Absolute numbers of lymphocytes subsets migrating through the compartments of the normal and transplanted rat spleen

Clever-1 contributes to lymphocyte entry into the spleen via the red pulp

Lymphocytes recirculate continuously between the blood and lymphoid organs, a process that is of fundamental importance for proper functioning of the immune system. The molecular mechanisms underlying lymphocyte trafficking to the spleen remain an enigma. Here, we show that lymphocytes enter the spleen preferentially from vessels in the red pulp rather than the marginal sinus or the vasculature in the white pulp. Ex vivo adhesion assays in mice and humans, together with genetic ablation of Clever-1 in mice, indicate that CD8+T cell and B220+B cell homing to the spleen via the red pulp is Clever-1 dependent. Moreover, absence of Clever-1 leads to down-regulation of the B cell attractant chemokine, CXCL13, on spleen endothelium. CXCL13 is known to guide B cell trafficking to lymphoid organs, and its lack may contribute to the observed decrease in B cell trafficking into the spleen as well. In summary, this study identifies Clever-1 as an important molecule controlling lymphocyte entry into the spleen, along with a critical role for the splenic red pulp in this regulated trafficking. Furthermore, the results demonstrate that location-specific homing-associated molecules guide lymphocyte entry into the spleen.

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.

Activated CD4+ T cells enter the splenic T-cell zone and induce autoantibody-producing germinal centers through bystander activation

CD4(+) T (helper) cells migrate in huge numbers through lymphoid organs. However, little is known about traffic routes and kinetics of CD4(+) T-cell subsets within different organ compartments. Such information is important because there are indications that CD4(+) T cells may influence the function of microenvironments depending on their developmental stage. Therefore, we investigated the migration of resting (naïve), activated, and recently activated (memory) CD4(+) T cells through the different compartments of the spleen. Resting and recently activated CD4(+) T cells were separated from thoracic duct lymph and activated CD4(+) T cells were generated in vitro by cross-linking the T-cell receptor and CD28. The present study shows that all three CD4(+) T-cell subsets selectively accumulate in the T-cell zone of the spleen. However, only activated T cells induce the formation of germinal centers (GCs) and autoantibodies in rats and mice. Our results suggest that in a two-step process they first activate B cells independent of the T-cell receptor repertoire and CD40 ligand (CD154) expression. The activated B cells then form GCs whereby CD154-dependent T-cell help is needed. Thus, activated T cells may contribute to the development of autoimmune diseases by activating autoreactive B cells in an Ag-independent manner.

Local lymphogenic migration pathway in normal mouse spleen

Although the immunological and hemodynamical significance of the spleen is of great importance, few reports detail the lymphatic vessels in this organ. We have used an immunohistochemical three-dimensional imaging technique to characterize lymphatic vessels in the normal mouse spleen and have successfully demonstrated their spatial relationship to the blood vascular system for the first time. Lymphatic markers, such as LYVE-1, VEGFR-3, and podoplanin, show different staining patterns depending on their location in the spleen. LYVE-1-positive lymphatic vessels run reverse to the arterial blood flow along the central arteries in the white pulp and trabecular arteries and exit the spleen from the hilum. These lymphatic vessels are surrounded by type IV collagen, indicating that they are collecting lymphatic vessels rather than lymphatic capillaries. Podoplanin is expressed not only in lymphatic vessels, but also in stromal cells in the white pulp. These podoplanin-positive cells form fine meshworks surrounding the lymphatic vessels and central arteries. Following intravenous transplantation of lymphocytes positive for green fluorescent protein (GFP(+)) into normal recipient mice, donor cells appear in the meshworks within 1 h and accumulate in the lymphatic vessels within 6 h after injection. The GFP(+) cells further accumulate in a draining celiac lymph node through the efferent lymphatic vessels from the hilum. These meshworks might therefore act as an extravascular lymphatic pathway and, together with ordinary lymphatic vessels, play a primary role in the cell traffic of the spleen, additional to the blood circulatory system.

