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

Local complement activation and modulation in mucosal immunity

The complement system is an evolutionarily conserved arm of innate immunity, which forms one of the first lines of host response to pathogens and assists in the clearance of debris. A deficiency in key activators/amplifiers of the cascade results in recurrent infection, whereas a deficiency in regulating the cascade predisposes to accelerated organ failure, as observed in colitis and transplant rejection. Given that there are over 60 proteins in this system, it has become an attractive target for immunotherapeutics, many of which are United States Food and Drug Administration-approved or in multiple phase 2/3 clinical trials. Moreover, there have been key advances in the last few years in the understanding of how the complement system operates locally in tissues, independent of its activities in circulation. In this review, we will put into perspective the abovementioned discoveries to optimally modulate the spatiotemporal nature of complement activation and regulation at mucosal surfaces.

Periodontitis increases the risk of gastrointestinal dysfunction: an update on the plausible pathogenic molecular mechanisms

Periodontitis is an immuno-inflammatory disease of the soft tissues surrounding the teeth. Periodontitis is linked to many communicable and non-communicable diseases such as diabetes, cardiovascular disease, rheumatoid arthritis, and cancers. The oral-systemic link between periodontal disease and systemic diseases is attributed to the spread of inflammation, microbial products and microbes to distant organ systems. Oral bacteria reach the gut via swallowed saliva, whereby they induce gut dysbiosis and gastrointestinal dysfunctions. Some periodontal pathogens like Porphyromonas. gingivalis, Klebsiella, Helicobacter. Pylori, Streptococcus, Veillonella, Parvimonas micra, Fusobacterium nucleatum, Peptostreptococcus, Haemophilus, Aggregatibacter actinomycetomcommitans and Streptococcus mutans can withstand the unfavorable acidic, survive in the gut and result in gut dysbiosis. Gut dysbiosis increases gut inflammation, and induce dysplastic changes that lead to gut dysfunction. Various studies have linked oral bacteria, and oral-gut axis to various GIT disorders like inflammatory bowel disease, liver diseases, hepatocellular and pancreatic ductal carcinoma, ulcerative colitis, and Crohn's disease. Although the correlation between periodontitis and GIT disorders is well established, the intricate molecular mechanisms by which oral microflora induce these changes have not been discussed extensively. This review comprehensively discusses the intricate and unique molecular and immunological mechanisms by which periodontal pathogens can induce gut dysbiosis and dysfunction.

Evidence for Genetic Causal Relationships Between Multiple Immune-Mediated Inflammatory Diseases and Age-Related Macular Degeneration: A Univariable and Multivariable Mendelian Randomization Study

INTRODUCTION

With the global aging population on the rise, age-related macular degeneration (AMD) poses a growing healthcare burden. Prior research hints at immune-mediated inflammatory diseases (IMIDs) potentially elevating AMD risk via diverse mechanisms. However, causality remains disputed as a result of confounding factors. Hence, our Mendelian randomization (MR) study aims to untangle this link, mitigating confounding effects to explore the IMID-AMD causal relationship. This study aims to investigate the causal relationship between IMIDs and AMD, providing new strategies for the prevention and treatment of AMD in clinical practice.

METHODS

This study was registered with PROSPERO, CRD42023469815. We obtained data on IMIDs and AMD from Genome-Wide Association Studies (GWAS) summary statistics and the FinnGen consortium. Rigorous selection steps were applied to screen for eligible instrumental single nucleotide polymorphisms (SNPs). We conducted univariate Mendelian randomization, inverse variance-weighted (IVW), weighted median, Mendelian randomization-Egger (MR-Egger), and multivariate Mendelian randomization (MVMR) analyses. Various sensitivity analysis methods were employed to assess pleiotropy and heterogeneity. The aim was to explore the causal relationships between IMIDs and AMD.

RESULTS

The MR analysis revealed that Crohn's disease (CD) (IVW: odd ratios (OR) 1.05, 95% CI (confidence interval) 1.01-1.10, p = 0.007), rheumatoid arthritis (RA) (IVW: OR 1.09, 95% CI 1.04-1.15, p = 0.0001), and type 1 diabetes (T1D) (IVW: OR 1.05, 95% CI 1.02-1.09, p = 0.001) were correlated with an elevated risk of AMD, while multiple sclerosis (MS) (IVW: OR 2.78E-18, 95% CI 2.23E-31 to 3.48E-05, p = 0.008) appeared to be protective against AMD. These findings were supported by an array of MR analysis methodologies and the MVMR approach.

