Appearance and phenotype of murine follicular dendritic cells expressing VCAM‐1

The architecture of lymphoid follicles is determined by a series of interactions between lymphoid and follicular stromal cells. A cardinal population in the non‐lymphoid compartment is the follicular dendritic cell (FDC), whose communication with resting and activated B cells involves various adhesive interactions. The FDC phenotype variably includes the display of vascular cell adhesion molecule (VCAM‐1). In this report we investigated the appearance and follicular tissue distribution of VCAM‐1 in murine peripheral lymphoid tissues, and compared VCAM‐1 with other FDC markers using immunohistochemistry. Correlating the appearance of VCAM‐1 with other murine FDC‐associated markers (CR1.2 [complement receptor 1.2 or CD35/21] and FDC‐M1) revealed that the display of VCAM‐1 is restricted to a subset of CR1.2‐positive FDCs. We found that the expression of VCAM‐1 antigen in the spleen or peripheral lymph nodes on FDCs requires antigenic stimulus, and that it coincides with germinal center formation. The VCAM‐1 expression is associated with the appearance of mucosal addressin cell adhesion molecule (MAdCAM‐1), with some slight differences in occurrence. The appearance of VCAM‐1 and MAdCAM‐1 antigens on FDCs may serve as indicators of FDC activation. Anat Rec 268:160–168, 2002. © 2002 Wiley‐Liss, Inc.     

Follicular dendritic cells in aging, a "bottle-neck" in the humoral immune response

Senescence leads to the appearance of atrophic follicular dendritic cells (FDCs) that trap and retain little immune complexes (IC), generate few memory B cells, and induce a reduced number of germinal centers (GC). Deficiencies in antibody responses to T cell dependent exogenous antigens such as pneumonia and influenza vaccines may reflect intrinsic FDC defects or altered FDC-B cell interactions. We recently studied antigen handling capacity and co-stimulatory activity of old FDCs and determined age-related changes in the expression or function of FcgammaRII or CR1 and 2 on FDCs. Here, we present an overview of FDC function in recall responses with known deficiencies in FDCs and GC development. Then, we review our recent work on aged FDCs and discuss age-related changes in molecular interactions between FDCs and B cells. We also discuss the causes underlying the impaired humoral immune response with respect to age-related molecular changes in FDC and B cell interactions. In vitro evidence suggests that FcgammaRII on aged FDCs is regulated abnormally and this in turn might cause the development of a defective FDC-network (reticulum) that retains few ICs, promotes ITIM signaling, prevents B cell proliferation and GC formation, and antibody production.     

FDC-specific functions of p55TNFR and IKK2 in the development of FDC networks and of antibody responses

The FDC-specific molecular signals required in the formation of FDC networks, B cell follicles, and germinal centers (GCs) have remained poorly understood. We used FDC-specific gene targeting to investigate the function of p55TNFR and IKK2 in lymphoid organ structure and function. Here we show that FDC-specific expression of p55TNFR is necessary and sufficient to promote FDC network and B cell follicle formation, restore the expression of CXCL13 and VCAM-1/ICAM-1 in FDCs, and lead to productive GCs. Notably, FDC-specific disruption of IKK2 does not affect formation of FDC networks. Yet, after antigen engagement or immune complex (IC) deposition, FDCs lacking IKK2 fail to upregulate VCAM-1 and ICAM-1, and GCs remain sterile. These findings demonstrate that IKK2-independent function of p55TNFR on FDCs is sufficient to support the development of FDC networks and GCs, while FDC-specific IKK2 is indispensable for the generation of efficient humoral immune responses.     

