Chemokines in lymphopoiesis and lymphoid organ development

Visualizing thymocyte motility using 2-photon microscopy

Our view of a thymocyte based on its behavior in tissue culture and appearance in fixed tissue sections was of a round sessile cell. Its travel through the thymus might occur slowly, perhaps even passively, leaving it in contact with the support cells that happened to be in its immediate environment. However, when we got our first look at the behavior of thymocytes in a 3D cellular stromal cell environment, that picture changed dramatically. Instead we found that thymocytes are actively crawling, allowing them to explore their environment over relatively long distances and interact with peptide-major histocompatibility complex (pMHC)-bearing thymic stromal cells in both dynamic and stable modes. In this review, we discuss the implications of thymocyte motility for T-cell repertoire selection and for the mechanisms that determine the spatial organization of thymocyte subsets within the thymus.

Dissecting the role of lymphotoxin in lymphoid organs by conditional targeting

Mice with inactivation of lymphotoxin beta receptor (LTbetaR) system have profound defects in the development and maintenance of peripheral lymphoid organs. As surface LT is expressed by lymphocytes, natural killer cells, and lymphoid tissue-initiating cells as well as by some other cell types, we dissected cell type-specific LT contribution into the complex LT-deficient phenotype by conditional gene targeting. B-LTbeta knockout (KO) mice displayed an intermediate phenotype in spleen as compared with mice with complete LTbeta deficiency. In contrast, T-LTbeta KO mice displayed normal structure of the spleen. However, inactivation of LTbeta in both T and B cells resulted in additional defects in the structure of the marginal zone and in the development of follicular dendritic cells in spleen. Structure of lymph nodes (LN) and Peyer's patches (PP) was normal in both B-LTbeta KO and T- and B-LTbeta KO mice, except that PPs were of reduced size. When compared across the panel of lymphocyte-specific LT KOs, the defects in antibody responses to T-cell-dependent antigens correlated with the severity of defects in spleen structure. Expression of CCL21 and CCL19 chemokines was not affected in spleen, LN and PP of B-LTbeta KO and T- and B-LTbeta KO mice, while CXCL13 was slightly reduced only in spleen. Collectively, our data suggest the following: (i). requirements for LT signaling to support architecture of spleen, LN and PP are different; (ii). LT complex expressed by B cells plays a major role in the maintenance of spleen structure, while surface LT expressed by T cells provides a complementary but distinct signal; and (iii). in a non-transgenic model, expression of lymphoid tissue chemokines is only minimally dependent on the expression of surface LT complex on B and T lymphocytes.

Organogenesis of peripheral lymphoid organs

The discovery that lymphotoxin alpha (LTalpha) knockout mice lack peripheral lymphoid tissues reformed the study of organogenesis of peripheral lymphoid tissues from a research field that was solely descriptive and dependent on histological methods to one requiring all modern technologies. The concepts of inducer cells for organogenesis of peripheral lymphoid tissues as a separate hematopoietic lineage and of mesenchymal organizer cells have been established through this progress. These discoveries led to the comprehension of the basic framework of the events during organogenesis of peripheral lymphoid tissues. However, many important questions remain unanswered. This review discusses those questions which have arisen from our studies on the organogenesis of Peyer's patches.

Regulation of secondary lymphoid organ development by the nuclear factor-kappaB signal transduction pathway

In primary lymphoid organs, such as thymus and bone marrow, B and T lymphocytes differentiate from lymphoid stem cells into mature albeit naïve effector cells. In contrast, secondary lymphoid organs, such as the spleen, lymph nodes, and Peyer's patches (PPs), provide an environment that enable lymphocytes to interact with each other, with accessory cells, and with antigens, resulting in the initiation of antigen-specific primary immune responses. Recently, the analysis of gene-knockout mice has shed light on the signaling pathways, cellular requirements, and molecular mechanisms involved in secondary lymphoid organ development. In particular, signals that converge on the nuclear factor-kappaB (NF-kappaB) pathway have been demonstrated to play an important role in both early developmental steps as well as maintenance of secondary lymphoid organ structures. Analysis of the histopathological changes in secondary lymphoid tissues of mice lacking individual Rel/NF-kappaB family members, upstream kinases, and receptors strongly indicates that activation of the recently described alternative NF-kappaB pathway by membrane-bound lymphotoxin, via p52-RelB heterodimers, plays a major role during initiation steps of secondary lymphoid organ development. Induction of the classical p50-RelA NF-kappaB activity, as exemplified by tumor necrosis factor receptor signaling, clearly also contributes, but seems to be involved primarily in later developmental step, such as the proper cellular and structural organization of B-cell follicles.

Chemokines on the move: control by the chemokine "interceptors" Duffy blood group antigen and D6

Chemokines drive and direct leukocyte trafficking across the biological barriers. Controlling the microanatomical localization and quantity of chemokines is of fundamental importance in regulating these migratory responses. Here we discuss the emerging roles that two atypical chemokine receptors, Duffy antigen and D6, may play in controlling chemokine movement and how this may impinge on chemokine function. Mechanistically, Duffy antigen and D6 represent a subclass of chemokine internalizing receptors, "interceptors," taking chemokines into nucleated cells in the apparent absence of signaling. The subsequent fate of chemokines, either transport or degradation, may ultimately depend on cell type-specific targeting within the endocytotic pathway.

