ONLY CORE PROTEIN
Thrombomodulin controls the complement arm of the innate immune system in a thrombin-dependent manner through activation of the TAFI that binds the EGF-like repeats 3-6 domains of thrombomodulin core protein. An intact thrombin-thrombomodulin complex is required for TAFI to suppress complement activation [Anastasiou et al., 2012; Koutsi et al., 2007]. When associated with
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thrombomodulin, the pro-inflammatory properties of thrombin are abrogated and in this way thrombomodulin become an indirect anti-inflammatory molecule. Instead the N-terminal C-type lectin-like domain of thrombomodulin has direct anti-inflammatory properties by mediating signals that interfere with MAPK and NFkB pathway [Anastasiou et al., 2012; Koutsi et al., 2007]. The complement control may happen also in a thrombin-independent constitutive manner via thrombomodulin lectin-like extracellular domain [Anastasiou et al., 2012].
PRELP play a foundamental role in immunological disease as modulator of the immune response. For example in rheumatoid arthritis, the cartilage tissue is destroyed and fragmented molecules, including PRELP, are released into the synovial fluid where they may interact with components of the complement system. In different studies PRELP is found to interact with the complement inhibitor C4-BP, which is suggested to locally down-regulate complement activation in joints during rheumatoid arthritis. It is shown that PRELP directly inhibits all pathways of complement by binding C9 and thereby prevents the formation of the membrane attack complex.
PRELP does not interfere with the interaction between C9 and already formed C5-8, but inhibits C9 polymerization thereby preventing formation of the lytic pore. The alternative pathway is moreover inhibited already at the level of C3-convertase formation due to an interaction between PRELP and C3. This suggests that PRELP may downregulate complement attack at basement membranes and on damaged cartilage and it therefore limit pathological complement activation in inflammatory disease such as rheumatoid arthritis.
ONLY GAG CHAINS
After the generation of serglycin knockout mouse models it become easy to study the role of this PG in immune system and, in particular, in the process of granulopoiesis. In the knockout model, the generation of secretory granules in mast cells shows to be compromised and there is a reduction of several important proteases, histamine and serotonin [Kolset and Tveit, 2008, Kolset and Pejler, 2011]. It is clear that also secretory granules of cytotoxic T lymphocytes, mast cells, neutrophils and platelets are compromised in knockout mice [Kolset and Tveit, 2008]. It is important to underline that granzyme A and B are not affected by the lack of serglycin, more precisely the storage of granzyme B but not A is affected rather than protease mRNA expression.
The same effect showed in serglycin knockout mice is seen also in NDST-2 knockout mice that lack the enzymes involved in GAG chains production. For this reason is possible to conclude that all the effect of serglycin on the granule storage may be attributed to GAG chains presence and binding [Kolset and Tveit, 2008; Kolset and Pejler, 2011]. In neutrophils only one type of granules, the azurophil ones, is affected in knockout models. Studies on Gram-negative bacteria infection of
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serglycin knockout mice and control mice, show that the first one model seems to be more susceptible to the infection, because the lack of serglycin may affect immune defence [Kolset et Tveit, 2008]. Serglycin is shown to bind some proteases, like tryptases, forming tetramers complex, and it covalently binds also MMP9 as it is demonstrated in monocyte-like THP-1 cells [Kolset and Tveit, 2008]. Interacting with them, serglycin is involved in the retention of proteases in storage granules in mast cells. Not all the granule components depend on serglycin presence, in fact granzyme A and cathepsin G are not affected by the lack of this PG [Kolset and Tveit, 2008].
