T-lymphocyte differentiation and the extracellular matrix: identification of a thymocyte subset that attaches specifically to fibronectin

The thymus road to a T cell: migration, selection, and atrophy

The thymus plays a pivotal role in generating a highly-diverse repertoire of T lymphocytes while preventing autoimmunity. Thymus seeding progenitors (TSPs) are a heterogeneous group of multipotent progenitors that migrate to the thymus via CCR7 and CCR9 receptors. While NOTCH guides thymus progenitors toward T cell fate, the absence or disruption of NOTCH signaling renders the thymus microenvironment permissive to other cell fates. Following T cell commitment, developing T cells undergo multiple selection checkpoints by engaging with the extracellular matrix, and interacting with thymic epithelial cells (TECs) and other immune subsets across the different compartments of the thymus. The different selection checkpoints assess the T cell receptor (TCR) performance, with failure resulting in either repurposing (agonist selection), or cell death. Additionally, environmental cues such as inflammation and endocrine signaling induce acute thymus atrophy, contributing to the demise of most developing T cells during thymic selection. We discuss the occurrence of acute thymus atrophy in response to systemic inflammation. The thymus demonstrates high plasticity, shaping inflammation by abrogating T cell development and undergoing profound structural changes, and facilitating regeneration and restoration of T cell development once inflammation is resolved. Despite the challenges, thymic selection ensures a highly diverse T cell repertoire capable of discerning between self and non-self antigens, ultimately egressing to secondary lymphoid organs where they complete their maturation and exert their functions.

Does furan affect the thymus in growing male rats?

Furan has been identified in foods such as heat-treated foods, including coffee, canned meat, hazelnuts, and infant foods and formulas. Children may be exposed to furan via either consumption of these foods or their derivatives. We evaluated the effects of furan on the thymus of weaning male rats in the present study. Five separate groups containing male rats were used: control, oil control, and three furan-treated groups. Furan was given orally to rats in the treatment groups at doses of 2, 4, and 8 mg/kg/day for 90 days. At the end of the experiment, thymus of the rats were examined morphologically, histopathologically, and immunohistochemically. We observed that absolute and relative weights of thymus were decreased significantly in rats treated with 4- and 8-mg/kg/day doses of furan. In histopathological examination, enlargement of interstitial connective tissue between the thymic lobules, lymphocyte depletion, and hemorrhage were observed. We detected an increase in apoptotic cell counts in thymus of the treatment groups. In addition, we found significant differences in the distribution of fibronectin and transforming growth factor-beta in the thymus of the treatment groups. In conclusion, we suggest that furan has affected the thymus in growing male rats.

Thymus organogenesis and development of the thymic stroma

T-cell development occurs principally in the thymus. Here, immature progenitor cells are guided through the differentiation and selection steps required to generate a complex T-cell repertoire that is both self-tolerant and has propensity to bind self major histocompatibility complex. These processes depend on an array of functionally distinct epithelial cell types within the thymic stroma, which have a common developmental origin in the pharyngeal endoderm. Here, we describe the structural and phenotypic attributes of the thymic stroma, and review current cellular and molecular understanding of thymus organogenesis.

Integrin functions play a key role in the differentiation of thymocytes in vivo

T cells express a variety of surface proteins as they develop to maturity in the thymus. In addition to the TCR-CD3 complex and the two major coreceptors, CD4 and CD8, other surface proteins expressed include receptors for cytokines, growth factors, counterreceptors, and extracellular matrix molecules. To determine the role of integrin adhesion receptors in T cell development, we have expressed a trans-dominant inhibitor of integrin function in the thymus. This inhibitor leads to a block of adhesion to fibronectin due to reduced activation of integrin receptors. This reduced adhesion leads to a partial block in differentiation from CD4-CD8- cells to CD4+CD8+ cells, after the CD25+ stage, suggesting that integrins are important during Lck-mediated differentiation. Furthermore, the overall production of CD4+ cells is reduced compared with that of CD8+ cells without changes in negative selection, suggesting that integrins may be involved in the determination of the fate of the cell as well. These results demonstrate that integrin receptor function is required for proper thymocyte development in vivo.

Laminin-8 (alpha4beta1gamma1) is synthesized by lymphoid cells, promotes lymphocyte migration and costimulates T cell proliferation

Laminins are a growing family of large heterotrimeric proteins with cell adhesive and signalling functions. They are major components of basement membranes and are found in many organs, including the vasculature and other compartments of bone marrow, thymus, lymph nodes and spleen. However, expression, recognition and use of laminin isoforms by lymphoid cells are poorly understood. In the present study, lymphoid T cells (Jurkat) were found to synthesize laminin alpha4, beta1 and gamma1 mRNAs and polypeptides and to assemble the chains into laminin-8. Lymphoblastoid B (NAD-20) cells, lymphoid NK (NKL) cells and blood lymphocytes also contained laminin-8 and, after cell permeabilization, practically all blood lymphocytes reacted with mAbs to laminin beta1 and gamma1 chains. Following stimulation, blood lymphocytes secreted laminin-8, and this laminin isoform, but not laminin-10/11(alpha5beta1gamma1/alpha5beta2gamma1), promoted chemokine-induced migration of the cells. In an activation-dependent manner, purified blood CD4 T cells adhered to immobilized laminin-8 and laminin-10/11 by using alpha6beta1 integrin, but minimally to laminin-1 (alpha1beta1gamma1). Accordingly, laminin-8 and laminin-10/11, but not laminin-1, strongly costimulated proliferation of the T cells via the same integrin. Thus, lymphoid cells are able to synthesize and secrete complete laminin molecules. In addition, synthesis of laminin-8 and recognition of laminin-8 and -10/11 by lymphocytes indicate relevance of these laminin isoforms in lymphocyte physiology.

Age-dependent immunolocalization of fibronectin and histological changes in the thymus of rats

Age-dependent variations in the immunolocalization of fibronectin (FN) in the thymus were investigated in 1-, 6-, 12- and 20-month-old male and female Swiss albino rats (Rattus rattus) at the light-microscopic level and the changes with ageing in the histological structure of the thymus were also studied. There were no significant differences in the age-dependent variations in the immunolocalization of fibronectin or in the histological structure of the thymus between male and female rats of the same age but there were increases with ageing in the fibronectin content of the thymic capsule, the connective tissue between the lobules, around blood vessels, and in the medulla and cortex of thymus. The connective-tissue content between lobules, fat cells, Hassall's corpuscles, the thickness of capsule and the ratio of the medulla to the cortex of the lobules showed age-dependent increases in the thymus of rats of both sexes. Decreases in the organ weight/body weight ratio were also observed with ageing.

