MAdCAM-1 Dependent Colonization of Developing Lymph Nodes Involves a Unique Subset of CD4+CD3- Hematolymphoid Cells

During fetal lymph node organogenesis in mice, lymph node postcapillary high endothelial venules briefly express the Peyer's patch addressin MAdCAM-1. This allows initial seeding by two unusual lymphocyte populations selectively expressing the Peyer's patch homing receptor integrin alpha4beta7: CD4 + CD3- oligolineage progenitors and TCR gammadelta + T cells. It was found that the CD4 + CD3- cells are lineage-restricted progenitors that express surface lymphotoxin-beta (LTbeta) and the chemokine receptor BLR1. They can differentiate into natural killer cells, dendritic antigen-presenting cells, and follicular cells of unknown outcome, but these cells do not become T or B lymphocytes.

In addition to LN, CD4 + CD3- cells can also be found in fetal spleen starting at 13.5 dpc, while absent from fetal liver. In view of the necessity of lymphotoxin in lymphoid organ development, it is thought that the novel subset of CD4 + CD3-LTbeta + fetal cells is instrumental in the development of lymphoid tissue architecture.     

Role of CXC chemokine ligand 13, CC chemokine ligand (CCL) 19, and CCL21 in the organization and function of nasal-associated lymphoid tissue

Nasal-associated lymphoid tissue (NALT) orchestrates immune responses to Ags in the upper respiratory tract. Unlike other lymphoid organs, NALT develops independently of lymphotoxin-alpha (LTalpha). However, the structure and function of NALT are impaired in Ltalpha(-/-) mice, suggesting a link between LTalpha and chemokine expression. In this study we show that the expression of CXCL13, CCL19, CCL21, and CCL20 is impaired in the NALT of Ltalpha(-/-) mice. We also show that the NALT of Cxcl13(-/-) and plt/plt mice exhibits some, but not all, of the structural and functional defects observed in the NALT of Ltalpha(-/-) mice. Like the NALT of Ltalpha(-/-) mice, the NALT in Cxcl13(-/-) mice lacks follicular dendritic cells, BP3(+) stromal cells, and ERTR7(+) lymphoreticular cells. However, unlike the NALT of Ltalpha(-/-) mice, the NALT of Cxcl13(-/-) mice has peripheral node addressin(+) high endothelial venules (HEVs). In contrast, the NALT of plt/plt mice is nearly normal, with follicular dendritic cells, BP3(+) stromal cells, ERTR7(+) lymphoreticular cells, and peripheral node addressin(+) HEVs. Functionally, germinal center formation and switching to IgA are defective in the NALT of Ltalpha(-/-) and Cxcl13(-/-) mice. In contrast, CD8 T cell responses to influenza are impaired in Ltalpha(-/-) mice and plt/plt mice. Finally, the B and T cell defects in the NALT of Ltalpha(-/-) mice lead to delayed clearance of influenza from the nasal mucosa. Thus, the B and T cell defects in the NALT of Ltalpha(-/-) mice can be attributed to the impaired expression of CXCL13 and CCL19/CCL21, respectively, whereas impaired HEV development is directly due to the loss of LTalpha.     

Differing activities of homeostatic chemokines CCL19, CCL21, and CXCL12 in lymphocyte and dendritic cell recruitment and lymphoid neogenesis

