intrinsic pathways that regulate the organogenesis, early development, and microenvironment of the thymus. However, identification of the transcription factors (TFs) involved in the late development of the thymus, particularly later than embryonic day 15.5 (E15.5), is progressing slowly. In this study, we used in situ hybridization to screen numerous expression patterns of the TFs involved in the development of the mouse thymus. More than 400 members, including unique TFs and some transcription co-factors, were tested. Among the screened TFs, 160 were found to be expressed in the thymus after E15.5, and 74 of these were expressed in restricted areas.

Immunological substance testing on human lymphatic micro-organoids in vitro

Pharmaceutical drugs and compounds used for consumer products may bear the risk of unexpected immuno-toxicological side effects, such as sensitization, allergy, anaphylaxis or immunogenicity. Modern biopharmaceuticals with high potency and target specificity, like antibodies and cytokines need to be tested for their therapeutical doses, their exposition regimens and their immune functionality prior to first-in-man applications. For the latter, existing in vitro tests and animal models do not sufficiently reflect the complexity and specificity of the human immune system. Even novel humanised animal models have limitations in their systemic reactions. Monolayer or suspended cell culture possesses neither tissue functionality nor organ physiology, and also cannot be used for long term culture and experiments. In contrast, solid tissue biopsies, e.g. tonsil preparations of tonsillitis patients typically show inflammatory artefacts and degrade in long term culture due to preparation-induced damage. The construction of tissue-like structures in vitro, so-called "micro-organoids", can overcome these limitations. Key structures of secondary lymphatic organs, e.g. lymph nodes or the spleen are the primary lymphatic follicles and germinal centres, in particular during the "activated state" of an inflammation or infection. To remodel lymphatic follicles, functional and structural cells, e.g. lymphoid cells derived from peripheral blood mononuclear cells (PBMCs) and stromal cells need to be combined with biogenic or artificial matrices and scaffolds to produce a suitable 3D tissue-mimicking environment. Therefore, a unique human lymph node model (HuALN) was designed to operate over several weeks, and allow long term and repeated drug exposure to induce and monitor both cellular and humoral immune responses. Cellular immunity is monitored, for example, by cytokine release patterns; humoral immunity is analysed, for example, by B cell activation, plasma cell formation and antibody secretion profiles (IgM and IgG). Moreover, cellular composition and micro-organoid formation are analysed by flow cytometry, histology and in situ imaging.

The lymph node revisited: development, morphology, functioning, and role in triggering primary immune responses

Scenarios have been proposed to explain how lymphoid components of a lymph node favor the encounter of a drained antigen with a circulating competent naïve lymphocyte to trigger a primary immune response. However, these scenarios rest on incorrect concepts about the organ. This situation resulted from a loss of interest for studies on in vivo lymphoid organs due to a widespread switch, decades ago, to work on suspended lymphoid cells. However, an in vivo holistic study of the organ continued in our laboratory. The present review synthesizes resulting knowledge on lymph node morphology and global functioning. We show that the opening of an afferent lymphatic vessel into the subcapsular sinus is the focal point from which the related portion of a lymph node-a node compartment-is developed. As to the formation of a compartment's lymphoid components, it is neonatally orchestrated by the dichotomic nature and distribution of antigens in this subcapsular sinus, which determines a dichotomic recruitment of circulating cells and the compartment's architectural complexity. The transport process of an antigen from a given tissue territory into restricted sites of the draining compartment further defines its local morphological features and activities, while providing the possibility to reduce the wandering of a short-lived naïve cell through innumerable target-devoid sites. We also explain that the nodal lymphoid components are not implicated in the triggering of primary responses, but are rather products of such responses. Scenarios for the triggering of primary responses, consistent with real node morphology and functioning, are proposed.

Toxoplasma gondii: effect of maternal infection in the development of lymphoid organs of BALB/c neonates

Toxoplasmosis is one of the worldwide parasitic zoonoses. Alterations in the lymphopoietic system are still poorly studied. We analyzed lymphoid organs of BALB/c mice neonates from Toxoplasma gondii-intraperitoneally-infected mothers on 19th day of gestation, with 30 tachyzoites of strain RH. Normal non-infected pregnant females were used as controls. At 7 days after birth, animals were classified as neonates from infected (NIM) and neonates from non-infected mothers (NNIM). Weight of the thymus and number of thymic cells in NIM were decreased, percentage of apoptosis was significantly increased. Decrease in lymphocytes and monocytes and an increase of plasma cells were observed in bone marrow of NIM. Peripheral blood of NIM showed an increase of monocytes and neutrophils and a decrease in lymphocytes. Infection of the mother during the last day of gestation provokes in the neonates changes in the lymphoid organs that could explain survival of 75% of them.

