Persistent complement activation in submucosal blood vessels of active inflammatory bowel disease: immunohistochemical evidence

The neoepitope of the complement C5b-9 Membrane Attack Complex is formed by proximity of adjacent ancillary regions of C9

The Membrane Attack Complex (MAC) is responsible for forming large β-barrel channels in the membranes of pathogens, such as gram-negative bacteria. Off-target MAC assembly on endogenous tissue is associated with inflammatory diseases and cancer. Accordingly, a human C5b-9 specific antibody, aE11, has been developed that detects a neoepitope exposed in C9 when it is incorporated into the C5b-9 complex, but not present in the plasma native C9. For nearly four decades aE11 has been routinely used to study complement, MAC-related inflammation, and pathophysiology. However, the identity of C9 neoepitope remains unknown. Here, we determined the cryo-EM structure of aE11 in complex with polyC9 at 3.2 Å resolution. The aE11 binding site is formed by two separate surfaces of the oligomeric C9 periphery and is therefore a discontinuous quaternary epitope. These surfaces are contributed by portions of the adjacent TSP1, LDLRA, and MACPF domains of two neighbouring C9 protomers. By substituting key antibody interacting residues to the murine orthologue, we validated the unusual binding modality of aE11. Furthermore, aE11 can recognise a partial epitope in purified monomeric C9 in vitro, albeit weakly. Taken together, our results reveal the structural basis for MAC recognition by aE11.

Local complement synthesis-A process with near and far consequences for ischemia reperfusion injury and transplantation

The model of the solid organ as a target for circulating complement deposited at the site of injury, for many years concealed the broader influence of complement in organ transplantation. The study of locally synthesized complement especially in transplantation cast new light on complement's wider participation in ischaemia-reperfusion injury, the presentation of donor antigen and finally rejection. The lack of clarity, however, has persisted as to which complement activation pathways are involved and how they are triggered, and above all whether the distinction is relevant. In transplantation, the need for clarity is heightened by the quest for precision therapies in patients who are already receiving potent immunosuppressives, and because of the opportunity for well-timed intervention. This review will present new evidence for the emerging role of the lectin pathway, weighed alongside the longer established role of the alternative pathway as an amplifier of the complement system, and against contributions from the classical pathway. It is hoped this understanding will contribute to the debate on precisely targeted versus broadly acting therapeutic innovation within the aim to achieve safe long term graft acceptance.

Upregulation of complement C1q reflects mucosal regeneration in a mouse model of colitis

Confirming mucosal healing is important in inflammatory bowel disease treatment. Complement C1q-mediated Wnt signaling activation has recently been suggested to mediate tissue repair and mucosal regeneration. We investigated the involvement of complement C1q and Wnt signaling in intestinal mucosal regeneration using a murine colitis model. The colitis model was established by providing C57BL/6J mice with 4% dextran sodium sulfate (DSS) for 1 week (inflammation phase) followed by regular water for 2 weeks (recovery phase). After 3 weeks, we investigated the relationship between C1q in serum and colonic tissue during the inflammation and recovery phases. We assessed Wnt signaling activity by evaluating β-catenin expression in mouse intestinal tissue. Serum C1q levels were elevated during the recovery phase. C1q-specific staining indicated high C1q expression in pathological intestinal tissue during the inflammation and recovery phases. C1q mRNA and protein expression was increased during both phases. Interestingly, C1q-expressing cells were consistent with macrophages (F4/80-positive cells). Moreover, the expression of β-catenin increased in the colonic tissues during the recovery phase of DSS-induced colitis but decreased during the inflammation phase of DSS-induced colitis. C1q expression may mediate Wnt signaling activity and intestinal epithelial regeneration.

Collectin-11 (CL-11) Is a Major Sentinel at Epithelial Surfaces and Key Pattern Recognition Molecule in Complement-Mediated Ischaemic Injury

The complement system is a dynamic subset of the innate immune system, playing roles in host defense, clearance of immune complexes and cell debris, and priming the adaptive immune response. Over the last 40 years our understanding of the complement system has evolved from identifying its presence and recognizing its role in the blood to now focusing on understanding the role of local complement synthesis in health and disease. In particular, the local synthesis of complement was found to have an involvement in mediating ischaemic injury, including following transplantation. Recent work on elucidating the triggers of local complement synthesis and activation in renal tissue have led to the finding that Collectin-11 (CL-11) engages with L-fucose at the site of ischaemic stress, namely at the surface of the proximal tubular epithelial cells. What remains unknown is the precise structure of the damage-associated ligand that participates in CL-11 binding and subsequent complement activation. In this article, we will discuss our hypothesis regarding the role of CL-11 as an integral tissue-based pattern recognition molecule which we postulate has a significant contributory role in complement-mediated ischaemic injury.

A novel protocol allowing oral delivery of a protein complement inhibitor that subsequently targets to inflamed colon mucosa and ameliorates murine colitis

While there is evidence of a pathogenic role for complement in inflammatory bowel disease, there is also evidence for a protective role that relates to host defence and protection from endotoxaemia. There is thus concern regarding the use of systemic complement inhibition as a therapeutic strategy. Local delivery of a complement inhibitor to the colon by oral administration would ameliorate such concerns, but while formulations exist for oral delivery of low molecular weight drugs to the colon, they have not been used successfully for oral delivery of proteins. We describe a novel pellet formulation consisting of cross-linked dextran coated with an acrylic co-polymer that protects the complement inhibitor CR2-Crry from destruction in the gastrointestinal tract. CR2-Crry containing pellets administered by gavage, were characterized using a therapeutic protocol in a mouse model of dextran sulphate sodium (DSS)-induced colitis. Oral treatment of established colitis over a 5-day period significantly reduced mucosal inflammation and injury, with similar therapeutic benefit whether or not the proton pump inhibitor, omeprazole, was co-administered. Reduction in injury was associated with the targeting of CR2-Crry to the mucosal surface and reduced local complement activation. Treatment had no effect on systemic complement activity. This novel method for oral delivery of a targeted protein complement inhibitor will reduce systemic effects, thereby decreasing the risk of opportunistic infection, as well as lowering the required dose and treatment cost and improving patient compliance. Furthermore, the novel delivery system described here may provide similar benefits for administration of other protein-based drugs, such as anti-tumour necrosis factor-α antibodies.

