Migration pathways of recirculating murine B cells and CD4+ and CD8+ T lymphocytes

Caspase-8 in endothelial cells maintains gut homeostasis and prevents small bowel inflammation in mice

The gut has a specific vascular barrier that controls trafficking of antigens and microbiota into the bloodstream. However, the molecular mechanisms regulating the maintenance of this vascular barrier remain elusive. Here, we identified Caspase-8 as a pro-survival factor in mature intestinal endothelial cells that is required to actively maintain vascular homeostasis in the small intestine in an organ-specific manner. In particular, we find that deletion of Caspase-8 in endothelial cells results in small intestinal hemorrhages and bowel inflammation, while all other organs remained unaffected. We also show that Caspase-8 seems to be particularly needed in lymphatic endothelial cells to maintain gut homeostasis. Our work demonstrates that endothelial cell dysfunction, leading to the breakdown of the gut-vascular barrier, is an active driver of chronic small intestinal inflammation, highlighting the role of the intestinal vasculature as a safeguard of organ function.

Goose Nephritic Astrovirus Infection of Goslings Induces Lymphocyte Apoptosis, Reticular Fiber Destruction, and CD8 T-Cell Depletion in Spleen Tissue

The emergence of a novel goose nephritic astrovirus (GNAstV) has caused economic losses to the Chinese goose industry. High viral load is found in the spleen of goslings infected with GNAstV, but pathological injuries to the spleen due to GNAstV are largely unknown. In this study, 50 two-day-old goslings were infected orally with GNAstV, and 50 goslings were treated with PBS as control. Spleens were collected at different times following infection to assess damage. GNAstV infection caused visceral gout and urate deposition in joints, and resulted in 16% mortality. GNAstV was found in the lymphocytes and macrophages within the spleen. Lymphocyte loss, especially around the white pulp, and destruction and decline in the number of reticular fibers was observed in GNAstV-infected goslings. Moreover, in GNAstV-infected goslings, ultrahistopathological examination found that splenic lymphocytes exhibited condensed chromatin and apoptotic bodies, and reticular cells displayed damage to plasma membrane integrity and swollen mitochondria. Furthermore, TUNEL staining confirmed apoptosis of lymphocytes, and the mRNA levels of Fas and FasL were significantly increased in the GNAstV-infected goslings. In addition, GNAstV infection reduced the number and protein expression of CD8. In conclusion, GNAstV infection causes lymphocyte depletion, reticular cell necrosis, reticular fiber destruction, lymphocyte apoptosis, and reduction in CD8 levels, which contribute to spleen injury.

Morphological characterization of postembryonic development of blood-spleen barrier in duck

The spleen is the largest peripheral lymphoid organ and an important site of immune response, in which the blood–spleen barrier (BSB) plays a significant role to resist various pathogens. The BSB structure of duck spleen is different from that of chicken and mammals. However, no information about the development of BSB after the postembryonic age has been reported in ducks. The current study observed the spleen of 1, 7, 14, 21, 35, and 60-day-old ducks by light and electron microscopy to analyze the cellular structural development. The results showed that the spleen index was continuously increased from 1 to 14-day-old ducks. During their early age, the spleen of ducks showed no definite zone of white and red pulp, but the area of the white pulp was large compared to that of the red pulp. The diameter of the ellipsoid was constantly increased in up to 35-day-old duck spleen, while the periellipsoidal lymphatic sheath (PELS) and periarterial lymphatic sheath continuously developed after 1 D. The reticular fibers developed with age; their branching reached the ellipsoidal wall to show a developed framework in the BSB of 14-day-old ducks. After 7 D, the endothelial cells of the sheathed capillary showed a typical cuboidal shape; between these cells, the gaps increased as age advanced, while the thickness of the basement membrane and collagen fibers increased in 35-day-old ducks. The mechanical filtration function of BSB by intravenous injection showed a 1-layer ring of carbon particles restricted in the white pulp in 1-day-old duck spleen; however, in 14 to 60 D, these particles were restricted in the ellipsoid and PELS, forming 2-layer rings of carbon particles. Collectively, the cellular features of the duck BSB developed up to 35 D of postembryonic age to perform their immune function.

Human spleen microanatomy: why mice do not suffice

The microanatomical structure of the spleen has been primarily described in mice and rats. This leads to terminological problems with respect to humans and their species-specific splenic microstructure. In mice, rats and humans the spleen consists of the white pulp embedded in the red pulp. In the white pulp, T and B lymphocytes form accumulations, the periarteriolar lymphatic sheaths and the follicles, located around intermediate-sized arterial vessels, the central arteries. The red pulp is a reticular connective tissue containing all types of blood cells. The spleen of mice and rats exhibits an additional well-delineated B-cell compartment, the marginal zone, between white and red pulp. This area is, however, absent in human spleen. Human splenic secondary follicles comprise three zones: a germinal centre, a mantle zone and a superficial zone. In humans, arterioles and sheathed capillaries in the red pulp are surrounded by lymphocytes, especially by B cells. Human sheathed capillaries are related to the splenic ellipsoids of most other vertebrates. Such vessels are lacking in rats or mice, which form an evolutionary exception. Capillary sheaths are composed of endothelial cells, pericytes, special stromal sheath cells, macrophages and B lymphocytes. Human spleens most probably host a totally open circulation system, as connections from capillaries to sinuses were not found in the red pulp. Three stromal cell types of different phenotype and location occur in the human white pulp. Splenic white and red pulp structure is reviewed in rats, mice and humans to encourage further investigations on lymphocyte recirculation through the spleen.