Stromal cells confer lymph node-specific properties by shaping a unique microenvironment influencing local immune responses

Lymph nodes (LN) consist not only of highly motile immune cells coming from the draining area or from the systemic circulation, but also of resident stromal cells building the backbone of the LN. These two cell types form a unique microenvironment which is important for initiating an optimal immune response. The present study asked how the unique microenvironment of the mesenteric lymph node (mLN) is influenced by highly motile cells and/or by the stromal cells. A transplantation model in rats and mice was established. After resecting the mLN, fragments of peripheral lymph node (pLN) or mLN were inserted into the mesentery. The pLN and mLN have LN-specific properties, resulting in differences of, for example, the CD103(+) dendritic cell subset, the adhesion molecule mucosal addressin cell adhesion molecule 1, the chemokine receptor CCR9, the cytokine IL-4, and the enzyme retinal dehydrogenase 2. This new model clearly showed that during regeneration stromal cells survived and immune cells were replaced. Surviving high endothelial venules retained their site-specific expression (mucosal addressin cell adhesion molecule 1). In addition, the low expression of retinal dehydrogenase 2 and CCR9 persisted in the transplanted pLN, suggesting that stromal cells influence the lymph node-specific properties. To examine the functional relevance of this different expression pattern in transplanted animals, an immune response against orally applied cholera toxin was initiated. The data showed that the IgA response against cholera toxin is significantly diminished in animals transplanted with pLN. This model documents that stromal cells of the LN are active players in shaping a unique microenvironment and influencing immune responses in the drained area.

CXCR5+ CCR7- CD8 T cells are early effector memory cells that infiltrate tonsil B cell follicles

Naive and central memory CD8 T cells use CCR7 to recirculate through T cell zones of secondary lymphoid organs where they can encounter antigen. Here we describe a subset of human CD8 T cells expressing CXCR5 which enables homing in response to CXCL13 produced within B cell follicles. CXCR5+ CD8 T cells were found in tonsil B cell follicles, and isolated cells migrated towards CXCL13 in vitro. They expressed CD27, CD28, CD45RO, CD69, and were CD7low, and produced IFN-gamma and granzyme A but lacked perforin, a functional profile suggesting that these cells are early effector memory cells in the context of contemporary T cell differentiation models. Receptors important in the interaction with B cells, including CD70, OX40 and ICOS, were induced upon activation, and CXCR5+ CD8 T cells could to some extent support survival and IgG production in tonsil B cells. Furthermore, CXCR5+ CD8 T cells expressed CCR5 but no CCR7, suggesting a migration pattern distinct from that of follicular CD4 T cells. The finding that a subset of early effector memory CD8 T cells use CXCR5 to locate to B cell follicles indicates that MHC class I-restricted CD8 T cells are part of the follicular T cell population.

Dendritic cell subsets in lymph nodes are characterized by the specific draining area and influence the phenotype and fate of primed T cells

Dendritic cells (DC) are important in differential T-cell priming. Little is known about the local priming by DC in the microenvironment of different lymph nodes and about the fate of the imprinted T cells. Therefore, freshly isolated rat DC from mesenteric lymph nodes (mLN) and axillary lymph nodes (axLN) were phenotyped and cultured with blood T cells in the presence of the superantigen Mycoplasma arthritidis mitogen (MAM). The phenotype, proliferation and apoptosis of the primed T cells were analysed. Our data show that a common DC population exists in both mLN and axLN. In addition, region-specific DC with an organotypical marker expression imprinted by the drained area were found. Coculture of T cells with DC from mLN or axLN resulted in a distinct shift in the CD4 and CD8 expression of T cells and their phenotype. Furthermore, when these differentially primed mLN and axLN T cells were injected into recipients, mLN-primed T cells survived longer in other lymphoid organs. The results show that the region-specific DC have a unique phenotype and an impact on the ratio of CD4 : CD8 T cells during an immune response in vivo.

Plasticity and heterogeneity of lymphoid organs. What are the criteria to call a lymphoid organ primary, secondary or tertiary?