Despite their widespread expression, the in vivo recruitment activities of CCL19 (EBV-induced molecule 1 ligand chemokine) and CXCL12 (stromal cell-derived factor 1) have not been established. Furthermore, although CXCL13 (B lymphocyte chemoattractant) has been shown to induce lymphoid neogenesis through induction of lymphotoxin (LT)alpha1beta2, it is unclear whether other homeostatic chemokines have this property. In this work we show that ectopic expression in pancreatic islets of CCL19 leads to small infiltrates composed of lymphocytes and dendritic cells and containing high endothelial venules and stromal cells. Ectopic CXCL12 induced small infiltrates containing few T cells but enriched in dendritic cells, B cells, and plasma cells. Comparison of CCL19 transgenic mice with mice expressing CCL21 (secondary lymphoid tissue chemokine) revealed that CCL21 induced larger and more organized infiltrates. A more significant role for CCL21 is also suggested in lymphoid tissues, as CCL21 protein was found to be present in lymph nodes and spleen at much higher concentrations than CCL19. CCL19 and CCL21 but not CXCL12 induced LTalpha1beta2 expression on naive CD4 T cells, and treatment of CCL21 transgenic mice with LTbetaR-Fc antagonized development of organized lymphoid structures. LTalpha1beta2 was also induced on naive T cells by the cytokines IL-4 and IL-7. These studies establish that CCL19 and CXCL12 are sufficient to mediate cell recruitment in vivo and they indicate that LTalpha1beta2 may function downstream of CCL21, CCL19, and IL-2 family cytokines in normal and pathological lymphoid tissue development.     

Expression of the lymphotoxin beta receptor on follicular stromal cells in human lymphoid tissues

The lymphotoxin beta receptor (LTbetaR), and its ligand, LTalpha1beta2, have been proposed to play a key role in the development and organization of lymphoid tissues. The LTbetaR is expressed on a variety of human primary and transformed cells, but strikingly absent on T or B lymphocytes and primary monocytes or peripheral dendritic cells, although LTbetaR is detected on some myeloid leukemic lines. In the developing thymus LTbetaR is prominent along the trabeculae and into the medulla upto corticomedullary junction. In the spleen, LTbetaR is prominently expressed by cells in the red pulp and along the borders of red and white pulp which colocalizes with reticular stromal cells. The LTbetaR is expressed on a human follicular dendritic cell line, FDC-1, and signals expression of CD54 when ligated with the LTalpha1beta2 complex. These results support the concept that directional interactions between LTalpha1beta2 bearing lymphocytes and LTbetaR bearing stromal cells are involved in the organization of lymphoid tissue.     

Cutting edge: ectopic expression of the chemokine TCA4/SLC is sufficient to trigger lymphoid neogenesis

To test whether accumulation of naive lymphocytes is sufficient to trigger lymphoid development, we generated mice with islet expression of the chemokine TCA4/SLC. This chemokine is specific for naive lymphocytes and mature dendritic cells (DC) which express the CCR7 receptor. Islets initially developed accumulations of T cells with DC, with scattered B cells at the perimeter. These infiltrates consolidated into organized lymphoid tissue, with high endothelial venules and stromal reticulum. Infiltrate lymphocytes showed a naive CD44low CD25- CD69- phenotype, though half were CD62L negative. When backcrossed to RAG-1 knockout, DC were not recruited. Interestingly, islet lymphoid tissue developed in backcrosses to Ikaros knockout mice despite the absence of normal peripheral nodes. Our results indicate that TCA4/SLC can induce the development and organization of lymphoid tissue through diffential recruitment of T and B lymphocytes and secondary effects on stromal cell development.     

Cutting edge: nonproliferating mature immune cells form a novel type of organized lymphoid structure in idiopathic pulmonary fibrosis

Ectopic formation of secondary lymphoid tissue is initiated by the local attraction of naive T and B cells. In this study, we describe a novel type of organized lymphoid structure in the lung of human idiopathic pulmonary fibrosis, with key features of lymphoid neogenesis, including: 1) recently activated CD40 ligand (CD40L)+ T cells; 2) variable numbers of activated CD40+/CD40L+ B cells, sometimes organized in follicles; 3) fully mature dendritic cells (DC) expressing CD40, CD83, CD86, and DC-lysosome-associated membrane protein; 4) the expression of the chemokine CCL21; 5) the presence of vessels with characteristics of high endothelial venules; and 6) a dense network of follicular DC. Surprisingly, these structures are devoid of CCR7+ naive T cells, proliferating lymphocytes, and germinal centers, suggesting that newly recruited activated DC and Ag-experienced lymphocytes can drive lymphoid neogenesis and that factors present within the lymphoid aggregates, such as CD40L, are essential to induce DC maturation.     