VCAM-1, but not ICAM-1 or MAdCAM-1, immunoblockade ameliorates DSS-induced colitis in mice

Adhesion molecule immunoneutralization is envisioned as a promising therapy for inflammatory bowel disease, but the relative value of selective blockade of different adhesion molecules has not been established. The aims of this study were to measure expression and functional relevance of endothelial intercellular adhesion molecule 1 (ICAM-1), vascular cell adhesion molecule 1 (VCAM-1), and mucosal addressin cell adhesion molecule 1 (MAdCAM-1) in leukocyte recruitment in experimental colitis and to compare the therapeutic effectiveness of their selective blockade. For this purpose, cell adhesion molecule expression was measured by the dual radiolabeled antibody technique in mice with dextran sulfate sodium-induced colitis and controls. Leukocyte-endothelial cell interactions were determined in colonic venules by fluorescence intravital microscopy. Therapeutic effects of chronic treatment with anti-ICAM-1, anti-VCAM-1, or anti-MAdCAM-1 antibodies were also assessed. Whereas colonic endothelial ICAM-1 was constitutively expressed and had a mild up-regulation in colitic animals, constitutive expression of VCAM-1 and MAdCAM-1 was low, but markedly increased after induction of colitis. Leukocyte adhesion was abrogated by immunoneutralization of VCAM-1 or MAdCAM-1 but not by treatment with an anti-ICAM-1 antibody. Chronic administration of anti-VCAM-1 antibody, but not anti-ICAM-1 or anti-MAdCAM-1, resulted in significant attenuation of colitis in terms of disease activity index, colon length, ratio of colon weight to length, and myeloperoxidase activity. In conclusion, VCAM-1 plays a central role in leukocyte recruitment in colitis and blockade of this adhesion molecule has higher therapeutic effect than immunoneutralization of ICAM-1 or MAdCAM-1 in this experimental model.     

Follicular dendritic cells share a membrane-bound protein with fibroblasts

The expression of a fibroblast antigen (AS02) on a proportion of CD21+ follicular dendritic cells (FDCs) provides evidence in support of their fibroblastic reticular origin. This antigen is expressed on the membrane of tissue fibroblasts but is absent from lymphocytes, macrophages or granulocytes. The distribution of AS02 in conjunction with other FDC markers (DRC-1, RFD3, CD23, IgM, and vitronectin) showed six types of FDCs. AS02 is present in the outer layers of primary and secondary follicles, but gradually decreases and disappears in the centre of germinal centres. In contrast, there is a progressive up-regulation of the other FDC markers. AS02 is re-expressed in involuting FDCs. Intermediate forms from fibroblastic to dendritic appearance are also apparent and occasionally FDC processes contain collagen type I and IV fibres, a characteristic feature of fibroblasts. In pathological follicles the normal differentiation pattern is disrupted, with persistence of the fibroblast marker, possibly due to altered interactions between FDCs and disrupted lymphocytic patterns. These findings provide new evidence for a local differentiation pathway of fibroblasts to mature FDCs.     

Expression of ICAM-1, VCAM-1 and ELAM-1 in angiofollicular lymph node hyperplasia (Castleman''s disease): evidence for dysplasia of follicular dendritic reticulum cells

The inducible adhesion molecules mediate important functions in the lymphoid tissues. We have investigated the expression of intercellular adhesion molecule 1 (ICAM-1), endothelial leucocyte adhesion molecule 1 (ELAM-1), vascular cell adhesion molecule 1 (VCAM-1), and platelet endothelial cell adhesion molecule (PECAM/CD31), using immunocytochemistry on cryostat sections of five lymph nodes from patients with Castleman's disease of the hyaline-vascular type. All five cases were characterized by marked hyperplasia of follicular dendritic reticulum cells, which were extensively present even in the mantle zone. Hyperplastic follicular dendritic reticulum cells showed marked expression of VCAM-1, and weak expression of ICAM-1. In two cases, several dysplastic giant cells with aberrant, polyploid nuclei showed aberrant expression of ELAM-1, an endothelium-restricted molecule. Dysplastic giant cells were positive with DRC-1 (an antibody to dendritic reticulum cells), VCAM-1 and occasionally ICAM-1, were negative for the endothelial cell markers factor VIII-related antigen and CD31 and were non-proliferating (Kl-67-). Cells positive for ICAM-1 or VCAM-1 were rare in the interfollicular areas. In all cases vascular hyperplasia was prominent, but endothelial cells were poorly activated in terms of expression of inducible adhesion molecules and of HLA-DR antigens. The possibility that dysplastic follicular dendritic reticulum cells have a pathogenetic role in Castleman's disease is discussed.     