Lymphoid organs are generally classified in a hierarchy with primary lymphoid organs such as the thymus and bone marrow for the production of receptor specific T and B lymphocytes, respectively, independent of antigens. In secondary lymphoid organs such as lymph nodes, spleen, and tonsils, the lymphocytes are expanded due to antigen exposure, producing memory T cells and effector B cells, resulting in plasma cells. Tertiary lymphoid tissues are often defined as aggregations of lymphoid cells in autoimmune diseases. It will be outlined that all these organs have a high plasticity and also the thymic medulla is included in the route of migrating mature T cells and the bone marrow, not only in the traffic of CD4+ but also of CD8+ lymphocytes. The mucosa-associated lymphoid organs depend to a much larger extent on microbial antigen and are much more diverse than often described. The role of structural elements as well as blood and lymphatic vessels as an entry and exit site of lymphocytes will be outlined. Using a precise terminology, taking account of the plasticity of these organs at different ages and considering species differences will reduce misunderstandings among immunologists.

Bone morphogenetic protein-2/4 signalling pathway components are expressed in the human thymus and inhibit early T-cell development

T-cell differentiation is driven by a complex network of signals mainly derived from the thymic epithelium. In this study we demonstrate in the human thymus that cortical epithelial cells produce bone morphogenetic protein 2 (BMP2) and BMP4 and that both thymocytes and thymic epithelium express all the molecular machinery required for a response to these proteins. BMP receptors, BMPRIA and BMPRII, are mainly expressed by cortical thymocytes while BMPRIB is expressed in the majority of the human thymocytes. Some thymic epithelial cells from cortical and medullary areas express BMP receptors, being also cell targets for in vivo BMP2/4 signalling. The treatment with BMP4 of chimeric human-mouse fetal thymic organ cultures seeded with CD34+ human thymic progenitors results in reduced cell recovery and inhibition of the differentiation of human thymocytes from CD4- CD8- to CD4+ CD8+ cell stages. These results support a role for BMP2/4 signalling in human T-cell differentiation.

Life cycle, migration and antigen presenting functions of spleen and lymph node dendritic cells: limitations of the Langerhans cells paradigm

The phenotypic and functional studies carried out in recent years on dendritic cells (DC) purified from spleen and lymph nodes has revealed the existence of heterogeneous populations with distinct life cycles, migratory properties and antigen presenting functions. A major subdivision can be made between "tissue derived" DC that migrate to the lymph nodes from peripheral tissues, both in the steady state and in the course of infections, and "blood-derived" DC, which reside in the spleen and lymph nodes throughout their life cycle. These two groups of DC can be subdivided into smaller subsets. The tissue-derived and the blood-derived DC also show fundamental differences in maturational status and antigen presenting capabilities. In this review, we summarize the roles played by the different DC types in the steady state and during pathogen infections, relating those roles to maintenance of peripheral tolerance and the induction of immunity. We point out the caveats of assuming that the DC that collect antigens are the ones involved in their presentation, emphasizing the phenomenon of antigen transfer as an important component of the immune response.

Immunohistochemical study of the reticular and vascular network of mouse lymph node using vibratome sections

The function of lymph nodes is greatly influenced by their unique microanatomy, in which distinct subpopulations of cells are compartmentalized by a meshwork of reticular cells and fibres, specialized blood and lymphatic vessels and nerves. Using antibodies against extracellular matrix (ECM) proteins (fibronectin, collagen IV and laminin), proteoglycan (perlecan), and a fibroblastic marker (ERTR-7), the distribution and molecular organization of the system of reticular fibres was investigated by three-dimensional (3D) reconstruction methods. Fibronectin, collagen IV and laminin are restricted to reticular fibres and have a similar distribution pattern, whereas perlecan is limited to the vascular system of the lymph node. Various compartments of the lymph node, such as the B-cell follicle, paracortex (including the high endothelial venules and paracortical cord), and medulla have been reconstructed to visualize their vasculature with respect to B and T cells. Since the morphology of lymph nodes may change significantly in pathological conditions, different compartments of reactive lymph node (after low-dose Listeria monocytogenes infection), especially germinal centres, were also investigated. The data presented here should facilitate our understanding of the 3D organization of non-immune cell components of lymph nodes, which is crucial for cell adhesion, migration, activation, and differentiation in normal and pathological conditions.