A critical role for dendritic cells in the formation of lymphatic vessels within tertiary lymphoid structures

Ectopic, or tertiary, lymphoid aggregates often form in chronically inflamed areas. Lymphatic vessels, as well as high endothelial venules, form within these lymphoid aggregates, but the mechanisms underlying their development are poorly understood. Overexpression of the chemokine CCL21 in the thyroid of transgenic mice leads to formation of lymphoid aggregates containing topologically segregated T and B lymphocytes, dendritic cells (DCs), and specialized vasculature, including Lyve-1(+)/Prox-1(+) lymphatic vessels. In this article, we show that adoptive transfer of mature CD4(+) T cells into animals expressing CCL21 in a RAG-deficient background promotes the influx of host NK cells and DCs into the thyroid and the formation of new lymphatic vessels within 10 d. This process is dependent on the expression of lymphotoxin ligands by host cells, but not by the transferred CD4(+) T cells. Ablation of host DCs, but not NK cells, reduces the formation of new lymphatic vessels in the thyroid. Taken together, these data suggest a critical role for CD11c(+) DCs in the induction of lymphangiogenesis in tertiary lymphoid structures.     

Dendritic cells and T lymphocyte interactions in patients with lymphoid malignancies

Dendritic cell (DC) vaccination is an attractive approach to the treatment of patients with lymphoid tumors. To evaluate its feasibility, we have tested the functional properties of DC and T-lymphocytes in patients with treated and untreated chronic lymphocytic leukemia (CLL) and follicular lymphoma (FL). Healthy volunteers were used both as controls and as a source of cells for allogeneic mixed leukocyte reaction (MLR). In these reactions, dendritic cells from both untreated and treated patients were comparable to dendritic cells from healthy volunteers. In all the untreated patients studied, autologous dendritic cells promoted the survival and proliferation of both CD4 and CD8 lymphocytes (though the proliferation response was much better in the CD4 subset), whereas only 3 out of 5 treated patients were able to mount this response with CD4 lymphocytes and 4 out of 5 with CD8 lymphocytes. In 3 out of 5 untreated patients, pulsing of DCs with tetanus toxoid promoted a better CD4 response than was achieved with unpulsed DCs, while none of 5 treated patients had an additional response after pulsing with tetanus toxoid. None of patients studied, either treated or untreated, had a better CD8 response to pulsed DCs than to unpulsed ones. During CD4 lymphocyte proliferation, more CD4(+)CD25(hi) lymphocytes were generated in both treated and untreated patients than in healthy controls. Poor proliferation of cytotoxic cells and preferential proliferation of CD4(+)CD25(hi) T-regulatory cells in response to self and/or foreign antigens might be one of the mechanisms responsible for immunosuppression and impaired tumor surveillance in patients with lymphoid malignancies.     

Cutting edge: organogenesis of nasal-associated lymphoid tissue (NALT) occurs independently of lymphotoxin-alpha (LT alpha) and retinoic acid receptor-related orphan receptor-gamma, but the organization of NALT is LT alpha dependent.

Peyer's patch and nasal-associated lymphoid tissue (NALT) are mucosal lymphoid tissues that appear similar in structure and function. Surprisingly, we found that NALT, unlike Peyer's patch, was formed independently of lymphotoxin (LT)alpha. Furthermore, using mice deficient in the retinoic acid receptor-related orphan receptor-gamma, we found that NALT was formed in the absence of CD4+CD3- cells, which are thought to be the embryonic source of LTalpha. However, we also found that NALT of LTalpha-/- animals was disorganized and lymphopenic, suggesting that the organization and recruitment of lymphocytes within NALT remained dependent on LTalpha. Finally, we demonstrated that both the structure and function of NALT were restored in LTalpha-/- animals upon reconstitution with normal bone marrow. These results demonstrate that the organogenesis of NALT occurs through unique mechanisms.     