LPAM-1 (integrin alpha 4 beta 7)-ligand binding: overlapping binding sites recognizing VCAM-1, MAdCAM-1 and CS-1 are blocked by fibrinogen, a fibronectin-like polymer and RGD-like cyclic peptides

The alpha 4 integrin LPAM-1 (alpha 4 beta 7) mediates lymphocyte attachment within the extracellular matrix (ECM) by adhering to the connecting segment (CS)-1 site of fibronectin (FN). Here we reveal that very late antigen (VLA)-4 LPAM-1+ T cell lymphoma TK-1 cells bind via LPAM-1 to multiple copies of the RGD sequence engineered within an FN-like polymer. Further, the small conformationally restrained RGD-like cyclic peptides 1-adamantaneacetyl-Cys-Gly-Arg-Gly-Asp-Ser-Pro-Cys and Arg-Cys-Asp-thioproline-Cys inhibit the adhesion of TK-1 cells to immobilized CS-1 peptide, and to endothelial counterreceptors for LPAM-1, namely mucosal addressin cell adhesion molecule (MAdCAM)-1 and vascular cell adhesion molecule (VCAM)-1. Spontaneous adhesion of the VLA-4- LPAM-1+ B lymphoma cell line RPMI 8866 to CS-1 was likewise inhibited, confirming a previously undocumented ability of LPAM-1 to recognize the RGD tripeptide. The RGD-binding site in LPAM-1 either overlaps or is identical to sites required for interaction with MAdCAM-1, VCAM-1, and the CS-1. The binding of LPAM-1 and VLA-4 to RGD-containing ligands may have relevance in vivo given that fibrinogen at physiological concentrations is able to partially block the binding of TK-1 cells to MAdCAM-1. Hence fibrinogen and other vascular RGD-containing proteins may have mild anti-inflammatory activity required for maintaining effective homeostasis, analogous to the anti-thrombogenic activity of the vascular endothelium.

Analysis of extracellular matrix proteins in malignant chicken cell lines

The adherent chicken cell line, MDCC-MSB1-41C, was highly transplantable and metastatic in vivo, compared with the parental non-adherent cell line MDCC-MSB1 from Marek's disease (MD) lymphoblastoid tumor. For clarification of differences in extracellular matrix proteins in MSB1-41C and MSB1 cells, examination was made of various components of extracellular matrix proteins. A detachment experiment indicated the protein(s) recognizing the Arg-Gly-Asp (RGD) sequence, the minimum structure required for recognition by the cell-surface receptors, is essential for the adherent character. Immunoblot assay using antibodies showed increased expression of fibronectin, fibronectin receptors, and vinculin on MSB1-41C cell lines. RGD-directed integrins mediate important cell-cell adhesive interaction and these interactions with extracellular matrix proteins may thus possibly be requisite for migration, proliferation and metastatic dissemination of MDCC-MSB1-41C cells. The RGD-containing peptide in the culture medium could cause detachment of cultured adhesive lymphoid leukosis LSCC-1104X5 cells from the dish too.

Extracellular matrix and lung inflammation

Lung injury triggers an acute inflammatory response characterized by increased expression and deposition of extracellular matrix (ECM) components such as fibronectin and collagen. Although the function of newly deposited matrices in injured lungs is unknown, their ability to affect the migration, proliferation, differentiation, and activation state of cells in vitro suggests an important role in the initiation and maintenance of the inflammatory response in vivo. Interactions between immune and nonimmune cells with the lung ECM are mediated via cell surface receptors of the integrin family which link the ECM with intracellular molecules involved in signal transduction. Activation of integrin-mediated intracellular signals may promote inflammation by facilitating leukocyte recruitment and cytokine expression.

Extracellular matrix and beta 1 integrin expression in nodal and extranodal T-cell lymphomas

Since non-Hodgkin's lymphoma (NHL) cells interact with surrounding structures similarly to their normal counterparts, micro-environmental changes and the aberrant expression of adhesion molecules are considered to be of importance in lymphomagenesis. In this immunohistochemical study, the composition of several extracellular matrix (ECM) components and the expression of their beta 1 integrin receptors were examined in nodal and extranodal T-cell NHLs. Except for the T-lymphoblastic NHLs, almost all T-NHLs displayed abundant deposition of matrix and considerable expression of the alpha 4 and beta 1 integrin chains. This is in contrast to B-cell NHLs, which show ECM patterns comparable to those in reactive lymphoid tissue or, in cases of high-grade malignancy, active matrix degradation and very low expression or absence of beta 1 integrins, as previously described. This difference is probably based on distinct cytokine production in B- and T-cell malignancies. As in B-NHL, nodal and extranodal T-NHLs of the same morphological subtype exhibit identical ECM patterns, which suggests that malignant lymphoid cells of both B and T origin create at least part of their own specific micro-environment.

Distribution of extracellular matrix components and their receptors in human lymphoid tissue and B-cell non-Hodgkin lymphomas

In this study the distribution patterns of various extracellular matrix components and their receptors (i.e. beta 1 integrins) in B-cell non-Hodgkin lymphomas were examined and compared to those in reactive lymphoid tissue. Neoplastic follicles within follicular lymphomas showed similar patterns to that observed in reactive follicles, which appeared to be strongly associated with the presence of follicular dendritic cells. Diffuse lymphomas of low and intermediate malignancy grade revealed features comparable to those of interfollicular areas of reactive lymphoid tissue, irrespective to which compartment the tumour cells were related. Highly malignant lymphomas, however, displayed unique extracellular matrix configurations, resulting from active matrix degradation by macrophages; this may support rapid tumour growth. Extranodal lymphomas showed virtually the same matrix patterns as their nodal counterparts, suggesting that (malignant) lymphoid cells generate (at least partly) their own specific microenvironment. In reactive lymphoid tissue beta 1 integrins were mainly found on resident cells and except for alpha 4, alpha 5 (and beta 1) the lymphoid cells expressed very little, if any, beta 1 integrins. In comparison, expression of these integrins on lymphoma cells was reduced (follicular lymphomas) or could not be detected at all (diffusely growing lymphomas); this might contribute to the growth pattern and metastatic properties of the tumours.