Neutralization of complement component C5 ameliorates the development of dextran sulfate sodium (DSS)-colitis in mice

The complement system is a potent effector of innate immunity. To elucidate the pathophysiological role of the complement system in inflammatory bowel disease, we evaluated the effects of anti-C5 antibodies on the development of dextran sulfate sodium-induced colitis in mice. Dextran sulfate sodium-colitis was induced in BALB/c mice with intraperitoneal administrations of anti-C5 antibodies (1 mg/body [DOSAGE ERROR CORRECTED]) every 48 h. Tissue samples were evaluated by standard histological procedures. The mucosal mRNA expression of the inflammatory cytokines was analyzed by real-time PCR. Body weight loss in the mice was completely blocked by the administration of anti-C5 antibody. The disease activity index was significantly lower in the anti-C5 antibody-treated mice than the dextran sulfate sodium mice. The colonic weight/length ratio, histological colitis score and mucosal myeloperoxidase activity were significantly lower in the anti-C5 antibody-treated mice than the dextran sodium sulfate mice. The administration of the anti-C5 antibody significantly reduced the mucosal expression of mRNAs for tumor necrosis factor-α, interleukin-1β and interleukin-6. In conclusion, the complement system plays a role in the development of dextran sodium sulfate-induced experimental colitis.

Complement-dependent injury and protection in a murine model of acute dextran sulfate sodium-induced colitis

Complement plays a key role in the pathophysiology of many inflammatory diseases, and in this study, we investigated the role of complement in the pathogenesis of inflammatory bowel disease. Compared to wild-type mice, mice deficient in C3 or factor B were protected from acute dextran sulfate sodium (DSS)-induced colitis. C1q/mannose-binding lectin (MBL) double-deficient mice, however, exhibited more severe colitis than wild-type mice. When mice were allowed to recover after DSS treatment, all C1q/MBL(-/-) mice died by day 2 of recovery period, and, surprisingly, all C3(-/-) and factor B(-/-) mice died by day 5. Serum endotoxin levels were significantly increased in complement-deficient mice prior to death, particularly in C1q/MBL(-/-) mice, and antibiotic treatment prevented the lethal effect of DSS in all complement-deficient mice. In contrast to complement deficiency, targeted complement inhibition with either complement receptor 2 (CR2)-Crry (blocks all pathways at C3 activation) or CR2-factor H (blocks alternative pathway) was highly protective at treating established acute colitis. Endotoxin levels remained low in complement-inhibited mice, and complement inhibition also reduced inflammatory cytokines, leukocyte infiltration, and tissue injury while improving wound repair and mucosal healing. CR2-factor H provided more effective protection than CR2-Crry. Thus, complement has both pathogenic and protective roles in acute DSS-induced colitis, and whereas the alternative pathway appears to play a key role in tissue inflammation and injury, the classical/lectin pathway provides important protection in terms of host defense and wound repair. Targeted inhibition of the alternative pathway may represent a therapeutic modality for treating acute phases of inflammatory bowel disease.

Human colonic epithelial cells detect and respond to C5a via apically expressed C5aR through the ERK pathway

Intestinal epithelial cells (IECs) exhibit numerous adaptations to maintain barrier function as well as play sentinel roles by expressing receptors for microbial products and antimicrobial peptides. The complement system is another important innate sensing and defense mechanism of the host against bacteria and increasing evidence shows that complement plays a role in colitis. The split component C5a is a potent proinflammatory molecule, and the C5a receptor (C5aR) CD88 has been reported on multiple cell types. Here, we examined the question of whether human colonic cell lines can detect activated complement via C5aR and what signaling pathway is critical in the subsequent responses. T84, HT29, and Caco2 cell lines all possessed mRNA and protein for C5aR and the decoy receptor C5L2. Polarized cells expressed the proteins on the apical cell membrane. C5a binding to the C5aR on human IECs activates the ERK pathway, which proved critical for a subsequent upregulation of IL-8 mRNA, increased permeability of monolayers, and enhanced proliferation of the cells. The fact that human IECs are capable of detecting complement activation in the lumen via this anaphylatoxin receptor highlights the potential for IECs to detect pathogens indirectly through complement activation and be primed to amplify the host response through heightened inflammatory mediator expression to further recruit immune cells.

A possible role for complement in the pathogenesis of chronic chagasic cardiomyopathy

The membrane attack complex (MAC) of complement participates in several inflammatory and proliferative processes by releasing pro-inflammatory cytokines and growth factors from target cells. Chronic Chagasic cardiomyopathy (CCH) is a parasitic dilated cardiopathy, characterized by severe fibrosis and inflammation, which differs from idiopathic dilated cardiomyopathy (DCM). Trypanosoma cruzi, the pathogenic organism of CCH, is a strong complement activator and can also induce alternative pathway activation by mammalian cells. This study explored whether the myocardium in CCH patients has increased MAC deposition, an expression of complement activation, compared to DCM patients. MAC was semi-quantified in endomyocardial human samples (29 CCH subjects, 18 DCM subjects, and four controls) by immunohistochemistry. MAC was present in the sarcolemma of 38% of CCH, 5.5% of DCM (p<0.02), and 0% of controls, and in interstitial inflammatory cells of CCH. No difference was observed in the expression of the complement regulatory protein CD59, indicating that increased MAC deposition is likely to be the result of complement activation rather than decreased protection. It is proposed that the increased MAC deposition found in CCH, but not in DCM or controls, may help to explain the diffuse myocardial fibrosis and inflammation characteristic of the disease.

