Soluble thrombospondin-1 suppresses T cell proliferation and enhances IL-10 secretion by antigen presenting cells stimulated with phytohemagglutinin

Tolerating CD47

Cluster of differentiation 47 (CD47) occupies the outer membrane of human cells, where it binds to soluble and cell surface receptors on the same and other cells, sculpting their topography and resulting in a pleiotropic receptor-multiligand interaction network. It is a focus of drug development to temper and accentuate CD47-driven immune cell liaisons, although consideration of on-target CD47 effects remain neglected. And yet, a late clinical trial of a CD47-blocking antibody was discontinued, existent trials were restrained, and development of CD47-targeting agents halted by some pharmaceutical companies. At this point, if CD47 can be exploited for clinical advantage remains to be determined. Herein an airing is made of the seemingly conflicting actions of CD47 that reflect its position as a junction connecting receptors and signalling pathways that impact numerous human cell types. Prospects of CD47 boosting and blocking are considered along with potential therapeutic implications for autoimmune diseases and cancer.

Tolerogenic dendritic cells generated with different immunosuppressive cytokines induce antigen-specific anergy and regulatory properties in memory CD4+ T cells

Dendritic cells (DCs) are professional APCs involved in the initiation of both immunity and immunological tolerance. In autoimmune diseases or graft rejections, most reactive lymphocytes are effector/memory cells. It is believed that memory T cells are more resistant to tolerance induction than naive lymphocytes; however, studies on mechanisms for their efficient tolerization are still scarce. In this study, we generated human monocyte-derived DCs by culture with GM-CSF and IL-4 (control DCs), as well as tolerogenic DCs (tDCs) by adding IL-10, IL-10/TGF-beta1, or IL-10/IL-6. Cells were maturated with TNF-alpha/PGE(2). Compared with control DCs, tDCs had similar expression of HLA-DR, CD80, and CD86, lower expression of CD40, higher levels of macrophage markers, enhanced endocytic ability, increased secretion of IL-6, IL-10 (only tDCs generated with IL-10 and tDCs generated with IL-10/IL-6), and PGE(2), and lower secretion of IL-12 and IL-23. In vitro, tDCs had the capacity to induce anergy in tetanus toxoid-specific memory CD4(+) T cells, whereas the proliferative response to an unrelated Ag was intact. Anergy could be reverted upon exposure to IL-2. tDC-primed T cells have low suppressive ability. Nevertheless, the generation of both anergic and regulatory T cells was more efficient with tDCs generated with IL-10/TGF-beta1. Microarray-based gene expression profiling reflected modulated expression of several transcripts in tDCs. Surface CLIP-HLA-DR complexes and intracellular thrombospondin-1 were increased in the three tDCs. CD39 was highly expressed only in tDC-TGF, which correlated with increased adenosine production. We propose that these molecules, together with IL-10 and prostanoids, are key factors to induce Ag-specific tolerance in memory T cells.

Immunological tolerance-related genes in a spontaneous tolerant model of rat liver transplantation explored by suppression subtractive hybridization

Natural immunological tolerance can be induced in certain types of allogeneic liver transplantation in rats. To screen for genes associated with the induction of tolerance, suppression subtractive hybridization was performed in the rat liver transplantation model between a DA donor and PVG recipient combination where spontaneous immunological tolerance is known to occur without any immunosuppressive treatment. As a result, 112 genes were cloned from a DA liver graft that survived for 20 days in the fully allogeneic PVG recipient. After confirmation of the expression intensity using an in-house manufactured DNA array with cDNAs from the DA graft, 36 genes were classified in the highly expressed group and 26 moderately expressed group. In the first group, there were 8 immunoglobulin-related genes and 6 MHC class II-related genes, suggesting the existence of an underlying rejection response. Among those genes, an antiapoptotic gene in the p38 MAP kinase pathway, heme oxygenase gene (HO-1), and a ras cascade gene, IQ motif containing GTPase activating protein 1 (Iqgapl), retained biological significance. The results suggested that the molecular response to a liver graft tends to be antiapoptotic and to terminate the rejection response. Unfortunately, there was no gene identified that qualified as a putative immunosuppressive protein, liver suppressor factor-1 (LSF-1). The panel of genes identified in the present work will be a useful panel of candidate genes to investigate the induction of spontaneous tolerance.

Thrombospondin orchestrates the tolerance-promoting properties of TGFβ-treated antigen-presenting cells

