Kaisa E. Happonen

Regulation of complement by cartilage oligomeric matrix protein allows for a novel molecular diagnostic principle in rheumatoid arthritis.

OBJECTIVE Cartilage oligomeric matrix protein (COMP) is a structural component of cartilage, where it catalyzes collagen fibrillogenesis. Elevated amounts of COMP are found in serum during increased turnover of cartilage associated with active joint disease, such as rheumatoid arthritis (RA) and osteoarthritis (OA). This study was undertaken to investigate the ability of COMP to regulate complement, a capacity that has previously been shown for some other cartilage proteins. METHODS Regulation of complement by COMP was studied using functional in vitro assays. Inter-actions between complement proteins and COMP were investigated by direct binding assay and electron microscopy. Circulating COMP and COMP-C3b complexes in serum and synovial fluid from RA and OA patients and healthy controls were measured with a novel enzyme-linked immunosorbent assay. RESULTS We found in vivo evidence of complement activation by released COMP in the general circulation of patients with RA, but not patients with OA. COMP induced activation and deposition of C3b and C9 specifically via the alternative pathway of complement, which was attributable to direct interaction between COMP and properdin. Furthermore, COMP inhibited the classical and the lectin complement pathways due to direct interaction with the stalk region of C1q and mannose-binding lectin, respectively. CONCLUSION COMP is the first extracellular matrix protein for which an active role in inflammation has been demonstrated in vivo. It can activate one complement pathway at the same time as it has the potential to inhibit another. The net outcome of these interactions is most likely determined by the type of released COMP fragments, which may be disease specific.

Complement inhibitor C4b-binding protein interacts directly with small glycoproteins of the extracellular matrix.

Components derived from cartilage have been suggested to maintain the inflammation in joints in arthritis. Small leucine-rich repeat proteins (SLRPs) are structural components of cartilage important in organizing the meshwork of extracellular matrix components. It has recently been shown that the SLRP fibromodulin interacts with complement initiator C1q, leading to complement activation. The complement response is limited since fibromodulin also interacts with the complement inhibitor factor H. We have now found that osteoadherin, chondroadherin, fibromodulin, and proline arginine-rich end leucine-rich repeat protein bind to the complement inhibitor C4b-binding protein (C4BP). Using direct binding assays with C4BP fragments and C4BP mutants lacking individual domains in combination with electron microscopy, we have demonstrated that mainly the central core of C4BP mediated binding to SLRPs. Binding of SLRPs to C4BP did not affect its ability to inhibit complement. Osteoadherin, fibromodulin, and chondroadherin, which bind C1q and activate complement, were found to cause significantly higher C9 deposition in C4BP-depleted serum compared with Igs, indicating that the level of complement activation initiated by SLRPs is regulated by simultaneous binding to C4BP. A similar dual binding of C1q and complement inhibitors was observed previously for other endogenous ligands (amyloid, prions, C-reactive protein, and apoptotic cells) but not for exogenous activators (bacteria-bound Igs). These interactions can be significant during inflammatory joint diseases, such as rheumatoid arthritis, where cartilage is degraded, and cartilage components are released into synovial fluid, where they can interact with factors of the complement system.

Interactions of the complement system with molecules of extracellular matrix: Relevance for joint diseases.

Rheumatoid arthritis (RA) is a highly disabling disease affecting all structures of the joint. Understanding the pathology behind the development of RA is essential for developing targeted therapeutic strategies as well as for developing novel markers to predict disease onset. Several molecules normally hidden within the cartilage tissue are exposed to complement components in the synovial fluid upon cartilage breakdown. Some of these have been shown to activate complement and toll-like receptors, which may enhance an already existing inflammatory response, thereby worsening the course of disease. Other cartilage-resident molecules have in contrast shown to possess complement-inhibitory properties. Knowledge about mechanisms behind pathological complement activation in the joints will hopefully lead to methods which allow us to distinguish patients with pathological complement activation from those where other inflammatory pathways are predominant. This will help to elucidate which patients will benefit from complement inhibitory therapies, which are thought to aid a specific subset of patients or patients at a certain stage of disease. Future challenges are to target the complement inhibition specifically to the joints to minimize systemic complement blockade.

Serglycin is implicated in the promotion of aggressive phenotype of breast cancer cells.