Defective development and function of Bcl10-deficient follicular, marginal zone and B1 B cells

Bcl10 is an intracellular protein essential for nuclear factor (NF)-kappaB activation after lymphocyte antigen receptor stimulation. Using knockout mice, we show that absence of Bcl10 impeded conversion from transitional type 2 to mature follicular B cells and caused substantial decreases in marginal zone and B1 B cells. Bcl10-deficient B cells showed no excessive apoptosis. However, both Bcl10-deficient follicular and marginal zone B cells failed to proliferate normally, although Bcl10-deficient marginal zone B cells uniquely failed to activate NF-kappaB efficiently after stimulation with lipopolysaccharide. Bcl10-deficient marginal zone B cells did not capture antigens, and Bcl10-deficient (Bcl10-/-) mice failed to initiate humoral responses, leading to an inability to clear blood-borne bacteria. Thus, Bcl10 is essential for the development of all mature B cell subsets.

Changes in the lymph node microenvironment induced by oncostatin M

Oncostatin M (OM) transforms the lymph node (LN) into a "super lymphoid organ" with 2 striking features: massive thymus-independent T-cell development and major expansion of the memory T-cell pool. We report that T-cell development in the LckOM LN is regulated by a cyclooxygenase-2 (COX-2)-dependent neoangiogenesis involving high endothelial venules (HEVs). That LN HEVs are particularlyrich in OM-receptor beta-chain provides aplausible explanation for the fact that extrathymic T-cell development in LckOM mice is limited to the LN. Moreover, we found that increased production of the CCL20 chemokine by LN stromal cells was instrumental in the expansion of the memory phenotype CD4 T-cell pool in LckOM mice. The generality of the latter finding was demonstrated by the fact that CCL20/CCR6 interactions increase the basal proliferation rate of CD62L(lo) CD4 T cells irrespective of their thymic (in non-OM-transgenic mice) or extrathymic (in LckOM mice) origin. To our knowledge, CCL20 is the first molecule found to increase the proliferation of memory phenotype CD4 T cells. These findings identify potential targets for the creation of thymic substitutes (LN HEVs) and for expansion of the CD4 memory T-cell compartment (CCL20).

Ectopic LT alpha beta directs lymphoid organ neogenesis with concomitant expression of peripheral node addressin and a HEV-restricted sulfotransferase

Lymph node (LN) function depends on T and B cell compartmentalization, antigen presenting cells, and high endothelial venules (HEVs) expressing mucosal addressin cell adhesion molecule (MAdCAM-1) and peripheral node addressin (PNAd), ligands for naive cell entrance into LNs. Luminal PNAd expression requires a HEV-restricted sulfotransferase (HEC-6ST). To investigate LT alpha beta's activities in lymphoid organogenesis, mice simultaneously expressing LT alpha and LT beta under rat insulin promoter II (RIP) control were compared with RIPLT alpha mice in a model of lymphoid neogenesis and with LT beta-/- mice. RIPLT alpha beta pancreata exhibited massive intra-islet mononuclear infiltrates that differed from the more sparse peri-islet cell accumulations in RIPLT alpha pancreata: separation into T and B cell areas was more distinct with prominent FDC networks, expression of lymphoid chemokines (CCL21, CCL19, and CXCL13) was more intense, and L-selectin+ cells were more frequent. In contrast to the predominant abluminal PNAd pattern of HEV in LT beta-/- MLN and RIPLT alpha pancreatic infiltrates, PNAd was expressed at the luminal and abluminal aspects of HEV in wild-type LN and in RIPLT alpha beta pancreata, coincident with HEC-6ST. These data highlight distinct roles of LT alpha and LT alpha beta in lymphoid organogenesis supporting the notion that HEC-6ST-dependent luminal PNAd is under regulation by LT alpha beta.

Sustained interleukin-6 signalling leads to the development of lymphoid organ-like structures in the lung

A variety of pathological changes are seen in lymphoproliferative disorders of the lung but the histogenesis of these abnormalities is not yet fully understood. We previously showed that adenovirus vector-mediated transient expression of both the human interleukin-6 (IL-6) and IL-6 receptor (IL-6R) genes, but not the IL-6 gene alone, in the rat lung induced lymphocytic alveolitis. In the present study, we explored the lung pathology of human IL-6 and IL-6R double transgenic mice to elucidate the effects of prolonged IL-6 signalling on the lung. The transgenic animals developed mononuclear cell accumulation in peribronchovascular regions, but little infiltration into alveolar spaces. Immunohistochemical analysis revealed that the cellular accumulations contained not only mixtures of inflammatory cells but also lymphoid tissue-like structures. As the expression of CXCL13/BLC, the indispensable chemokine for lymphoid organogenesis, was recognized in the B cell follicles of the pulmonary lesions, we speculate that this chemokine plays an inductive role in the development of the lymphoid tissue-like structures. These structures were distinguished from bronchus-associated lymphoid tissues (BALTs) by their location and by the lack of lymphoepithelium, which is a characteristic of BALT. These findings imply that IL-6 signalling may play a role in the pathogenesis of lymphoproliferative disorders of the lung.