Leukocyte trafficking in a mouse model for leukocyte adhesion deficiency II/congenital disorder of glycosylation IIc

Leukocyte adhesion deficiency II (LAD II), also known as congenital disorder of glycosylation IIc (CDG-IIc), is a human disease in which a defective GDP-fucose transporter (SLC35C1) causes developmental defects and an immunodeficiency that is based on the lack of fucosylated selectin ligands. Since the study of in vivo leukocyte trafficking in patients with LAD II is experimentally limited, we analyzed this process in mice deficient for Slc35c1. We found that E-, L-, and P-selectin-dependent leukocyte rolling in cremaster muscle venules was virtually absent. This was accompanied by a strong but not complete decrease in firm leukocyte adhesion. Moreover, neutrophil migration to the inflamed peritoneum was strongly reduced by 89%. Previous reports showed surprisingly normal lymphocyte functions in LAD II, which indicated sufficient lymphocyte trafficking to secondary lymphoid organs. We now found that while lymphocyte homing to lymph nodes was reduced to 1% to 2% in Slc35c1(-/-) mice, trafficking to the spleen was completely normal. In accordance with this, we found a defect in the humoral response to a T cell-dependent antigen in lymph nodes but not in the spleen. Taken together, Slc35c1(-/-) mice show strongly defective leukocyte trafficking but normal lymphocyte homing to the spleen, which may explain normal lymphocyte functions in LAD II.

Natural killer cells and B lymphocytes in L-selectin and Mac-1/LFA-1 knockout mice: marker-dependent, but not cell lineage-dependent changes in the spleen and bone marrow

The majority of B lymphocytes, virgin T lymphocytes and a subpopulation of memory T cells express the addressin, L-selectin. Natural killer (NK) cells in rodents and humans also express L-selectin. We have shown that a similar proportion (40%) of NK cells in mouse spleen also express the integrin, CD18Mac-1, and moreover, that NK cells express both the addressin and the integrin constitutively. It was the aim of the present study to quantify, in knock-out mice deficient for either the L-selectin addressin, or the CD18:Mac-1/LFA-1 integrins, NK cells and B cells in both the spleen and their bone marrow birth site. These cells, in both organs, were immunophenotypically stained with FITC-conjugated anti-NK1.1 (to identify NK cells), and FITC-conjugated anti-mouse B220 (to identify B lymphocytes) and subjected to flow-cytometric analysis using a FACScan equipped with a doublet discrimination module. From the known total organ (spleen, femurs) cellularity, obtained by means of an electronic cell counter, at the time of extraction of each organ, the absolute numbers of NK cells and B lymphocytes from each mouse were obtained. The results revealed that there are significantly more NK cells and B lymphocytes in the spleens of CD18:Mac-1/LFA-1 knockout mice than in control (same strain) mice. Moreover, in L-selectin knockout mice spleens, NK cells and B lymphocytes were elevated by 26.2% and 17.8% respectively. NK cells and B lymphocytes in the bone marrow of the integrin knockout showed no difference from control, however, both cell types in the bone marrow of the L-selectin knockout mice fell to only 3/4 their control levels. Collectively, the results demonstrated that there are organ-specific, but not cell lineage-specific differences in the absolute numbers of NK cells and B lymphocytes, in integrin-deficient (CD18:Mac-1/LFA-1 knockout) mice and addressin-deficient (L-selectin knockout) mice.

Social stress affects migration of blood T cells into lymphoid organs

The effect of social confrontation stress on the normal distribution of blood T cells into lymphoid organs and some other body tissues was studied. Social stress was induced by placing a male Fischer 344 (F334) rat into the home cage of a resident opponent. 51Cr-labeled blood T cells isolated from syngeneic donor, were intravenously injected into recipients immediately before confrontation. The accumulation of 51Cr-labeled T cells in the spleen, mesenteric and cervical lymph nodes 24 h after injection was about 30% lower in socially stressed than in control males. Substantially higher localization was observed in the bone marrow of socially stressed males.