Eye-derived antigen-presenting cells (APCs) are known to contribute to the immune privilege status of the eye by inducing a form of peripheral tolerance that deviates T(h)1 type of pro-inflammatory immune responses. Similar systemic tolerance can also be induced by non-ocular APCs exposed to transforming growth factor beta (TGFbeta) in vitro. Such APCs were found to express enhanced levels of thrombospondin (TSP)-1, an extracellular matrix (ECM) protein. In this report, we analyzed the significance of TSP-1 in conferring tolerance-inducing properties on APCs. While TSP-treated APCs matched TGFbeta-treated APCs in their functional ability to induce systemic tolerance, a deficiency of TSP-1 or its receptor CD36 prevented APCs from becoming tolerogenic in response to TGFbeta. Exogenous TSP-1 restored tolerogenic ability of TGFbeta-treated TSP-1 null APCs. Both TGFbeta-treated TSP-1 null and CD36 knockout APCs failed to inhibit IL-12 secretion. Furthermore, TGFbeta-treated TSP-1 null APCs, unlike similarly treated wild-type APCs, failed to increase secretion of active TGFbeta. Similar to TGFbeta, TSP could also up-regulate expression of MIP-2, TGFbeta2 and tumor necrosis factor alpha-all of which are required for tolerance induced by TGFbeta-treated APCs. We conclude that TSP-1, an ECM protein induced by TGFbeta treatment, orchestrates the changes in APC functional programs that equip these cells to promote tolerance of the eye-derived type.

Murine mammary carcinoma exosomes promote tumor growth by suppression of NK cell function

Many tumor cells shed specialized membrane vesicles known as exosomes. In this study, we show that pretreatment of mice with exosomes produced by TS/A or 4T.1 murine mammary tumor cells resulted in accelerated growth of implanted tumor cells in both syngeneic BALB/c mice and nude mice. As implanted TS/A tumor cells grew more rapidly in mice that had been depleted of NK cells, we analyzed the effects of the tumor-derived exosomes on NK cells. The tumor-derived exosomes inhibit NK cell cytotoxic activity ex vivo and in vitro as demonstrated by chromium release assays. The treatment of mice with TS/A tumor exosomes also led to a reduction in the percentages of NK cells, as determined by FACS analysis, in the lungs and spleens. Key features of NK cell activity were inhibited, including release of perforin but not granzyme B, as well as the expression of cyclin D3 and activation of the Jak3-mediated pathways. Human tumor cell lines also were found to produce exosomes that were capable of inhibiting IL-2-stimulated NK cell proliferation. Exosomes produced by dendritic cells or B cells did not. The presentation of tumor Ags by exosomes is under consideration as a cancer vaccine strategy; however, we found that pretreatment of mice with tumor exosomes blunted the protective effect of syngeneic dendritic cells pulsed ex vivo with tumor exosomes. We propose that tumor exosomes contribute to the growth of tumors by blocking IL-2-mediated activation of NK cells and their cytotoxic response to tumor cells.

Thrombospondin 2 functions as an endogenous regulator of angiogenesis and inflammation in rheumatoid arthritis

Thrombospondin 2 (TSP2), a matricellular protein with a primary role in modulating cell-matrix interactions, has been implicated in tissue repair and foreign body responses. Here we show that TSP2 has regulatory function in the chronic inflammatory lesions of rheumatoid arthritis. Tissue TSP2, produced by synovial fibroblasts, endothelial cells, and macrophages correlated not only with the intensity of angiogenesis but also with the architecture of lymphoid infiltrates. Synovial tissues with diffuse inflammatory infiltrates had high levels of TSP2, whereas synovial tissues with ectopic germinal center reactions and T cell-B cell aggregates produced low levels. Cell-based gene therapy with TSP2 was used to examine the in vivo effects of the matrix protein on neoangiogenesis and lymphoid organization. Human synovium-severe combined immunodeficiency (SCID) mouse chimeras were treated with TSP2-transfected fibroblasts deposited into the peritoneum. Overexpression of TSP2 led to the accumulation of TSP2 protein in the inflamed synovium and resulted in a prompt inhibition of lesional vascularization. Beside its anti-angiogenic activity, TSP2 also suppressed the production of the proinflammatory mediators, interferon-gamma and tumor necrosis factor-alpha, and induced the depletion of tissue-residing T cells. We propose that TSP2 is an endogenous regulator of angiogenesis and autoimmune inflammation in the synovium and represents a protective mechanism preventing ectopic lympho-organogenesis and persistent inflammation in this tissue site.

Functional regulation of T lymphocytes by modulatory extracellular matrix proteins

In addition to the major structural molecules, which are constitutively present in extracellular matrices, several proteins appear in the extracellular matrix only at specific stages in development or in association with specific pathological conditions. These proteins include thrombospondin-1 and -2, tenascin C, osteopontin, members of the cysteine-rich 61/connective tissue growth factor/nephroblastoma overexpressed family, and secreted protein acidic and rich in cysteine (osteonectin). These proteins play important roles in regulating cell fate during development and in the pathogenesis of several diseases in adult animals. We will review the interactions of T cells with this class of molecules and their resulting effects on T cell behavior. Receptors and signal transduction pathways that mediate the actions of matricellular proteins on T cells are beginning to be defined. Transgenic mice are providing new insights into the functions of these proteins in vivo and are yielding insights into the significance of their reported dysregulation in several human diseases.

