Synthesis of the third component of complement (C3) by lectin-activated and HTLV-infected human T-cells

Complement C3 overexpression activates JAK2/STAT3 pathway and correlates with gastric cancer progression

Background

Localized C3 deposition is a well-known factor of inflammation. However, its role in oncoprogression of gastric cancer (GC) remains obscured. This study aims to explore the prognostic value of C3 deposition and to elucidate the mechanism of C3-related oncoprogression for GC.

Methods

From August to December 2013, 106 GC patients were prospectively included. The regional expression of C3 and other effectors in gastric tissues were detected by WB, IHC, qRT-PCR and other tests. The correlation of localized C3 deposition and oncologic outcomes was determined by 5-year survival significance. Human GC and normal epithelial cell lines were employed to detect a relationship between C3 and STAT3 signaling pathway in vitro experiments.

Results

C3 and C3a expression were markedly enhanced in GC tissues at both mRNA and protein levels compared with those in paired nontumorous tissues. According to IHC C3 score, 65 (61.3%) and 41 (38.7%) patients had high and low C3 deposition, respectively. C3 deposition was negatively correlated with plasma levels of C3 and C3a (both P < 0.001) and positively correlated with pathological T and TNM stages (both P < 0.001). High C3 deposition was identified as an independent prognostic factor of poor 5-year overall survival (P = 0.045). In vitro C3 administration remarkably enhanced p-JAK2/p-STAT3 expression in GC cell lines but caused a reduction of such activation when pre-incubated with a C3 blocker. Importantly, C3 failed to activate such signaling in cells pre-treated with a JAK2 inhibitor.

Conclusions

Localized C3 deposition in the tumor microenvironment is a relevant immune signature for predicting prognosis of GC. It may aberrantly activate JAK2/STAT3 pathway allowing oncoprogression.

Trial registration

ClinicalTrials.gov, NCT02425930, Registered 1st August 2013.

Intracellular complement activation-An alarm raising mechanism?

It has become increasingly apparent that the complement system, being an ancient defense mechanism, is not operative only in the extracellular milieu but also intracellularly. In addition to the known synthetic machinery in the liver and by macrophages, many other cell types, including lymphocytes, adipocytes and epithelial cells produce selected complement components. Activation of e.g. C3 and C5 inside cells may have multiple effects ranging from direct antimicrobial defense to cell differentiation and possible influence on metabolism. Intracellular activation of C3 and C5 in T cells is involved in the maintenance of immunological tolerance and promotes differentiation of T helper cells into Th1-type cells that activate cell-mediated immune responses. Adipocytes are unique in producing many complement sensor proteins (like C1q) and Factor D (adipsin), the key enzyme in promoting alternative pathway amplification. The effects of complement activation products are mediated by intracellular and cell membrane receptors, like C3aR, C5aR1, C5aR2 and the complement regulator MCP/CD46, often jointly with other receptors like the T cell receptor, Toll-like receptors and those of the inflammasomes. These recent observations link complement activation to cellular metabolic processes, intracellular defense reactions and to diverse adaptive immune responses. The complement components may thus be viewed as intracellular alarm molecules involved in the cellular danger response.

Complement Component C4 Regulates the Development of Experimental Autoimmune Uveitis through a T Cell-Intrinsic Mechanism

In addition to its conventional roles in the innate immune system, complement has been found to directly regulate T cells in the adaptive immune system. Complement components, including C3, C5, and factor D, are important in regulating T cell responses. However, whether complement component C4 is involved in regulating T cell responses remains unclear. In this study, we used a T cell-dependent model of autoimmunity, experimental autoimmune uveitis (EAU) to address this issue. We compared disease severity in wild-type (WT) and C4 knockout (KO) mice using indirect ophthalmoscopy, scanning laser ophthalmoscopy, spectral-domain optical coherence tomography, and histopathological analysis. We also explored the underlying mechanism by examining T cell responses in ex vivo antigen-specific recall assays and in in vitro T cell priming assays using bone marrow-derived dendritic cells, splenic dendritic cells, and T cells from WT or C4 KO mice. We found that C4 KO mice develop less severe retinal inflammation than WT mice in EAU and show reduced autoreactive T cell responses and decreased retinal T cell infiltration. We also found that T cells, but not dendritic cells, from C4 KO mice have impaired function. These results demonstrate a previously unknown role of C4 in regulating T cell responses, which affects the development of T cell-mediated autoimmunity, as exemplified by EAU. Our data could shed light on the pathogenesis of autoimmune uveitis in humans.