Local and systemic autonomic nervous effects on cell migration to the spleen

This work is based on the hypothesis that sympathetic nerves regulate the uptake of circulating cells by the spleen by affecting splenic blood flow and that the quantity of cells sequestered depends on whether changes in noradrenergic transmission occur at local or systemic levels. Fluorescently labeled lymphoid cells were injected into rats, and organ blood flow was measured by the microsphere method. Increased retention of cells in the spleen paralleled by increased blood flow was detected after local denervation of this organ or administration of bacterial endotoxin. A comparable enhanced splenic blood flow was observed after general sympathectomy. However, the redistribution of blood perfusion during general vasodilatation resulted in deviation of leukocyte flow from the spleen, thus resulting in reduced uptake of cells by this organ. These results indicate that, although the uptake of cells by the spleen depends on arterial blood supply, enhanced perfusion does not always result in increased cell sequestration because general vasodilatation reduces cell uptake by this organ and even overrides stimulatory effects of endotoxin.

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.

Splenectomy of rats selectively reduces lymphocyte function-associated antigen 1 and intercellular adhesion molecule 1 expression on B-cell subsets in blood and lymph nodes

Splenectomy increases the number of B cells in the blood of humans and animals. It is unknown whether this is due to changes in migration, proliferation, or both. The numbers of naïve (IgD(+)IgM(+)), memory (IgD(-)IgM(high)), newly formed (IgM(high)CD90(high)), early recirculating follicular (IgM(low)CD90(high)), recirculating follicular (IgM(low)CD90(-)), and marginal zone (IgM(high)CD90(-)) phenotype B cells were determined in control and splenectomized rats by flow cytometry. All subsets increased significantly in the blood after splenectomy. Because surface molecules are involved in the regulation of migration and proliferation, their expression (lymphocyte function-associated antigen 1 [LFA-1], intercellular adhesion molecule 1 (ICAM-1), L-selectin, alpha4-integrins, CD44, major histocompatability complex class II, interleukin 2 receptor-alpha chain) was determined on B- and T-cell subsets of both groups. B cells, but not T cells, showed a significantly reduced LFA-1 and ICAM-1 expression in blood and lymph nodes, whereas the expression of the other surface molecules analyzed remained unchanged. The down-regulation of these molecules did not influence the adherence of B cells to high endothelial venules in vitro. In vivo, however, ICAM-1(low)-expressing B cells migrated significantly faster through lymph nodes (ICAM-1(low) 41 +/- 5 hours versus ICAM-1(high) 58 +/- 3 hours), whereas proliferation of B cells in bone marrow, lymph node, and blood remained unchanged. Thus, the presence of one organ is necessary for appropriate expression of LFA-1 and ICAM-1 on B cells in other, distant organs. The more rapid transit of ICAM-1(low) B cells through lymph nodes may be responsible for the increased B-cell number in the blood after splenectomy.

Highly restricted spread of HIV-1 and multiply infected cells within splenic germinal centers

The tremendous dynamics of HIV infection finds expression in the tempo of sequence diversification. Genetic diversity calculations require the clearance of a majority of infected cells, the obvious predator being anti-HIV immune responses. Indeed, infiltration of germinal centers (GCs) by HIV-specific CD8(+) cytotoxic T lymphocytes has been described. A corollary to this description would be limited diffusion of virus within lymphoid structures. HIV efficiently infects and replicates mainly in activated CD4(+) T lymphoblasts. These cells are found within GCs after their activation in the adjacent periarteriolar lymphoid sheath (PALS). Here GCs and PALS have been dissected from consecutive 10-micrometer sections through splenic tissue from three HIV-1-infected patients. Nested PCR amplification of the two first hypervariable regions of the env gene indicated that 38-78% of sections contained HIV-infected cells. Since there are several hundred CD4(+) T cells per GC section, approximately 0.09-0.64% harbor proviral DNA. Such a low frequency not only suggests that virions on the follicular dendritic cell surfaces do not readily infect adjacent T cells but also indicates highly restricted spread of HIV within GCs and the PALS. Sections were heavily infiltrated by CD8(+) cells, which, together with a large body of extant data, suggests that the majority of infected cells are destroyed by HIV-specific cytotoxic T lymphocytes before becoming productively infected. Finally, sequence analysis revealed that those HIV-positive cells were multiply infected, which helps explain widespread recombination despite a low overall frequency of infected cells.

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.