Discovery of distinctive gene expression profiles in rheumatoid synovium using cDNA microarray technology: evidence for the existence of multiple pathways of tissue destruction and repair

Rheumatoid arthritis (RA) is a heterogeneous disease. We used cDNA microarray technology to subclassify RA patients and disclose disease pathways in rheumatoid synovium. Hierarchical clustering of gene expression data identified two main groups of tissues (RA-I and RA-II). A total of 121 genes were significantly higher expressed in the RA-I tissues, whereas 39 genes were overexpressed in the RA-II tissues. Among the 121 genes overexpressed in RA-I tissues, a relative majority of nine genes are located on chromosome 6p21.3. An interpretation of biological processes that take place revealed that the gene expression profile in RA-I tissues is indicative for an adaptive immune response. The RA-II group showed expression of genes suggestive for fibroblast dedifferentiation. Within the RA-I group, two subgroups could be distinguished; the RA-Ia group showed predominantly immune-related gene activity, while the RA-Ib group showed an additional higher activity of genes indicative for the classical pathway of complement activation. All tissues except the RA-Ia subgroup showed elevated expression of genes involved in tissue remodeling. These results confirm the heterogeneous nature of RA and suggest the existence of distinct pathogenic mechanisms that contribute to RA. The differences in expression profiles provide opportunities to stratify patients based on molecular criteria.

Identification of chemokines and a chemokine receptor in cichlid fish, shark, and lamprey

Chemokines are small, inducible, structurally related proteins that guide cells expressing the right chemokine receptors to sites of immune response. They have been identified and studied extensively in mammals, but little is known about their presence in other vertebrate groups. Here we describe seven new chemokines in bony fish and one in a cartilaginous fish, as well as one chemokine receptor in a jawless vertebrate. All eight chemokines belong to the SCYA (CC) subfamily characterized by four conserved cysteine residues of which the first two are adjacent. The chemokine receptor is of the CXCR4 type. Phylogenetic analysis does not reveal any clear evidence of orthology of fish and human chemokines. Although the divergence of the subfamilies began before the fish-tetrapod split, much of the divergence within the subfamilies took place separately in the two vertebrate groups. The existence of a chemokine receptor in the lamprey indicates that chemokines are apparently also present in the Agnatha.

Sustained activation of cell adhesion is a differentially regulated process in B lymphopoiesis

It is largely unknown how hematopoietic progenitors are positioned within specialized niches of the bone marrow microenvironment during development. Chemokines such as CXCL12, previously called stromal cell-derived factor 1, are known to activate cell integrins of circulating leukocytes resulting in transient adhesion before extravasation into tissues. However, this short-term effect does not explain the mechanism by which progenitor cells are retained for prolonged periods in the bone marrow. Here we show that in human bone marrow CXCL12 triggers a sustained adhesion response specifically in progenitor (pro- and pre-) B cells. This sustained adhesion diminishes during B cell maturation in the bone marrow and, strikingly, is absent in circulating mature B cells, which exhibit only transient CXCL12-induced adhesion. The duration of adhesion is tightly correlated with CXCL12-induced activation of focal adhesion kinase (FAK), a known molecule involved in integrin-mediated signaling. Sustained adhesion of progenitor B cells is associated with prolonged FAK activation, whereas transient adhesion in circulating B cells is associated with short-lived FAK activation. Moreover, sustained and transient adhesion responses are differentially affected by pharmacological inhibitors of protein kinase C and phosphatidylinositol 3-kinase. These results provide a developmental cell stage-specific mechanism by which chemokines orchestrate hematopoiesis through sustained rather than transient activation of adhesion and cell survival pathways.

Absence of CCR8 does not impair the response to ovalbumin-induced allergic airway disease

Interaction of chemokines with their specific receptors results in tight control of leukocyte migration and positioning. CCR8 is a chemokine receptor expressed mainly in CD4(+) single-positive thymocytes and Th2 cells. We generated CCR8-deficient mice (CCR8(-/-)) to study the in vivo role of this receptor, and describe in this study the CCR8(-/-) mouse response in OVA-induced allergic airway disease using several models, including an adoptive transfer model and receptor-blocking experiments. All CCR8(-/-) mice developed a pathological response similar to that of wild-type animals with respect to bronchoalveolar lavage cell composition, peripheral blood and bone marrow eosinophilia, lung infiltrates, and Th2 cytokine levels in lung and serum. The results contrast with a recent report using one of the OVA-induced asthma models studied here. Similar immune responses were also observed in CCR8(-/-) and wild-type animals in a different model of ragweed allergen-induced peritoneal eosinophilic inflammation, with an equivalent number of eosinophils and analogous increased levels of Th2 cytokines in peritoneum and peripheral blood. Our results show that allergic diseases course without critical CCR8 participation, and suggest that further work is needed to unravel the in vivo role of CCR8 in Th2-mediated pathologies.