Complexes of serglycin and proteases are released from mast cells and serglycin is implicated in the secretion of different granule molecules, like TNF-α from macrophages, MMP9 from monocytes or chemokines from HIV-infected T cells. So it is possible to conclude that serglycin is essential for the regulation of the secretion of an impressive repertoire of molecules important in inflammatory reactions [Kolset and Tveit, 2008]. The secretion of the serglycin-ligand complexes is linked to the cell activation. After secretion, the change in microenvironment pH conditions causes the release of serglycin partner molecules [Kolset and Tveit, 2008; Kolset and Pejler, 2011]. So it is clear that serglycin has also important roles after immune cell secretion. In particular its heparin chains are important for the protection of enzymes inactivation because protease inhibitors are not able to bind heparin. Moreover heparin chains may mediate encounter between proteases and their substrates and are involved in activation and processing of proteases. Serglycin is also necessary to protect its partner molecules during their transport, for example proteases are only released to perform its function when the complex has reached its final destination [Kolset and Tveit, 2008; Kolset and Pejler, 2011]. Thanks to studies on serglycin knockout mice and NDST-2 knockout mice, it is clear that GAG chains linked to serglycin core protein are cell type-dependent. In particular in connective tissue-type mast cells serglycin bind prevalently heparin chains and this is the only cell type capable to synthesize the highly sulfated GAG heparin whereas a multitude of other cells like macrophages synthesize HS. The mucosal mast cells subtype produce serglycin binding CS chains, carrying O-sulfate groups at the 4- and 6-position. In cell type other than mast cells, for example in T lymphocytes, lower sulphated GAG species of CS type are predominantly attached to serglycin [Kolset and Pejler, 2011]. The composition of serglycin structure and the GAG chain type attached may depend on immunological signals. In fact it is shown that in eosinophils serglycin carries prevalently CS-4 sulfated chains after stimulation of cells with IL-3 or GM-CSF [Kolset and Pejler, 2011]. Serglycin is involved in granules control in most hematopoietic malignancies where its aggregation with the partner molecules may prevent extensive diffusion outside inflamed tissues. In human multiple myeloma is demonstrated an high cell surface localization of serglycin that binds cell surface by its GAG chains [Kolset and Tveit, 2008; Kolset and Pejler, 2011]. Myeloma-derived
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serglycin interfered with bone mineralization providing a possible explanation for osteoporosis commonly in multiple myeloma patients. Serglycin is highly produced in acute myeloid leukemia but not in acute lymphoblastic leukemia [Kolset and Pejler, 2011].
Syndecans are able to bind most chemokines and participate in immune responses. In particular syndecans 1 is directly involved in inflammation as suggested by increased leukocyte-endothelial interactions in syndecans 1 null mice. It also has an important role in forming complex with IL-8, that control syndecans 1 shedding, and in chemokine gradient formation for trans-endothelial and trans-epithelial migration of neutrophils [Tkachenko et al., 2005]. For example, syndecan-1 binds through its heparan sulfate chains to the CC chemokines CCL7, CCL11, and CCL17 implicated in allergic diseases, resulting in inhibition of CC chemokine mediated T cell migration and suppression of allergen induced accumulation of Th2 cells in the lung [Choi et al., 2011]. Syndecan 1 and syndecan 4 bind the chemokine RANTES on monocyte derived macrophages, facilitating the subsequent interaction of RANTES with its receptor CCR5. The interaction of syndecans with chemokines play a pivotal role in cell survival. For example SDF-1 associates with chemokine (C-X-C motif) receptor 4 (CXCR4) and syndecan 4 at the plasma membrane of Huh7 cells, inducing the growth of Huh7 cells by promoting their entry into the cell cycle and inhibiting tumor necrosis factor-α-mediated apoptosis [Choi et al., 2011]. Syndecans possess the unique ability to interact with pathogens. All members of the syndecan family possess similar capacities to interact with HIV-1. Enzymatic removal of the heparan sulfate chains abrogates the ability of syndecans to capture HIV-1, indicating the importance of the linear anionic heparan sulfate chains for HIV-1 binding [Choi et al., 2011]. Like HIV-1, a great number of other pathogens use syndecans as attachment receptors to facilitate either entry into host cells or persistence in the hostile host environment [Choi et al., 2011].
COOPERATION
T-cell activation during immune response not only requires the recognition of antigen-loaded major histocompatibility complex by the T-cell receptor, but also the interaction of additional surface receptors expressed by T-cells with their corresponding counter-receptors on APC cells. Agrin is one of these important additional receptors and it tooks part in the T-cell-APC cells interaction, inducing the cluster of the surface receptors [Bezakova et al., 2003; Jury and Kabouridis, 2010]. Its splicing at the amino N-terminus produces either secreted or membrane-anchored proteins, whereas splicing at two additional sites closer to the carboxy C-terminal, named y and z in the mammalian system, produces proteins that either lack or include two small exons. On the T-cells only the transmembrane agrin isoform is expressed, it accumulates to distinct areas of
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the cell membrane and T-cell activation causes its redistribution post-translationally modification to a form with an increased ability to aggregate receptors on T-cells. Although if the nature of this modification is unknown, it is shown that glycosylated agrin is dominant in resting lymphocytes and deglycosylation form is present in activated lymphocytes [Bezakova et al., 2003; Jury and Kabouridis, 2010].
Collagen type IX NC4 domain binds both C4-BP and factor H. C4-BP is the major soluble inhibitor of the classical and lectin pathway for the complement activation; factor H inhibits the alternative route. Binding these two molecules, collagen type IX play an important role in degrading C4b and C3b activating complement factors slowing down the complement activation process [Kalchishkova et al., 2011]; NC4 domain can inactivate complement directly inhibiting C9 polymerization and membrane attack complex formation [Kalchishkova et al., 2011].
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