NKX2.3 is required for MAdCAM-1 expression and homing of lymphocytes in spleen and mucosa-associated lymphoid tissue

Targeted disruption of the transcription factor NKX2.3 gene in mice results in anatomical defects of intestine and secondary lymphoid organs. Here, we report that spleen and Peyer's patches of NKX2. 3-deficient mice are considerably reduced in size and lack the ordered tissue architecture. T and B cells are misplaced within the spleen and mesenteric lymph nodes and fail to segregate into the appropriate T and B cell areas. Furthermore, splenic marginal zones, characterized by specific B cells and various types of macrophage-derived cells around the marginal sinus, are absent in mutants. Homozygous NKX2.3 mutants lack the mucosal addressin cell adhesion molecule-1 (MAdCAM-1) that is normally expressed in mucosa-associated lymphoid tissue (MALT) and spleen. We provide evidence that NKX2.3 can activate MAdCAM-1 transcription directly, suggesting that MAdCAM-1 is at least partly responsible for the migration and homing defects of lymphocytes and macrophages in mutants. Therefore, expression of MAdCAM-1 seems to be required for building functional structures in spleen and MALT, a prerequisite for unimpaired migration and segregation of B and T cells to and within these organs.

Immunohistochemical study of lymphoid tissues in adjuvant arthritis (AA) by image analysis; relationship with synovial lesions

The aim of this study was to examine leucocyte populations in lymphoid organs during AA and to ascertain the relationship with lesions in synovial joints. Popliteal lymph nodes, spleen and knee synovial membranes were removed from both healthy and AA rats at intervals of 3-4 days over a 3-week period. Cryostat sections were stained with MoAbs directed against lymphocyte and macrophage subpopulations, and studied by image analysis. Throughout the arthritic period, high numbers of ED1+ and ED3+ macrophages were seen in both lymphoid compartments and intercellular adhesion molecule-1 (ICAM-1) expression also increased in some zones of lymph nodes and spleen. The percentages of CD4+ and CD8+ cells rose in the splenic zones studied but fell in the lymph node cortex. Very few natural killer (NK) cells were found in lymphoid tissues, but the number rose after AA induction. In synovia from AA rats, ED2+ macrophages proliferated but alpha/beta T cell infiltration was only occasionally observed, accompanied by ED1+ cells and ICAM-1 expression. In conclusion, synovitis developing after AA induction seems to be caused directly by macrophages and indirectly by lymphocytes placed both in popliteal lymph nodes and spleen.

The role of CCR7 in TH1 and TH2 cell localization and delivery of B cell help in vivo

Subsets of murine CD4+ T cells localize to different areas of the spleen after adoptive transfer. Naïve and T helper 1 (TH1) cells, which express the chemokine receptor CCR7, are home to the periarteriolar lymphoid sheath, whereas activated TH2 cells, which lack CCR7, form rings at the periphery of the T cell zones near B cell follicles. Retroviral transduction of TH2 cells with CCR7 forces them to localize in a TH1-like pattern and inhibits their participation in B cell help in vivo but not in vitro. Thus, differential expression of chemokine receptors results in unique cellular migration patterns that are important for effective immune responses.

Norepinephrine stimulates lymphoid cell mobilization from the perfused rat spleen via beta-adrenergic receptors

The possibility that norepinephrine (NE) influences lymphoid cell outflow independently of its vasoconstrictor action was investigated in the perfused rat spleen. Using agents that affect the vasoconstrictor tonus of the spleen, we observed an inverse correlation between flow resistance and splenic cell output. The curve obtained served as a reference for evaluating effects of different treatments on the number of cells that are mobilized at defined levels of flow resistance. Perfusion of the beta-adrenergic blocker propranolol either alone or in combination with NE lowered splenic leukocyte outflow clearly beyond the number of cells expected at the corresponding flow resistance. No comparable effects were observed when the alpha-adrenergic blocker phentolamine was perfused. When the vasoconstrictor effect of NE was counteracted by papaverine, splenic cell outflow was significantly higher than expected for the level of flow resistance attained. Furthermore, when NE was perfused together with endotoxin, which does not inhibit the vasoconstriction induced by catecholamines, splenic cell mobilization was severalfold higher than expected at increased flow resistance. Propranolol abrogated this effect to a large extent. Furthermore, perfusion of the beta-agonist isoproterenol stimulated lymphoid cell outflow from the spleen despite increased flow resistance. These studies show a dual effect of NE on cell mobilization from the spleen: cell retention by decreasing blood flow and stimulation of cell output by a beta-adrenergically mediated, smooth muscle-independent mechanism.