Podoplanin (d2-40): a new immunohistochemical marker for reactive follicular dendritic cells and follicular dendritic cell sarcomas

The diagnosis of follicular dendritic cell (FDC) sarcoma can be challenging because of its morphologic overlaps with many other spindle cell neoplasms and, therefore, new phenotypic markers will be helpful in its differential diagnosis. Podoplanin is a mucin-type transmembrane glycoprotein that has recently been detected in reactive FDCs. In this study, we investigated the expression patterns of podoplanin using a new mouse monoclonal antibody D2-40, and compared them with CD21, a well-established FDC marker, in a comprehensive panel of cases. The panel included 4 FDC sarcomas, 38 spindle cell neoplasms of other types, 25 reactive lymphoid hyperplasia, and 117 lymphoid and 5 myeloid malignant hematopoietic neoplasms. Our study revealed that D2-40 strongly stained 3 of 4 FDC sarcomas. In contrast, D2-40 stained only 2/38 other spindle cell neoplasms tested. Furthermore, we observed that D2-40 highlighted more FDC meshworks than CD21 in Castleman's disease, follicular lymphoma, nodular lymphocyte predominance Hodgkin lymphoma, and residual reactive germinal centers in a variety of lymphoma types. D2-40 and CD21 stained an equal number of cases of reactive lymphoid hyperplasia, progressively transformed germinal centers and angioimmunoblastic T-cell lymphoma. No expression of podoplanin was detected in normal or neoplastic lymphoid and myeloid cells. We conclude that podoplanin (D2-40) is a sensitive and specific FDC marker, which is superior or equal to CD21 in evaluating both reactive and neoplastic FDCs. In addition, our results suggest that podoplanin (D2-40) can be used to support the diagnosis of FDC sarcoma.     

Mucosal addressin expression and binding-interactions with naive lymphocytes vary among the cranial,oral, and nasal-associated lymphoid tissues

The head and neck lymph nodes (LN)--or cranial, oral, and nasal-associated lymphoid tissue (CONALT)--help disseminate activated lymphocytes to produce salivary immune responses, especially after intranasal immunization. To elucidate the mechanisms that induce immunity at these sites, we investigated the interactions between addressins and homing-receptors that allow for lymphocyte binding to high endothelial venules (HEV) of CONALT. In vivo lymphocyte trafficking to CONALT was mediated primarily through interactions between peripheral node addressin (PNAd) and L-selectin, whereas interactions between mucosal addressin cell adhesion molecule-1 (MAdCAM-1) and alpha 4 beta 7 played a role in retention in cervical LN (CLN). Upon immunofluorescent staining for PNAd and MAdCAM-1, nearly all HEV in CONALT expressed PNAd, with varying MAdCAM-1 expression among these LN. The parotid gland LN (PRLN) and submaxillary gland LN (SMLN) rely exclusively upon PNAd-L-selectin interactions for naive-lymphocyte binding, whereas the CLN utilize PNAd-L-selectin interactions and MAdCAM-1-alpha 4 beta 7 interactions for binding. Intense staining of non-HEV-expressed vascular cell adhesion molecule-1 (VCAM-1) was observed in PRLN, whereas SMLN and CLN displayed less VCAM-1 but showed intense staining for diffuse MAdCAM-1. This study suggests that though PNAd-L-selectin interactions play an important role in the trafficking of lymphocytes throughout CONALT, varying MAdCAM-1 and VCAM-1 addressin expression and usage impart important differences among the PRLN, SMLN, and CLN.     