Mannan-binding lectin (MBL) gene polymorphisms in ulcerative colitis and Crohn's disease

The inflammatory bowel diseases (IBD), Crohn's disease (CD), and ulcerative colitis (UC), are complex multifactorial traits involving both environmental and genetic factors. Mannan-binding lectin (MBL) plays an important role in non-specific immunity and complement activation. Point mutations in codons 52, 54 and 57 of exon 1 of the MBL gene are associated with decreased MBL plasma concentrations and increased susceptibility to various infectious diseases. If these MBL mutations could lead to susceptibility to putative IBD-etiological microbial agents, or could temper the complement-mediated mucosal damage in IBD, MBL could function as the link between certain microbial, immunological and genetic factors in IBD. In this study, we investigated the presence of the codon 52, 54 and 57 mutations of the MBL gene in 431 unrelated IBD patients, 112 affected and 141 unaffected first-degree relatives, and 308 healthy control individuals. In the group of sporadic IBD patients (n = 340), the frequency of the investigated MBL variants was significantly lower in UC patients when compared with CD patients (P = 0.01) and with controls (P = 0.02). These results suggest that MBL mutations which decrease the formation of functional MBL could protect against the clinical development of sporadic UC, but not of CD. This could be explained by the differential T-helper response in both diseases.

Cellular localization of complement C3 and C4 transcripts in intestinal specimens from patients with Crohn's disease

It has been suggested that the increase in C3 and C4 levels in jejunal perfusates of patients with Crohn's disease (CD) results from local intestinal synthesis of complement. The present study evaluated the expression of these complement genes in inflamed tissues from patients with CD. Surgically resected specimens from patients with CD and control tissue obtained from subjects with adenocarcinoma of the colon were evaluated for C3 and C4 gene expression by the use of 35S-labelled anti-sense RNA probes. All tissue samples, diseased and normal tissue, expressed C4 mRNA throughout in the intestinal epithelium. C3 mRNA was not detected in epithelial cells in histologically normal tissue, but in diseased specimens there was a focal distribution of C3 mRNA in epithelial cells of the crypts, but not in villous epithelium. Focal C3 gene expression correlated with crypt abscess formation and the presence of polymorphonuclear leucocytes in the lumen of the crypts. In addition, C3 mRNA was also found in macrophages of the submucosa. These macrophages were CD68+, fusiform with faint cytoplasm and morphologically different from the large rounded lamina propria macrophages, which do not express C3 mRNA. Multinucleated giant cells did not express either C3 or C4 genes. In addition to its presence in intestinal epithelium, C4 mRNA was also expressed in mast cells, which however did not express C3 mRNA. These observations identify cells in the intestinal wall expressing complement genes and support the hypothesis that there is local regulated production of complement in the intestine of patients with CD, and subsequent complement activation may contribute to the inflammatory process.

Counter-regulatory effect of sodium butyrate on tumour necrosis factor-alpha (TNF-alpha)-induced complement C3 and factor B biosynthesis in human intestinal epithelial cells

The various biological activities of butyrate have been well documented. In this study, we tested the effects of butyrate on TNF-alpha-induced complement C3 and factor B biosynthesis in human intestinal epithelial cells. The biosynthesis of C3, factor B and IL-8 was evaluated at the protein and mRNA levels. To evaluate transcriptional activation, the nuclear run-on assay was performed. The transcription factor-DNA binding activity was assessed by an electrophoretic gel mobility shift assay (EMSA). In the intestinal epithelial cell lines HT-29, T84 and Caco-2, sodium butyrate enhanced TNF-alpha-induced C3 secretion, but suppressed TNF-alpha-induced factor B and IL-8 secretion. Nuclear run-on assay revealed that transcriptional regulatory mechanisms are involved in the effects of sodium butyrate. The EMSAs indicated that sodium butyrate suppressed TNF-alpha-induced nuclear factor (NF)-kappaB- and activation protein (AP)-1-DNA binding activity, but enhanced TNF-alpha-induced activation of CCAAT/enhancer-binding protein (C/EBP)beta (NF-IL-6)-DNA binding activity. Sodium butyrate induced a counter-regulatory effect on TNF-alpha-induced C3 and factor B biosynthesis in human intestinal epithelial cells. Butyrate action has been discussed with its activity to induce histone hyperacetylation, but its counter-regulatory effect on complement biosynthesis may be closely associated with the modulation of transcription factor activation.

Endotoxemia in mice stimulates production of complement C3 and serum amyloid A in mucosa of small intestine

We examined the effect of endotoxemia in mice on protein and mRNA levels for the acute phase proteins complement C3 and serum amyloid A (SAA) in jejunal mucosa. Endotoxemia was induced in mice by the subcutaneous injection of 250 microg lipopolysaccharide per mouse. Control mice were injected with saline. C3 and SAA were measured by ELISA. Messenger RNA levels were determined by Northern blot analysis or competitive PCR. Immunohistochemistry was performed to determine in which cell type(s) C3 and SAA were present. Mucosal C3 and SAA protein and mRNA levels were increased in endotoxemic mice. Immunohistochemistry showed that C3 was present in both enterocytes and cells of the lamina propria, whereas SAA was seen mainly in lamina propria cells. Results suggest that endotoxemia stimulates production of C3 and SAA in small intestinal mucosa. The response may be regulated at the transcriptional level and probably reflects increased synthesis of the acute phase proteins in both enterocytes and cells of the lamina propria.

Autoimmunity to tropomyosin isoforms in ulcerative colitis (UC) patients and unaffected relatives

Autoimmunity to cytoskeletal protein tropomyosin (TM) has been demonstrated in UC. However, the TM isoforms involved in this IgG-mediated autoimmune response in UC and the possible presence of serum IgG antibodies against TM (hTMs IgG) in unaffected UC relatives are unknown. The aim of this study was to investigate the human TM (hTM) isoforms recognized by serum IgG from UC and to explore whether hTM IgG antibodies are present in healthy UC relatives. We studied 33 UC patients with 58 unaffected relatives, 31 Crohn's disease (CD) patients with 31 unaffected relatives and 20 controls (C). Serum IgG against four recombinant hTM isoforms (hTM1, 2, 3, 5) were tested by ELISA. p-ANCA were tested by ELISA and immunofluorescence. Serum hTM1 and hTM5 IgG were higher in UC patients than in CD and C (P<0.005). Among UC patients 52% were seropositive for hTM1 and 64% for hTM5 (P<0.001 versus CD and C). In UC, hTM5 IgG were higher in p-ANCA+ than in ANCA- patients (P=0.04). In UC relatives hTM1 IgG were higher than in CD relatives and C (P<0.01). UC relatives were more frequently seropositive for hTM1 than hTM5 IgG (P=0.001). while probands were more frequently seropositive for hTM5 IgG (P=0.008). We conclude that autoimmunity to hTMI and hTM5 is a feature of UC, while hTM1 IgG differentiate UC relatives from controls. A genetic susceptibility to immune recognition of hTM isoforms in UC is suggested.