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.

Foxp3+ regulatory T-cell homeostasis quantitatively differs in murine peripheral lymph nodes and spleen

Regulatory T (Treg) cells are essential for maintaining self-tolerance and modulating inflammatory immune responses. Treg cells either develop within the thymus or are converted from CD4(+) naive T (Tnaive) cells in the periphery. The Treg-cell population size is tightly controlled and Treg-cell development and homeostasis have been intensively studied; however, quantitative information about mechanisms of peripheral Treg-cell homeostasis is lacking. Here we developed the first mathematical model of peripheral Treg-cell homeostasis, incorporating secondary lymphoid organs as separate entities and encompassing factors determining the size of the Treg-cell population, namely thymic output, homeostatic proliferation, peripheral conversion, transorgan migration, apoptosis, and the Tnaive-cell population. Quantitative data were collected by monitoring Tnaive-cell homeostasis and Treg-cell rebound after selective in vivo depletion of Treg cells. Our model predicted the previously unanticipated possibility that Treg cells regulate migration of Tnaive cells between spleen and peripheral lymph nodes (LNs), whereas migration of Treg cells between these organs can largely be neglected. Furthermore, our simulations suggested that peripheral conversion significantly contributed to the maintenance of the Treg-cell population, especially in LNs. Hence, we provide the first estimation of the peripheral Treg-cell conversion rate and propose additional facets of Treg-cell-mediated immune regulation that may previously have escaped attention.

The microstructure of secondary lymphoid organs that support immune cell trafficking

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

Access to the spleen microenvironment through lymph shows local cytokine production, increased cell flux, and altered signaling of immune cells during lipopolysaccharide-induced acute inflammation

The spleen is involved in fluid volume regulation, immune responses, and hematopoiesis. Yet, the composition of the fluid phase within the spleen microenviroment, the migratory routes of lymphocytes as well as the splenic response to bacterial endotoxin is incomplete. To address these issues, we isolated postnodal lymph in rats by cannulating an efferent lymphatic draining the spleen, and assessed the secretion of signaling substances during a septic response induced by LPS. Spleen lymph flow increased 8-fold after LPS exposure. The spleen exhibited a permeable microvasculature with low sieving of macromolecules that was absent after exposure to LPS. Furthermore, after LPS exposure the spleen contributed significantly to the production of pro- and anti-inflammatory cytokines, and experiments in splenectomized rats suggested it may induce a protracted inflammation because of a dominant role in IL-6 production. A significant amount of lymphocytes exited via lymphatics draining the spleen in control rats. LPS-induced inflammation resulted in increased T cell and reduced B cell subset fractions, and gave a significant increase in CD4(+) and CD8(+) subset T cell efflux and a reduced B cell efflux in spleen lymph. Exposure of leukocytes to the spleen microenvironment affected their signaling status, and by phosphorylation specific flow cytometry we could identify STAT3 and CREB as important mediators in the cellular signaling occurring during endotoxemia. We conclude that analysis of spleen lymph may unravel immune cell migration patterns and local signaling, and immune cells exit via lymph having acquired specific activation signatures after exposure to the spleen microenvironment.

Stromal cell contributions to the homeostasis and functionality of the immune system

A defining characteristic of the immune system is the constant movement of many of its constituent cells through the secondary lymphoid tissues, mainly the spleen and lymph nodes, where crucial interactions that underlie homeostatic regulation, peripheral tolerance and the effective development of adaptive immune responses take place. What has only recently been recognized is the role that non-haematopoietic stromal elements have in many aspects of immune cell migration, activation and survival. In this Review, we summarize our current understanding of lymphoid compartment stromal cells, examine their possible heterogeneity, discuss how these cells contribute to immune homeostasis and the efficient initiation of adaptive immune responses, and highlight how targeting of these elements by some pathogens can influence the host immune response.

Geography and plumbing control the T cell response to infection

The orchestrated movement of cells of the immune system is essential to generation of productive responses leading to protective memory development. Recent advances have allowed the direct microscopic visualization of lymphocyte and antigen-presenting cell migration and interaction during immune response initiation and progression. These studies have defined important characteristics of the microanatomy of lymphocyte movement, particularly in the lymph node. Moreover, the ability to track endogenous antigen-specific T cells has revealed a coordinated pathway of CD8 T cell movement in the spleen following primary and secondary infection. As a consequence, the local anatomy of secondary lymphoid tissues during infection has emerged as a critical regulator of immunity. While some of the factors responsible for the migratory cues instructing immune cell movement have been identified, much remains to be learned. Here, we provide a brief overview of studies examining CD8 T cell localization during the immune response to infection in the context of our current understanding of immune system structure.

