Complement Cls, a classical enzyme with novel functions at the endochondral ossification center: immunohistochemical staining of activated Cls with a neoantigen-specific antibody

Degradation of collagen I by activated C1s in periodontal Ehlers-Danlos Syndrome

Periodontal Ehlers-Danlos syndrome (pEDS) is an autosomal dominant disorder characterized by early-onset periodontitis leading to premature loss of teeth, lack of attached gingiva and thin and fragile gums leading to gingival recession. Connective tissue abnormalities of pEDS typically include easy bruising, pretibial plaques, distal joint hypermobility, hoarse voice, and less commonly manifestations such as organ or vessel rupture. pEDS is caused by heterozygous missense mutations in C1R and C1S genes of the classical complement C1 complex. Previously we showed that pEDS pathogenic variants trigger intracellular activation of C1r and/or C1s, leading to extracellular presence of activated C1s. However, the molecular link relating activated C1r and C1s proteases to the dysregulated connective tissue homeostasis in pEDS is unknown. Using cell- and molecular-biological assays, we identified activated C1s (aC1s) as an enzyme which degrades collagen I in cell culture and in in vitro assays. Matrix collagen turnover in cell culture was assessed using labelled hybridizing peptides, which revealed fast and comprehensive collagen protein remodeling in patient fibroblasts. Furthermore, collagen I was completely degraded by aC1s when assays were performed at 40°C, indicating that even moderate elevated temperature has a tremendous impact on collagen I integrity. This high turnover is expected to interfere with the formation of a stable ECM and result in tissues with loose compaction a hallmark of the EDS phenotype. Our results indicate that pathogenesis in pEDS is not solely mediated by activation of the complement cascade but by inadequate C1s-mediated degradation of matrix proteins, confirming pEDS as a primary connective tissue disorder.

Complement C1s and C4d as Prognostic Biomarkers in Renal Cancer: Emergence of Noncanonical Functions of C1s

The complement system plays a complex role in cancer. In clear cell renal cell carcinoma (ccRCC), local production of complement proteins drives tumor progression, but the mechanisms by which they do this are poorly understood. We found that complement activation, as reflected by high plasma C4d or as C4d deposits at the tumor site, was associated with poor prognosis in two cohorts of patients with ccRCC. High expression of the C4-activating enzyme C1s by tumor cells was associated with poor prognosis in three cohorts. Multivariate Cox analysis revealed that the prognostic value of C1s was independent from complement deposits, suggesting the possibility of complement cascade-unrelated, protumoral functions for C1s. Silencing of C1s in cancer cell lines resulted in decreased proliferation and viability of the cells and in increased activation of T cells in in vitro cocultures. Tumors expressing high levels of C1s showed high infiltration of macrophages and T cells. Modification of the tumor cell phenotype and T-cell activation were independent of extracellular C1s levels, suggesting that C1s was acting in an intracellular, noncanonical manner. In conclusion, our data point to C1s playing a dual role in promoting ccRCC progression by triggering complement activation and by modulating the tumor cell phenotype and tumor microenvironment in a complement cascade-independent, noncanonical manner. Overexpression of C1s by tumor cells could be a new escape mechanism to promote tumor progression.See related Spotlight by Magrini and Garlanda, p. 855. See article by Daugan et al., p. 909 (40).

Complement activation links inflammation to dental tissue regeneration

OBJECTIVES

Complement is an efficient plasma immune surveillance system. It initiates inflammation by inducing vascular modifications and attracting immune cells expressing Complement receptors. Investigating Complement receptors in non-immune cells pointed out Complement implication in the regeneration of tissue such as liver, skin, or bone. This review will shed the light on Complement implication in the initial steps of dental tissue regeneration.

MATERIALS AND METHODS

Review of literature was conducted on Complement local expression and implication in oral tissue regeneration in vivo and in vitro.