Prolonged impairment of very late activating antigen-mediated T cell proliferation via the CD3 pathway after T cell-depleted allogeneic bone marrow transplantation

One of the major obstacles in allogeneic bone marrow transplantation (allo-BMT) is prolonged T cell dysfunction resulting in a variety of infectious complications in the months to years after hematologic engraftment. We previously showed that immobilized extracellular matrix (ECM) proteins such as fibronectin (FN), the CS-1 domain of FN, or collagen (CO) acted synergistically with immobilized anti-CD3 to induce T cell proliferation. In addition, the comitogenic effect of ECMs could be mimicked by immobilized mAb reactive with a common beta 1 chain (CD29) of very late activating (VLA) antigens which include ECM receptors. Since the interaction of T cells with ECMs appears to play an important role in the process of T cell reconstitution following allo-BMT, we examined the expression of VLA antigens (alpha 1-alpha 6, beta 1) and their functional roles in CD3-mediated T cell proliferation at various times after T cell depleted allo-BMT. VLA beta 1 as well as VLA alpha 4, alpha 5, and alpha 6 expression was lower than normal controls during the first 3 mo after allo-BMT and auto-BMT, whereas these expressions returned to normal levels by 4 mo after allo-BMT and auto-BMT. Although alpha 1 and alpha 2 were not expressed on lymphocytes from normal controls, these antigens were expressed on lymphocytes at the detectable levels (5-15%) from patients after allo-BMT and auto-BMT. Both CD29 and CD3 were expressed at normal levels on lymphocytes from patients > 3 mo after allo-BMT, whereas T cell interaction with ECM through VLA proteins or crosslinking of VLA beta 1 expressed by T cells with anti-CD29 mAb results in poor induction of CD3-mediated T cell proliferation for a prolonged period (> 1 yr) after allo-BMT. In contrast, T cell proliferation induced by crosslinking of anti-CD2 or anti-CD26 with anti-CD3 was almost fully recovered by 1 yr post-allo-BMT. After autologous BMT, impaired VLA-mediated T cell proliferation via the CD3 pathway after auto-BMT returned to normal levels within 1 yr despite no significant difference in CD3 and CD29 expression following either allo- or auto-BMT. The adhesion of T cells from post-allo-BMT patients to FN-coated plate was normal or increased compared to that of normal controls. Moreover, the induction of the tyrosine phosphorylation of pp105 protein by the ligation of VLA molecules was not impaired in allo-BMT patients. These results suggest that there are some other defects in the process of VLA-mediated signal transduction in such patients. Our results imply that disturbance of VLA function could explain, at least in part, the persistent immunoincompetent state after allo-BMT and may be involved in susceptibility to opportunistic infections after allo-BMT.

Fibronectin and lymphocytes in inflammatory tissue. Studies of blood and synovial fluid lymphocytes from patients with rheumatoid arthritis and other inflammatory arthritides

Lymphocytes infiltrating tissues under chronic inflammatory conditions are often surrounded by deposits of fibronectin. We have studied the possibility that this reflects capacity of lymphocytes to synthesize fibronectin and compared lymphocytes from blood and synovial fluid with respect to fibronectin interactions. In vitro activated blood lymphocytes exhibited synthesis of a fibronectin-like molecule. Synovial fluid cells appeared to synthesize the same high molecular weight component spontaneously. Activated blood lymphocytes have cell surface fibronectin and surface components of lower molecular weight which could be immunoprecipitated with anti-fibronectin antibodies as well as by insolubilized collagen. Synovial fluid cells showed cell surface fibronectin as revealed by immunocytochemical detection but seemed to lack or have relatively small amounts of the low-molecular weight fibronectin-like surface components. Synovial fluid T cells from arthritis patients showed adhesion to fibronectin. Immunocytochemistry demonstrated presence of alpha 4 and alpha 5 beta 1 integrins at the surface of the synovial fluid T cells and RGD and LDV peptides inhibited adhesion of the cells to fibronectin. Noteworthy, a portion of synovial fluid cells with lymphocyte markers also bound to plastic. Blood lymphocytes from the same arthritis patients displayed relatively poor or negligible adhesion to fibronectin unless activated to blast transformation and did not attach to plastic. Taken together these results suggest that activated lymphocytes from blood and synovial fluid may use fibronectin of exogenous or endogenous origin when interacting with tissues during inflammatory processes. Furthermore, the presence at the lymphocyte surface of components of different molecular weight precipitated by anti-fibronectin antibodies suggests that fibronectin or its fragments can bind to the lymphocyte surface.

Extracellular matrix components of the mouse thymus microenvironment. IV. Modulation of thymic nurse cells by extracellular matrix ligands and receptors

Extracellular matrix (ECM) proteins can influence cell migration and differentiation in a variety of cell systems. Within the thymus, these molecules are heterogeneously distributed, and their physiological role is poorly understood. This prompted us to carry out in vitro studies using the thymic nurse cell (TNC) model. We observed that fibronectin and laminin accelerate spontaneous in vitro release of thymocytes from TNC, whereas anti-ECM antibodies exhibited a blocking effect. Similar results were obtained with anti-ECM receptor reagents. Moreover, these antibodies abrogated in vitro reconstitution of TNC complexes and thymocyte adhesion to TNC-derived epithelial cultures. Our results indicate that lymphocyte traffic in TNC (comprising both entrance into and exit from the epithelial structure) is affected by interactions involving extracellular matrix ligands and receptors. In this respect, the dynamic analysis of thymic nurse cell complexes should be regarded as a relevant in vitro tool for functional studies of distinct adhesion molecules in intrathymic lymphocyte traffic.

The possible role of fibronectin in multiple myeloma

Fibronectin (FN) has an active role in the immune response, interacting with a number of different cells and components. It has been implicated in the formation of cryoprecipitates in rheumatic diseases and is present in tissues where under pathological conditions immune complexes are deposited. Under physiological conditions of pH and ionic strength both heavy and light chain of all multiple myeloma and normal IgG show affinity to FN. FN binds to both B and T cells and is shown to inhibit thrombin and collagen-induced platelet aggregation. We have found elevated levels of FN in the plasma of multiple myeloma patients tested compared to a group of normal subjects. Even though the level of FN did not correlate with the level of the paraprotein, our findings raise the possibility that FN might be implicated in some of the clinical symptoms of multiple myeloma.