Modeling emergent tissue organization involving high-speed migrating cells in a flow equilibrium

There is increasing interest in the analysis of biological tissue, its organization and its dynamics with the help of mathematical models. In the ideal case emergent properties on the tissue scale can be derived from the cellular scale. However, this has been achieved in rare examples only, in particular, when involving high-speed migration of cells. One major difficulty is the lack of a suitable multiscale simulation platform, which embeds reaction diffusion of soluble substances, fast cell migration and mechanics, and, being of great importance in several tissue types, cell flow homeostasis. In this paper a step into this direction is presented by developing an agent-based mathematical model specifically designed to incorporate these features with special emphasis on high-speed cell migration. Cells are represented as elastic spheres migrating on a substrate in lattice-free space. Their movement is regulated and guided by chemoattractants that can be derived from the substrate. The diffusion of chemoattractants is considered to be slower than cell migration and, thus, to be far from equilibrium. Tissue homeostasis is not achieved by the balance of growth and death but by a flow equilibrium of cells migrating in and out of the tissue under consideration. In this sense the number and the distribution of the cells in the tissue is a result of the model and not part of the assumptions. For the purposes of demonstration of the model properties and functioning, the model is applied to a prominent example of tissue in a cellular flow equilibrium, the secondary lymphoid tissue. The experimental data on cell speed distributions in these tissues can be reproduced using reasonable mechanical parameters for the simulated cell migration in dense tissue.

One technique, two approaches, and results: thoracic duct cannulation in small laboratory animals

Experimental studies in immunology, pharmacology, or hematology require the sampling of the total thoracic duct lymph in awake and unrestrained rats or mice. Several approaches have been described for cannulation of the thoracic duct, but they are characterized by a modest reproducibility and a low lymph flow rate. An improved technique for obtaining thoracic duct lymph is described here, emphasizing the similarities and differences concerning both rats and mice (average weights of 305 and 15 g, respectively). Rats yielded a mean of 55.6 ml/day thoracic duct lymph, while lymph output in mice reached unexpected volumes of 29.3 ml/day. The use of an operating microscope and silicone cannula, and maintenance of mobility of the animals during lymph collection, offer a reliable method for a high and constant output of thoracic duct lymph. Relevant aspects of the murine thoracic duct anatomy are also identified.

The strict regulation of lymphocyte migration to splenic white pulp does not involve common homing receptors

Although the spleen is the largest secondary lymphoid organ, little is known about the regulation of lymphocyte migration towards its different compartments of red and white pulp, in contrast to the well-studied mechanisms of lymphocyte homing to lymph nodes. Here we show that short-term trypsin treatment of lymphocytes cleaved off molecules involved in entry into lymph nodes, while homing to the splenic white pulp was unaltered. Prolonged trypsin treatment also abolished the ability of lymphocytes to enter the white pulp. Analysis of affected cell surface molecules and adoptive transfer studies in combination with blocking antibodies revealed that l-selectin, CD44, PSGL-1 and the alpha4 integrins are not required for migration to the white pulp. Although lymphocyte function-associated antigen-1 (LFA-1) is critical for entry into lymph nodes, we show here that in the absence of functional LFA-1 molecules, lymphocytes can still enter the white pulp, in spite of the high expression of intercellular adhesion molecule-1 on sinus lining cells in the marginal zone. The data indicate that adhesion molecules involved in lymphocyte homing to lymph nodes are not essential for migration towards the splenic white pulp, but that additional, trypsin-sensitive, and so far unidentified, molecules are required.

A transmembrane CXC chemokine is a ligand for HIV-coreceptor Bonzo

We describe a protein with the hallmarks of a chemokine, designated CXCL16, that is made by dendritic cells (DCs) in lymphoid organ T cell zones and by cells in the splenic red pulp. CXCL16 contains a transmembrane domain and both membrane-bound and soluble forms are produced. Naïve CD8 T cells, natural killer T cells and a subset of memory CD4 T cells bind CXCL16, and activated T cells migrated chemotactically to the soluble chemokine. By expression cloning, Bonzo (also known as STRL33 and TYMSTR) was identified as a CXCL16 receptor. CXCL16 may function in promoting interactions between DCs and CD8 T cells and in guiding T cell movements in the splenic red pulp. CXCL16 was also found in the thymic medulla and in some nonlymphoid tissues, indicating roles in thymocyte development and effector T cell trafficking.