In situ binding assay for studying chemokine interactions with endothelial cells

The association of chemokines with endothelial cells (EC) and extracellular matrices is required for the prototypical pro-emigratory and pro-migratory in vivo activity of these molecules, respectively. In order to investigate chemokine binding to intact microanatomical structures, e.g. venular EC, we have developed an in situ binding assay. This is an autoradiographic morphological method in which the saturable binding of radiolabeled chemokines is studied in vitro in pieces of viable tissues. This article discusses the general applicability, advantages and shortcomings of the in situ binding assay in comparison with the other techniques available for visualizing chemokine receptor binding by cells in the tissues. We used this assay to demonstrate: (a) selective specific binding of CXC and CC chemokines to the EC of postcapillary venules but not capillaries or arteries; (b) selective specific binding of CC chemokines to the EC of afferent lymphatic vessels; and (c) selective specific binding of inflammatory chemokines to the EC lining high endothelial venules (HEV) in lymph nodes. The assessment of ligand cross-competition provided a fingerprint of chemokine-binding specificity of the EC in each of these microanatomical sites. This fingerprint could be paralleled with the chemokine-binding profiles of two non-signaling chemokine-binding molecules, Duffy antigen receptor for chemokines (DARC) and D6, present in venular and lymphatic EC, respectively. These observations allowed us to put forward the hypotheses regarding the involvement of EC DARC and D6 in chemokine transport and presentation by the EC.

Comparative DNA sequence analysis of mouse and human CC chemokine gene clusters

The CC chemokines are a closely related subfamily of the chemokine superfamily. Most of the CC chemokine genes form a cluster on chromosome 11 in mice and chromosome 17 in humans. To date, 11 and 16 functional genes have been localized within the mouse and human clusters, respectively. Notably, some of the genes within these clusters appear to have no counterparts between the two species, and the orthologous relationships of some of the genes are difficult to establish solely on the basis of amino acid similarity. In this study, we have taken a comparative genomic approach to reveal some of the features that may be involved in the dynamic evolution of these gene clusters. We sequenced a 122-kb region containing five chemokine genes of the mouse CC cluster. This mouse sequence was combined with those determined by the Mouse Genome Sequencing Project, and the entire sequence of the mouse CC cluster was compared with that of the corresponding cluster in the human genome by percent identity plot and dot-plot analyses. Although no additional chemokine genes have been found in these clusters, our analysis has revealed that numerous gene rearrangements have occurred even after the diversification of rodents and primates, resulting in several species-specific chemokine genes and pseudogenes. In addition, phylogenetic analysis and comparison of the genomic sequences unambiguously identified the orthologous relationships of some of the chemokine genes in the mouse and human CC gene clusters.

BOB.1/OBF.1 deficiency affects marginal-zone B-cell compartment

Marginal-zone (MZ) B cells represent a first line of defense against particulate blood-borne antigens. Together with the B1 cells, they are responsible for the early response against type II T-independent antigens. The molecular pathways controlling the development of MZ B cells are only poorly understood. We found that these cells are virtually absent in mice deficient in the BOB.1/OBF.1 coactivator. Loss of these B cells was demonstrated by the lack of cells showing the appropriate cell surface phenotype but also by histological analyses and tri-nitro-phenol-Ficoll capturing. The lack of these cells is a B-cell-intrinsic defect, as shown by bone marrow complementation experiments. We also show that the expression of BOB.1/OBF.1 in peripheral B cells is required for the development of MZ B lymphocytes. Our analysis of BOB.1/OBF.1-deficient splenic B cells reveals alterations in cell motility, tumor necrosis factor receptor expression, and B-cell receptor (BCR) signaling. These changes could contribute to the loss of MZ B lymphocytes by altering the maturation of the cells. Interestingly, development of and BCR signaling in B1 B cells are completely normal in BOB.1/OBF.1 mutant mice.

CCR5-binding chemokines modulate CXCL12 (SDF-1)-induced responses of progenitor B cells in human bone marrow through heterologous desensitization of the CXCR4 chemokine receptor

Although the SDF-1 (CXCL12)/CXCR4 axis is important for B-cell development, it is not yet clear to what extent CC chemokines might influence B lymphopoiesis. In the current study, we characterized CC chemokine receptor 5 (CCR5) expression and function of primary progenitor B-cell populations in human bone marrow. CCR5 was expressed on all bone marrow B cells at levels between 150 and 200 molecules per cell. Stimulation of bone marrow B cells with the CCR5-binding chemokine macrophage inflammatory protein 1beta (MIP-1beta; CCL4) did not cause chemotaxis, but CCL4 was able to trigger potent calcium mobilization responses and activation of the mitogen-activated protein kinase (MAPK) pathway in developing B cells. We also determined that CCR5-binding chemokines MIP-1alpha (CCL3), CCL4, and RANTES (CCL5), specifically by signaling through CCR5, could affect all progenitor B-cell populations through a novel mechanism involving heterologous desensitization of CXCR4. This cross-desensitization of CXCR4 was manifested by the inhibition of CXCL12-induced calcium mobilization, MAPK activation, and chemotaxis. These findings indicate that CCR5 can indeed mediate biologic responses of bone marrow B cells, even though these cell populations express low levels of CCR5 on their cell surface. Thus, by modulation of CXCR4 function, signaling through CCR5 may influence B lymphopoiesis by affecting the migration and maturation of B-cell progenitors in the bone marrow microenvironment.