Yersinia has a tropism for B and T cell zones of lymph nodes that is independent of the type III secretion system

Pathogenic Yersinia have a pronounced tropism for lymphatic tissues and harbor a virulence plasmid that encodes a type III secretion system, pTTSS, that transports Yops into host cells. Yops are critical virulence factors that prevent phagocytosis by macrophages and neutrophils and Yersinia mutants lacking one or more Yops are defective for survival in lymphatic tissues, liver, and gastrointestinal tract. However, here we demonstrate that Y. pseudotuberculosis (Yptb) mutants lacking the pTTSS survived as well as or better than wild-type (WT) Yptb in the mesenteric lymph nodes (MLN). Infection with pTTSS mutants caused lymphadenitis with little necrosis, whereas infection with WT Yptb provoked lymphadenitis with multiple necrotic suppurative foci. Gentamicin protection assays and microscopic examination of the MLN revealed that pTTSS mutants resided extracellularly adjacent to B and T lymphocytes in the cortex and paracortex. WT Yptb was found extracellularly adjacent to neutrophils and macrophages in necrotic areas and adjacent to B and T lymphocytes in less-inflamed areas. To determine whether lymphocytes protected pTTSS mutants from phagocytic cells, Rag1(-/-) mice were infected with pTTSS mutants or WT Yptb. pTTSS mutants but not WT, were impaired for survival in MLN of Rag1(-/-) mice, suggesting that lymphocyte-rich regions constitute a protective niche for pTTSS mutants. Finally, we show that invasin and the chromosomally encoded TTSS were not required for Yptb survival in MLN. In summary, chromosomally encoded factors are sufficient for Yptb replication in the cortex and paracortex of MLN; the pTTSS enables Yersinia to survive within phagocyte-rich areas of lymph nodes, and spread to other tissues.

B cells as antigen presenting cells

Several characteristics confer on B cells the ability to present antigen efficiently: (1) they can find T cells in secondary lymphoid organs shortly after antigen entrance, (2) BCR-mediated endocytosis allows them to concentrate small amounts of specific antigen, and (3) BCR signaling and HLA-DO expression direct their antigen processing machinery to favor presentation of antigens internalized through the BCR. When presenting antigen in a resting state, B cells can induce T cell tolerance. On the other hand, activation by antigen and T cell help converts them into APC capable of promoting immune responses. Presentation of self antigens by B cells is important in the development of autoimmune diseases, while presentation of tumor antigens is being used in vaccine strategies to generate immunity. Thus, detailed understanding of the antigen presenting function of B cells can lead to their use for the generation or inhibition of immune responses.

Cell-mediated adaptive immune defense of the lungs

Cell-mediated adaptive immune responses contribute to defense against all classes of pulmonary pathogens and are essential against viruses, mycobacteria, and fungi, including Pneumocystis carinii. Adaptive responses depend on sequential pairwise interactions between three cell types: T cells, natural killer (NK) cells, and dendritic cells (DC). Differential expression of specific adhesion molecules and chemokines regulates the location and timing of these interactions. Primary adaptive responses are triggered by immature myeloid DC, which carry antigen from the lungs to regional lymph nodes. Antigen presentation by these mature DC is required to activate naive CD4 T cells, which are essential to generate polarized type 1 or type 2 effector responses and for robust immunologic memory. Inflammation recruits NK cells and DC that interact in a contact- and tumor necrosis factor-alpha-dependent fashion within injured tissues to initiate immune response polarization. NK cells exposed to IL-12 favor survival of DC that prime for Th1 responses, whereas NK cells exposed to IL-4 do not exert DC selection, leading to tolerogenic or Th2 responses. Naive alphabeta T cells, NK cells, and DC also amplify secondary adaptive responses to previously encountered pathogens. However, secondary responses are accelerated because memory T cells can migrate directly to infected tissues where they can be activated without strenuous costimulatory requirements. Additionally, previous pulmonary infections or immune responses increase numbers of lung DC and populate the lungs with clones of memory B cells and T cells that are immediately available to respond to infections.