Thymic extracellular matrix in human malnutrition

Previous studies have shown that malnutrition severely affects both lymphoid and epithelial components of the thymus. Yet, few data are available concerning the extracellular matrix (ECM) of the thymic microenvironment in malnutrition. We studied by histological, ultrastructural, and immunohistochemical means thymuses obtained in necropsies from 19 malnourished children. We observed a consistent increase in the intralobular ECM-containing network which could be ascertained histologically by the dense reticulin staining. This abnormally dense ECM network contained fibronectin, laminin, and type IV collagen. Importantly, the enhancement of thymic ECM in malnourished individuals positively correlated with the degree of thymocyte depletion. This correlation may represent a cause-effect relationship in which the contact of thymocytes with abnormally high amounts of thymic ECM triggers and/or enhances programmed cell death.

Interactions of promonocytic U937 cells with proteins of the extracellular matrix

Monocyte interaction with proteins of the extracellular matrix (ECM) is regulated by expression of specific cell-surface receptors. 12-O-tetradecanoyl phorbol-13-acetate (TPA) has been shown to induce the promonocytic cell line U937 to a more differentiated monocyte-like state. In this study we have analysed the attachment of U937 cells to ECM proteins and the effects of treatment with TPA on this process. Non-induced U937 cells attach to fibronectin- and Matrigel-coated surfaces without TPA stimulation, but TPA further increases adherence to these substrates as measured by an enhanced binding and by the lower concentration of proteins needed in the substrate to achieve 50% of maximal cell adhesion. Attachment to type I collagen was seen only with activated U937 cells, whereas no measurable attachment to bovine serum albumin, vitronectin, and type IV collagen was detected. TPA-activated U937 cells showed a two-fold increase in the expression of the RGD-dependent integrin receptors alpha 3 and alpha 5, and a reduction in the expression of alpha 4, another fibronectin-specific receptor, whereas the common beta 1 chain was unchanged. Attachment of U937 cells to fibronectin was primarily mediated by the alpha 3 and alpha 5 integrins, as revealed by the ability of GRGDS peptides to inhibit attachment, whereas the CS-1 peptide, containing the alpha 4 binding site, was largely ineffective in blocking attachment.

Monoclonal antibody 4C7 recognizes an endothelial basement membrane component that is selectively expressed in capillaries of lymphoid follicles

In order to define compartment-related structures within the extracellular matrix of human lymphoid organs, monoclonal antibodies (MAbs) were generated by immunizing mice with stromal fragments of human tonsils. One MAb (4C7) was selected which recognized an endothelial basal membrane component that is selectively expressed in capillaries of lymphoid follicles. The epitope was also present in follicles within chronically inflamed synovial membrane and in a hyperplastic thymus of a patient with myasthenia gravis. B-cell non-Hodgkin's lymphomas with a follicular growth pattern expressed the antigen in neoplastic follicles, whereas diffuse growing lymphomas lacked the antigen. The restricted distribution pattern suggests involvement of the 4C7-defined antigen in the organization of the follicular compartment within human lymphoid tissue.

Tissue distribution of very late activation antigens-1/6 and very late activation antigen ligands in the normal thymus and in thymoma

The expression of very late activation antigens (VLAs)-1/6 was correlated with that of the VLA ligands fibronectin, laminin, collagen, and vascular cell adhesion molecule-1 in sections of normal thymus, in thymocyte suspensions, and in 10 cases of thymoma. Capsular epithelial cells are VLA-2+, VLA-3+, and VLA-6+ and face the thymic basement membrane, which is rich in fibronectin, laminin, and collagen type IV. Cortical epithelial cells are VLA-2+ and are embedded in a reticular meshwork of nonorganized extracellular matrix (ECM) that is rich in fibronectin. Cortical thymocytes, identified as CD3dim cells by using immunofluorescence in suspension, are highly positive for VLA-4, a fibronectin ligand. Most cortical macrophages are positive for vascular cell adhesion molecule-1, a molecule recognized by VLA-4. Medullary epithelial cells are VLA-2+/VLA-3+ and are codistributed with fibrous strands of organized ECM that are positive for fibronectin, collagen, and laminin. Medullary thymocytes, identified as CD3bright cells, are positive for VLA-4 and VLA-6, a ligand for laminin. Our findings suggest that intrathymic thymocyte maturation is associated with changes in expression of VLA molecules, which are apparently correlated with the presence of VLA ligands in the tissue microenvironment. Thymomas were classified as cortical (three), common (five), or medullary (two) type. Expression of VLA molecules and distribution of ECM in the three histological subtypes were reminiscent of those observed in the respective regions of the normal thymus. All cases of thymoma were characterized by overexpression of VLA molecules on neoplastic cells, which was associated with increased deposition of organized ECM rich in fibronectin, laminin, and collagen.

Fibronectin synthesis in the harderian gland of the chicken

Immunoelectron microscopic staining with a monoclonal antibody against fibronectin demonstrated the presence of this extracellular matrix glycoprotein in the avian Harderian gland. Fibronectin was detected as a component of the electron-dense material, which has been observed both between the epithelial cells lining the ducts of the gland and between the lymphoid cells within the subepithelial lymphoid tissue. Additionally, intracellular fibronectin was detected in the rough endoplasmic reticulum, Golgi complex, and secretory vacuoles in the cytoplasm of a cell, showing the ultrastructural features of a myofibroblast. These findings indicate that the Harderian gland myofibroblasts secrete and release fibronectin into the extracellular matrix.