RESULTS

Recent data reported expression of Complement receptors and soluble proteins in dental tissues. Cultured pulp fibroblasts secrete all Complement components. Complement C3b and MAC have been shown to control bacteria growth in the dental pulp while C3a and C5a are involved in the initial steps of pulp regeneration. Indeed, C3a induces pulp stem cell/fibroblast proliferation, and fibroblast recruitment, while C5a induces neurite growth, guides stem cell recruitment, and odontoblastic differentiation. Similarly, cultured periodontal ligament cells produce C5a which induces bone marrow mesenchymal stem cell recruitment.

CONCLUSIONS

Overall, this review highlights that local Complement synthesis in dental tissues plays a major role, not only in eliminating bacteria but also in the initial steps of dental tissue regeneration, thus providing a link between dental tissue inflammation and regeneration.

CLINICAL RELEVANCE

Complement provides an explanation for understanding why inflammation preceeds regeneration. This may also provide a biological rational for understanding the reported success conservative management of mature permanent teeth with carious pulp exposure.

Growth plate expression profiling: Large and small breed dogs provide new insights in endochondral bone formation

The difference in the adult height of mammals, and hence in endochondral bone formation, is not yet fully understood and may serve to identify targets for bone and cartilage regeneration. In line with this hypothesis, the intra-species disparity between the adult height of Great Danes and Miniature Poodles was investigated at a transcriptional level. Microarray analysis of the growth plate of five Great Danes and five Miniature Poodles revealed 2,981 unique genes that were differentially expressed, including many genes with an unknown role in skeletal development. A signaling pathway impact analysis indicated activation of the cell cycle, extracellular matrix receptor interaction and the tight junction pathway, and inhibition of pathways associated with inflammation and the complement cascade. In additional validation steps, the gene expression profile of the separate growth plate zones for both dog breeds were determined. Given that the BMP signaling is known for its crucial role in skeletal development and fracture healing, and BMP-2 is used in orthopaedic and spine procedures for bone augmentation, further investigations concentrated on the BMP pathway.The canonical BMP-2 and BMP-6 signaling pathway was activated in the Great Danes compared to Miniature Poodles. In conclusion, investigating the differential expression of genes involved in endochondral bone formation in small and large breed dogs, could be a game changing strategy to provide new insights in growth plate development and identify new targets for bone and cartilage regeneration. © 2017 The Authors. Journal of Orthopaedic Research® published by Wiley Periodicals, Inc. on behalf of the Orthopaedic Research Society. J Orthop Res 36:138-148, 2018.

Reduced bone loss in a murine model of postmenopausal osteoporosis lacking complement component 3

The growing field of osteoimmunology seeks to unravel the complex interdependence of the skeletal and immune systems. Notably, we and others have demonstrated that complement signaling influences the differentiation of osteoblasts and osteoclasts, the two primary cell types responsible for maintaining bone homeostasis. However, the net effect of complement on bone homeostasis in vivo was unknown. Our published in vitro mechanistic work led us to hypothesize that absence of complement component 3 (C3), a central protein in the complement activation cascade, protects against bone loss in the ovariectomy-based model of postmenopausal osteoporosis. Indeed, we report here that, when compared to their C57BL/6J (WT) counterparts, ovariectomized C3 deficient mice experienced reduced bone loss at multiple sites and increased stiffness at the femoral neck, the latter potentially improving mechanical function. WT and B6;129S4-C3^tm1Crr /J (C3^-/- ) mice were either ovariectomized or sham-operated at 6 weeks of age and euthanized at 12 weeks. MicroCT on harvested bones revealed that the trabecular bone volume fraction in the metaphyses of both the proximal tibiae and distal femora of ovariectomized C3^-/- mice is significantly greater than that of their WT counterparts. Lumbar vertebrae showed significantly greater osteoid content and mineral apposition rates. Mechanical testing demonstrated significantly greater stiffness in the femoral necks of ovariectomized C3^-/- mice. These results demonstrate that C3 deficiency reduces bone loss at ovariectomy and may improve mechanical properties. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:118-128, 2018.