Murine extramedullary erythropoiesis induced by tick infestation

Tick saliva contains molecules that modulate the haemostasis, pain/itch responses, wound healing and immune defences of the host. Using BALB/c mice that were each infested with 10 nymphs of Dermacentor andersoni Stiles (Acari: Ixodidae), an attempt has now been made to determine the influence of tick infestation on the expression of leucocyte adhesion molecules in the host. The ticks became fully engorged by the fourth to sixth day of infestation. On the fourth day of infestation, the results of flow cytometry indicated that 2% of the host's splenocytes were expressing high levels of CD49 (alpha4 integrin of VLA-4) and low levels of CD11a (alphaL subunit of the integrin LFA-1). By the eighth day of infestation, 30% of the hosts' splenocytes had this phenotype and were negative for the lineage markers CD3e (T-lymphocytes), DX5 (natural-killer cells of a BALB/c lineage), B220 (B-lymphocytes), CD11b (monocytes/macrophages, granulocytes, natural-killer cells, activated T-lymphocytes, and B-1 cells) and CD11c (myeloid and splenic dendritic cells). Histological examination of the spleens from infested mice revealed disruption of the white-pulp/red-pulp demarcations and the presence of a large number of basophilic normoblasts. The CD11a(lo) population of splenocytes from the tick-infested mice was positive for TER-119 but negative for CD3, B220, CD11b and Gr, confirming that the splenocytes were members of the erythroid lineage. These results indicate that, within 8 days of their initiation, the tick infestations induced extramedullary erythropoiesis in the spleens of their murine hosts.

Adipose tissue and the immune system

Adipocytes anatomically associated with lymph nodes (and omental milky spots) have many special properties including fatty acid composition and the control of lipolysis that equip them to interact locally with lymphoid cells. Lymph node lymphocytes and tissue dendritic cells acquire their fatty acids from the contiguous adipocytes. Lymph node-derived dendritic cells suppress lipolysis in perinodal adipocytes but those that permeate the adipose tissue stimulate lipolysis, especially after minor, local immune stimulation. Inflammation alters the composition of fatty acids incorporated into dendritic cells, and that of node-containing adipose tissue, counteracting the effects of dietary lipids. Thus these specialised adipocytes partially emancipate the immune system from fluctuations in the abundance and composition of dietary lipids. Prolonged, low-level immune stimulation induces the local formation of more adipocytes, especially adjacent to the inflamed lymph node. This mechanism may contribute to hypertrophy of the mesentery and omentum in chronic inflammatory diseases such as HIV-infection, and in smokers. Paracrine interactions between adipose and lymphoid tissues are enhanced by diets rich in n-6 fatty acids and attentuated by fish oils. The latter improve immune function and body conformation in animals and people. The partitioning of adipose tissue in many depots, some specialised for local, paracrine interactions with other tissues, is a fundamental feature of mammals.

Endocrine-like cells in the chick embryo thymus express ultrastructural features of piecemeal degranulation

The endocrine-like cells of the chick embryo thymus were studied by transmission electron microscopy and a highly characteristic pattern of cell secretion referred to as piecemeal degranulation (PMD) was identified. This is the first description of PMD in embryonic cells.

Site-Specific Differences in Fatty Acid Composition of Dendritic Cells and Associated Adipose Tissue in Popliteal Depot, Mesentery, and Omentum and Their Modulation by Chronic Inflammation and Dietary Lipids

BACKGROUND

This study explores the role of lymphatics-associated adipocytes in determining the lipid composition of dendritic cells.

METHODS AND RESULTS

Adult male rats were fed plain chow, or chow supplemented with 20% sunflower or fish oil. Chronic local inflammation was induced by subcutaneous injection of 20 microg lipopolysaccharide three times a week for 2 weeks near the popliteal lymph nodes. Chemokine-stimulated dendritic cells were collected over 4 hours from popliteal and mesenteric lymph nodes, and perinodal and other samples of mesenteric, popliteal, and omental adipose tissue. Fatty acids extracted from triacylglycerols and/or phospholipids were separated and quantified by gas chromatography from each sample of dendritic cells and intracellular lipids, membranes, stroma and isolated adipocytes from the adipose tissue. Dendritic cells from lymph nodes and adipose tissue samples differed in fatty acid composition, and were modulated by diet. The site-specific differences of dendritic cells correlated with those of the contiguous adipocytes. Chronic mild stimulation altered the lipid composition of dendritic cells near the inflamed site and elsewhere; changes were minimal after the fish-oil diet. The composition of adipocyte triacylglycerol and phospholipid fatty acids also changed near the stimulation site in ways that counteracted alterations induced by the experimental diets.

CONCLUSIONS

Fatty acids in dendritic cells differed with anatomical site, and were determined by the adjacent adipocytes, which actively regulated their own lipid composition. These findings demonstrated functional bases for the anatomical associations between adipose and lymphoid tissues and may be a mechanism by which dietary lipids modulate the immune system.