In vivo tracking and protective properties of Yersinia-specific intestinal T cells

After invasion via M cells enteropathogenic Yersinia enterocolitica subsequently establish an infection at three different sites: (i) Peyer's patches (PP), (ii) mesenteric lymph nodes (MLN), and after systemic dissemination in (iii) spleen, liver and lung. In order to characterize protective properties of intestinal T cells at the different sites of Y. enterocolitica infection, PP and MLN T cells were isolated from Y. enterocolitica-infected C57B1/6 mice and Yersinia-specific T cell lines were generated. These T cells exhibited the phenotype of CD4 Th1 cells. The adoptive transfer of Yersinia-specific Th1 cells from PP and MLN conferred protection against a lethal orogastric inoculum with Y. enterocolitica as revealed by survival post-infection. However, determination of bacterial counts in infected organs revealed that the transfer of PP T cells conferred protection in spleen but not in MLN and PP, whereas the transfer of T cells from MLN reduced bacterial counts in both spleen and MLN but not in PP. To elucidate the different protection pattern we wanted to track the transferred cells in vivo. For this purpose the cells were labelled with the stable green fluorescent cell linker PKH2-GL prior to the adoptive transfer. In vivo tracking of these cells revealed that the distribution pattern of transferred T cells in spleen, MLN and PP correlated closely with the protection pattern observed after Yersinia infection. Thus, most cells were recovered from the spleen, while only few cells were recovered from MLN and PP. In keeping with these results a rapid and significant increase in interferon-gamma (IFN-gamma) production in the spleen of mice after adoptive transfer of T cell lines was observed. Taken together, the present results demonstrate that intestinal CD4 Th1 cells from PP and MLN may be involved in the defence against Y. enterocolitica at different sites of the infection, and that PKH2-GL labelling is a suitable tool to characterize T cell functions in vivo.

Regeneration of implanted splenic tissue in the rat: re-innervation is host age-dependent and necessary for tissue development

The loss of spleen may lead to fatal bacterial infections. To prevent this, splenic autotransplantation has been performed in humans and experimental animals. However, there is still controversy about the protective function of this procedure. Since innervation plays an important role in splenic function, we investigated whether splenic regenerates are re-innervated, and whether this depends on the donor and host age. Splenic tissue (30 mg) was implanted into the greater omentum of either young (2 days) or old (12 months) rats, from either young or old syngeneic animals. After 3 months of regeneration, the weight of the regenerates was determined, PGP+ nerve fibers were revealed by immunohistology, and subdivided into nerve fibers of sympathetic (TH+, NPY+) or sensory (SP+, CGRP+) origin. In addition, proliferating (Ki-67 proliferation antigen+) and apoptotic cells (TUNEL technique+) were likewise investigated. No innervation of splenic regenerates was observed after implantation into old hosts, correlating with poorly developed splenic compartments. In contrast, almost normal re-innervation occurred in young hosts after implantation of both young and old splenic tissue. These regenerates showed well-developed splenic compartments and a normal number and tissue distribution of proliferating and apoptotic cells. However, after the implantation of young tissue, the final size of splenic regenerates was three times larger (140 +/- 30 vs. 40 +/- 10 mg). Thus, re-innervation of splenic implants is necessary for their subsequent development. It is determined by host age, whereas the final size of the splenic regenerates is regulated by donor age-dependent factors. This model is useful for studying both the process leading to initial innervation and the consequences of this innervation.