C5aR inhibition in the early inflammatory phase does not affect bone regeneration in a model of uneventful fracture healing

Background

Recent studies were able to demonstrate involvement of the complement cascade in bone biology. Further studies analyzed the role of complement in traumatic injuries and demonstrated negative effects after excessive systemic activation of the inflammatory response with early abrogation of complement activation after application of a C5aR-antagonist exerting beneficial effects upon bone regeneration. In contrast, own fracture healing experiments with complement-deficient animals implied a crucial role of the complement cascade for sufficient fracture healing.

Methods

To analyze the effect of a short abrogation of the complement system in the local process of fracture healing, a fracture healing experiment with wild-type mice (C57BL6), femoral osteotomy, consecutive external fixation for 21 days and blockade of the early complement activation (C5aRA) directly after trauma and after 12 h was performed. Control animals received a peptide without any biological effects. After 1–3 days, the inflammatory response was monitored with IL-6 immunostaining, serum analyses of C5a and after 3 days with histological evaluation of PMN. Fracture healing was examined with biomechanical, radiological and histological methods after 21 days.

Results

While a decrease of the early inflammatory response was seen on day 1 of the C5aRA-treated group regarding immunostaining for IL-6 and after 3 days in the histological evaluation of PMN, no significant differences were demonstrated between both experimental groups after 21 days in the biomechanical, radiological and histological evaluation.

Conclusions

The present results demonstrate that the short-term inhibition of complement activation immediately after fracture does not significantly affect bone regeneration in an experimental model of regular fracture healing. Whereas other studies demonstrated that the early posttraumatic blockade of the C5aR improves fracture healing in a scenario of combined trauma, the present findings implicate that the same treatment has no effect in uneventful bone healing.

Complement C3 and C5 deficiency affects fracture healing

There is increasing evidence that complement may play a role in bone development. Our previous studies demonstrated that the key complement receptor C5aR was strongly expressed in the fracture callus not only by immune cells but also by bone cells and chondroblasts, indicating a function in bone repair. To further elucidate the role of complement in bone healing, this study investigated fracture healing in mice in the absence of the key complement molecules C3 and C5. C3^-/- and C5^-/- as well as the corresponding wildtype mice received a standardized femur osteotomy, which was stabilized using an external fixator. Fracture healing was investigated after 7 and 21 days using histological, micro-computed tomography and biomechanical measurements. In the early phase of fracture healing, reduced callus area (C3^-/-: -25%, p=0.02; C5^-/-: -20% p=0.052) and newly formed bone (C3^-/-: -38%, p=0.01; C5^-/-: -52%, p=0.009) was found in both C3- and C5-deficient mice. After 21 days, healing was successful in the absence of C3, whereas in C5-deficient mice fracture repair was significantly reduced, which was confirmed by a reduced bending stiffness (-45%; p=0.029) and a smaller callus volume (-17%; p=0.039). We further demonstrated that C5a was activated in C3^-/- mice, suggesting cleavage via extrinsic pathways. Our results suggest that the activation of the terminal complement cascade in particular may be crucial for successful fracture healing.

Fetal mesenchymal stromal cells differentiating towards chondrocytes acquire a gene expression profile resembling human growth plate cartilage