Detection of complement C3 and factor B gene expression in normal colorectal mucosa, adenomas and carcinomas

Local secretion of complement components in the human intestine has been previously reported. However, the cellular source has not been identified. In this study, we demonstrate complement C3 and factor B mRNA expression in the normal colonic mucosa by in situ hybridization analysis. C3 and factor B genes were found to be expressed at high levels in the epithelial cells of the lower parts of the crypts in colonic mucosa, and this expression decreased gradually from the crypt base to the luminal surface. At the upper crypt and the luminal surface, these genes almost disappeared. C3 and factor B genes were expressed in all crypts at the same level. Furthermore, C3 and factor B gene expression was also identified in adenomas and carcinomas. In these neoplastic tissues, C3 and factor B genes were expressed uniformly, and the polarized distribution observed in the normal crypts was not detected. It is likely that complement components are locally synthesized in the intestine, and that these complement components may actively participate in normal immune and inflammatory responses over the enormous surface area of the intestinal mucosa.

Beta2 integrin/ICAM expression in Crohn's disease

We have previously reported on the expression of the beta2 integrin family of adhesion molecules and their ligands, the ICAM molecules, in the normal human intestine. These molecules likely have a role to play in the inflammatory response and, therefore, were studied in a group of patients with Crohn's disease. A comprehensive study was undertaken in both colon (n = 8) and ileum (n = 10) specimens from 15 patients who underwent surgical resections. Immunohistochemistry was performed for CD18, CD11a, CD11b, CD11c, alphad, ICAM-1, ICAM-2, and ICAM-3. Each of the mucosal, submucosal, muscle, and adventitial layers were scored for expression. Specimens from normal colon (n = 15), normal ileum (n = 6), and ulcerative colitis (n = 7) were used for comparisons. Compared with normal, the expression in the colon mucosa and submucosa in Crohn's disease was increased for all beta2 integrins. Mucosal CD11c expression was significantly greater in Crohn's disease than in ulcerative colitis. In the colon muscle and adventitial layers the expression in Crohn's disease was similar to normal but increased compared with ulcerative colitis. In Crohn's disease ileum, the beta2 integrin mucosal and submucosal expression was similar to normal; however, muscle and adventitial expression was increased, particularly for CD11c. Colon ICAM-1, ICAM-2, and ICAM-3 expression in Crohn's disease was similar to that seen in ulcerative colitis. ICAM-1 was predominantly expressed on endothelium but in the inflammatory bowel diseases was also evident on mucosal mononuclear cells. ICAM-1 and ICAM-2 expression was increased in Crohn's disease colon and ileum compared with normals. This was most notable in ileal mucosa since ICAM-2 is typically absent in normal ileal mucosa. In summary, we are reporting a comprehensive immunohistochemical study of the differential expression of beta2 integrins, including the newly described alphad molecule, and the ICAM molecules in all layers of the colon and ileum from patients with Crohn's disease. The increased expression of these molecules may have implications for therapeutic interventions in Crohn's disease.

The central role of chemokines (chemotactic cytokines) in the immunopathogenesis of ulcerative colitis and Crohn's disease

The final composition of leukocytes present in a site of inflammation in response to chemokine stimulation and activation may depend on both the nature of the secreted chemokines as well as the relative expression of the multitude of specific chemokine cell surface receptors on many different cell types. Because related receptors with different affinities and cross-reactive binding capabilities are present on each type of leukocyte, relative differences in receptor distribution and receptor affinity for specific chemokines may significantly influence which cells are ultimately attracted to and activated by each individual chemokine. Production of IL-8, MCP-1, and ENA-78 by endothelial cells, LPMNC, and epithelial cells in IBD could establish a chemotactic gradient capable of influencing the increased migration of monocytes/macrophages, granulocytes, and lymphocytes from the blood stream through the endothelium into both the mucosa and submucosa during chronic IBD. The ability of chemokines to induce chemotaxis, leukocyte activation, granule exocytosis, increased production of metalloenzymes, and up-regulation of respiratory burst activity indicates that there may be a variety of different mechanisms by which chemokines could markedly increase chronic inflammation and chronic intestinal tissue destruction in IBD.

Complement activation of electrogenic ion transport in isolated rat colon

The complement cascade is an important component in many immune and inflammatory reactions and may contribute to both the diarrhoea and inflammation associated with inflammatory bowel disease. Isolated rat colonic mucosae were voltage clamped in Ussing chambers. Basolateral addition of zymosan-activated whole human serum (ZAS) induced a rapid onset, transient inward short circuit current (SCC). This response was concentration dependent and was significantly attenuated by pre-heating ZAS at 60 degrees C for 30 min. Depletion of complement from normal human serum with cobra venom factor (CVF) significantly lowered SCC responses. Chloride was the primary charge carrying ion as responses to ZAS were abolished in the presence of the loop diuretic bumetanide. The complement component C3a stimulated ion transport but not to the same extent as whole serum. Exogenous C5 was without effect. The cyclooxygenase inhibitor piroxicam significantly attenuated the response to ZAS. These findings support the possibility that complement activation may contribute to the pathophysiology of secretory diarrhoea since activation of electrogenic chloride secretion converts intestinal epithelia to a state of net fluid secretion.