Cutting edge: selective impairment of CD8+ T cell function in mice lacking the TNF superfamily member LIGHT

Interactions of LIGHT and its receptors, herpesvirus entry mediator on T cells and lymphotoxin beta receptor on stromal cells, are implicated in the regulation of lymphoid organogenesis, costimulation of T cells, and activation of dendritic cells. In this work we report that LIGHT-deficient mice had normal lymphoid organs with T cells and APCs that normally responded to Ag stimulation and normally stimulated T cells. Although the number of Vbeta8(+) T cells in naive LIGHT(+/+) and LIGHT(-/-) mice was identical, Vbeta8(+)CD8(+) T cell proliferation in response to staphylococcal enterotoxin B was significantly lower in LIGHT(-/-) mice. Consistently, induction and cytokine secretion of CD8(+) CTL to MHC class I-restricted peptide was also reduced in LIGHT(-/-) mice. However, the proliferative response of Vbeta8(+)CD4(+) T cells to staphylococcal enterotoxin B was comparable in LIGHT(-/-) and LIGHT(+/+) mice. Our results suggest that LIGHT is required for activation of normal CD8(+) T cells but not CD4(+) T cells.     

Expression of low levels of peripheral lymph node-associated vascular addressin in mucosal lymphoid tissues: possible relevance to the dissemination of passaged AKR lymphomas

Lymphoid tumors display a wide variety of growth patterns in vivo, from that of a solitary extralymphoid tumor, to a general involvement of all lymphoid organs. Normal lymphocytes are uniquely mobile cells continuously recirculating between blood and lymph throughout much of their life cycle. Therefore, it is reasonable to propose that disseminating malignant lymphocytes may express recirculation characteristics or homing properties consistent with that of their normal lymphoid counterparts. Trafficking of lymphocytes involves the expression and recognition of both lymphocyte homing receptors and their opposing receptors on endothelium, the vascular addressins. These cell surface elements direct the tissue-selective localization of lymphocyte subsets in vivo into organized lymphoid organs and sites of chronic inflammation where specific binding events occur between lymphocytes and the endothelium of specialized high endothelial venules (HEV). In a recent murine study of 13 lymphoma lines, we found that lymphomas that bind well to high endothelial venules, in the Stamper-Woodruff in vitro assay (an assay of lymphocyte binding to venules in frozen sections of peripheral lymph nodes or Peyer's patches), spread hematogenously to all high endothelial venule bearing lymphoid organs, whereas non-binding lymphomas did not. In some cases lymphomas that bound with a high degree of selectivity to peripheral lymph node (PLN) high endothelial venules exhibited only limited organ preference of metastasis, involving the mucosal lymphoid organs Peyer's patches (PP) in addition to the peripheral lymph nodes of adoptive recipients. Here we demonstrate that Peyer's patch high endothelial venules express a low but functional level of peripheral lymph node addressin (MECA-79) that can be recognized by lymphomas expressing the peripheral lymph node homing receptor (MEL-14 antigen).(ABSTRACT TRUNCATED AT 250 WORDS)     

The molecular basis of lymphoid architecture and B cell responses: implications for immunodeficiency and immunopathology

Immune responses usually take place in secondary lymphoid organs such as spleen and lymph nodes. Most lymphocytes within these organs are in transit, yet lymphoid organ structure is highly organized; T and B cells segregate into separate regions. B cell compartments include na?ve cells within follicles, marginal zones and B-1 cells. Interactions between TNF family molecules on hematopoietic cells and their receptors on mesenchymal cells guide the initial phase of lymphoid organogenesis, and regulate chemokine secretion that mediates subsequent T-B cell segregation. Recruitment of B cells into different compartments depends on both the milieu established during organogenesis, and the threshold for B cell receptor signaling, which is modulated by numerous coreceptors. Novel intrafollicular (germinal center) and extrafollicular (plasma cell) compartments are established when B cells respond to antigen. These divergent B cell responses are mediated by different patterns of gene expression, and influenced again by BCR signaling threshold and cellular interactions that depend on normal lymphoid architecture. Aberrant B cell responses are reviewed in the light of these principles taking into account the molecular and architectural aspects of immunopathology. Histological features of immunodeficiency reflect defects of B cell recruitment or differentiation. B cell hyper-reactivity may arise from altered BCR signaling thresholds (autoimmunity), defects in stimuli that guide differentiation in response to antigen (follicular hyperplasia vs plasmacytosis), or defective B cell gene expression. Interestingly, in diseases such as rheumatoid arthritis, Sjogren's syndrome and Hashimoto's thyroiditis lymphoid organogenesis may be recapitulated in non-lymphoid parenchyma, under the influence of molecular interactions similar to those that operate during embryogenesis.     