We used human fetal bone marrow-derived mesenchymal stromal cells (hfMSCs) differentiating towards chondrocytes as an alternative model for the human growth plate (GP). Our aims were to study gene expression patterns associated with chondrogenic differentiation to assess whether chondrocytes derived from hfMSCs are a suitable model for studying the development and maturation of the GP. hfMSCs efficiently formed hyaline cartilage in a pellet culture in the presence of TGFβ3 and BMP6. Microarray and principal component analysis were applied to study gene expression profiles during chondrogenic differentiation. A set of 232 genes was found to correlate with in vitro cartilage formation. Several identified genes are known to be involved in cartilage formation and validate the robustness of the differentiating hfMSC model. KEGG pathway analysis using the 232 genes revealed 9 significant signaling pathways correlated with cartilage formation. To determine the progression of growth plate cartilage formation, we compared the gene expression profile of differentiating hfMSCs with previously established expression profiles of epiphyseal GP cartilage. As differentiation towards chondrocytes proceeds, hfMSCs gradually obtain a gene expression profile resembling epiphyseal GP cartilage. We visualized the differences in gene expression profiles as protein interaction clusters and identified many protein clusters that are activated during the early chondrogenic differentiation of hfMSCs showing the potential of this system to study GP development.

Does complement play a role in bone development and regeneration?

The skeletal and the immune system are not two independent systems, rather, there are multifaceted and complex interactions between the different cell types of both systems and there are several shared cytokines. As a part of the innate immunity, the complement system was found to be an important link between bone and immunity. Complement proteins appear to be involved in bone development and homeostasis, and specifically influence osteoblast and osteoclast activity. This review describes the complex mutual regulation of the two systems, and indicates some of the negative side effects as a result of inappropriate or excessive complement activation.

Molecular mechanisms of inflammation and tissue injury after major trauma--is complement the "bad guy"?

Trauma represents the leading cause of death among young people in industrialized countries. Recent clinical and experimental studies have brought increasing evidence for activation of the innate immune system in contributing to the pathogenesis of trauma-induced sequelae and adverse outcome. As the "first line of defense", the complement system represents a potent effector arm of innate immunity, and has been implicated in mediating the early posttraumatic inflammatory response. Despite its generic beneficial functions, including pathogen elimination and immediate response to danger signals, complement activation may exert detrimental effects after trauma, in terms of mounting an "innocent bystander" attack on host tissue. Posttraumatic ischemia/reperfusion injuries represent the classic entity of complement-mediated tissue damage, adding to the "antigenic load" by exacerbation of local and systemic inflammation and release of toxic mediators. These pathophysiological sequelae have been shown to sustain the systemic inflammatory response syndrome after major trauma, and can ultimately contribute to remote organ injury and death. Numerous experimental models have been designed in recent years with the aim of mimicking the inflammatory reaction after trauma and to allow the testing of new pharmacological approaches, including the emergent concept of site-targeted complement inhibition. The present review provides an overview on the current understanding of the cellular and molecular mechanisms of complement activation after major trauma, with an emphasis of emerging therapeutic concepts which may provide the rationale for a "bench-to-bedside" approach in the design of future pharmacological strategies.

The anaphylatoxin receptor C5aR is present during fracture healing in rats and mediates osteoblast migration in vitro

BACKGROUND

There is evidence that complement components regulate cytokine production in osteoblastic cells, induce cell migration in mesenchymal stem cells, and play a regulatory role in normal enchondral bone formation. We proved the hypothesis that complement might be involved in bone healing after fracture.

METHODS

We investigated the expression of the key anaphylatoxin receptor C5aR during fracture healing in rats by immunostaining after 1, 3, 7, 14, and 28 days. C5aR expression was additionally analyzed in human mesenchymal stem cells (hMSC) during osteogenic differentiation, in human primary osteoblasts, and osteoclasts by reverse transcriptase polymerase chain reaction and immunostaining. Receptor functionality was proven by the migratory response of cells to C5a in a Boyden chamber.

RESULTS

C5aR was expressed in a distinct spatial and temporal pattern in the fracture callus by differentiated osteoblast, chondroblast-like cells in cartilaginous regions, and osteoclasts. In vitro C5aR was expressed by osteoblasts, osteoclasts, and hMSC undergoing osteogenic differentiation. C5aR was barely expressed by undifferentiated hMSC but was significantly induced after osteogenic differentiation. C5aR activation by C5a induced strong chemotactic activity in osteoblasts, and in hMSC, which had undergone osteogenic differentiation, being abolished by a specific C5aR antagonist. In hMSC, C5a induced less migration reflecting their low level of C5aR expression.