Regulation of fibronectin and laminin binding activity in cultured human lymphoblastic cell lines

The current study shows that a clonal derivative of the Jurkat cell line up-regulates both the avidity and density of the alpha 6/beta 1 receptor in response to phorbol 12-myristate 13-acetate (PMA). This derivative attaches to fibronectin and, to a lesser degree, laminin constitutively. Adhesion and spreading are dramatically up-regulated following treatment with PMA. The response on fibronectin peaks within 4 hours, is insensitive to cyclohexamide, can be blocked by monoclonal antibodies (Mabs) to the beta 1 and alpha 5 subunits of the beta 1 family of integrins, and is not associated with increased expression of the alpha 5 or beta 1 epitopes at the cell surface. In contrast, the response on laminin is biphasic. The early phase parallels the response on fibronectin. The second phase peaks after 48-72 hours of treatment with PMA, is sensitive to cycloheximide, can be blocked by Mabs to the beta 1 and alpha 6 subunits, and is associated with increased expression of the alpha 6 epitope. Both the density independent and dependent responses to PMA in Jurkat cells are blocked by the protein kinase inhibitor staurosporine. The HSB-2, CEM, Molt-4, and HPB-ALL T-lymphoblastic cell lines also up-regulate attachment to fibronectin and laminin following treatment with PMA. All four lines constitutively attach to fibronectin and show rapid up-regulation of attachment following treatment with PMA. None of the lines attach to laminin prior to PMA treatment; however, specific adhesion developed after 4-120 hours of treatment. The most mature lines (Jurkat and HPB-ALL) up-regulated adhesion on laminin more rapidly than the less phenotypically mature lines (CEM, Molt-4, and HSB-2). In summary, clonal derivatives of the Jurkat cell line up-regulated attachment to laminin through protein kinase dependent increases in alpha 6/beta 1 receptor avidity and density. In addition, the expression of functional receptors for laminin is linked to developmental maturity in a series of T-lymphoblastic cell lines.

Ligation of VLA-4 on T cells stimulates tyrosine phosphorylation of a 105-kD protein

The VLA/integrins are a family of heterodimeric adhesion receptors shown to be involved in cell-to-cell and cell-to-extracellular matrix (ECM) interactions. Given recent evidence that VLA molecules can synergize with the CD3/T cell receptor (TCR) pathway to activate T cells, it is important to identify biochemical event(s) generated by these molecules. Here, we report that the engagement of VLA-4 on T cells with specific antibodies or its ligand activates protein-tyrosine kinase (PTK) activity as detected by antiphosphotyrosine immunoblotting. The crosslinking of VLA-beta 1 (CD29) with a specific monoclonal antibody (mAb) (anti-4B4) plus anti-mouse immunoglobulin resulted in the rapid tyrosine phosphorylation of a 105-kD protein (pp105) in the human T cell line H9, as well as in peripheral resting T cells. The increase in tyrosine phosphorylation of pp105 was specifically mediated by VLA-4, since mAbs against alpha 4, but not against other VLA alpha chains, could induce this phosphorylation. In addition, the binding of T cells with the CS1 alternatively spliced segment of fibronectin (the binding site recognized by VLA-4) induced pp105 tyrosine phosphorylation. Crosslinking the CD3 complex or VLA-4 molecules with mAbs demonstrated that each of these molecules stimulated the tyrosine phosphorylation of unique sets of proteins with different kinetics, suggesting that these two receptor systems are coupled to distinct PTKs. Since tyrosine phosphorylation of cellular proteins has been shown to be a crucial biochemical event in cell growth, our findings suggest that the induction of pp105 tyrosine phosphorylation via VLA-4 may play a role in the transduction of activation signals through this molecule.

Human natural killer cells express different integrins and spread on fibronectin

Human natural killer (NK) cells adhered and most of them also actively spread on cellular fibronectin (cFn), plasma Fn (pFn) and its Mr 120,000-140,000 or Mr 105,000 cell-binding proteolytic Fn-fragments as well as on heparin-binding Fn-fragments containing the alternative cell binding site. The cells did not spread on vitronectin, laminin or collagens. Adhesion on Mr 105,000 Fn fragment containing the cell binding site, could be prevented by the synthetic peptide GRGDS but not by an inactive peptide, whereas adhesion on heparin-binding Fn fragments was unaffected by the peptide. Spreading of the NK cells led to a distinct reorganization of F-actin. Immunoprecipitation with monoclonal antibodies (MoAb) against the beta 1 integrin subunit of radioactively surface-labelled cells revealed a broad polypeptide band of Mr 140,000 under reducing conditions and a polypeptide doublet of Mr 160,000 and Mr 110,000 under non-reducing conditions. Identical polypeptides, corresponding to the alpha- and beta-subunits of the Fn-receptor complex, were bound to the Mr 105,000 chymotryptic Fn-fragment coupled to Sepharose. Similar experiments with small lymphocytes did not reveal any polypeptides. Immunofluorescence results with McAbs suggested that among the alpha-subunits of integrins, the alpha 3, alpha 4, and alpha 5 subunits are expressed in NK cells. The present results suggest that non-activated NK cells, but not small lymphocytes, express beta 1-integrins, and that at least the Fn-receptors alpha 4 beta 1 and alpha 5 beta 1 may function in the adhesion and migration of NK cells.

Beta 1 integrin-mediated lymphocyte adherence to extracellular matrix is enhanced by phorbol ester treatment

The interaction of the human T cell leukemia, Jurkat, with the extracellular matrix components collagen and fibronectin was examined. These cells displayed constitutive binding to fibronectin and low levels of adherence to collagen which were enhanced following stimulation with phorbol esters. The relevant binding structures were identified as members of the CD29/beta 1 integrin family of adhesion molecules. Adherence to collagen and to fibronectin was mediated by alpha 2 beta 1 and alpha 5 beta 1, respectively. The enhancement of adherence by phorbol esters did not involve up-regulation of receptor expression but appeared to derive from the increased functionality of structures which were expressed on the cell surface.

A monoclonal antibody (8H3) that binds to rat T lineage cells and augments in vitro proliferative responses

A murine monoclonal antibody, designated 8H3, recognizes a cell surface antigen expressed exclusively on rat T lineage cells. 8H3 antibody immunoprecipitated 180-, 120-, and 90-kD components from rat thymocytes as well as splenic T cells under nonreducing conditions. 8H3 antibody specifically inhibited the binding of thymocytes to fibronectin. Furthermore, binding of rat thymocytes to immobilized synthetic peptide Gly-Arg-Gly-Asp-Ser-Pro-Cys-BSA was inhibited by 8H3 antibody as was Gly-Arg-Gly-Asp-Ser-Pro-Cys, but not by Gly-Arg-Ala-Asp-Ser-Pro-Lys or Gly-Arg-Gly-Glu-Ser-Pro. Crosslinking of 8H3 antigen on double-negative thymocytes and adult thymocytes, as well as splenic T lymphocytes by 8H3 antibody and F(ab')2 fragments of goat antibodies to mouse immunoglobulin, led to an increase in the concentration of cytoplasmic free Ca2+ due to the release of Ca2+ from intracellular stores as well as the influx of Ca2+ from extracellular sources. Expression of interleukin 2 receptor and subsequently cell proliferation was observed upon incubation of thymocytes and splenic T cells with 8H3 antibody. Furthermore, 8H3 antibody induced the proliferation of double-negative thymocytes. These data collectively indicated that a cell surface antigen, 8H3, is involved in not only cell adhesion but also involved in the expression of immature as well as mature thymocytes.