The role of chemokines in the microenvironmental control of T versus B cell arrest in Peyer's patch high endothelial venules

Chemokines have been hypothesized to contribute to the selectivity of lymphocyte trafficking not only as chemoattractants, but also by triggering integrin-dependent sticking (arrest) of circulating lymphocytes at venular sites of extravasation. We show that T cells roll on most Peyer's patch high endothelial venules (PP-HEVs), but preferentially arrest in segments displaying high levels of luminal secondary lymphoid tissue chemokine (SLC) (6Ckine, Exodus-2, thymus-derived chemotactic agent 4 [TCA-4]). This arrest is selectively inhibited by functional deletion (desensitization) of CC chemokine receptor 7 (CCR7), the receptor for SLC and for macrophage inflammatory protein (MIP)-3beta (EBV-induced molecule 1 ligand chemokine [ELC]), and does not occur in mutant DDD/1 mice that are deficient in these CCR7 ligands. In contrast, pertussis toxin-sensitive B cell sticking does not require SLC or MIP-3beta signaling, and occurs efficiently in SLC(low/-) HEV segments in wild-type mice, and in the SLC-negative HEVs of DDD/1 mice. Remarkably, sites of T and B cell firm adhesion are segregated in PPs, with HEVs supporting B cell accumulation concentrated in or near follicles, the target domain of most B cells entering PPs, whereas T cells preferentially accumulate in interfollicular HEVs. Our findings reveal a fundamental difference in signaling requirements for PP-HEV recognition by T and B cells, and describe an unexpected level of specialization of HEVs that may allow differential, segmental control of lymphocyte subset recruitment into functionally distinct lymphoid microenvironments in vivo.

Epstein-Barr virus-induced molecule 1 ligand chemokine is expressed by dendritic cells in lymphoid tissues and strongly attracts naive T cells and activated B cells

Movement of T and B lymphocytes through secondary lymphoid tissues is likely to involve multiple cues that help the cells navigate to appropriate compartments. Epstein-Barr virus- induced molecule 1 (EBI-1) ligand chemokine (ELC/MIP3beta) is expressed constitutively within lymphoid tissues and may act as such a guidance cue. Here, we have isolated mouse ELC and characterized its expression pattern and chemotactic properties. ELC is expressed constitutively in dendritic cells within the T cell zone of secondary lymphoid tissues. Recombinant ELC was strongly chemotactic for naive (L-selectinhi) CD4 T cells and for CD8 T cells and weakly attractive for resting B cells and memory (L-selectinlo) CD4 T cells. After activation through the B cell receptor, the chemotactic response of B cells was enhanced. Like its human counterpart, murine ELC stimulated cells transfected with EBI-1/CC chemokine receptor 7 (CCR7). Our findings suggest a central role for ELC in promoting encounters between recirculating T cells and dendritic cells and in the migration of activated B cells into the T zone of secondary lymphoid tissues.

CD4+ T cells of both the naive and the memory phenotype enter rat lymph nodes and Peyer's patches via high endothelial venules: within the tissue their migratory behavior differs

It is thought that naive T cells predominantly enter lymphoid organs such as lymph nodes (LN) and Peyer's patches (PP) via high endothelial venules (HEV), whereas memory T cells migrate mainly into non-lymphoid organs. However, direct evidence for the existence of these distinct migration pathways in vivo is incomplete, and nothing is known about their migration through the different compartments of lymphoid organs. Such knowledge would be of considerable interest for understanding T cell memory in vivo. In the present study we separated naive and memory CD4+ T cells from the rat thoracic duct according to the expression of the high and low molecular weight isoforms of CD45R, respectively. At various time points after injection into congenic animals, these cells were identified by quantitative immunohistology in HEV, and T and B cell areas of different LN and PP. Three major findings emerged. First, both naive and memory CD4+ T cells enter lymphoid organs via the HEV in comparable numbers. Second, naive and memory CD4+ T cells migrate into the B cell area, although in small numbers and continuously enter established germinal centers (GC) with a bias for memory CD4+ T cells. Third, memory CD4+ T cells migrate faster through the T cell area of lymphoid organs than naive CD4+ T cells. Thus, our study shows that memory CD4+ T cells are not excluded from the HEV route. In addition, "memory" might depend in part on the ability of T cells to specifically enter the B cell area and GC and to screen large quantities of lymphoid tissues in a short time.

Immunohistochemical and structural characteristics of the reticular framework of the white pulp and marginal zone in the human spleen

BACKGROUND

The reticular framework of the white pulp (WP) and marginal zone (MZ) consists of reticulum cells and reticulin fibers. The antigenic heterogeneity of the reticular framework is well documented in the mouse and rat spleen. The aim of the present study is to characterize the reticular framework of the WP and MZ of the human spleen.

METHODS

Nine surgically resected human spleens were investigated. Five of the nine spleens were perfused. Formalin-fixed materials were embedded in paraffin and serial sections prepared for hematoxylin-eosin, silver staining, and immunohistochemical examination. Electron and immuno-electron microscopy were also applied. Using confocal laser scanning microscopy, the reticular framework was analyzed three-dimensionally.