CONCLUSIONS

Our in vitro and in vivo results demonstrated the presence of C5aR in bone forming osteoblasts and bone resorbing osteoclasts. It is suggested that C5aR might play a regulatory role in fracture healing in intramembranous and in enchondral ossification, one possible function being the regulation of cell recruitment.

Complement C3a and C5a modulate osteoclast formation and inflammatory response of osteoblasts in synergism with IL-1β

There is a tight interaction of the bone and the immune system. However, little is known about the relevance of the complement system, an important part of innate immunity and a crucial trigger for inflammation. The aim of this study was, therefore, to investigate the presence and function of complement in bone cells including osteoblasts, mesenchymal stem cells (MSC), and osteoclasts. qRT-PCR and immunostaining revealed that the central complement receptors C3aR and C5aR, complement C3 and C5, and membrane-bound regulatory proteins CD46, CD55, and CD59 were expressed in human MSC, osteoblasts, and osteoclasts. Furthermore, osteoblasts and particularly osteoclasts were able to activate complement by cleaving C5 to its active form C5a as measured by ELISA. Both C3a and C5a alone were unable to trigger the release of inflammatory cytokines interleukin (IL)-6 and IL-8 from osteoblasts. However, co-stimulation with the pro-inflammatory cytokine IL-1β significantly induced IL-6 and IL-8 expression as well as the expression of receptor activator of nuclear factor-kappaB ligand (RANKL) and osteoprotegerin (OPG) indicating that complement may modulate the inflammatory response of osteoblastic cells in a pro-inflammatory environment as well as osteoblast-osteoclast interaction. While C3a and C5a did not affect osteogenic differentiation, osteoclastogenesis was significantly induced even in the absence of RANKL and macrophage-colony stimulating factor (M-CSF) suggesting that complement could directly regulate osteoclast formation. It can therefore be proposed that complement may enhance the inflammatory response of osteoblasts and increase osteoclast formation, particularly in a pro-inflammatory environment, for example, during bone healing or in inflammatory bone disorders.

New insights of an old defense system: structure, function, and clinical relevance of the complement system

The complement system was discovered a century ago as a potent defense cascade of innate immunity. After its first description, continuous experimental and clinical research was performed, and three canonical pathways of activation were established. Upon activation by traumatic or surgical tissue damage, complement reveals beneficial functions of pathogen and danger defense by sensing and clearing injured cells. However, the latest research efforts have provided a more distinct insight into the complement system and its clinical subsequences. Complement has been shown to play a significant role in the pathogenesis of various inflammatory processes such as sepsis, multiorgan dysfunction, ischemia/reperfusion, cardiovascular diseases and many others. The three well-known activation pathways of the complement system have been challenged by newer findings that demonstrate direct production of central complement effectors (for example, C5a) by serine proteases of the coagulation cascade. In particular, thrombin is capable of producing C5a, which not only plays a decisive role on pathogens and infected/damaged tissues, but also acts systemically. In the case of uncontrolled complement activation, “friendly fire” is generated, resulting in the destruction of healthy host tissue. Therefore, the traditional research that focuses on a mainly positive-acting cascade has now shifted to the negative effects and how tissue damage originated by the activation of the complement can be contained. In a translational approach including structure-function relations of this ancient defense system, this review provides new insights of complement-mediated clinical relevant diseases and the development of complement modulation strategies and current research aspects.

Local production of complement proteins in rheumatoid arthritis synovium

OBJECTIVE

Complement has been repeatedly implicated in the pathogenesis of rheumatoid arthritis (RA) based on studies showing reduced levels of native complement components and increased levels of complement metabolites in plasma, synovial fluid (SF), and synovial tissue (ST) of RA patients. However, there is limited information on local production and activation of key factors of the complement cascade in RA synovium and their potential modulation by novel anticytokine therapies. This study was undertaken to characterize the expression of complement proteins and receptors in RA SF and ST.