Serglycin is a proteoglycan expressed by some malignant cells. It promotes metastasis and protects some tumor cells from complement system attack. In the present study, we show for the first time the in situ expression of serglycin by breast cancer cells by immunohistochemistry in patients’ material. Moreover, we demonstrate high expression and constitutive secretion of serglycin in the aggressive MDA-MB-231 breast cancer cell line. Serglycin exhibited a strong cytoplasmic staining in these cells, observable at the cell periphery in a thread of filaments near the cell membrane, but also in filopodia-like structures. Serglycin was purified from conditioned medium of MDA-MB-231 cells, and represented the major proteoglycan secreted by these cells, having a molecular size of ∼250 kDa and carrying chondroitin sulfate side chains, mainly composed of 4-sulfated (∼87%), 6-sulfated (∼10%) and non-sulfated (∼3%) disaccharides. Purified serglycin inhibited early steps of both the classical and the lectin pathways of complement by binding to C1q and mannose-binding lectin. Stable expression of serglycin in less aggressive MCF-7 breast cancer cells induced their proliferation, anchorage-independent growth, migration and invasion. Interestingly, over-expression of serglycin lacking the glycosaminoglycan attachment sites failed to promote these cellular functions, suggesting that glycanation of serglycin is a pre-requisite for its oncogenic properties. Our findings suggest that serglycin promotes a more aggressive cancer cell phenotype and may protect breast cancer cells from complement attack supporting their survival and expansion.

Serglycin inhibits the classical and lectin pathways of complement via its glycosaminoglycan chains: Implications for multiple myeloma

Serglycin (SG) is a proteoglycan expressed by hematopoietic cells and is constitutively secreted by multiple myeloma (MM) cells. SG participates in the regulation of various inflammatory events. We found that SG secreted by human MM cell lines inhibits both the classical and lectin pathways of complement, without influencing alternative pathway activity. The inhibitory effect of SG is due to direct interactions with C1q and mannose‐binding lectin (MBL). C1q‐binding is mediated through the glycosaminoglycan moieties of SG, whereas MBL binds additionally to SG protein core. Interactions between SG and C1q as well as MBL are diminished in the presence of chondroitin sulfate type E. In addition, we localized the SG‐binding site to the collagen‐like stalk of C1q. Interactions between SG and C1q as well as MBL are ionic in character and only the interaction with MBL was found to be partially dependent on the presence of calcium. We found the serum levels of SG to be elevated in patients with MM compared to healthy controls. Moreover, we found that SG expressed from myeloma plasma cells protects these cells from complement activation induced by treatment with anti‐thymocyte immunoglobulins. This might protect myeloma cells during immunotherapy and promote survival of malignant cells.

PRELP Protein Inhibits the Formation of the Complement Membrane Attack Complex

Background: PRELP binds the complement inhibitor C4b-binding protein and may have other complement regulatory functions. Results: PRELP inhibits the formation of the membrane attack complex and thereby inhibits all three pathways of complement. Conclusion: PRELP regulates complement responses at several levels of the cascade. Significance: PRELP may act as a local complement inhibitor at basement membranes or at sites with exposed cartilage. PRELP is a 58-kDa proteoglycan found in a variety of extracellular matrices, including cartilage and at several basement membranes. In rheumatoid arthritis (RA), 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 a previous study, PRELP was found to interact with the complement inhibitor C4b-binding protein, which was suggested to locally down-regulate complement activation in joints during RA. Here we show that PRELP directly inhibits all pathways of complement by binding C9 and thereby prevents the formation of the membrane attack complex (MAC). PRELP does not interfere with the interaction between C9 and already formed C5b-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 down-regulate complement attack at basement membranes and on damaged cartilage and therefore limit pathological complement activation in inflammatory disease such as RA. The net outcome of PRELP-mediated complement inhibition will highly depend on the local concentration of other complement modulating molecules as well as on the local concentration of available complement proteins.

Serum COMP-C3b complexes in rheumatic diseases and relation to anti-TNF-α treatment

IntroductionCartilage oligomeric matrix protein (COMP) is found at elevated concentrations in sera of patients with joint diseases such as rheumatoid arthritis (RA) and osteoarthritis (OA). We recently showed that COMP activates complement via the alternative pathway and that COMP-C3b complexes are present in sera of RA patients, but not in healthy controls. We now set out to elaborate on the information provided by this marker in a variety of diseases and larger patient cohorts.MethodsCOMP-C3b levels in sera were measured by using an enzyme-linked immunosorbent assay (ELISA) capturing COMP and detecting C3b. Serum COMP was measured by using ELISA.ResultsCOMP-C3b levels were significantly elevated in patients with RA as well as in systemic lupus erythematosus (SLE), compared with healthy controls. SLE patients with arthritis had significantly higher COMP-C3b levels than did those without. COMP-C3b was furthermore elevated in patients with ankylosing spondylitis (AS), psoriatic arthritis (PsA), reactive arthritis, systemic sclerosis, and OA. COMP-C3b did not correlate with COMP in any of the patient groups. COMP-C3b correlated with disease activity in RA, but not in other diseases. COMP-C3b levels in RA patients decreased on treatment with tumor necrosis factor (TNF)-α inhibitors, whereas the levels increased in patients with AS or PsA. The changes of COMP-C3b did not parallel the changes of C-reactive protein (CRP).ConclusionsCOMP-C3b levels are elevated in several rheumatologic diseases and correlate with inflammatory measures in RA. COMP-C3b levels in RA decrease during TNF-α inhibition differently from those of CRP, suggesting that formation of COMP-C3b relates to disease features not reflected by general inflammation measures.