Regulated expression and binding of three VLA (beta 1) integrin receptors on T cells

Regulated adhesion of T cells to extracellular matrix (ECM) proteins is likely to be essential in T cell migration. Constitutive binding of various other cell types to ECM components is mediated by members of the VLA (very late antigen) subfamily of integrins. We describe here the regulated binding of resting CD4+ human T cells to ECM through three VLA integrins: VLA-4 and VLA-5 binding to fibronectin (FN), and a novel pathway of VLA-6 binding to laminin (LN). Binding to ECM is regulated in two ways. First, unlike other VLA-mediated interactions, VLA binding activity of the T cells is rapidly and dramatically augmented with cell activation without change in level of expression of the VLA molecules. Second, binding is regulated with T-cell differentiation; memory T cells express three- to four-fold more VLA-4, VLA-5, and VLA-6 than do naive cells, and bind more efficiently through them to FN and LN.

Roles of adhesion molecules in T-cell recognition: fundamental similarities between four integrins on resting human T cells (LFA-1, VLA-4, VLA-5, VLA-6) in expression, binding, and costimulation

This review summarizes our recent work on expression and function of 4 integrins on resting human CD4+ T cells. Three themes are highlighted: multiplicity of molecular pathways of adhesion, regulation of adhesion, and costimulation by adhesion molecules. Four distinct receptor/ligand interactions have been elucidated: LFA-1/ICAM-1, VLA-5/fibronection, VLA-4/fibronectin, and VLA-6/laminin. Our studies indicate fundamental similarities in function and regulation of these four receptor/ligand interactions: 1) acute activation of the T cell (by CD3/TCR crosslinking or by PMA) induces rapid but transient integrin binding function; and 2) higher expression of each integrin on memory T cells compared to naive T cells results in greater binding of memory cells to each ligand. The identification of T-cell integrins (VLA-4, VLA-5, VLA-6) which interact with ECM components directs attention to the potential importance of T-cell interactions with ECM components which either may be immobilized in ECM or which may act as molecular bridges between cells. The existence of multiple adhesion pathways, of multiple ligands for a single receptor (such as LFA-1/ICAM-1 and LFA-1/ICAM-2), multiple receptors for a single ligand (such as VLA-4/FN and VLA-5/FN), and regulation of ligand expression (ICAM-1) provides opportunities for co-operativity, rebundancy and diversity which the T cell utilizes to exquisitely regulate its adhesive interactions. The thesis that adhesion molecules can be multifunctional receptors that also influence signalling is demonstrated by our findings that each of these integrin receptor/ligand interactions is capable of providing a potent costimulatory signal to CD3-mediated T-cell activation. The importance of interactions of T-cell integrins with their cell surface and ECM ligands is discussed with respect to T-cell migration, differentiation and recognition. Analysis of the precise mechanisms by which T cells regulate and exploit these multiple receptor/ligand interactions and the resulting functional consequences of those interactions will be exciting areas of future research.

Adhesion of human T-lymphoid cells to fibronectin is mediated by two different fibronectin domains

Lymphocyte adhesion to components of extracellular matrices (i.e. fibronectin) is important for their proper localization in tissues and inflammatory sites. We have studied the attachment of the human cell line HUT-78 (mature T lymphocytes) to fibronectin and to several tryptic fragments of fibronectin. HUT-78 cells effectively adhered to surfaces coated with two Hep II domain-containing fragments of 38,000 and 58,000 MW derived from the A and B chains of fibronectin, respectively. Cells also bound to an 80,000 MW fragment containing the RGDS sequence of fibronectin. Cell adhesion to the 38,000 MW fragment was completely inhibited (100%) by cell preincubation with the soluble 38,000 MW fragment; it was partially inhibited (30-37%) by preincubation with the 58,000 MW fragment or with a synthetic peptide CS-1, comprising the first 25 amino acid residues of the alternatively spliced connecting segment (IIICS), which is present in the A chain of fibronectin and in the 38,000 MW fragment. Cell preincubation with RGDS-containing synthetic peptides or with the 80,000 MW fragment, did not affect attachment to 38,000 MW-coated surfaces. Moreover, preincubation of HUT-78 cells with 38,000 MW fragment had no effect on cell adhesion to 80,000 MW-coated wells, while preincubation with 80,000 MW fragment completely inhibited cell attachment to these surfaces. These results strongly suggest the involvement of two different cell surface receptors which recognize the Hep II/IIICS site and the RGDS site independently. Preincubation with either 38,000 or 80,000 MW fragments prevented cell attachment to fibronectin, indicating that adhesion to the intact molecule requires interaction with both regions. Therefore T-lymphocyte adherence to fibronectin-containing matrices may be regulated by the co-expression of both receptors at the cell surface.

The levels of cytochrome c oxidase subunit II mRNA change during the rat T-cell development

Steady-state levels of the mtRNA encoding the subunit II of the cytochrome c oxidase were determined at several stages of rat T-cell differentiation. Our results showed that its abundance was higher in cells representing the early steps of T-cell development, decreasing in mature T-cells. The possible implications of these findings are discussed.