Increased expression of mucosal addressin cell adhesion molecule-1 (MAdCAM-1) and lymphocyte recruitment in murine gastritis induced by Helicobacter pylori

Although T cell involvement in Helicobactor pylori-induced gastritis is known, mechanism about T cell recruitment is not understood. In this study we examined how mucosal addressin cell adhesion -molecule-1 (MAdCAM-1) is involved in lymphocyte recruitment in murine chronic gastritis induced by H. pylori. C57 BL/6 mice were infected with Sydney strain (SS1). Six months after infection, the stomach was removed. The expression of adhesion molecules, MAdCAM-1, intercellular adhesion molecule-1 (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1), and the cell surface antigens CD4, CD8, CD45R/B220 or beta7-integrin were determined by immunohistochemistry. A significant increase in CD4 lymphocytes was observed in the body portion of stomach in SS1-infected mice and most of these CD4 cells express beta7-integrin, a known counter ligand for MAdCAM-1 molecule. Strong MAdCAM-1 expression was observed adjacent to these cells in the lamina propria as well as in the submucosa of SS1-infected stomach. Quantitative analysis showed that the area of MAdCAM-1 expression well correlated with the infiltration of beta7-integrin positive lymphocytes. On the other hand, expression of ICAM-1 or VCAM-1 in the lamina propria was few even in the SS1-infected stomach. Increased expression of MAdCAM-1 was well correlated to the location of lymphocytes, which express CD4 and beta7-integrin. These results suggest the possibility that MAdCAM-1 may be largely involved in the lymphocyte recruitment in the gastritis mucosa with H. pylori.     

Selective antibody blockade of lymphocyte migration to mucosal sites and mast cell adhesion.

The integrins alpha4beta7 and alpha4beta1 mediate adhesion to the mucosal addressin cell adhesion molecule-1 (MAdCAM-1) and the vascular cell adhesion molecule-1 (VCAM-1) and are important in T cell and allergic inflammatory reactions in the rat. The relative contributions of alpha4beta7 and alpha4beta1 in these reactions is unknown. To examine the role of alpha4beta7 in the rat a new mAb, TA-6, was developed. TA-6 inhibited adhesion to MAdCAM-1 but not to VCAM-1, a characteristic of alpha4beta7 adhesion, and immunofluorescence and immunoprecipitation studies were compatible with binding to alpha4beta7. TA-6 blocked rat lymphocyte adhesion to mesenteric lymph nodes and T cell migration to mucosal lymphoid tissues and it bound to rat mucosal mast cells. TA-6 did not inhibit lymphocyte adhesion to peripheral lymph nodes and T cell migration to peripheral lymphoid tissues or cutaneous inflammatory sites, and was not expressed on connective tissue mast cells.     

The adhesion molecules, l-selectin and sialyl lewis x, relate to the formation of the follicular dendritic cell-lymphocyte cluster in the mantle zone

The follicular dendritic cell (FDC)-lymphocyte cluster is rich in the follicular light zone of the secondary lymphoid follicles (LFs). Although, the mantle zone (MZ) also has FDC-lymphocyte cluster, it has not known about what kind of adhesion molecules relates to cluster formation. In the present study, we investigated whether the adhesion molecules, L-selectin (CD62L) and sialyl Lewis x (CD15s) can mediate the formation of the cluster in human tonsillar LFs. The MZ only expressed both the adhesion molecules in the secondary LF. Isolated FDC-lymphocyte clusters were composed of CD62L(+) lymphocytes and CD15s(+) FDCs. Stamper-Woodruff binding assay revealed that the binding of IgD(+) lymphocytes was significantly inhibited by pretreatment with anti-CD62L antibody or with anti-CD15s antibody. These results indicate that CD62L on MZ lymphocytes and CD15s on FDCs may play a role of the cluster formation, unlike the clusters in the other parts of LFs.     

Novel expression of vascular cell adhesion molecule-1 (CD106) by squamous epithelium in experimental acute graft-versus-host disease