The versatile functions of complement C3-derived ligands

The complement system is a major component of immune defense. Activation of the complement cascade by foreign substances and altered self-structures may lead to the elimination of the activating agent, and during the enzymatic cascade, several biologically active fragments are generated. Most immune regulatory effects of complement are mediated by the activation products of C3, the central component. The indispensable role of C3 in opsonic phagocytosis as well as in the regulation of humoral immune response is known for long, while the involvement of complement in T-cell biology have been revealed in the past few years. In this review, we discuss the immune modulatory functions of C3-derived fragments focusing on their role in processes which have not been summarized so far. The importance of locally synthesized complement will receive special emphasis, as several immunological processes take place in tissues, where hepatocyte-derived complement components might not be available at high concentrations. We also aim to call the attention to important differences between human and mouse systems regarding C3-mediated processes.

Generation of complement protein C3 deficient pigs by CRISPR/Cas9-mediated gene targeting

Complement protein C3 is the pivotal component of the complement system. Previous studies have demonstrated that C3 has implications in various human diseases and exerts profound functions under certain conditions. However, the delineation of pathological and physiological roles of C3 has been hampered by the insufficiency of suitable animal models. In the present study, we applied the clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated (Cas) system to target the C3 gene in porcine fetal fibroblasts. Our results indicated that CRISPR/Cas9 targeting efficiency was as high as 84.7%, and the biallelic mutation efficiency reached at 45.7%. The biallelic modified colonies were used as donor for somatic cell nuclear transfer (SCNT) technology to generate C3 targeted piglets. A total of 19 C3 knockout (KO) piglets were produced and their plasma C3 protein was undetectable by western blot analysis and ELISA. The hemolytic complement activity and complement-dependent cytotoxicity assay further confirmed that C3 was disrupted in these piglets. These C3 KO pigs could be utilized as a valuable large animal model for the elucidation of the roles of C3.

Complement in basic processes of the cell

The complement system is reemerging in the last few years not only as key element of innate immunity against pathogens, but also as a main regulator of local adaptive responses, affecting dendritic cells as well as T and B lymphocytes. We review data showing that leucocytes are capable of significant autocrine synthesis of complement proteins, and express a large range of complement receptors, which in turn regulate their differentiation and effector functions while cross talking with other innate receptors such as Toll-like receptors. Other unconventional roles of complement proteins are reviewed, including their impact in non-leukocytes and their intracellular cleavage by vesicular proteases, which generate critical cues required for T cell function. Thus, leucocytes are very much aware of complement-derived information, both extracellular and intracellular, to elaborate their responses, offering rich avenues for therapeutic intervention and new hypothesis for conserved major histocompatibility complex complotypes.

Targeting complement at the time of transplantation

Complement activation occurs in at least two phases when an organ is transplanted into a naive recipient: during reperfusion with recipient blood particularly when the donor organ has undergone a significant period of ischaemia and then during acute rejection once the recipient immune system has recognised the donor tissue as non-self. Both of these reactions are most obvious in the extravascular compartment of the transplanted organ and involve local synthesis of some of the key complement components as well as loss of controls that limit the activation of the pivotal component C3. In contrast, sensitised individuals with pre-existing circulating antibodies have an immediate reaction against the transplant organ that is also complement dependent but is enacted in the intravascular space. All three types of injury (ischaemia-reperfusion, acute rejection, hyperacute rejection) have a critical effect on transplant outcome. Here we discuss therapeutic strategies that are designed to overcome the impact of these factors at the start of transplantation with the aim of improving long-term transplant outcomes. These include the concept of treating the donor organ with modified therapeutic regulators that are engineered to be retained by the donor organ after transplantation and prevent inflammatory injury during the critical early period. By targeting the donor organ with anchored therapeutic proteins, the systemic functions of complement including host defence remain intact. The control of complement activation during the first stages of transplantation, including the possibility that this will reduce the capacity of the graft for stimulating the adaptive immune system, offers an important prospect for increasing the longevity of the transplant and offsetting demand on the limited supply of donor organs. It also provides a model in which the benefits and indications for localised therapy to maximise therapeutic efficiency and minimise the systemic disturbance may be instructive in other complement-related disorders.