Defective expression of gp180, a novel CD8 ligand on intestinal epithelial cells, in inflammatory bowel disease

Previous studies support a role for intestinal epithelial cells (IEC) as antigen-presenting cells in mucosal immune responses. T cells activated by IEC are CD8+, suppressor in function, and dependent upon CD8-associated p56lck activation. A 180-kD glycoprotein (gp180) recognized by mAbs B9 and L12 has been identified and shown to be important in CD8+ T cell activation by IEC. Since IEC derived from patients with inflammatory bowel disease (IBD) are incapable of activating CD8+ T cells, we asked whether this correlated with gp180 expression. While frozen sections of normal bowel revealed bright gp180 staining on all IEC, both inflamed and uninflamed ulcerative colitis (UC) specimens showed patchy staining. In Crohn's disease (CD), staining was faint to absent. Flow cytometry confirmed immunohistochemical data. The staining patterns correlated with the ability of IEC to activate CD8-associated p56lck. Normal IEC induced phosphorylation of p56lck in CD8alpha but not CD4+ transfectants. In contrast, both UC and CD IEC activated CD4 and, to a much lesser extent, CD8-associated p56lck. Thus, gp180 expression by IBD IEC appears to be altered, and correlates with a functional alteration of lck activation. This defect may reflect a more proximal event in the pathogenesis of IBD.

Alterations of the mucosal immune system in inflammatory bowel disease

The normal intestinal immune system is under a balance in which proinflammatory and anti-inflammatory cells and molecules are carefully regulated to promote a normal host mucosal defense capability without destruction of intestinal tissue. Once this careful regulatory balance is disturbed, nonspecific stimulation and activation can lead to increased amounts of potent destructive immunologica and inflammatory molecules being produced and released. The concept of balance and regulation of normal mucosal immune and inflammatory events is indicative of how close the intestine is to developing severe inflammation. The normal intestinal mucosal immune system is constantly stimulated by lumenal contents and bacteria. The stimulatory molecules present in the intestinal lumen that activate and induce subsequent mucosal immunologic and inflammatory events include bacterial cell wall products, such as peptidoglycans and lipopolysaccharides, as well as other chemotactic and toxic bacterial products that are produced by the many different types of bacteria within the gastrointestinal tract. These highly stimulatory bacterial cell wall products are capable of activating macrophages and T lymphocytes to release potent proinflammatory cytokines, including interleukin-1 (IL-1), interleukin-6 (IL-6), and tumor necrosis factor alpha (TNF-alpha). IL-1, IL-6, and TNF-alpha increase the presence of human leukocyte antigen (HLA) class II antigen-presenting molecules on the surfaces of epithelial cells, endothelial cells, macrophages, and B cells, thus increasing their ability to present lumenal antigens and bacterial products. The proinflammatory cytokines IL-1 and TNF-alpha also increase the ability of epithelial cells, endothelial cells, macrophages, and fibroblasts to secrete potent chemotactic cytokines, such as interleukin-8 (IL-8) and monocyte chemoattractant protein-1 (MCP-1), which serve to increase the movement of macrophages and granulocytes from the circulation into the inflamed mucosa. Thus, through lumenal exposure to potent, nonspecific stimulatory bacterial products, the state of activation of the intestinal immune system and mucosal inflammatory pathways are markedly up-regulated. This raises the question of whether there is a deficiency in effective down-regulation through the absence of normally suppressive cytokines such as interleukin-10 (IL-10), transforming growth factor-beta (TGF-beta), interleukin-4 (IL-4), and IL-1 receptor antagonist. Normally, the turning off of the active and destructive immunologic and inflammatory events should occur following the resolution of a bacterial or viral infection that has been appropriately defended against and controlled by the mucosal immune system. In inflammatory bowel disease (IBD), however, the down-regulatory events and processes that should turn off the immunologic and inflammatory protective processes, once the pathogenic agent has been cleared, appear to be deficient or only partially effective. We may find that we ultimately are dealing with disease processes that have more than one genetic or cellular basis. The improved understanding of the immunopathophysiology of IBD will allow exploration of novel immunologic and genetic approaches, such as gene replacement therapy, administration of a suppressor cytokine or an altered cell surface antigen, the administration of humanized monoclonal antibodies directed against proinflammatory cytokines, or the development of newer strategies against fundamental cell biologic mechanisms such as adhesion molecules.

Regulation of complement C3 synthesis by interleukin-1 and transforming growth factor-beta in rat non-transformed intestinal epithelial cell line, IEC-6

Intestinal epithelial cells are an important source of many biologically active molecules that modulate immune responses in the mucosa. The purpose of this study was to demonstrate the synthesis of complement C3 component in the rat non-transformed crypt-like intestinal epithelial cell line, IEC-6. Unstimulated IEC-6 cells secreted a low level of C3 protein and showed weak expression of C3 mRNA. The addition of interleukin (IL)-1 beta induced a dose- and time-dependent increase in C3 production. These effects of IL-1 beta were observed at a concentration as low as 0.01 ng/ml and reached a plateau at a concentration of 5 ng/ml. The effects were observed at the mRNA level as early as 6 h after the beginning of incubation. Transforming growth factor (TGF)-beta alone had no effect. However, TGF-beta at low concentrations (0.001-1 ng/ml) enhanced the effect of IL-1 beta in increasing C3 production; this enhancement was not observed at high concentrations (5-10 ng/ml). These effects of TGF-beta were also observed at the mRNA level. The present findings indicate that intestinal epithelial cells are indeed capable of synthesizing complement C3 in response to IL-1 beta and TGF-beta.

Cytokines and gastrointestinal disease mechanisms

Cytokines mediate immune responses and are detectable in the normal gastrointestinal mucosa. It is unclear how cytokines are physiologically regulated but in inflammatory enteropathies their expression is often greatly increased and may account for the tissue damage observed. T-cells may be sub-divided according to the pattern of cytokines which they secrete. TH1 cytokine expression is increased in delayed type IV cell mediate immune responses whereas TH2 cytokines are raised in diseases in which humoral mechanisms are more important. Cytokines are secreted by macrophages in relatively greater amounts than from T-cells. They are non-specific products of inflammation and may account for the majority of tissue damage seen in mucosal disease. The pattern of cytokine secretion may determine the immunopathogenesis of an inflammatory disorder. The ultimate goal of cytokine research is the development of therapeutic measures based on a better understanding of their actions which may be achieved with a better understanding of the molecular immune-microenvironment in inflammatory enteropathies. Studies with transgenic mice and gene targeted mice have important implications to the understanding of the immune system and its role in intestinal diseases.