CCR7 expression and memory T cell diversity in humans

CCR7, along with L-selectin and LFA-1, mediates homing of T cells to secondary lymphoid organs via high endothelial venules (HEV). CCR7 has also been implicated in microenvironmental positioning of lymphocytes within secondary lymphoid organs and in return of lymphocytes and dendritic cells to the lymph after passage through nonlymphoid tissues. We have generated mAbs to human CCR7, whose specificities correlate with functional migration of lymphocyte subsets to known CCR7 ligands. We find that CCR7 is expressed on the vast majority of peripheral blood T cells, including most cells that express adhesion molecules (cutaneous lymphocyte Ag alpha(4)beta(7) integrin) required for homing to nonlymphoid tissues. A subset of CD27(neg) memory CD4 T cells from human peripheral blood is greatly enriched in the CCR7(neg) population, as well as L-selectin(neg) cells, suggesting that these cells are incapable of homing to secondary lymphoid organs. Accordingly, CD27(neg) T cells are rare within tonsil, a representative secondary lymphoid organ. All resting T cells within secondary lymphoid organs express high levels of CCR7, but many activated cells lack CCR7. CCR7 loss in activated CD4 cells accompanies CXC chemokine receptor (CXCR)5 gain, suggesting that the reciprocal expression of these two receptors may contribute to differential positioning of resting vs activated cells within the organ. Lymphocytes isolated from nonlymphoid tissues (such as skin, lung, or intestine) contain many CD27(neg) cells lacking CCR7. The ratio of CD27(neg)/CCR7(neg) cells to CD27(pos)/CCR7(pos) cells varies from tissue to tissue, and may correlate with the number of cells actively engaged in Ag recognition within a given tissue.     

Intestinal cryptopatch formation in mice requires lymphotoxin alpha and the lymphotoxin beta receptor

Interactions between lymphotoxin (LT)alpha(1)beta(2) on inducer cells and the lymphotoxin beta receptor (LTbetaR) on stromal cells initiate development of lymph nodes and Peyer's patches. In this study, we assessed the contributions of LTalpha and LTbetaR to the development of cryptopatches (CP), aggregates of T cell precursors in the mouse small intestine. Mice genetically deficient in LTalpha or LTbetaR lacked CP. Bone marrow from LTalpha-deficient mice was unable to initiate development of CP or isolated lymphoid follicles (ILF) after transfer to CD132-null mice lacking CP and ILF. However, LTalpha-deficient bone marrow-derived cells contributed to CP formed in CD132-null mice receiving a mixture of wild-type and LTalpha-deficient bone marrow cells. Transfer of wild-type bone marrow into irradiated LTalpha-deficient mice resulted in reconstitution of both CP and ILF. However, the LT-dependent formation of CP was distinguished from the LT-dependent formation of ILF and Peyer's patches by not requiring the presence of an intact NF-kappaB-inducing kinase gene. CP but not ILF were present in the small intestine from NF-kappaB-inducing kinase-deficient alymphoplasia mice, indicating that the alternate NF-kappaB activation pathway required for other types of LTbetaR-dependent lymphoid organogenesis is dispensable for CP development. In addition, we identified VCAM-1(+) cells within both CP and ILF that are candidates for the stromal cells involved in receiving LT-dependent signals from the hemopoietic precursors recruited to CP. These findings demonstrate that interactions between cells expressing LTalpha(1)beta(2) and LTbetaR are a shared feature in the development of all small intestinal lymphoid aggregates.     