The fibroblast: sentinel cell and local immune modulator in tumor tissue

Development and progression of epithelial malignancies are frequently accompanied by complex phenotypic alterations of resident tissue fibroblasts. Some of these changes, such as myofibroblastic differentiation and an oncofetal extracellular matrix (ECM) expression profile, are also implicated in inflammation and tissue repair. Studies over the past decade revealed the relevance of reciprocal interactions between tumor cells and tumor-associated host fibroblasts (TAF) in the malignant process. In many tumors, a considerable fraction of the inflammatory infiltrate is located within the fibroblast- and ECM-rich stromal compartment. However, while fibroblasts are known as "sentinel cells" in various nonneoplastic diseases, where they often regulate the composition and function of recruited leucocytes, they are hardly considered active participants in the inflammatory host response in tumors. This article focuses on the functional impact of TAF on immune cells. The complex network of immune-modulating effects transduced by TAF and TAF-derived factors is highlighted, and recent reports that support the hypothesis that TAF are involved in the inflammatory response and immune suppression in tumors are reviewed. The role of TAF-dependent ECM remodeling and TAF-derived peptide growth factors, cytokines, and chemokines in the immune modulation is stressed and the idea of TAF as an important therapeutic target is emphasized.

Opposite effects of thrombospondin-1 via CD36 and CD47 on homotypic aggregation of monocytic cells

Thrombospondin-1 (TSP-1), an extracellular matrix protein, has a multimodular structure and each domain specifies a distinct biological function through interaction with a specific ligand. In this study we found that exogenously added TSP-1 inhibits phorbol myristate acetate (PMA)/LPS-induced homotypic aggregation of human monocytic U937 cells, whereas the 70-kDa fragment of TSP-1 generated by the proteolytic cleavage of the intact molecule promotes the homotypic aggregation. The aggregation was also inhibited by anti-CD47 mAb or the 4N1K peptide, of which sequence is derived from the CD47-binding site of TSP-1 and absent in the 70-kDa fragment. In contrast, the augmented cell aggregation by the 70-kDa fragment was hampered by anti-CD36 mAb or antibody against the CD36-binding site of TSP-1. The cell aggregation of U937 cells was completely blocked, even in the presence of the 70-kDa fragment, by mAb against leukocyte function associated antigen-1 (LFA-1) or intercellular adhesion molecule-1 (ICAM-1). We therefore propose that TSP-1 may regulate LFA-1/ICAM-1-mediated cell adhesion of monocytes/macrophages by either the inhibitory effect through CD47 or the promoting effect through CD36 depending on which domain/fragment is functional in a given biological setting.

Interactions of thrombospondins with alpha4beta1 integrin and CD47 differentially modulate T cell behavior

Thrombospondin (TSP)-1 has been reported to modulate T cell behavior both positively and negatively. We found that these opposing responses arise from interactions of TSP1 with two different T cell receptors. The integrin alpha4beta1 recognizes an LDVP sequence in the NH2-terminal domain of TSP1 and was required for stimulation of T cell adhesion, chemotaxis, and matrix metalloproteinase gene expression by TSP1. Recognition of TSP1 by T cells depended on the activation state of alpha4beta1 integrin, and TSP1 inhibited interaction of activated alpha4beta1 integrin on T cells with its counter receptor vascular cell adhesion molecule-1. The alpha4beta1 integrin recognition site is conserved in TSP2. A recombinant piece of TSP2 containing this sequence replicated the alpha4beta1 integrin-dependent activities of TSP1. The beta1 integrin recognition sites in TSP1, however, were neither necessary nor sufficient for inhibition of T cell proliferation and T cell antigen receptor signaling by TSP1. A second TSP1 receptor, CD47, was not required for some stimulatory responses to TSP1 but played a significant role in its T cell antigen receptor antagonist and antiproliferative activities. Modulating the relative expression or function of these two TSP receptors could therefore alter the direction or magnitude of T cell responses to TSPs.

Thrombospondin-1 differentially regulates release of IL-6 and IL-10 by human monocytic cell line U937

We investigated possible regulatory effects of thrombospondin-1 (TSP-1), a multifunctional extracellular matrix protein, on cytokine release from macrophages. Immobilized TSP-1 enhanced IL-6 release from the human monocytic U937 cells stimulated with phorbol myristate acetate and LPS, whereas it inhibited IL-10 release. The 70-kDa fragment of TSP-1 containing the type 1 repeats showed the same regulatory effects. The enhanced IL-6 release by TSP-1 was inhibited by anti-CD36 antibody or antibody against the sequence of the binding site to CD36 in the type 1 repeats of TSP-1. Conversely, the decrease in IL-10 release by TSP-1 was strengthened by the blocking of the interaction between CD36 and TSP-1. Furthermore, the involvement of TGF-beta1 in the inhibition of IL-10 release by TSP-1 was indicated by the facts that (i) TSP-1 induced activation of TGF-beta1 produced by the U937 cells, (ii) exogenously added TGF-beta1 inhibited IL-10 release, and (iii) antibody against TGF-beta1 blocked the inhibition of IL-10 release by TSP-1. Together, the present findings suggest that TSP-1 enhances IL-6 release from macrophages by interaction with CD36, whereas IL-10 release is regulated by the balance between the enhancing effect of TSP-1 via CD36 and the suppressive effect by TSP-1-activated TGF-beta1.