Differential expression of complement proteins and regulatory decay accelerating factor in relation to differentiation of cultured human colon adenocarcinoma cell lines

Self protection of host cells against inadvertent injury resulting from attack by autologous complement proteins is well reported for vascular epithelium. In intestinal epithelium, the expression of C complement proteins and regulatory proteins remains currently poorly reported. This study looked at the distribution of C complement proteins and regulatory decay accelerating factor (DAF) in four cultured human intestinal cell lines of embryogenic or colon cancer origins. C3 and C4 proteins and DAF were widely present in human colon adenocarcinoma T84, HT-29 glc-/+ cells compared with human embryonic INT407 cells. In contrast, no expression of C5, C5b-9, and CR1 was seen for any of the cell lines. Taking advantage of the Caco-2 cells, which spontaneously differentiate in culture, it was seen that the C3, C4, and DAF were present in undifferentiated cells and that their expression increased as a function of the cell differentiation. These results, taken together with other reports on the presence of C complement proteins and DAF in the intestinal cells infer that the expression of regulatory C complement proteins develops in parallel with the expression of C proteins to protect these cells against the potential injury resulting from the activation of these local C proteins. Moreover, the finding that the pathogenic C1845 Escherichia coli binds to the membrane bound DAF in the cultured human intestinal cells synthetising locally C proteins and regulatory C proteins supports the hypothesis that E coli could promote inflammatory disorders by blocking local regulatory protein function.

Physiology and pathophysiology of complement: progress and trends

The complement system comprises a family of at least 20 plasma and membrane proteins that interact in a tightly regulated cascade system to destroy invading bacteria and prevent the deposition of immune complexes in the tissues. This brief review addresses the basic mechanisms of complement activation and control and describes the active fragments produced during complement activation. The biological importance of the complement system is amply illustrated in patients with complement deficiencies, who are susceptible to bacterial infections and immune complex diseases. The involvement of complement in other immunological diseases is an expanding area of clinical research, supported by the development of new assays for the identification of complement activation. This area is discussed here with particular reference to neurological diseases. A promising new prospect involves the use of complement inhibitory molecules in therapy of complement-mediated disease and this exciting area is also discussed. Novel physiological roles of complement also are being revealed and new evidence that complement and complement receptors play an important role in reproduction is summarized. It is hoped that this brief overview will convey some of the enthusiasm currently pervading research in this underappreciated area of immunology.

Alterations in the mucosal immune system in ulcerative colitis and Crohn's disease

Emphasis is now being placed upon obtaining a better understanding of the regulatory cytokines that normally downregulate acute intestinal inflammation. These inhibitory cytokines appear to be missing or not functioning properly in patients with inflammatory bowel disease (IBD), thereby leading to perpetuation of inflammation. As we obtain an increased understanding of immune and inflammatory regulatory processes in the intestine, we will be able to devise better future therapeutic strategies for use in our IBD patients.

New concepts of allergy to cow's milk

CMA is a diagnostic and therapeutic problem in the field of pediatrics. The response to oral cow's milk challenge was evaluated in patients with suspective CMA. To investigate immune mechanisms in different subtypes of CMA, suppressor activity, in vitro IFN-gamma and IL-4 generation, eosinophil activation and humoral immune response were measured. Also, changes in the function of the immune system were evaluated when patients had clinically recovered from CMA. The results of different measurements were correlated with the patients' clinical response to cow's milk challenge. The prechallenge suppressor activity was found to be low in patients with challenge-proven CMA when compared with those clinically negative to oral cow's milk challenge but had normalized after a 4-18 month milk-elimination period in patients who had recovered from CMA. IFN-gamma generation was low in patients with active CMA but had clearly enhanced in patients recovering from CMA. The serum concentration of ECP increased significantly during oral cow's milk challenge in patients with cutaneously manifested CMA. Measured by the ELISPOT method, the patients with active CMA mounted a high, non-antigen-specific immune response but were unable to direct the antigen-specific response, especially in the IgA class. After a follow-up of a mean 13.5 months on cow's milk elimination diet, the immune response to cow's milk antigens had developed in patients who had recovered from CMA. The development in the capacity to mount an adequate immune response to specific protein antigens, especially in the IgA class, can be interpreted as deriving from the development in T-lymphocyte regulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Aetiology and pathogenesis of chronic inflammatory bowel disease

While Crohn's disease and ulcerative colitis are both conditions characterized by intestinal inflammation, with some overlap in their clinical and histological features, they are essentially different in pathogenesis. Crohn's disease appears to be primarily a condition of chronic T-lymphocyte activation, with tissue damage induced by secondary macrophage activation. What activates the T-cells is unknown. In this chapter we look at the evidence for and against cell-wall deficient mycobacteria species, viral infection of vascular endothelium and luminal contents as potential mechanisms of chronic activation. In ulcerative colitis, by contrast, there is no strong evidence for T-cell activation, and humoral mechanisms predominate. While the finding of atypical anti-neutrophil cytoplasmic antibodies (P-ANCAs) may be useful in screening, the only novel pathogenetic discovery is the co-localization of a 40 kD colonic autoantibody with immunoglobulins and complement on the apical enterocyte surface. Despite the fundamental differences in initiating mechanisms, the two conditions have many 'downstream' inflammatory processes in common. We discuss the evidence for local production of cytokines, arachidonic acid metabolites and reactive oxygen and nitrogen radicals, highlighting the potential adverse consequences for intestinal vascular integrity.

Location of tumour necrosis factor alpha by immunohistochemistry in chronic inflammatory bowel disease

This study determined the location and tissue density of cells immunoreactive for tumour necrosis factor alpha (TNF alpha) in intestinal specimens from 24 patients with chronic inflammatory bowel disease (15 with Crohn's disease, nine with ulcerative colitis) and 11 controls. There was significantly increased density of TNF alpha immunoreactive cells in the lamina propria of both ulcerative colitis and Crohn's disease specimens, although the distribution of these cells differed in the two conditions. In ulcerative colitis most of the TNF alpha immunoreactivity was seen in the subepithelial macrophages, with comparatively less in the deep lamina propria, while in Crohn's disease immunoreactive cells were distributed evenly throughout the lamina propria. Increased submucosal immunoreactivity was found only in Crohn's disease, in which TNF alpha positive macrophages tended to cluster around arterioles and venules, often infiltrating and disrupting vascular endothelium. It is suggested that this degree of TNF alpha production probably contributes significantly to the pathogenesis of both Crohn's disease and ulcerative colitis, by impairing the integrity of epithelial and endothelial membranes, increasing inflammatory cell recruitment, and by prothrombotic effects on the vascular endothelium.