Tenascin, an extracellular matrix protein, exerts immunomodulatory activities

Tenascin is a nonubiquitous extracellular matrix protein mainly expressed during morphogenesis in embryonal life. In adults it reappears in malignant tumors and during inflammation and tissue repair. Extracellular matrix proteins can alter cell morphology, adhesion, motility, differentiation, and growth. Since cells of the immune system can express receptors for extracellular matrix, we investigated the effects of tenascin on human monocytes and T and B lymphocytes. Tenascin inhibited monocyte adhesion to fibronectin and enhanced the LFA-1 (lymphocyte function-associated antigen 1)-dependent clustering of Epstein-Barr virus-transformed B cells. The physiological consequences of the effects of tenascin were studied in several T-cell activation models. Tenascin inhibited T-cell activation induced by a soluble antigen (tetanus toxoid), alloantigens, or the mitogen concanavalin A. However, T-cell activation with phytohemagglutinin, crosslinked anti-CD3 antibody, or a mixture of ionomycin and phorbol ester was not inhibited by tenascin. Tenascin did not prevent interleukin 2-dependent T-cell growth or the cytolytic activity of an antigen-specific CD4+ T-cell clone. These results suggest that tenascin alters the adhesion properties of human monocytes, B cells, and T cells. The in vitro immunosuppressive activity of tenascin might be due to abrogation of an accessory cell function at an early stage of the interaction between antigen-presenting cells and T cells.

Activation of CD4 cells by fibronectin and anti-CD3 antibody. A synergistic effect mediated by the VLA-5 fibronectin receptor complex

In this study, fibronectin synergized with anti-CD3 antibody to promote CD4 cell proliferation in a serum-free culture system. The cell-adhesive domain plus additional regions of the fibronectin molecule are involved in this synergy. Anti4B4(CDw29) antibody blocked the activation of CD4 cells in this system. Furthermore, it is the VLA-5 protein within the set of molecules recognized by anti-4B4 that serves as a fibronectin receptor on the CD4 lymphocytes. The VLA-5 fibronectin receptor was mainly expressed on CD4+ CD45R-CDw29+ cells and may in part contribute to the unique function of these cells.

The heterogeneity of the reticulum of rat peripheral lymphoid organs identified by monoclonal antibodies

We have developed a panel of six monoclonal antibodies, ED10-ED15, directed against reticular cells in peripheral lymphoid organs. Immunohistochemistry revealed prominent differences between these antibodies with regard to their tissue distribution in lymphoid and non-lymphoid organs. Furthermore, the determinants recognized by ED10-ED13 were found to be differentially expressed by reticular cells occupying the various specialized compartments present in peripheral lymphoid organs. The reactivity patterns of these antibodies observed during the ontogenetic development of the spleen suggest that they recognize differentiation antigens expressed by reticular cells. In contrast, ED14 and ED15 were found to have a relatively ubiquitous tissue distribution recognizing reticular cells in each compartment, with a constitutive reactivity during splenic ontogeny. The present results indicate that reticular cells form a heterogeneous population within the lymphoid organs.

Expression of the fibronectin gene

Fibronectin (FN) is an extracellular matrix protein that acts as a substrate for cell migration and adhesion during development. FN adheres to cells through a dimeric membrane protein, the FN receptor. Antibodies to FN and synthetic peptides that inhibit FN-receptor interaction inhibit gastrulation, block neural crest cell migration, arrest cardiac development, and block the fusion of myoblasts to form myotubes. FN and its receptor also appear to be important for lung development, where their expression coincides with the onset of branching morphogenesis, but drops to barely detectable levels in adult lung, indicating developmental specificity. FN expression is generally low in most adult tissues. However, synthesis is drastically increased during injury and wound healing, a process that in many ways mimics development. FN synthesis is also drastically increased in fibroproliferative lung lesions associated with major architectural changes in the lung. Expression of FN is regulated by a variety of growth factors and hormones. Several of these inducers (cAMP, transforming growth factor-beta, epidermal growth factor, platelet-derived growth factor, glucocorticoids, and vitamin D3) have themselves been implicated in developmental processes, and both cAMP and transforming growth factor-beta are known to stimulate expression of other matrix genes. One role of these hormones and growth factors in development may be to control expression of matrix genes, thereby controlling cell migration and adhesion. In the following report, the effect of hormones and growth factors on expression of the FN gene is reviewed.

Lymphocyte extracellular matrix interactions. Induction of interferon by connective tissue components

Limiting dilution analysis was performed in the presence of interleukin 2 (IL-2) on lymphocytes isolated from the synovial fluid (SF) and peripheral blood (PB) of patients with rheumatoid arthritis (RA) and PB of normal donors. Clones of these 'spontaneously' IL-2-responsive cells from PB and SF were compared for their reactivity with components of the extracellular matrix (i.e. native or denatured type I or type II collagen and proteoglycan). It was determined that all clones from both PB and SF were activated to produce interferon (IFN) in the presence of any of the connective tissue components (CTC). Clones derived from normal PB behaved in a similar fashion but produced lower IFN-gamma levels. There was a synergy between the CTC and serum or plasma fibronectin, which was more apparent when soluble CTC were used as the stimuli rather than immobilized CTC. The fibronectin alone was unable to induce IFN-gamma production under any of the conditions tested (i.e. soluble or immobilized). These results demonstrate that clones of IL-2-responsive T cells can be activated by interactions with connective tissue components to produce IFN-gamma.

Tenascin mediates cell attachment through an RGD-dependent receptor

Tenascin is an extracellular matrix glycoprotein expressed in association with mesenchymal-epithelial interactions during development and in the neovasculature and stroma of undifferentiated tumors. This selective expression of tenascin indicates a specific role in cell matrix interactions. We now show that tenascin can support the adhesion of a variety of cell types, including various human tumor cells, normal fibroblasts, and endothelial cells, all of which can attach to a substrate coated with tenascin. Detailed studies on the mechanism of the tenascin-promoted cell attachment were carried out with the human glioma cell line U251MG. The attachment of these cells and others to tenascin were inhibited specifically by peptides containing the RGD cell attachment signal. Affinity chromatography procedures similar to those that have been used to isolate other adhesion receptors yielded a heterodimeric cell surface protein which bound to a tenascin affinity matrix in an RGD-dependent fashion. One of the subunits of this putative tenascin receptor comigrates with the beta subunit of the fibronectin receptor in SDS-PAGE and cross reacts with antibodies prepared against the fibronectin receptor in immunoblotting. These results identify the tenascin receptor as a member of the fibronectin receptor family within the integrin superfamily of receptors. The cell attachment response on tenascin is distinctly different from that seen on fibronectin, suggesting that cell adhesion and motility may be modulated at those sites where tenascin is expressed in the extracellular matrix.