A method for tracking the migration of blood lymphocytes

A method has been devised for labeling whole blood with the fluorescent dye fluorescein isothiocyanate (FITC) so the migration of blood lymphocytes can be studied in the sheep. Although lymphocytes can be purified from blood using density gradient media or elutriation it is difficult to obtain a large number of cells, because many cells are usually lost during the purification steps. It is desirable to label at least 10(8)-10(9) lymphocytes for lymphocyte tracking studies, because a smaller number is difficult to subsequently detect and quantitate in blood and lymph even using flow cytometry. Also, it is desirable to minimize the in vitro manipulation of lymphocytes, because dead or damaged lymphocytes will not recirculate. By labeling all the cellular components of a sample of whole blood rather than first purifying the lymphocytes we have been able to satisfy both of these criteria. Although labeled blood cells of all types are reinjected into the animal, the lymphocytes are easily distinguishable from other cells using a flow cytometer. In these studies between 2.4-12.4 x 10(8) lymphocytes were injected intravenously, and they were detectable in the blood and lymph for at least 10 days. The recovery of FITC-labeled (FITC+) lymphocytes in efferent lymph is comparable to that of lymphocytes labeled with other fluorescent or radioactive markers. The presence of labeled non-lymphoid cells in the animal makes this technique impractical for studies of lymphocyte localization within histologic sections. However, it is useful for studies in animals in which lymphatic vessels can be cannulated and the blood-to-lymph recirculation of labeled lymphocytes monitored, and it also may be applicable for studies in which lymphoid organ suspensions are analyzed using flow cytometry.

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.

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.

Complement-mediated follicular localization of T-independent type-2 antigens: the role of marginal zone macrophages revisited

In this study we demonstrate a hitherto undescribed phenomenon, namely that thymus-independent type-2 antigens (TI-2 Ag) localize in splenic follicles within 1 h after administration. The follicular localization of 2,4,6-trinitrophenyl (TNP)-Ficoll was not antibody mediated. In addition in case of high-dose administration we observed a relatively large amount of TI-2 Ag in marginal zone macrophages. However, after low-dose administration we observed a preferential localization of TNP-Ficoll in the splenic follicles. Detection of TNP-haptenated Ag in cryostat sections of murine spleens was performed with a high-affinity TNP-specific monoclonal antibody conjugated to beta-galactosidase. Within minutes after injection the TI-2 Ag localized in the marginal zone, attached to marginal zone macrophages and B cells. Twenty minutes after injection the Ag was also detected in the follicles and gradually accumulated there until 7 h after injection. Thereafter, the amount of follicular Ag gradually decreased but was still detectable up to 14 days after immunization. The follicular localization of TNP-Ficoll was complement dependent in contrast to the binding to and uptake by marginal zone macrophages. Double staining revealed that Ag was bound by macrophages, B cells and follicular dendritic cells. Haptenated thymus-dependent (TD) Ag localized exclusively in the red pulp macrophages. In vivo macrophage elimination drastically increased the amount of TNP-Ficoll in the follicles, and enhanced the humoral immune response at low doses of Ag. Moreover, complement deprivation of mice abrogated the localization of TI-2 Ag in the follicles, and led to a decreased humoral TI-2 immune response. In conclusion, we demonstrate for the first time that TI-2 Ag localize in follicles. Moreover, the presented results provide further evidence that B cells and follicular localized Ag play an important role in the induction of humoral TI-2 immune responses.

Lymphocyte subsets in the blood: a diagnostic window on the lymphoid system?

The assessment of peripheral blood lymphocyte numbers and ratios has been performed in almost every conceivable disease state, but do these values give any useful information about immune status? Jürgen Westermann and Reinhard Pabst try to answer this question and to put peripheral blood population assessments into the whole-body context.

The in vitro lymphocyte/endothelium binding assay. An improved method employing light microscopy

The in vitro lymphocyte binding assay (HEV assay) has proved to be a useful approach for examining the first step of lymphocyte migration, i.e., homing to organs containing high endothelial venules (HEVs). Since fluorescence-labelled standard lymphocytes are usually included in each assay to account for day-to-day variations, HEV preparations have to be evaluated by fluorescence microscopy. Thus no counterstaining can be performed and HEVs without adherent lymphocytes cannot easily be recognized. Because the preparations are not suitable for storage they must be evaluated within a short time. In this study an improved technique is described which permits HEV preparations made with fluorescence-labelled standard lymphocytes to be evaluated by light microscopy in counterstained sections. The phenotypes of the sample lymphocytes can be determined by staining for surface antigens on the same slides and the preparations obtained are permanent.