The lymphotoxin-beta receptor induces different patterns of gene expression via two NF-kappaB pathways

The lymphotoxin-beta receptor (LTbetaR) plays critical roles in inflammation and lymphoid organogenesis through activation of NF-kappaB. In addition to activation of the classical NF-kappaB, ligation of this receptor induces the processing of the cytosolic NF-kappaB2/p100 precursor to yield the mature p52 subunit, followed by translocation of p52 to the nucleus. This activation of NF-kappaB2 requires NIK and IKKalpha, while NEMO/IKKgamma is dispensable for p100 processing. IKKbeta-dependent activation of canonical NF-kappaB is required for the expression but not processing of p100 and for the expression of proinflammatory molecules including VCAM-1, MIP-1beta, and MIP-2 in response to LTbetaR ligation. In contrast, IKKalpha controls the induction by LTbetaR ligation of chemokines and cytokines involved in lymphoid organogenesis, including SLC, BLC, ELC, SDF1, and BAFF.

Chemokines and dendritic cells: a crucial alliance

Dendritic cells (DC) are bone marrow-derived professional antigen-presenting cells that function as sentinels of the immune system. Their importance in immunity resides in their unique ability to prime or tolerize T lymphocytes, thereby initiating or inhibiting immune responses. They reside in all tissues and organs and upon appropriate activation, migrate to secondary lymphoid organs to present antigen to T lymphocytes in the T cell zones. Because of this central role in T cell activation, there is a great deal of interest in using DC therapeutically to deliver positive or negative signals to the immune system. The DC system is critically dependent on the ability of DC at different stages of maturation to respond to a range of soluble and cell-bound signals, including members of the chemokine gene superfamily. This review will describe the interactions between DC and the chemokine system.

CD4+CD3- cells induce Peyer's patch development: role of alpha4beta1 integrin activation by CXCR5

CD4+CD3- cells are the predominant hematopoietic cells found in mouse fetal intestine. We prove their role as Peyer's patch (PP)-inducing cells by transfer into neonatal PP-deficient mice. To test the requirement of chemokines and adhesion molecules in induction of PP, we studied mice deficient in CXCR5 and/or alpha4beta1 integrin-mediated adhesion. CXCR5-/- mice have CD4+CD3- cells, which are inefficient in inducing PP formation. We show here that CXCR5/CXCL13 signaling activates alpha4beta1 integrin on CD4+CD3- cells. Blocking of beta1 integrin or VCAM-1, the ligand of alpha4beta1 integrin, inhibits PP formation. This study demonstrates the link between chemokine receptors and adhesion molecules that regulates stromal/hematopoietic cell interaction leading to PP formation.

RGS13 regulates germinal center B lymphocytes responsiveness to CXC chemokine ligand (CXCL)12 and CXCL13

Normal lymphoid tissue development and function depend upon directed cell migration. Providing guideposts for cell movement and positioning within lymphoid tissues, chemokines signal through cell surface receptors that couple to heterotrimeric G proteins, which are in turn subject to regulation by regulator of G protein signaling (RGS) proteins. In this study, we report that germinal center B lymphocytes and thymic epithelial cells strongly express one of the RGS family members, RGS13. Located between Rgs1 and Rgs2, Rgs13 spans 42 kb on mouse chromosome 1. Rgs13 encodes a 157-aa protein that shares 82% amino acid identity with its 159-aa human counterpart. In situ hybridization with sense and antisense probes localized Rgs13 expression to the germinal center regions of mouse spleens and Peyer's patches and to the thymus medulla. Affinity-purified RGS13 Abs detected RGS13-expressing cells in the light zone of the germinal center. RGS13 interacted with both Gialpha and Gqalpha and strongly impaired signaling through G(i)-linked signaling pathways, including signaling through the chemokine receptors CXCR4 and CXCR5. Prolonged CD40 signaling up-regulated RGS13 expression in human tonsil B lymphocytes. These results plus previous studies of RGS1 indicate the germinal center B cells use two RGS proteins, RGS1 and RGS13, to regulate their responsiveness to chemokines.

The development of primary and secondary lymphoid tissues in the nurse shark Ginglymostoma cirratum: B-cell zones precede dendritic cell immigration and T-cell zone formation during ontogeny of the spleen

Secondary lymphoid tissue and immunoglobulin (Ig) production in mammals is not fully developed at birth, requiring time postnatally to attain all features required for adaptive immune responses. The immune system of newborn sharks - the oldest vertebrate group having adaptive immunity - also displays immature characteristics such as low serum IgM concentration and high levels of IgM1gj, an innate-like Ig. Primary and secondary lymphoid tissues in sharks and other cartilaginous fish were identified previously, but their cellular organization was not examined in detail. In this study of nurse shark lymphoid tissue, we demonstrate that the adult spleen contains well-defined, highly vascularized white pulp (WP) areas, composed of a central T-cell zone containing a major histocompatibility complex (MHC) class II+ dendritic cell (DC) network and a small number of Ig+ secretory cells, surrounded by smaller zones of surface Ig+ (sIg+) B cells. In neonates, splenic WPs are exclusively B-cell zones containing sIgM+-MHC class IIlow B cells; thus compartmentalized areas with T cells and DCs, as well as surface Ig novel antigen receptor (sIgNAR)-expressing B cells are absent at birth. Not until the pups are 5 months old do these WP areas become adult-like; concomitantly, sIgNAR+ B cells are readily detectable, indicating that this Ig class requires a 'mature immune-responsive environment'. The epigonal organ is the major site of neonatal B lymphopoiesis, based on the presence of developing B cells and recombination-activating gene 1 (RAG1)/terminal deoxynucleotidyl transferase (TdT) expression, indicative of antigen receptor rearrangement; such expression persists into adult life, whereas the spleen has negligible lymphopoietic activity. In adults but not neonates, many secretory B cells reside in the epigonal organ, suggesting, like in mammals, that B cells home to this primary lymphoid tissue after activation in other areas of the body.