Human T cell derived, cell-bound complement iC3b is integrally involved in T cell activation

Although the complement system is thought to be mainly involved in innate immunity and in the humoral arm of adaptive responses, evidence implicating that complement impacts T cell responses are accumulating recently. The role of the various activation products of the major complement component C3 were mainly studied so far in animal systems, and investigations regarding the effect of different C3-fragments on human T cells are sparse. Here we show that anti-CD3 activated human T lymphocytes derived from the blood and tonsil of healthy individuals produce C3, and the major cleavage fragment that appears on the T cell surface is iC3b. Based on studies carried out in allogenic system we demonstrate that the T cell membrane bound iC3b binds to the CR3 and probably to CR4 receptors expressed on monocyte-derived dendritic cells, and this interaction leads to significantly enhanced T-cell proliferation. Since neither C3aR and nor C3a binding could be detected on the membrane of anti-CD3 activated T cells, our findings indicate that in humans – in contrast to mice – the C3a peptide is most probably not involved directly in the T cell activation process.

The relative importance of local and systemic complement production in ischaemia, transplantation and other pathologies

Besides a critical role in innate host defence, complement activation contributes to inflammatory and immunological responses in a number of pathological conditions. Many tissues outside the liver (the primary source of complement) synthesise a variety of complement proteins, either constitutively or response to noxious stimuli. The significance of this local synthesis of complement has become clearer as a result of functional studies. It revealed that local production not only contributes to the systemic pool of complement but also influences local tissue injury and provides a link with the antigen-specific immune response. Extravascular production of complement seems particularly important at locations with poor access to circulating components and at sites of tissue stress responses, notably portals of entry of invasive microbes, such as interstitial spaces and renal tubular epithelial surfaces. Understanding the relative importance of local and systemic complement production at such locations could help to explain the differential involvement of complement in organ-specific pathology and inform the design of complement-based therapy. Here, we will describe the lessons we have learned over the last decade about the local synthesis of complement and its association with inflammatory and immunological diseases, placing emphasis on the role of local synthesis of complement in organ transplantation.

Complement C5a receptor is essential for the optimal generation of antiviral CD8+ T cell responses

The complement system has been long regarded as an important effector of the innate immune response. Furthermore, complement contributes to various aspects of B and T cell immunity. Nevertheless, the role of complement in CD8(+) T cell antiviral responses has yet to be fully delineated. We examined the CD8(+) T cell response in influenza type A virus-infected mice treated with a peptide antagonist to C5aR to test the potential role of complement components in CD8(+) T cell responses. We show that both the frequency and absolute numbers of flu-specific CD8(+) T cells are greatly reduced in C5aR antagonist-treated mice compared with untreated mice. This reduction in flu-specific CD8(+) T cells is accompanied by attenuated antiviral cytolytic activity in the lungs. These results demonstrate that the binding of the C5a component of complement to the C5a receptor plays an important role in CD8(+) T cell responses.