Vascular cell adhesion molecule-1 (VCAM-1; CD106), the receptor for VLA-4, is an important mediator of adhesive and co-stimulatory interactions that govern cutaneous immune responses. Initial studies designed to elucidate temporal aspects of endothelial adhesion molecule induction in murine acute graft-versus-host disease (aGVHD) revealed unexpected and novel VCAM-1 expression by cutaneous and mucosal epithelial cells. Immunohistochemical techniques confirmed VCAM-1 staining as early as 7 days after transplantation in a distinctive subpopulation of squamous epithelial cells that normally occupy focal domains within the epidermal basal cell layer, the follicular infundibulum, and the dorsal lingual epithelium. Specifically, VCAM-1 expression was intimately associated with rete ridge-like prominences in footpad epidermis and in dorsal lingual epithelium. VCAM-1, as evaluated by serial section-labeling techniques, was preferentially expressed at sites of early epithelial infiltration by CD4(+) T cells. Western blot analysis confirmed expression of the 110-kd isoform of VCAM-1 in epithelium isolated from aGVHD animals, and immunoelectron microscopy demonstrated VCAM-1 reactivity restricted exclusively to epithelial cell plasma membranes. It is concluded that VCAM-1 is selectively expressed by discrete squamous epithelial subpopulations in murine aGVHD. As such, VCAM-1 may play a previously unrecognized role in mediating interactions between donor effector T lymphocytes and host epithelial cell targets.     

Expression of molecules involved in B lymphocyte survival and differentiation by synovial fibroblasts

The synovitis of rheumatoid arthritis (RA) is one of few pathological lesions in which B lymphocyte accumulation progresses to the extent of germinal centre formation. The present study was designed to assess the ability of synovial fibroblasts to express molecules implicated in B lymphocyte survival and differentiation, both in vivo, and in response to cytokines in vitro. Normal and diseased synovia were examined by indirect immunofluorescence. In all tissues synovial intimal fibroblasts showed co-expression of vascular cell adhesion molecule-1 (VCAM-1) and complement decay-accelerating factor (DAF) comparable to that of follicular dendritic cells (FDC), but not complement receptor 2 (CR2). In rheumatoid synovia, subintimal cells showed variable expression of VCAM-1 and DAF, with bright co-expression of VCAM-1, DAF and CR2 in lymphoid follicle centres. B lymphocytes, some of which were proliferating cell nuclear antigen-positive, were present in contact with subintimal cells expressing VCAM-1 with or without DAF or CR2. B lymphocytes were rarely present in the intimal layer, and, where present, showed fragmentation. In vitro, synovial fibroblasts exposed to tumour necrosis factor-alpha (TNF-α) in combination with interferon-gamma (IFN-γ) showed enhanced expression of VCAM-1, in comparison with fibroblasts from skin and lung and, unlike skin and lung fibroblasts, also expressed DAF and CR2. These findings support the hypothesis that synovial targeting in RA involves an enhanced ability of synovial fibroblasts to support B lymphocyte survival. This appears to be dependent, not on the constitutive expression of VCAM-1 and DAF on intimal cells, but on the increased ability of subintimal cells to respond to proinflammatory cytokines, perhaps critically in the expression of VCAM-1.     

Isolation of functionally active murine follicular dendritic cells

Biochemical, genetic, and immunological studies of follicular dendritic cells (FDCs) have been hampered by difficulty in obtaining adequate numbers of purified cells in a functional state. To address this obstacle, we enriched FDCs by irradiating mice to destroy most lymphocytes, excised the lymph nodes, and gently digested the nodes with an enzyme cocktail to form single cell suspensions. The FDCs in suspension were selected using the specific mAb FDC-M1 with magnetic cell separation technology. We were able to get nearly a million viable lymph node FDCs per mouse at about 90% purity. When examined under light and transmission electron microscopy, the cytological features were characteristic of FDCs. Furthermore, the cells were able to trap and retain immune complexes and were positive for important phenotypic markers including FDC-M1, CD21/35, CD32, CD40, and CD54. Moreover, the purified FDCs exhibited classical FDC accessory activities including: the ability to co-stimulate B cell proliferation, augment antibody responses induced by mitogens or antigens, maintain B cell viability for weeks, and protect B lymphocytes from anti-FAS induced apoptosis. In short, this combination of methods made it possible to obtain a substantial number of highly enriched functional murine FDCs.     