Intrathymic T-cell migration: a combinatorial interplay of extracellular matrix and chemokines?

Cell migration is crucial for intrathymic T-cell differentiation. Chemokines and extracellular matrix proteins per se induce thymocyte migration, and recent data suggest a combinatorial role for these molecules in this event. For example, thymocyte migration induced by fibronectin plus CXCL12/SDF1-alpha (stromal cell-derived factor1-alpha) is higher than that elicited by the chemokine alone. If such interactions are relevant in the thymus, abnormal expression of any of these ligands and/or their corresponding receptors will lead to defects in thymocyte migration. At least in the murine model of Chagas disease, this seems to be the case. Therefore a better knowledge of this complex biological circuitry will provide new clues for understanding thymus physiology and designing therapeutic strategies targeting developing T cells.

CXCR4/CCR5 down-modulation and chemotaxis are regulated by the proteasome pathway

Chemokines and their receptors play a critical role in host immune surveillance and are important mediators of human immunodeficiency virus (HIV) pathogenesis and inflammatory response. The chemokine receptors CCR5 and CXCR4, which act as co-receptors along with CD4 for HIV docking and entry, are down-modulated by their respective ligands, MIP-1beta/SDF-1alpha or by the HIV envelope protein, gp120. We have studied the role of the proteasome pathway in the down-regulation of these receptors. Using the yeast and mammalian two-hybrid systems, we observed that the CCR5 receptor is constitutively associated with the zeta subunit of proteasome. Immunoprecipitation studies in CCR5 L1.2 cells revealed that this association was increased with MIP-1beta stimulation. The proteasome inhibitors, lactacystin and epoxomicin, attenuated MIP-1beta induced CCR5 down-modulation as detected by fluorescence-activated cell sorter analysis and confocal microscopy. The proteasome inhibitors also inhibited the SDF-1alpha and gp120 protein-induced down-modulation of the CXCR4 receptor in Jurkat cells. However, the inhibitors had no significant effect on the gp120-induced internalization of the CD4 receptor. These inhibitors also blocked cognate ligand-mediated chemotaxis but had no effect on SDF-1alpha-induced p44/42 MAP kinase or MIP-1beta-induced p38 kinase activities, thus indicating differential effects of the inhibitors on signaling mediated by these receptors. These results indicate that the CCR5 and CXCR4 receptor down-modulation mechanism and chemotaxis mediated by these receptors are dependent upon proteasome activity.

Molecular cloning and sequencing of 25 different rhesus macaque chemokine cDNAs reveals evolutionary conservation among C, CC, CXC, AND CX3C families of chemokines

Chemokines are small chemoattractant cytokines involved in normal and pathological immune processes. Although extensive nucleotide sequence data are available for human and murine chemokine cDNA sequences, very few data are currently available regarding rhesus macaque sequences. To increase our understanding of immune function in nonhuman primates, we have used reverse-transcription polymerase chain reaction (RT-PCR) to clone and sequence rhesus macaque cDNAs from each of the C, CC, CXC, and CX3C groups of chemokines. Relative to the respective human chemokines, these 25 chemokine cDNA sequences were from 77% to 98% identical. Of the amino acid differences between the rhesus macaque and human chemokines, 51% were species-specific when compared together with the respective murine chemokine sequences. These studies of rhesus macaque chemokine sequences demonstrate that chemokine genes are highly conserved across species, and provide a large foundation for the study of chemokine biology and genetics in nonhuman primates.

The thymus gland: a target organ for growth hormone

Increasing evidence has placed hormones and neuropeptides among potent immunomodulators, in both health and disease. Herein, we focus on the effects of growth hormone (GH) upon the thymus. Exogenous GH enhances thymic microenvironmental cell-derived secretory products such as cytokines and thymic hormones. Moreover, GH increases thymic epithelial cell (TEC) proliferation in vitro, and exhibits a synergistic effect with anti-CD3 in stimulating thymocyte proliferation, which is in keeping with the data showing that transgenic mice overexpressing GH or GH-releasing hormone exhibit overgrowth of the thymus. GH also influences thymocyte traffic: it increases human T-cell progenitor engraftment into the thymus; augments TEC/thymocyte adhesion and the traffic of thymocytes in the lymphoepithelial complexes, the thymic nurse cells; modulates in vivo the homing of recent thymic emigrants, enhancing the numbers of fluroscein isothiocyanate (FITC)+ cells in the lymph nodes and diminishing them in the spleen. In keeping with the effects of GH upon thymic cells is the detection of GH receptors in both TEC and thymocytes. Additionally, data indicate that insulin-like growth factor (IGF)-1 is involved in several effects of GH in the thymus, including the modulation of thymulin secretion, TEC proliferation as well as thymocyte/TEC adhesion. This is in keeping with the demonstration of IGF-1 production and expression of IGF-1 by TEC and thymocytes. Also, it should be envisioned as an intrathymic circuitry, involving not only IGF-1, but also GH itself, as intrathymic GH expression is seen both in TEC and in thymocytes, and that thymocyte-derived GH could enhance thymocyte proliferation. Finally, the possibility that GH improve thymic functions, including thymocyte proliferation and migration, places this molecule as a potential therapeutic adjuvant in immunodeficiency conditions associated with thymocyte decrease and loss of peripheral T cells.