RESULTS

The reticulin fibers of the framework were immunostained for type IV collagen in the WP and MZ. The WP was three-dimensionally delimited by the alpha-smooth muscle actin (alpha-SMA)-positive reticulum cells. In the WP, the distribution of alpha-SMA-positive reticulum cells formed the reticular framework of the periarteriolar lymphoid sheath (PALS). They also ensheathed the reticulin fibers. Interdigitating cells (IDCs) were scattered throughout the framework. A few IDCs attached to the framework. In the lymph follicle (LF), reticulum cells were not alpha-SMA-positive. The mesh of follicular dendritic cells (FDCs) was found in the germinal center. In places, the reticulin fibers were involved in the mesh of the FDCs and covered by the cytoplasm of FDCs. In the MZ, alpha-SMA-positive reticulum cells were arranged in a mesh pattern and ensheathed the fine reticulin fibers.

CONCLUSION

The reticular framework of the PALS, LF, and MZ is specialized into heterogeneous components in the human spleen. The heterogeneity of the framework may induce the segregation of T and B lymphocytes.

Unique site of IgG2a and rheumatoid factor production in MRL/lpr mice

MRL/lpr (Fas-deficient) mice develop an autoimmune syndrome associated with excessive production of autoantibodies. A significant portion of these autoantibodies are IgG2a molecules specific for many of the autoantigens recognized by the sera of patients with systemic lupus erythematosus. In addition, MRL/lpr mice make exceedingly high titers of IgG or IgA rheumatoid factors (RF) specific for autologous IgG2a. The microenvironment of the IgG2a-producing B cells as well as the prototypic RF autoantibodies was determined by a combination of immunohistochemical and in situ hybridization techniques. In contrast to the antibody-producing cells present in mice responding to conventional foreign antigens, both IgG2a+ and RF+ B cells were found to be densely clustered in the T-cell-rich inner periarteriolar lymphatic sheath of the spleen. These results suggest that conventional antibody and autoantibody production in MRL/lpr mice may be mechanistically distinct processes.

A putative chemokine receptor, BLR1, directs B cell migration to defined lymphoid organs and specific anatomic compartments of the spleen

We describe the phenotype of gene-targeted mice lacking the putative chemokine receptor BLR1. In normal mice, this receptor is expressed on mature B cells and a subpopulation of T helper cells. Blr1 mutant mice lack inguinal lymph nodes and possess no or only a few phenotypically abnormal Peyer's patches. The migration of lymphocytes into splenic follicles is severely impaired, resulting in morphologically altered primary lymphoid follicles. Furthermore, activated B cells fail to migrate from the T cell-rich zone into B cell follicles of the spleen, and despite high numbers of germinal center founder cells, no functional germinal centers develop in this organ. Our results identify the putative chemokine receptor BLR1 as the first G protein-coupled receptor involved in B cell migration and localization of these cells within specific anatomic compartments.

The fate of self-reactive B cells depends primarily on the degree of antigen receptor engagement and availability of T cell help

Self-reactive B cells from tolerant double-transgenic (Dbl-Tg) mice coexpressing hen egg lysozyme (HEL) and rearranged anti-HEL immunoglobulin genes have a relatively short life span when compared to normal B cells, irrespective of whether they are exposed to antigen in multivalent membrane-bound form (mHEL-Dbl-Tg mice) or soluble form (sHEL-Dbl-Tg mice). The factors responsible for determining the fate of these B cells after encounter with self-antigen were investigated using a cell-tracking technique in which anti-HEL Ig-Tg spleen cells were labeled with the intracellular dye 5-carboxyfluorescein diacetate-succinimidyl ester (CFSE) and injected either into non-Tg recipients or a variety of HEL-Tg hosts. In non-Tg recipients, HEL-binding B cells persisted in the circulation and could be detected in the follicles of the spleen for at least 5 d. On transfer into either mHEL-Tg or sHEL-Tg hosts, they underwent activation and then rapidly disappeared from the blood and spleen over the next 3 d, consistent with the short life span reported previously. Immunohistology of spleens from sHEL-Tg recipients indicated that the transferred B cells had migrated to the outer margins of the periarteriolar lymphoid sheath (PALS), where they were detectable for 24 h before being lost. The positioning of B cells in the outer PALS depended on a critical threshold of Ig receptor binding corresponding to a serum HEL concentration between 0.5 and 15 ng/ml, but was not restricted to endogenously expressed HEL in that the same migratory pattern was observed after transfer into non-Tg recipients given exogenous (foreign) HEL. Moreover, bone marrow-derived immature Ig-Tg B cells homed to the outer PALS of sHEL-Tg mice and then disappeared at the same rate as mature B cells, indicating that the stage of maturation did not influence the fate of self-reactive B cells in a tolerant environment. On the other hand, HEL-binding B cells transferred into sHEL-Dbl-Tg recipients persisted over the 3-d period of study, apparently due to insufficient availability of antigen, as indicated by the fact that the degree of Ig receptor downregulation on the transferred B cells was much less than in sHEL-Tg recipients. If T cell help was provided to Ig-Tg B cells at the time of transfer into sHEL-Tg recipients in the form of preactivated CD4+ T cells specific for major histocompatibility complex-peptide complexes on the B cell surface, HEL-binding B cells migrated through the outer PALS of the spleen to the follicle, where they formed germinal centers, or to adjacent red pulp, where they formed proliferative foci and secreted significant amounts of anti-HEL antibody. Taken together, these results indicated that the outcome of the interaction between self-antigen and B cells is largely determined by a combination of the degree of receptor engagement and availability of T cell help.