Epithelial cells bearing class II molecules stimulate allogeneic human colonic intraepithelial lymphocytes

HLA-DR+ gut epithelial cells may present antigen to intraepithelial lymphocytes (IEL). This study aimed to isolate an IEL population from the human colon to activate CD3 + IEL by a human colonic epithelial cell line (HT-29), bearing different concentrations of class II antigen (HLA-DR). IEL were isolated by a mechanical method from six patients with ulcerative colitis (UC) and from 14 control patients. IEL were cocultured with HT-29 which had been induced to express class II molecules by gamma-interferon (IFN-gamma) in a dose dependent manner. The phenotype and the subsequent expression of activation markers by the IEL were determined to two colour flow cytometry. The IEL population had a CD4/CD8 ratio similar to that seen in tissue sections. In the mixed cell culture, the degree of IEL activation showed a positive correlation with the degree of HLA-DR expression by the HT29 cells and the IEL secreted a IFN-gamma like factor that in turn stimulated the HT-29. Thus, depending on their expression of HLA molecules, colonic epithelial cells are able to activate CD3+CD8+IEL.

Mucosal capillary thrombi in rectal biopsies

We studied the initial rectal biopsy from 46 patients in whom subsequent follow-up established the diagnosis of either self-limited colitis or inflammatory bowel disease. An additional 12 non-inflamed rectal biopsies were also studied. There was between 2 and 8 years of follow-up in each of these cases. Staining for fibrin (MSB, fibrinogen), platelets (factor XIIIA, Y2/51), and capillary basement membrane (reticulin, collagen 4) was performed to identify thrombotic material within capillaries. Mucosal capillary thrombi were best identified by staining for factor XIIIA; thrombi were observed in 8/13 cases of ulcerative colitis, 4/10 cases of Crohn's disease, 1/3 cases of unspecified inflammatory bowel disease and 5/20 cases of self-limited colitis. The presence of capillary thrombi was not related to the severity of inflammation, but none of the control biopsies showed capillary thrombi. Their presence seems of little diagnostic value in distinguishing inflammatory bowel disease from self-limited colitis. The pathogenetic significance of these mucosal capillary thrombi is uncertain.

Circulating von Willebrand factor in inflammatory bowel disease

Raised circulating von Willebrand factor is a recognised marker of vascular injury. To evaluate the role of vascular injury in the pathogenesis of inflammatory bowel disease, serum von Willebrand factor in Crohn's disease, ulcerative colitis, confirmed bacterial diarrhoea, and healthy subjects was measured. von Willebrand factor values were raised in 9/14 patients (p = 0.007) with active Crohn's disease, 15/28 (p = 0.0004) with inactive Crohn's disease, 16/23 (p = 0.0003) with active ulcerative colitis, 9/27 (p = 0.04) with inactive ulcerative colitis, and 15/17 (p = 0.0001) patients with bacterial diarrhoea. Serum von Willebrand factor was unrelated to disease activity in Crohn's disease but was significantly raised in active (p = 0.02) compared with inactive ulcerative colitis. In contrast to controls, the detection of von Willebrand factor from inflammatory bowel disease sera and that from fractured endothelial cells was significantly inhibited by the reducing agent, dithiothreitol, suggesting the presence of an additional dithiothreitol sensitive form of the molecule derived from injured endothelial cells in inflammatory bowel disease. That serum von Willebrand factor is raised in quiescent as well as active Crohn's disease is compatible with the proposal that vascular injury is a fundamental abnormality in this disorder. The raised von Willebrand factor values in active inflammatory bowel disease and bacterial diarrhoea could be caused by either vascular injury, occurring secondary to bowel inflammation, or to an acute phase response resulting from endothelial cell stimulation by mediators released during the inflammatory process. Raised circulating von Willebrand factor could contribute to the increased risk of thrombosis associated with active inflammatory bowel disease.

Association of subepithelial deposition of activated complement and immunoglobulin G and M response to gluten in celiac disease

Patients with celiac disease produce not only immunoglobulin A (IgA) but also immunoglobulin G (IgG) and M (IgM) antibodies to gluten. Intake of dietary gluten may hence induce local complement activation and mucosal damage. Jejunal tissue sections from adult patients with celiac disease were examined by immunofluorescence with monoclonal antibodies to activation neoepitopes in C3b and the terminal complement complex (TCC). Subepithelial deposition of TCC was observed in 93% of 28 untreated and in 57% of 23 partly treated study subjects. The immunofluorescence staining intensity was well correlated with the serum level of gluten-specific IgG and IgM (but not IgA), the number of mucosal IgG-producing cells, and the degree of villous atrophy. Similar immune deposits were not observed in 5 successfully treated patients with celiac disease, 5 patients with dermatitis herpetiformis without jejunal villous atrophy, and 90% of 21 control patients with histologically normal jejunal mucosa. Gluten challenge increased the amount of subepithelial TCC and produced additional C3b deposition, suggesting recent complement activation. Ingested gluten might thus, via Ig-mediated subepithelial complement activation, damage the surface epithelium in celiac disease and induce compensatory crypt hyperplasia.