Role of lymphotoxin alpha in T-cell responses during an acute viral infection

The importance of lymphotoxin alpha (LTalpha) in lymphoid organogenesis is well established. Although LTalpha has been implicated in the pathogenesis of T-cell-mediated immunopathologies, the requirement for LTalpha in T-cell activation and effector function in vivo is not well understood. To determine the role of LTalpha in T-cell activation in vivo, we compared the generation of antigen-specific T-cell responses between wild type (+/+) and LTalpha-deficient (LTalpha(-/-)) mice during an acute infection with lymphocytic choriomeningitis virus (LCMV). Our studies showed that LCMV-infected LTalpha(-/-) mice had a profound impairment in the activation and expansion of virus-specific CD8 T cells in the spleen, as determined by cytotoxicity assays, intracellular staining for gamma interferon, and staining with major histocompatibility complex class I tetramers. Further, the nonlymphoid organs of LTalpha(-/-) mice also contained substantially lower number of LCMV-specific CD8 T cells than those of +/+ mice. Greatly reduced virus-specific CD8 T-cell responses in LTalpha(-/-) mice led to a defect in LCMV clearance from the tissues. In comparison to that in +/+ mice, the activation of LCMV-specific CD4 T cells was also significantly attenuated in LTalpha(-/-) mice. Adoptive transfer experiments were conducted to determine if abnormal lymphoid architecture in LTalpha(-/-) mice caused the impairment in the activation of LCMV-specific T-cell responses. Upon adoptive transfer into +/+ mice, the activation and expansion of LCMV-specific LTalpha(-/-) T cells were restored to levels comparable to those of +/+ T cells. In a reciprocal cell transfer experiment, activation of +/+ T cells was significantly reduced upon transfer into LTalpha(-/-) mice. These results showed that impairment in the activation of LCMV-specific T cells in LTalpha(-/-) mice may be due to abnormal lymphoid architecture and not to an intrinsic defect in LTalpha(-/-) T cells.     

Mice with a targeted mutation in lymphotoxin-alpha exhibit enhanced tumor growth and metastasis: impaired NK cell development and recruitment.

Mice deficient in lymphotoxin (LT)-alpha lack peripheral lymph nodes and Peyer's patches and have profound defects in development of follicular dendritic cell networks, germinal center formation, and T/B cell segregation in the spleen. Although LTalpha is known to be expressed by NK cells as well as T and B lymphocytes, the requirement of LTalpha for NK cell functions is largely unknown. To address this issue, we have assessed NK cell functions in LTalpha-deficient mice by evaluating tumor models with known requirements for NK cells to control their growth and metastasis. Syngeneic B16F10 melanoma cells inoculated s.c. grew more rapidly in LTalpha-/- mice than in the wild-type littermates, and the formation of experimental pulmonary metastases was significantly enhanced in LTalpha-/- mice. Although LTalpha-/- mice exhibited almost a normal total number of NK cells in spleen, they showed an impaired recruitment of NK cells to lung and liver. Additionally, lytic NK cells were not efficiently produced from LTalpha-/- bone marrow cells in vitro in the presence of IL-2 and IL-15. These data suggest that LTalpha signaling may be involved in the maturation and recruitment of NK cells and may play an important role in antitumor surveillance.     

Lymphotoxin-alpha 1 beta 2 and LIGHT induce classical and noncanonical NF-kappa B-dependent proinflammatory gene expression in vascular endothelial cells