Lymphocyte traffic through lymph nodes and Peyer's patches of the rat: B- and T-cell-specific migration patterns within the tissue, and their dependence on splenic tissue

The migration routes of lymphocyte subsets through organ compartments are of importance when trying to understand the local events taking place during immune responses. We have therefore studied the traffic of B, T, CD4(+), and CD8(+ )lymphocytes through lymph nodes and Peyer s patches. At various time points after injection into the rat, labeled lymphocytes were localized, and their phenotype characterized in cryostat sections using immunohistochemistry. Morphometry was also performed, and the recovery of 51Cr-labeled lymphocytes in these organs was determined. B and T lymphocytes entered the lymph nodes via the high endothelial venules in similar numbers. Most B lymphocytes migrated via the paracortex (T cell area) into the cortex (B cell area), and then back in substantial numbers into the paracortex. In contrast, T lymphocytes predominantly migrated into the paracortex and were rarely seen in the cortex. No obvious differences were seen between various lymph nodes and Peyer s patches and the routes of CD4(+) and CD8(+)lymphocytes. After injection of lymphocytes into animals with autotransplanted splenic tissue, the number of B lymphocytes that had migrated into the B cell area of lymph nodes and of Peyer s patches was significantly decreased, whereas CD4(+) lymphocytes migrated in larger numbers into the T cell area of both organs.

Antigen-induced exclusion from follicles and anergy are separate and complementary processes that influence peripheral B cell fate

Anergic self-reactive B cells competing within a polyclonal B cell repertoire fail to migrate into primary follicles and die after 1-3 days residence in T cell zones. Transfer of anergic HEL-specific B cells to recipients lacking HEL autoantigen and continuous bromodeoxyuridine labeling in mixed bone marrow chimeras confirms that follicular exclusion and cell death in 1-3 days is not an intrinsic characteristic of anergic cells but results from competition with B cells bearing other specificities together with continued binding of autoantigen. When naive (nontolerant) HEL-specific B cells were transferred into mice expressing HEL autoantigen, they were also excluded from follicles and their lifespan was dramatically shortened, although they became activated to express CD86 (B7-2/B70). In the presence of helper T cells, activated B cells but not anergic B cells were rescued from death and formed large extrafollicular foci to autoantibody-secreting cells. Antigen-induced exclusion from follicles is therefore an independent process from anergy that prevents self-reactive B cells from recirculating in the long-lived repertoire and may foster interactions with T cells during immune responses. By contrast, anergy prevents self-reactive B cells from collaborating with helper T cells and secreting autoantibody while trapped in the T zone.

Intravital demonstration of sequential migration process of lymphocyte subpopulations in rat Peyer's patches

BACKGROUND & AIMS

Although recirculation of lymphocytes through Peyer's patches is important for specific immune defense, the intraorgan migration of lymphocyte subpopulations has not been clearly understood. The aim of this study was to compare the spatial distributions of labeled lymphocytes among various subpopulations in rat Peyer's patches.

METHODS

Lymphocytes collected from intestinal lymph were separated into CD4+, CD8+, and T and B cells, labeled with a fluorochrome carboxyfluorescein diacetate succinimidyl ester, and injected into the jugular vein. Peyer's patches of recipient rats were observed by intravital fluorescence microscopy.

RESULTS

No significant difference was found in the percentage of lymphocytes in transit or in the rolling velocity among different subpopulations. Lymphocytes sticking to the venules increased in number at 10-20 minutes, with preferential adherence of CD4+ cells to venules of 25-50 microns and preferential adherence of B cells to the venules of a wider size range. After 30 minutes, extravasated lymphocytes moved into the interstitium. B cells migrated from venules more quickly than CD4+ cells. CD8+ cells showed an intermediate pattern between CD4+ and B cells in sticking and migratory behaviors. Subsequently, CD4+ and CD8 cells preferentially appeared in parafollicular microlymphatics.

CONCLUSIONS

Significant differences were observed among lymphocyte subpopulations in terms of spatial distribution of lymphocytes sticking to venules, migration into the interstitium, and their lymphatic transport.

Pertussis toxin inhibits migration of B and T lymphocytes into splenic white pulp cords

The normal migration route of B cells into follicular areas of spleen and lymph nodes is altered in the case of autoreactive cells that have bound self-antigen. To begin characterizing the molecular requirements for B cell migration into follicles, cells were treated with pertussis toxin (PTX), an inhibitor of signaling by many G protein-coupled chemokine receptors. Lymphocyte accumulation in the spleen is not inhibited by PTX and, therefore, the distribution of transferred cells was examined in this tissue. In contrast to untreated cells that localized predominantly in follicular areas within white pulp cords, PTX-treated B cells failed to enter white pulp areas altogether and accumulated in the splenic red pulp. T cells were also excluded from white pulp cords and in the case of both cell types, the adenosine diphosphate-ribosylating subunit of the toxin was required to block white pulp entry. These findings implicate a G protein-coupled receptor in lymphocyte migration into splenic white pulp cords. Exclusion of PTX-treated cells from all organized areas of secondary lymphoid tissues raises the possibility that the association observed between PTX treatment and predisposition to autoimmune disease results from inhibition of tolerance mechanisms that normally operate within secondary lymphoid tissues.