B cell diversity and longevity in systemic autoimmunity

Despite much investigation, the nature of the primary disturbances that culminate in the production of pathogenic autoantibodies remains imprecise. However, major advances in the understanding of the genetics, the cellular and the molecular basis of pathogenic autoreactivity have been achieved in recent years. Not only B cells play a paramount role in systemic autoimmunity, but their role is not limited to secretion of autoantibodies. Under certain experimental conditions, B cells can activate memory T cells, and can process and present self-antigens to naive T cells, implying the existence of an antibody-independent mechanism for tissue injury in systemic autoimmune diseases, such as lupus. In both the mouse and the human disease, B cells secreting autoantibodies exhibit features which suggest that they are selected by specific autoantigens. Factors, such as BAFF, that support differentiation of selected B cells into mature long-lived B cells may be critical in generating deleterious autoimmune responses, at least in experimental animals. During these selection processes, the amount of signals received by the B cells are fine-tuned for optimal transmission, and kinases and phosphatases control most activities. Since a tight regulation of signaling pathways is required to prevent overt autoimmunity, faulty cell signaling may cause or exacerbate disorders of the immune system. Several observations showing altered expression of signaling molecules in T and B lymphocytes from patients with human lupus suggest that the subversion of immune receptor signaling could account for the hyperproduction of autoantibodies.

Regulated expression of human histocompatibility leukocyte antigen (HLA)-DO during antigen-dependent and antigen-independent phases of B cell development

Human histocompatibility leukocyte antigen (HLA)-DO, a lysosomal resident major histocompatibility complex class II molecule expressed in B cells, has previously been shown to be a negative regulator of HLA-DM peptide loading function. We analyze the expression of DO in human peripheral blood, lymph node, tonsil, and bone marrow to determine if DO expression is modulated in the physiological setting. B cells, but not monocytes or monocyte-derived dendritic cells, are observed to express this protein. Preclearing experiments demonstrate that approximately 50% of HLA-DM is bound to DO in peripheral blood B cells. HLA-DM and HLA-DR expression is demonstrated early in B cell development, beginning at the pro-B stage in adult human bone marrow. In contrast, DO expression is initiated only after B cell development is complete. In all situations, there is a striking correlation between intracellular DO expression and cell surface class II-associated invariant chain peptide expression, which suggests that DO substantially inhibits DM function in primary human B cells. We report that the expression of DO is markedly downmodulated in human germinal center B cells. Modulation of DO expression may provide a mechanism to regulate peptide loading activity and antigen presentation by B cells during the development of humoral immune responses.

Alteration of lymphocyte trafficking by sphingosine-1-phosphate receptor agonists

Blood lymphocyte numbers, essential for the development of efficient immune responses, are maintained by recirculation through secondary lymphoid organs. We show that lymphocyte trafficking is altered by the lysophospholipid sphingosine-1-phosphate (S1P) and by a phosphoryl metabolite of the immunosuppressive agent FTY720. Both species were high-affinity agonists of at least four of the five S1P receptors. These agonists produce lymphopenia in blood and thoracic duct lymph by sequestration of lymphocytes in lymph nodes, but not spleen. S1P receptor agonists induced emptying of lymphoid sinuses by retention of lymphocytes on the abluminal side of sinus-lining endothelium and inhibition of egress into lymph. Inhibition of lymphocyte recirculation by activation of S1P receptors may result in therapeutically useful immunosuppression.

Chemokines in cancer

Chemokines participate, by regulating cell trafficking and controlling angiogenesis, in the host response during infection and inflammation. Most of these mechanisms are also operating in cancer. The stimulation of angiogenesis and tumor growth--directly or indirectly through the recruitment of tumor-associated macrophages--are typical situations where chemokines promote tumor development. On the other hand, chemokines could be used to the benefit of cancer patients as they act in the recruitment of dendritic cells (DC) or/and effector cells or for their angiostatic properties. However, chemokine-mediated recruitment of immature DC within tumors, due to factors produced by the tumor milieu, could lead to the induction of immune tolerance and, therefore, novel strategies to eradicate tumors based on chemokines should attempt to avoid this risk.

Chemokine-mediated thymopoiesis is regulated by a mammalian Polycomb group gene, mel-18

Multiple chemokines are made in the thymus, and they are likely to function in the fine control of cellular migration and regulation of thymic T cell development. Mice lacking the gene mel-18, a member of the mammalian Polycomb group genes, displayed impaired thymic T cell development. Here we report that expression of chemokine receptors CXCR4 and CCR9 are regulated by mel-18 and that CXCL12/SDF-1- and CCL25/TECK-mediated chemotactic activities are also affected by the loss of mel-18. In mel-18-/- mice, high expression of CXCR4 on CD4-CD8- cells might lead to trapping in the SDF-1 rich subcapsular region, while low expression of CCR9 on CD4+CD8+ cells might reduce cell migration to the medulla. Therefore, this member of the Polycomb group genes plays a role in thymic T cell migration and differentiation via the chemokine system.