Macrophage-derived complement component C4 can restore humoral immunity in C4-deficient mice

Mice with a disrupted C4 locus (C4(-/-)) have an impaired immune response to thymus-dependent Ags. To test the role of bone marrow-derived C4 in humoral immunity, we reconstituted deficient animals with wild-type bone marrow or an enriched fraction of bone marrow-derived macrophages. C4 chimeras were immunized with 4-hydroxy-3-nitrophenyl(5) conjugated to keyhole limpet hemocyanin (NP(5)- KLH) or infected with HSV-1, and the Ab response was evaluated. Wild-type bone marrow rescued the humoral immune response to both Ags, i.e., the soluble Ag and HSV-1, demonstrating that local C4 production is sufficient for humoral responses. Although the C4 chimeric animals lacked detectable C4 in their sera, C4 mRNA was identified in splenic sections by in situ hybridization, and C4 protein deposits were identified in the germinal center areas of splenic follicles by immunofluorescence staining. Macrophages derived from bone marrow produced sufficient C4 protein to restore the humoral response to NP(5)-KLH in C4-deficient animals when administered along with Ag. Cell-sorting experiments, followed by C4-specific RT-PCR, identified splenic macrophages (CD11b(+), CD11c(-)) as a cellular source for C4 synthesis within the spleen.

Sequential cellular and molecular kinetics in acutely rejecting renal allografts in rats

The initial (0-24 hr), early (3-5 days), and late (7-14 days) events occurring in LBNF1 renal allografts transplanted into Lew recipients were examined to define precisely the sequential cellular and molecular kinetics during acute rejection. Grafts and spleens were harvested at 3, 6, 12, and 24 hr, and at 3, 5, 7, and 14 days and processed for morphology, immunohistology, and reverse transcriptase-polymerase chain reaction. Various factors (mRNA) were up-regulated sequentially in the allografts over time. In the initial phase, E-selectin and complement (C1 and C3) expression was noted within 6 hr, peaking by 24 hr. RANTES (regulated upon activation, normal T cell expressed and secreted) increased within 6 hr, and then again between 3 and 6 days. By immunohistology, MHC class II was up-regulated consistently after day 1. Intercellular adhesion molecule-1 expression increased after day 3; lymphocyte function-associated antigen-1+ infiltrating leukocytes peaked at day 5. Infiltrating CD8+ T lymphocytes increased strikingly between days 1 and 3, peaking at day 5; CD4+ cells infiltrated more slowly until day 5. The kinetics of ED1+ macrophages were similar to those of lymphocyte function-associated antigen-1+ cells. The CD4+ T cell-derived product, interleukin (IL)-2, peaked at 7 days. Interferon-gamma increased progressively up to 14 days. By 3 days, the macrophage-associated factor, transforming growth factor-beta, peaked; this was followed by increased IL-6 expression by day 5. IL-1, tumor necrosis factor-alpha, and inducible nitric oxide synthase increased slowly until day 7, declining thereafter. Endothelin increased progressively over the 14-day follow-up period. Cytokine dynamics occurring in host spleen were similar to those noted in the allografts. Although acute rejection is primarily T cell mediated, adhesion molecules, macrophages, and their associated products may influence initial and later changes. The brisk expression of complement, E-selectin, and RANTES within the first few hours after engraftment may occur secondary to ischemic injury and trigger subsequent immunological events. Macrophages and their products may play a larger role in the process than hitherto appreciated.

Tissue synthesis of complement as an immune regulator

Evidence is accumulating that a variety of tissues produce complement components, and that production in each tissue is differentially regulated by inflammatory cytokines. This locally produced complement could have protective or injurious actions, depending upon local circumstances. Techniques for analysing separately the contributions of local complement synthesis and complement derived from the circulation are now becoming available. We argue that an appreciation of the role of local complement synthesis may help to explain many features of organ- and tissue-specific immunological disease.

Complement biosynthesis in the central nervous system

Complement is an important effector arm of the human immune response. Binding of proteolytic fragments derived from activation of complement by specific receptors leads to responses as diverse as inflammation, opsonization, and B-cell activation. The importance of characterizing the expression and regulation of complement in the CNS is highlighted by growing evidence that complement plays a significant role in the pathogenesis of a variety of neurological diseases, such as multiple sclerosis and Alzheimer's disease. In vitro studies have demonstrated that astrocytes, the predominant glial cell type in the brain, are capable of expressing or producing a majority of the components of the complement system. Expression of many complement proteins synthesized by astrocytes is regulated by both pro- and anti-inflammatory cytokines, many of which are also produced by several cell types in the CNS. In addition to astrocytes, ependymal cells, endothelial cells, microglia, and neurons have recently been shown to synthesize various complement proteins or express complement receptors on their cell surfaces. Together, these studies demonstrate that several cell types throughout the brain have the potential to express complement and, in many cases, increase expression in response to mediators of the acute phase response. These studies suggest that complement may play a greater role in CNS immune responses than previously thought, and pave the way for better understanding of the dynamics of complement expression and regulation in vivo. Such understanding may lead to therapeutic manipulation of complement host defense functions in a variety of inflammatory and degenerative diseases in the CNS.