Regeneration of splenic stromal elements

Splenic regeneration represents an interesting phenomenon both in relation to its role as a model system (to study the development of the complex three-dimensional architecture of an immunological organ) and because of the clinical application, namely autotransplantation of spleen. The latter is one of the attempts to restore splenic functions after splenectomy, which is known to increase a life-long risk of fatal sepsis. However, splenic functions of autotransplanted splenic tissue are known to be highly dependent on the recovery of the complex microenvironment and immunoarchitecture of the splenic compartments during the regeneration processes, but the elements inducing splenic reorganization are still unknown. Therefore, the present work investigates whether splenic stroma depleted of cells is able to induce regenerative processes after implantation. In addition, we tried to recombine stromal tissue with selected cell populations to study their influence. Cell-free stromal tissue induced angiogenesis and to a lesser extent also attracted the immigration of lymphocytes during the first 60 days of regeneration. However, after this period of regeneration, the transplants began to degenerate and were resorbed. The recombination of stromal tissue with mitogen-stimulated spleen cells only resulted in intensifying the degenerative processes, and all implants were resorbed after 120 days. Except that in the first 30 days there were some accumulations of lymphocytes that resembled primitive follicles, no splenic compartments such as red pulp, periarteriolar lymphoid sheath, or marginal zone could be detected in any of the transplants. From these results it can be concluded that splenic stroma can induce the primary events of splenic regeneration (like angiogenesis), but is not able to provide an appropriate microenvironment and immunoarchitecture for a correct repopulation and differentiation of cells. Furthermore, the recombination experiments point to a minor role of T-cells and possibly an important role for accessory cells in splenic regeneration.

Random entry of circulating lymphocyte subsets into peripheral lymph nodes and Peyer's patches: no evidence in vivo of a tissue-specific migration of B and T lymphocytes at the level of high endothelial venules

Lymphocytes continuously migrate through the body and thus immune competent cells are constantly delivered to most tissues. They interact with high endothelial venules (HEV) via specific homing receptors and vascular addressins, and these molecules seem to be the reason for a preferential homing of B lymphocytes into Peyer's patches and of T lymphocytes into peripheral lymph nodes. When lymphocytes derived from lymph node cell suspensions were applied in the in vitro lymphocyte/endothelium binding assay, the well-known preference of mouse lymph node B lymphocytes for Peyer's patch HEV compared to peripheral lymph node HEV was confirmed in the rat (2.8 times). When in the same in vitro assay thoracic duct lymphocytes (TDL) were used this preference was far less obvious (1.4 times). However, by injecting rat TDL intravenously and by tracing them directly in HEV, B, T, CD4+ and CD8+ lymphocytes are seen to enter Peyer's patches and peripheral lymph nodes in vivo without preference. Thus, in contrast to lymphocytes from lymph node cell suspensions, no evidence was found of a tissue-specific migration of thoracic duct B, T, CD4+ and CD8+ lymphocytes at the HEV level. This finding demonstrates the importance of considering both experimental conditions and the cell source used when investigating lymphocyte traffic.

Regeneration of autotransplanted splenic tissue at different implantation sites

Inbred animals (Lewis rats) were used to investigate the regeneration of autologously implanted splenic tissue at intra-omental and subcutaneous sites. Quantitative immunohistology with monoclonal antibodies against lymphocytes and macrophages was performed to analyse the cell density of red pulp (RP), periarteriolar lymphoid sheath (PALS), marginal zone (MZ) and follicle, 7-180 days after transplantation. Antigenic, allogeneic and mitogenic stimulation and Northern blotting were also performed. Transplant groups differed from spleen only in the reduced size of PALS; however, quantitative analysis demonstrated subtle differences between spleen and transplants. The cell density of B-cells and ED-1+ macrophages was reduced in the RP, Tsupp/cyt-cells were decreased and B-cells increased in PALS, and B-cells and Thelper-cells reduced in the MZ. No differences could be detected between the transplant groups. Flow-cytometric analysis of cell suspensions from spleen and transplants revealed a reduction of T-cells (OX-19+), MHC-I and transferrin-receptor-bearing cells in both transplant groups, and a decrease in the number of Thelper-cells and ED-3+ macrophages in subcutaneous transplants. Both transplant groups were defective regarding the allogeneic and pokeweed mitogen response. Aberration of the lipopolysaccharide response was restricted to subcutaneous transplants, which additionally showed abnormal expression of interferon-gamma, interleukin-5 and interleukin-6 mRNA. Thus, subtle alterations of the newly developed microenvironment and/or lymphocyte-homing may influence the regeneration of splenic tissue; the implantation site may represent an important parameter in functional reorganisation.