Local complement activation in inflammatory bowel disease

To trace sites for local complement activation in inflammatory bowel disease, an indirect two-colour immunofluorescence method was applied on prewashed and directly ethanol-fixed mucosal specimens from patients with ulcerative colitis, Crohn's colitis, or terminal ileitis. Monoclonal antibodies to the IgG subclasses and to neoepitopes of activated complement C3b and the terminal complement complex (TCC) were used in combination with rabbit antiserum to immunoglobulins and various complement components. Deposits of activated C3b were found on the luminal face of the surface epithelium in the most affected ulcerative colitis specimens from 91% of 23 studied patients, together with cytolytic TCC in 81%. Furthermore, there was a selective deposition of the immunoglobulin G subclass 1 (IgG1) within the epithelial immune complexes in 63% of 11 studied patients. These results suggested that IgG1 autoantibodies to brush-border antigen(s) induce a complement-mediated attack on the epithelium in ulcerative colitis. The epithelial complement deposition seen in Crohn's disease tended to be more granular and was observed in 5 of 10 patients with colitis and in 4 of 10 with ileitis. No co-localization of IgG was observed, suggesting that complement activation had been induced by the alternative pathway. Type III immune reaction may, in addition, take place in both diseases since there was evidence of continuous vascular complement activation in submucosal blood vessels.

Chemotaxis of germinal center B cells in response to C5a

An infiltrate of B cells and plasma cells is characteristic of certain chronic inflammatory lesions. However, mechanisms involved in the local accumulation of these cells have not been established. Efforts to demonstrate that B cells from normal animals can migrate in response to inflammation-induced chemoattractants have been inconclusive. The objective of this study was to determine if murine germinal center (GC) B cells could respond chemotactically to a C5a gradient. On successive days after secondary immunization, draining lymph nodes were harvested and the activated GC B cells isolated. These GC B cells were placed in modified Boyden chambers, incubated for 3 h and the distance the leading front of cells migrated through the filters was determined. The results show that GC B cells migrated to factors in zymosan- and lipopolysaccharide-activated serum. The migratory response demonstrated distinct kinetics. Cells isolated between 2 to 4 days after secondary immunization migrated, whereas cells isolated at day 0 and beyond day 6 did not. Checkerboard analysis revealed that the migratory response was attributable to both chemokinesis and chemotaxis. Anti-C5 inhibited the migration of day-3 GC B cells implicating C5 in the migration mechanism. Studies using recombinant C5a established that this C5 fragment was chemotactically active. In conclusion, GC B cells generally were not chemotactically active. However, at a particular stage of maturation B cells in the GC become responsive to C5a as a chemotactic agent. Thus, B cells from normal animals may respond chemotactically, and C5a may play a role in recruitment of recently activated B cells into inflammatory sites.

Expression of leukocyte adhesion molecules by mucosal mononuclear phagocytes in inflammatory bowel disease

Leukocyte adhesion molecules are important in cell-cell interactions of the immune system. Lymphocyte function-associated antigen 1 (cluster designation 11a) mediates interactions between T cells and mononuclear phagocytes through its ligand, the intercellular adhesion molecule 1 (CD54), whereas complement receptors 3 (CD 11b) and 4 (CD11c) are involved in complement-mediated phagocytosis. Expression of CD11 molecules and intercellular adhesion molecule 1 was studied in colonic biopsy specimens from 20 patients with inflammatory bowel disease and 10 normal controls. In normal colon, few mononuclear phagocytes expressed lymphocyte function-associated antigen 1 and intercellular adhesion molecule 1 at high densities. The major adhesion molecule was CD11c. Thus, the largest population of normal colonic mononuclear phagocytes was represented by quiescent, resident macrophages with likely phagocytic function. In inflammatory bowel disease, mononuclear phagocytes showed only a slight increase in CD11a expression and no significant change in expression of CD11b and CD11c. By contrast, the percentage of mononuclear phagocytes expressing intercellular adhesion molecule 1 was increased from 6.9% +/- 3.9% in controls to 69.2% +/- 12.8% in ulcerative colitis (P less than 0.001) and to 45.7% +/- 22.8% in Crohn's disease (P less than 0.01), showing a close relationship with histological activity. The increased expression of intercellular adhesion molecule 1 in inflammatory bowel disease indicates a state of immunological activation induced by local release of inflammatory cytokines. Such induction of intercellular adhesion molecule 1 on mononuclear phagocytes may be important in the maintenance of chronic inflammation by facilitating interactions with T cells and T-cell antigen recognition.

Infants and children with cow milk allergy/intolerance. Investigation of the uptake of cow milk protein and activation of the complement system

Seventeen children with challenge-verified cow milk allergy/intolerance (CMAI), age 3-78 months, median 12 months, were re-challenged with cow milk in increasing doses. All subjects developed symptoms, such as bronchospasm, rhinitis, diarrhoea, erythema or eczema. Blood samples were taken before and up to 24 h after the start of the challenge. The cow milk protein beta-lactoglobulin (BLG) was determined in serum with ELISA (lower detection limit 0.3 micrograms/l). BLG was detectable in five children at low levels (below 2 micrograms/l). Analysis of the size distribution of the BLG by size exclusion chromatography indicated immunoreactive material as small aggregates. Plasma samples were analysed by electroimmunoassay for complement factor split product C3d, which was not demonstrable above background values in any of the cases. CMAI in infants and children may not be related to systemic activation of the complement system and may be elicited without considerable amounts of immunoreactive BLG in the circulation.

Epithelial deposition of immunoglobulin G1 and activated complement (C3b and terminal complement complex) in ulcerative colitis

The epithelial destruction seen in ulcerative colitis remains unexplained. Complement activation has been proposed to be involved, but no definite evidence has been available to this end. In the present study, we examined immunohistochemically ulcerative colitis lesions with monoclonal antibodies to activation neoepitopes in the complement component C3b and in the cytolytically active terminal complement complex. Colonic tissue specimens from 23 patients with ulcerative colitis were examined by indirect two-color immunofluorescence staining with monoclonal antibodies to the four human immunoglobulin G subclasses and to activated complement C3b or terminal complement complex. All except two patients had activated C3b deposited apically on the surface epithelium of involved mucosa. Immunoglobulin G1 was found on the epithelium in extensively prewashed specimens from 7 of 11 patients, and a striking colocalization of immunoglobulin G1, C3b, and terminal complement complex was observed in 4. Immune deposits were not observed in 31 noninflamed specimens from the same ulcerative colitis patients. Only 1 of 44 histologically normal mucosae from 17 controls and 1 of 10 colonic adenomas contained some epithelial complement deposits. It is concluded that activated complement is often deposited along the brush border of the surface epithelium in active ulcerative colitis lesions and may be associated with immunoglobulin G1 autoantibody.