Activation of the classical and noncanonical NF-kappaB pathways by ligation of the lymphotoxin (LT)-beta receptor (LTbetaR) plays a crucial role in lymphoid organogenesis and in the generation of ectopic lymphoid tissue at sites of chronic inflammation. Within these microenvironments, LTbetaR signaling regulates the phenotype of the specialized high endothelial cells. However, the direct effects of LTbetaR ligation on endothelial cells remain unclear. We therefore questioned whether LTbetaR ligation could directly activate endothelial cells and regulate classical and noncanonical NF-kappaB-dependent gene expression. We demonstrate that the LTbetaR ligands LIGHT and LTalpha1beta2 activate both NF-kappaB pathways in HUVECs and human dermal microvascular endothelial cells (HDMEC). Classical pathway activation was less robust than TNF-induced signaling; however, only LIGHT and LTalpha1beta2 and not TNF activated the noncanonical pathway. LIGHT and LTalpha1beta2 induced the expression of classical NF-kappaB-dependent genes in HUVEC, including those encoding the adhesion molecules E-selectin, ICAM-1, and VCAM-1. Consistent with this stimulation, LTbetaR ligation up-regulated T cell adhesion to HUVEC. Furthermore, the homeostatic chemokine CXCL12 was up-regulated by LIGHT and LTalpha1beta2 but not TNF in both HUVEC and HDMEC. Using HUVEC retrovirally transduced with dominant negative IkappaB kinase alpha, we demonstrate that CXCL12 expression is regulated by the noncanonical pathway in endothelial cells. Our findings therefore demonstrate that LTbetaR ligation regulates gene expression in endothelial cells via both NF-kappaB pathways and we identify CXCL12 as a bona fide noncanonical NF-kappaB-regulated gene in these cells.     

Intravital microscopy comparing T lymphocyte trafficking to the spleen and the mesenteric lymph node

Lymphocyte rolling velocity is determined largely by interactions between leukocyte alpha(4)-integrin (CD49d) and L-selectin and mucosal addressin cell adhesion molecule-1 (MAdCAM-1) in mesenteric postcapillary venules and Peyer's patch high endothelial venules (HEVs). The role of these interactions in other tissue sites of lymphocyte emigration is not known. With the use of real-time intravital confocal microscopy, we found that rolling velocities of T lymphocytes in the murine mesenteric lymph node (MLN) HEV also depend on L-selectin and CD49d. However, in the murine spleen, rolling velocities of T lymphocytes are not influenced by the loss of L-selectin and CD49d. With the use of FITC-dextran and TIE2-GFP mice, we further defined the microvascular compartments of the spleen and showed that adherence of T cells is localized to regions in the white pulp that are not lined by endothelial cells and have shear rates similar to bone marrow sinusoids. These results establish that T cell trafficking to the spleen differs from trafficking to other secondary lymphoid organs and suggest that the mechanical properties of the blood-filtering role of the spleen are important in T cell accumulation in the organ.     

Tissue distribution of the C3d/EBV-receptor: CD21 monoclonal antibodies reactive with a variety of epithelial cells,medullary thymocytes,and peripheral T-cells

Summary The CD21 antigen has been described to represent CR2, the receptor for the complement fragment C3d and also the receptor for the Epstein-Barr virus (EBV). Monoclonal antibodies B2, HB5, and B-ly4 belong to the CD21 cluster, recognizing different epitopes of the CD21-molecule. Immunohistology of lymphoid tissues employing these antibodies showed the known staining of B cells and dendritic reticulum cells. Surprisingly, B2, but not HB5 or B-ly4, stained a distinct spot in the cytoplasm of a major proportion of medullary thymocytes, in almost all peripheral blood lymphocytes, and in a substantial amount of cells in T-cell areas of peripheral lymphoid tissues. This distinct cytoplasmic B2 staining was confirmed by immuno-electronmicroscopy. A similar B2+ cytoplasmic dot was observed in B-lymphoblastic lymphomas. Staining of non-lymphoid tissues showed reactivity with all three CD21 mAb with epithelial cells of skin, lung, esophagus, jejunum, colon, pancreas, tonsil, adrenal cortex, renal tubuli, and parotid glands, and with hepatocytes and tongue muscle. In addition, endothelial cells of small vessels showed B2 staining. One possible explanation for our results is, that apart from the presence of B cells and follicular dendritic cells, a CD21-molecule may be expressed by other cell types. However, a maybe more likely explanation may be that the recognized epitopes are not exclusively associated with the C3d/EBV-receptor, but also with other structures. In particular should the possibility be recognized of cross-reactivity with CR2-related proteins, encoded by the large gene family, to which CR2 belongs.