The extent of clonal structure in different lymphoid organs

To gain insight into the clonal organization of lymphoid organs, we studied the distribution in situ of donor-derived cells in near-physiological chimeras. We introduced RT7b fetal liver cells into nonirradiated congenic RT7a neonatal rats. The chimerism 6-20 wk after injection ranged from 0.3 to 20%. The numbers of cell clones simultaneously contributing to cell generation in a particular histological feature were deduced from the variance in donor cell distribution. In bone marrow and thymus, donor-derived lymphoid cells were found scattered among host cells, indicating a high mobility of cells. In bone marrow, donor cells were evenly distributed over the entire marrow, even at low chimerism. This indicates that leukopoiesis is maintained by the proliferation of many clones. In the thymus, the various lobules showed different quantities of donor-derived lymphoid cells. Mathematical analysis of these differences indicated that 17-18 cell division cycles occur in the cortex. In spleen, the distribution of donor-derived cells over the germinal centers indicated that 5 d after antigenic stimulation, germinal centers develop oligoclonally. The main conclusions of this work are that (a) bone marrow and thymus are highly polyclonal; (b) 17-18 divisions occur between prothymocyte and mature T cell; and (c) lymphoid cells disperse rapidly while proliferating and differentiating.

Cells in the marginal zone of the spleen

The marginal zone of the spleen forms an intriguing area in which a variety of cell types are combined. Several of these cell types seem to have a fixed position in the marginal zone, such as the marginal zone macrophages, the marginal metallophilic macrophages at the inner border, and, to a lesser extent, the marginal zone B cells. For other cell types--T lymphocytes, small B cells, and dendritic cells--the marginal zone is only a temporary residence. It is this combination of relatively sessile cell populations and the continuous influx and passing of bloodborne immunocompetent cells that turn the marginal zone into a dynamic area, particularly apt for antigen processing and recognition. In no other lymphoid organ can such a unique combination of cells and functions be found. The opening of the arterial blood stream in the marginal sinuses results in a reduction of the velocity of the blood stream, and antigens are initially screened in the marginal zone. To this, extremely potent phagocytic cells, the marginal zone macrophages, are present which can take up and phagocytize large foreign particles, such as bacteria and effete red blood cells. Further filtration of the blood takes place in the filtration beds of the red pulp. The marginal zone macrophages express membrane receptors for bacterial polysaccharides which lead to efficient phagocytosis, probably even in the absence of prior opsonization. Antigenic fragments produced this way can be taken up by dendritic cells that enter the spleen by the blood as part of a mobile surveillance immune system. Dendritic cells present antigen to T cells in the outer area of the T cell-dependent PALS, leading to clustering and enrichment of antigen-specific T cells. Antigens in the marginal zone can also directly associate with memory B cells thought to reside here for longer times, having intimate contact with the marginal zone macrophages. B memory cells then migrate into the PALS and present antigen to T cells. The marginal zone therefore functions not only as an area of initial filtration and phagocytosis of antigens from the blood, but also as a site of lymphocyte emigration. Some of the incoming T and B lymphocytes in the recirculating pool enter the white pulp from the marginal zone. The underlying force and selective molecular mechanisms that guide this migration are unknown. Both B and T lymphocytes recirculate through the outer PALS area on their way to the follicles and the inner PALS, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

The spleen in malaria: the role of barrier cells

I believe that my laboratory has developed a construct of the spleen useful in understanding its range of normal and pathologic functions. The elements in the construct include recognition of an anatomically open vasculature with the interposition of reticular cell-reticular fiber filtration beds between terminal arterial vessels and proximal venules. The central function of the spleen, moreover--selective clearance of cells, microbes and other particles from the blood--depends upon these filtration beds. Such functions of the spleen as phagocytosis, immunologic reactivity, hematopoiesis, and blood cell storage derive from its clearance capacities. The reticular filtration beds offer but modest levels of basal clearance. The wide ranges of filtration that characterize the stressed spleen depend upon arming or augmenting the basic reticular filtration beds with responsive cells which can rapidly appear, and rapidly disappear. These include macrophages, salient phagocytic cells of rich repertoire, which have been accorded the major, even exclusive, role in splenic clearance. But other stromal cells participate in splenic clearance. I have identified a system of fibroblastic, contractile, granulated cells which fuse to form complex, branched syncytial sheets which, deployed as diverse barriers, augment the basic reticular filtration beds. Hence, I term these cells barrier cells. Barrier cells effectively interact with macrophages, reticular cells, other stromal and blood cells, contributing to the extraordinary range of splenic clearance capacities. Barrier cells may be elicited by a variety of infectious processes, damaged blood cells and hematopoietic factors. Interleukin-1-alpha evokes a strong barrier cell response, and may be the common denominator in splenic stress, stimulated by activated macrophages.(ABSTRACT TRUNCATED AT 250 WORDS)