The Gas6-Axl Interaction Mediates Endothelial Uptake of Platelet Microparticles

Upon activation, platelets release plasma membrane-derived microparticles (PMPs) exposing phosphatidylserine on their surface. The functions and clearance mechanism of these microparticles are incompletely understood. As they are pro-coagulant and potentially pro-inflammatory, rapid clearance from the circulation is essential for prevention of thrombotic diseases. The tyrosine kinase receptors Tyro3, Axl, and Mer (TAMs) and their ligands protein S and Gas6 are involved in the uptake of phosphatidylserine-exposing apoptotic cells in macrophages and dendritic cells. Both TAMs and their ligands are expressed in the vasculature, the functional significance of which is poorly understood. In this study, we investigated how vascular TAMs and their ligands may mediate endothelial uptake of PMPs. PMPs, generated from purified human platelets, were isolated by ultracentrifugation and labeled with biotin or PKH67. The uptake of labeled microparticles in the presence of protein S and Gas6 in human aortic endothelial cells and human umbilical vein endothelial cells was monitored by flow cytometry, Western blotting, and confocal/electron microscopy. We found that both endothelial cell types can phagocytose PMPs, and by using TAM-blocking antibodies or siRNA knockdown of individual TAMs, we show that the uptake is mediated by endothelial Axl and Gas6. As circulating PMP levels were not altered in Gas6−/− mice compared with Gas6+/+ mice, we hypothesize that the Gas6-mediated uptake is not a means to clear the bulk of circulating PMPs but may serve to locally phagocytose PMPs generated at sites of platelet activation and as a way to effect endothelial responses.

Sphingosine 1-phosphate and its carrier apolipoprotein M in human sepsis and in Escherichia coli sepsis in baboons

Sphingosine 1‐phosphate (S1P) is an important regulator of vascular integrity and immune cell migration, carried in plasma by high‐density lipoprotein (HDL)‐associated apolipoprotein M (apoM) and by albumin. In sepsis, the protein and lipid composition of HDL changes dramatically. The aim of this study was to evaluate changes in S1P and its carrier protein apoM during sepsis. For this purpose, plasma samples from both human sepsis patients and from an experimental Escherichia coli sepsis model in baboons were used. In the human sepsis cohort, previously studied for apoM, plasma demonstrated disease‐severity correlated decreased S1P levels, the profile mimicking that of plasma apoM. In the baboons, a similar disease‐severity dependent decrease in plasma levels of S1P and apoM was observed. In the lethal E. coli baboon sepsis, S1P decreased already within 6–8 hrs, whereas the apoM decrease was seen later at 12–24 hrs. Gel filtration chromatography of plasma from severe human or baboon sepsis on Superose 6 demonstrated an almost complete loss of S1P and apoM in the HDL fractions. S1P plasma concentrations correlated with the platelet count but not with erythrocytes or white blood cells. The liver mRNA levels of apoM and apoA1 decreased strongly upon sepsis induction and after 12 hr both were almost completely lost. In conclusion, during septic challenge, the plasma levels of S1P drop to very low levels. Moreover, the liver synthesis of apoM decreases severely and the plasma levels of apoM are reduced. Possibly, the decrease in S1P contributes to the decreased endothelial barrier function observed in sepsis.

Dominant suppression of inflammation by glycan-hydrolyzed IgG

A unique anti-inflammatory property of IgG, independent of antigen specificity, is described. IgG with modification of the heavy-chain glycan on asparagine 297 by the streptococcal enzyme endo-β-N-acetylglucosaminidase (EndoS) induced a dominant suppression of immune complex (IC)-mediated inflammation, such as arthritis, through destabilization of local ICs by fragment crystallizable–fragment crystallizable (Fc-Fc) interactions. Small amounts (250 µg) of EndoS-hydrolyzed IgG were sufficient to inhibit arthritis in mice and most effective during the formation of ICs in the target tissue. The presence of EndoS-hydrolyzed IgG disrupted larger IC lattice formation both in vitro and in vivo, as visualized with anti-C3b staining. Neither complement binding in vitro nor antigen–antibody binding per se was affected.