Reduction of marginal zone B cells in CD22-deficient mice

CD22 is a B cell-specific member of the immunoglobulin superfamily and binds to sialic acid. CD22 inhibits B cell receptor signaling. Mice deficient for CD22 show a largely normal B cell development. Here, we have performed a detailed analysis of the splenic B cell population and found that the subset of marginal zone (MZ) B cells was selectively reduced in CD22-deficient mice. CD22-deficient mice showed a lack of TNP-ficoll capturing cells in the MZ and a reduced response to TNP-ficoll, particularly when the antigen was applied intravenously. CD22-deficient B cells showed both enhanced motility as well as enhanced chemotaxis to certain chemokines. The altered chemokine responsiveness or the higher signaling capacity of CD22-deficient B cells may lead to the compromised MZ B cell compartment, as both processes have previously been shown to affect MZ composition.

Recruitment of lymphocytes to the human liver

This review discusses the function and localisation of lymphocytes resident within the human liver, under both physiological and pathological conditions. Through description of the mechanisms that mediate lymphocyte recruitment into tissues, this article explains how hepatic endothelial and epithelial cells regulate the recruitment of specific lymphocyte subpopulations. We illustrate that the expression of adhesion molecules and chemokines is crucial to the control of lymphocyte adhesion. Thus, in the normal liver, adhesion molecules such as vascular adhesion protein-1 (VAP-1), intercellular adhesion molecule-1 (ICAM-1) and intercellular adhesion molecule-2 (ICAM-2), and chemokines such as regulated on activation, normal T cell expressed and secreted (RANTES), monocyte chemoattractant protein-1 (MCP-1), macrophage inflammatory protein-1alpha (MIP-1alpha), interferon gamma inducible protein-10 (IP-10), MIG and interferon inducible T-cell alpha chemoattractant (ITAC) are involved in lymphocyte binding to different endothelial compartments. However, in response to inflammation or injury, additional expression of adhesion molecules such as VCAM-1, p-selectin and e-selectin, as well as higher levels of chemokines, permits the attraction and retention of specific effector populations of lymphocytes. We also discuss the expression and function of a newly defined adhesion protein, (VAP-1), and suggest that the unique functions of this protein may provide therapeutic potential for the treatment of liver disease.

Cell migration in development and disease

A recent meeting at the Max Delbrück Center in Berlin, Germany provided a forum to discuss the molecular mechanisms of cell migration in a broad range of contexts including chemotaxis, development, immunity, and cancer.

CXCL13 is required for B1 cell homing, natural antibody production, and body cavity immunity

B1 cells are a predominant cell type in body cavities and an important source of natural antibody. Here we report that in mice lacking the chemokine, CXCL13, B1 cells are deficient in peritoneal and pleural cavities but not in spleen. CXCL13 is produced by cells in the omentum and by peritoneal macrophages, and in adoptive transfers, B1 cells home to the omentum and the peritoneal cavity in a CXCL13-dependent manner. CXCL13(-/-) mice are deficient in preexisting phosphorylcholine (PC)-specific antibodies and in their ability to mount an anti-PC response to peritoneal streptococcal antigen. These findings provide insight into the mechanism of B1 cell homing and establish a critical role for B1 cell compartmentalization in the production of natural antibodies and for body cavity immunity.

Splenic T zone development is B cell dependent

The factors regulating growth and patterning of the spleen are poorly defined. We demonstrate here that spleens from B cell-deficient mice have 10-fold reduced expression of the T zone chemokine, CCL21, a threefold reduction in T cell and dendritic cell (DC) numbers, and reduced expression of the T zone stromal marker, gp38. Using cell transfer and receptor blocking approaches, we provide evidence that B cells play a critical role in the early postnatal development of the splenic T zone. This process involves B cell expression of lymphotoxin (LT)alpha1beta2, a cytokine that is required for expression of CCL21 and gp38. Introduction of a B cell specific LTalpha transgene on to the LTalpha-deficient background restored splenic CCL21 and gp38 expression, DC numbers, and T zone size. This work also demonstrates that the role of B cells in T zone development is distinct from the effect of B cells on splenic T cell numbers, which does not require LTalpha1beta2. Therefore, B cells influence spleen T zone development by providing: (a) signals that promote T cell accumulation, and: (b) signals, including LTalpha1beta2, that promote stromal cell development and DC accumulation. Defects in these parameters may contribute to the immune defects associated with B cell deficiency in mice and humans.

Formation of Peyer's patches

Formation of Peyer's patches requires complex interactions between the gut epithelium, the mesenchyme, and bone-marrow-derived hematopoietic progenitors. The first Peyer's patches anlage appear around embryonic day 15.5, when the endoderm has undergone transition to a simple epithelium, the lymphatic vessels have reached the intestinal mucosa, and mesenchymal cells have started to form clusters. Recent data using knockout mice provide insight into the molecular nature of the signals that mediate Peyer's patch ontogeny. These include members of the tumor-necrosis factor family and homeostatic chemokines.

Immunology. Long live the mature B cell--a baffling mystery resolved

What determines whether transitional B cells newly emerged from the bone marrow will differentiate further to become mature, long-lived, circulating B lymphocytes? In a Perspective, Waldschmidt and Noelle discuss new findings showing that the TNF family ligand BAFF and its receptor BAFF-R are crucial for selecting transitional B cells into the mature B cell pool (Thompson et al., Schiemann et al.).