Trefoil peptide TFF2 treatment reduces VCAM-1 expression and leukocyte recruitment in experimental intestinal inflammation

There is evidence for a beneficial effect of trefoil peptides in animal models of gastric damage and intestinal inflammation, but the optimal treatment strategy and the mechanistic basis have not been explored thoroughly. It has been suggested that these proteins may modulate the inflammatory response. The aims of this study were to compare the protective and curative value of systemic and topical trefoil factor family (TFF)2 administration in dextran sulfate sodium-induced experimental colitis and to investigate the relationship between the therapeutic effects of TFF2 and modulation of leukocyte recruitment and expression of cell adhesion molecules. Clinical and morphologic severity of colitis was evaluated at the end of the study (Day 10). Leukocyte-endothelial cell interactions were determined in colonic venules by fluorescence intravital microscopy. The expression of cell adhesion molecules vascular cell adhesion molecule 1 (VCAM-1) and mucosal addressin cell adhesion molecule 1 (MAdCAM-1) was measured by the dual radiolabeled monoclonal antibody technique. Pretreatment with TFF2 by subcutaneous or intracolonic (ic) route ameliorated the clinical course of colitis, and the luminal route had a significantly superior effect. This beneficial effect was correlated with significant reductions in endothelial VCAM-1 but not MAdCAM-1 expression and leukocyte adhesion to intestinal venules, which returned to levels similar to those of controls. In established colitis, ic TFF2 treatment did not modify the severity of colonic lesions. In conclusion, TFF2 is useful in the treatment of colitis, and topical administration is superior to the systemic route. Reduction in adhesion molecule expression and leukocyte recruitment into the inflamed intestine contributes to the beneficial effect of this treatment.     

The pathobiology of HIV infection.

The follicular dendritic cells (FDCs) of the germinal center are known to absorb antigens in the form of immune complexes and to express them on the cell surface for long periods of time. Here, Cecil Fox and Michele Cottler-Fox propose that, as a result of FDC binding of immune-complexed viruses, lymphoid organs are the major reservoirs of HIV, and that FDCs play a key role in infection of CD4+ T cells.     

Lymphotoxin α1β2 expression on B cells is required for follicular dendritic cell activation during the germinal center response

CD19‐deficient mice were used as a model to study follicular dendritic cell (FDC) activation because these mice have normal numbers of FDC‐containing primary follicles, but lack the ability to activate FDCs or form GCs. It was hypothesized that CD19 expression is necessary for B‐cell activation and upregulation of membrane lymphotoxin (mLT) expression, which promotes FDC activation. Using VCAM‐1 and FcγRII/III as FDC activation markers, it was determined that the adoptive transfer of CD19⁺ wild‐type B cells into CD19‐deficient hosts rescued GC formation and FDC activation, demonstrating that CD19 expression on B cells is required for FDC activation. In contrast, CD19⁺ donor B cells lacking mLT were unable to induce VCAM‐1 expression on FDCs, furthermore FcγRII/III upregulation was impaired in FDCs stimulated with mLT‐deficient B cells. VCAM‐1 expression on FDCs, but not FcγRII/III, was rescued when CD19‐deficient B cells expressing transgenic mLT were cotransferred into recipient mice with CD19⁺, mLT‐deficient B cells, suggesting that FDC activation requires the CD19‐dependent upregulation of mLT on activated B cells. Collectively, these data demonstrate that activated B cells are responsible for the initiation of FDC activation resulting in a microenvironment supportive of GC development and maintenance.     

ACETYL CHOLINESTERASE IS EXPRESSED IN THE FOLLICULAR DENDRITIC CELLS OF GERMINAL CENTRES: DIFFERENCES BETWEEN NORMAL AND NEOPLASTIC FOLLICLES

Acetyl cholinesterase (AcChE) was demonstrated by histochemistry in the follicular dendritic cells (FDCs) of the germinal centres of lymph nodes, tonsils, and bowel lymphoid tissue. Its presence in the FDCs was confirmed by double immunostaining for CD21 or DRC-1. AcChE-positive FDCs are concentrated in the inner portion of the light zone of the germinal centre, being absent from the dark zone. In the lymphoid tissue surrounding the germinal centres are AcChE-positive blood vessels; double staining shows that the AcChE is present in the pericytes surrounding the endothelium of the blood vessels. In contrast to the reactive follicle, the AcChE reactivity in FDCs of follicle centre lymphoma is absent or minimally expressed, although the dense FDC mesh is well stained with CD21 or DRC-1. This suggests that the AcChE is not constitutively expressed in FDCs but that its expression is influenced by the state and activity of the lymphoid cells in the germinal centre. The reduced level of AcChE staining can be profitably employed in the diagnosis of follicle centre lymphoma.     