METHODS

Using in situ hybridization, immunohistochemistry, and Western blot techniques, we assessed the presence of complement proteins C3, factor B (FB), and C5b-9, as well as the expression of complement receptors C3aR and C5aR in rheumatoid synovium. C3 and FB levels in SF were determined by enzyme-linked immunosorbent assay. Functional assessment was performed by examining the effects of soluble tumor necrosis factor receptor (sTNFR) p55 gene transfer in the SCID mouse model of RA.

RESULTS

Complement proteins and receptors could be localized in all RA synovial specimens, whereas in osteoarthritis (OA) synovium, only a few, single cells expressed complement proteins and receptors. No differences were noted in the concentration of C3 between RA and OA in SF; however, FB levels were markedly reduced in RA versus OA SF. In RA synovium, in contrast to OA synovium, local expression of complement factor and complement receptor messenger RNA was found throughout the various ST compartments, suggesting that activation of the complement cascade occurs in all parts of the rheumatoid synovium. Moreover, C5aR expression was up-regulated following overexpression of sTNFR p55 by adenovirus-based gene transfer.

CONCLUSION

In summary, local complement production and activation may play an important role in RA, and specific modulation and inhibition of local complement production could be an attractive therapeutic target for RA.

Complement C1s activation in degenerating articular cartilage of rheumatoid arthritis patients: immunohistochemical studies with an active form specific antibody

OBJECTIVE

The first complement component C1s was reported to have novel functions to degrade matrix components, besides its activities in the classic complement pathway. This study explores participation of C1s in articular cartilage degradation in rheumatoid arthritis (RA).

METHODS

Normal articular cartilage (n = 6) and cartilage obtained from joints with RA (n = 15) and osteoarthritis (OA, n = 10) were immunostained using anti-C1s monoclonal antibodies PG11, which recognises both active and inactive C1s, and M241, which is specifically reactive to activated C1s. The effects of inflammatory cytokines on C1s production by human articular chondrocytes were also examined by sandwich ELISA.

RESULTS

In normal articular cartilage, C1s was negative in staining with both PG11 and M241. In contrast, degenerating cartilage of RA was stained with PG11 (14 of 15 cases), and in most of the cases (13 of 15 cases) C1s was activated as revealed by M241 staining. In OA, C1s staining was restricted in severely degrading part of cartilage (5 of 10 cases), and even in that part C1s was not activated. In addition, C1s production by chondrocytes in vitro was increased by an inflammatory cytokine, tumour necrosis factor alpha.

CONCLUSION

These results suggest that C1s activated in degenerative cartilage matrix of RA but not in that of OA. C1s is thought to participate in the pathogenesis of RA through its collagenolytic activity in addition to the role in the classic cascade.

Inhibition of bFGF activity by complement C1s: covalent binding of C1s with bFGF

The first complement component C1s formed large aggregates with bFGF when bFGF and C1s were incubated at 37 degrees C overnight. Under non-reducing conditions, a part of the aggregates did not penetrate into 5% polyacrylamide gel in the presence of SDS, and the rest penetrated into 5% gel but not into 12% gel. The aggregates were dissociated into monomers by reducing with 2-mercaptoethanol. Both active and inactive C1s formed aggregates with bFGF. In addition, a portion of bFGF was degraded by active C1s but not by inactive C1s. Aggregates were not formed when 2-mercaptoethanol (2 mM) was added to the incubation mixture. After the incubation with C1s the growth-stimulating activity of bFGF was measured by using human umbilical vein endothelial cells (HUVEC) as indicator cells. The aggregate formation between C1s and bFGF significantly reduced the activity of bFGF.