Preferential expression of fibronectin receptors on immature thymocytes

Fibronectin-adherent (FNR+) thymocytes are enriched for immature (CD4-8-) and large (CD4+8+) cells, and depleted of mature (CD4-8+ and CD4+8-) and nonmature small (CD4+8+) cells. Among purified CD4-8- thymocytes, cells with the surface marker J11d and the IL-2 receptor, which can give rise to all other thymocyte subsets, showed selective attachment to fibronectin. Analysis of FNR+ thymocytes showed that such cells are greatly enriched for cells in cycle. Additionally, FNR+ cells expressed low levels of T cell receptor. These results suggest a role for the fibronectin receptor during the early, proliferative phase of thymocyte differentiation. The data suggest that loss of the fibronectin receptor is a hallmark of cells that have become committed either to functional maturation or to programmed cell death.

Ultrastructural localization of fibronectin in bone marrow of the embryonic chick and its relationship to granulopoiesis

Fibronectin was immunolocated in embryonic chick bone marrow by the use of both a direct peroxidase conjugated antiserum and an indirect Streptavidin bridge technique. Fibronectin is located in the extravascular granulopoietic compartment and, to a lesser extent, in the vascular, erythropoietic compartment. There is no evidence of fibronectin being associated with blood-stromal cell interactions involving either erythropoiesis or thrombopoiesis. However, mature thrombocytes display a substantial surface coat containing fibronectin. Much of the fibronectin appears to be situated on surfaces of those fibroblastic stromal cells which support granulopoiesis. Fibronectin containing extracellular material connects surfaces of developing granulocytes with surfaces of stromal cells. Fibronectin is a surface component of granulocytes as well as nearby stromal cells. However, there appear to be fewer ferritin particles per unit of surface on granulocytic cells. Many of the ferritin particles are not clearly associated with amorphous matrix material at cell surfaces. Immunocytochemical attempts to identify laminin were unsuccessful. These studies indicate that fibronectin is situated at sites where it could mediate adhesive interaction between granulopoietic cells and their stromal cells. Furthermore, cell surface-matrix interaction involving fibronectin could mediate migration of blood cells within the extravascular spaces.

Mitogen stimulation promotes human T lymphocyte adhesion to fibronectin

Purified human peripheral blood T lymphocytes were shown to adhere to growth substrata coated with purified human plasma fibronectin (pFn) and its Mr 120,000-140,000 proteolytic fragments containing the cell-binding site. In contrast, significant binding to laminin- or type I collagen-coated surfaces could not be demonstrated. Binding of T cells to pFn could be inhibited by the synthetic peptide Arg-Gly-Asp-Ser. Activation of T lymphocytes with concanavalin A (Con A) and a phorbol ester, 12-O-tetradecanoyl-phorbol-13-acetate (TPA), led to a higher adherence to pFn than in unstimulated, resting T cells. Activation with only Con A in the presence of accessory cells also promoted binding. Increased adherence of T cells to pFn could be demonstrated as early as 2 h after the onset of stimulation and reached its maximum in 2-3 days. Furthermore, activated but not resting T cells actively spread on pFn-coated surfaces and displayed an altered F-actin organization. In an overlay assay of electrophoretically separated polypeptides of activated T lymphocytes, pFn bound to a high molecular weight polypeptide of Mr 190,000, suggesting that the cells bind to pFn via a receptor-like molecule. Thus, adhesion ot pFn may be a two-stage process. At the first stage cells bind to Fn. Activated T cells proceed to the second stage, where cells begin to spread on pFn. This may be due to an altered relationship between Fn receptors and microfilaments.

New perspectives in cell adhesion: RGD and integrins

Rapid progress has been made in the understanding of the molecular interactions that result in cell adhesion. Many adhesive proteins present in extracellular matrices and in the blood contain the tripeptide arginine-glycine-aspartic acid (RGD) as their cell recognition site. These proteins include fibronectin, vitronectin, osteopontin, collagens, thrombospondin, fibrinogen, and von Willebrand factor. The RGD sequences of each of the adhesive proteins are recognized by at least one member of a family of structurally related receptors, integrins, which are heterodimeric proteins with two membrane-spanning subunits. Some of these receptors bind to the RGD sequence of a single adhesion protein only, whereas others recognize groups of them. The conformation of the RGD sequence in the individual proteins may be critical to this recognition specificity. On the cytoplasmic side of the plasma membrane, the receptors connect the extracellular matrix to the cytoskeleton. More than ten proved or suspected RGD-containing adhesion-promoting proteins have already been identified, and the integrin family includes at least as many receptors recognizing these proteins. Together, the adhesion proteins and their receptors constitute a versatile recognition system providing cells with anchorage, traction for migration, and signals for polarity, position, differentiation, and possibly growth.

Identification of fibronectin receptors on T lymphocytes

We report the identification of fibronectin receptors on thymocytes and T lymphoma cells. Affinity chromatography of extracts of the T cell lymphoma, WR16.1, on a fibronectin-Sepharose column combined with specific elution using a synthetic peptide containing the cell attachment-promoting sequence, arginine-glycine-aspartic acid, yielded two polypeptide components having apparent molecular masses of approximately 160 kD reduced and 175 and 150 kD nonreduced. Immunoprecipitations from surface-iodinated WR16.1 cells or fibronectin-adherent thymocytes using a rabbit antiserum raised against the fibronectin receptor that is present on human fibroblasts revealed, in each case, the same two radiolabeled components. In contrast, immunoprecipitation from fibronectin-nonadherent T lymphoma cells, designated WR2.3, revealed the presence of only the smaller subunit. Although the lymphocyte receptor and the fibronectin receptor identified on fibroblasts share immunologic determinants, they differ in that the molecular mass of the lymphocyte protein is larger. Moreover, trypsinization of either thymocytes or the WR16.1 T lymphoma cells resulted in a subsequent loss of their ability to adhere to fibronectin-coated substrates and a reduction in the electrophoretic mobility of each of the polypeptide chains of the fibronectin receptor present on their surfaces. These changes, however, were not observed with normal rat kidney fibroblasts or mouse 3T3 fibroblasts in response to trypsinization. The data establish the existence on normal lymphocytes of fibronectin receptors that are quite similar to those found on fibroblasts. The possible function of this molecule on thymocytes is discussed.