Distinct binding specificities of integrins {alpha}4{beta}7 (LPAM-1), {alpha}4{beta}1 (VLA-4), and {alpha}IEL{beta}7

Integrin receptors are Important for regulating lymphocyte reclrculatlonand recruitment to sites of inflammation. Transfoctants of theB cell lymphoma 38C13 were generated that differ exclusivelyin the expression of integrin ß1 or ß7 subunltsallowing for a functional comparison of lymphocyte Peyer's patchHEV adhesion molecule 1 (LPAM-1) (4ß7) and very lateantigen 4 (VLA-4) (4ß1) in an Identical cellular environment.Whereas 38-ß7 transfectants bound to purified andcellular mucosal addressin cell adhesion molecule (MAdCAM-1),unstlmulated 38-ß1 cells failed to bind MAdCAM-1.Treatment of 38-ß1 cells with Mn²⁺ but not with PMAinduced low level binding to MAdCAM-1. MAdCAM-1 adhesion of38-ß7 cells was constitutive and not enhanced by Mn²⁺treatment. Similarly, MAdCAM-1-dependent adhesion to mucosalhigh endothellal venules was shown for 38-ß7 but notfor 38-ß1 cells. The results therefore establish theLPAM-1 - MAdCAM-1 Interaction as the functionally dominant adhesionpathway for regulating lymphocyte homing to mucosal sites. Nonetheless,the activated VLA-4 on some lymphocytes may be involved in MAdCAM-1recognition or promote binding to MAdCAM-1 In other tissues.By contrast, 38-ß7 and 38-ß1 transfectantsdid not differ in their binding capacity for vascular cell adhesionmolecule 1 (VCAM-1) or fibronectin and LPAM-1 did not displayany preference for interacting with either MAdCAM-1 or VCAM-1.LPAM-1 may therefore contribute significantly to cellular functionspreviously attributed to VLA-4. Interestingly, functional analysisof the intraepithellal lymphocyte integrin IELß7 whichIs structurally related to LPAM-1 did not reveal detectablebinding activity for MAdCAM-1, VCAM-1, or fibronectin.     

Marginal zone B cells transport and deposit IgM-containing immune complexes onto follicular dendritic cells

Secreted IgM and complement are important mediators in the optimal initiation of primary T-dependent humoral immune responses. Secreted IgM serves as a natural adjuvant by enhancing the immunogenicity of protein antigens, perhaps as a result of IgM's ability to facilitate antigen deposition onto follicular dendritic cells (FDCs) and promote rapid germinal center (GC) formation. To understand how IgM enhances adaptive immune responses, we investigated the mechanism by which IgM-containing immune complexes (IgM-IC) are transported to FDCs as a first step in GC formation. We demonstrate that IgM-IC localize first to the splenic marginal zone (MZ) where the IgM-IC bind MZ B cells in a complement and complement receptor (CR1/2) dependent process. MZ B cells then transport the IgM-IC into the follicle for deposition onto FDCs. Mice with reduced numbers of MZ B cells trap IgM-IC on FDC less efficiently, whereas mice with reduced numbers of follicular B cells trap IgM-IC normally. The functional elimination of MZ B cells abrogates the ability of FDCs to trap IgM-IC. Transfer of B cells with associated IgM-IC into naive mice results in deposition of IgM-IC onto FDC by MZ B cells. The results demonstrate an IgM and complement-dependent role for MZ B cells in the fate of antigen early in the initial phases of T-dependent immune responses. The data also establish an important role for CR1/2 on MZ B cells in the efficient binding and transport of IgM-IC to FDCs, which we suggest is an important first step in initiating adaptive immune responses.