Hereditary deficiency of C3 in animals and humans

Inherited deficiency of complement C3 has been described in guinea pigs, dogs and 20 humans. Homozygous deficiency of C3 is associated with recurrent pyogenic infections by encapsulated bacteria, especially H. influenzae, S. pneumoniae and N. meningitidis. In dogs and humans there is also an association with development of glomerulonephritis of the mesangiocapillary type. Some patients also develop transient erythematous rashes in association with pyogenic infections, with histology showing predominantly neutrophil infiltration and small vessel vasculitis. Studies of antibody responses, mainly in experimental animals have shown impaired primary and secondary responses to both thymus-dependent and -independent antigens at low immunizing doses, with a reduced switch from IgM to IgG production. The molecular basis of C3 deficiency has been established in two humans with C3 deficiency. In one it was due to a splice junction mutation and in another, to a partial gene deletion. These mutations are not compatible with the production of functional C3 in any tissue. Such patients with absolute C3 deficiency are a valid model for understanding the physiological role of C3 in vivo.

The secretion of the third component of complement (C3) by human polymorphonuclear leucocytes from both normal and systemic lupus erythematosus cases

Recently, murine peritoneal exudate polymorphonuclear leucocytes (PMNs) have been proved to secrete complement C3. In this report we show the secretion of C3 by normal human blood PMNs. ELISA assay was used to detect secreted C3 in culture supernatants of PMNs, while immunoperoxidase staining was used for intracellular C3 detection. 12-o-tetradecanoyl phorbol 13 acetate (TPA) had a flushing effect on C3 secretion by PMNs but not macrophages, suggesting a special C3 storing capability in PMNs. Dioctanoyl glycerol, mezerein and calcium ionophore A23187 caused the same marked increase in C3 secretion by PMNs. This suggests the contribution of protein kinase C and the calmodulin pathway in the mechanism of C3 secretion, similar to murine peritoneal exudate PMNs. In some cases of systemic lupus erythematosus, C3 secretion by blood PMNs was increased but no similar response to TPA could be detected.

The role of C3 in the immune response

The complement system, particularly the third component, plays an important modulatory role in the inductive phase of the immune response. As discussed here by Anna Erdei and colleagues, the picture that is emerging is that immobilized C3 split products facilitate the cooperation between immunocompetent cells and are co-stimulatory molecules in T- and B-cell activation, probably as a result of their ability to promote cell-cell adhesion. In contrast, soluble C3 products inhibit lymphocyte proliferation.

Covalent binding of non-proteolysed C3 to Jurkat T cells

Purified C3 binds covalently to Jurkat T cells upon incubation at neutral pH. This binding does not appear to involve proteolysis of C3; it leads to high-molecular-weight associations, preferentially through ester linkages, which are disrupted upon incubation with hydroxylamine at alkaline pH. Part of the association also appears to involve disulfide links between C3 and Jurkat cells. Similarly, plasma membranes purified from these cells bind C3 with no evidence for proteolysis of C3. Binding of C3 appears to be "catalysed" by Jurkat cells, and is not due to the well-known spontaneous hydrolysis of C3. Binding of C3 involves hydrolysis of its thioester bond, as titratable--SH groups are available in soluble C3 after incubation of purified C3 with Jurkat plasma membranes; loss of C3 haemolytic activity confirms this finding. These observations give evidence for the binding of C3b-like C3 to Jurkat cells, conferring on these cells the potential to interact with other complement receptor-bearing cells such as B cells.