CONCLUSION

Our study results, based on MR, provide genetic evidence indicating a causal relationship between specific IMIDs and AMD. CD, RA, and T1D are factors increasing the risk of AMD, while MS may have a protective effect.

Complement C3-Deficiency-Induced Constipation in FVB/N-C3em1Hlee/Korl Knockout Mice Was Significantly Relieved by Uridine and Liriope platyphylla L. Extracts

Complement component 3 (C3) deficiency has recently been known as a cause of constipation, without studies on the therapeutic efficacy. To evaluate the therapeutic agents against C3-deficiency-induced constipation, improvements in the constipation-related parameters and the associated molecular mechanisms were examined in FVB/N-C3em1Hlee/Korl knockout (C3 KO) mice treated with uridine (Urd) and the aqueous extract of Liriope platyphylla L. (AEtLP) with laxative activity. The stool parameters and gastrointestinal (GI) transit were increased in Urd- and AEtLP-treated C3 KO mice compared with the vehicle (Veh)-treated C3 KO mice. Urd and AEtLP treatment improved the histological structure, junctional complexes of the intestinal epithelial barrier (IEB), mucin secretion ability, and water retention capacity. Also, an improvement in the composition of neuronal cells, the regulation of excitatory function mediated via the 5-hydroxytryptamine (5-HT) receptors and muscarinic acetylcholine receptors (mAChRs), and the regulation of the inhibitory function mediated via the neuronal nitric oxide synthase (nNOS) and inducible NOS (iNOS) were detected in the enteric nervous system (ENS) of Urd- and AEtLP-treated C3 KO mice. Therefore, the results of the present study suggest that C3-deficiency-induced constipation can improve with treatment with Urd and AEtLP via the regulation of the mucin secretion ability, water retention capacity, and ENS function.

A prospective phase 2 clinical trial of a C5a complement inhibitor for acute GVHD with lower GI tract involvement

Involvement of lower gastrointestinal tract (LGI) occurs in 60% of patients with graft-versus-host-disease (GVHD). Complement components C3 and C5 are involved in GVHD pathogenesis. In this phase 2a study, we evaluated the safety and efficacy of ALXN1007, a monoclonal antibody against C5a, in patients with newly diagnosed LGI acute GVHD receiving concomitant corticosteroid. Twenty-five patients were enrolled; one was excluded from the efficacy analysis based upon negative biopsy. Most patients (16/25, 64%) had acute leukemia; 52% (13/25) had an HLA-matched unrelated donor; and 68% (17/25) received myeloablative conditioning. Half the patients (12/24) had a high biomarker profile, Ann Arbor score 3; 42% (10/24) had high-risk GVHD per Minnesota classification. Day-28 overall response was 58% (13/24 complete response, 1/24 partial response), and 63% by Day-56 (all complete responses). Day-28 overall response was 50% (5/10) in Minnesota high-risk and 42% (5/12) in high-risk Ann Arbor patients, increasing to 58% (7/12) by Day-56. Non-relapse mortality at 6-months was 24% (95% CI 11-53). The most common treatment-related adverse event was infection (6/25, 24%). Neither baseline complement levels (except for C5), activity, nor inhibition of C5a with ALXN1007 correlated with GVHD severity or responses. Further studies are needed to evaluate the role of complement inhibition in GVHD treatment.

The Complement Pathway: New Insights into Immunometabolic Signaling in Diabetic Kidney Disease

Significance: The metabolic disorder, diabetes mellitus, results in microvascular complications, including diabetic kidney disease (DKD), which is partly believe to involve disrupted energy generation in the kidney, leading to injury that is characterized by inflammation and fibrosis. An increasing body of evidence indicates that the innate immune complement system is involved in the pathogenesis of DKD; however, the precise mechanisms remain unclear. Recent Advances: Complement, traditionally thought of as the prime line of defense against microbial intrusion, has recently been recognized to regulate immunometabolism. Studies have shown that the complement activation products, Complement C5a and C3a, which are potent pro-inflammatory mediators, can mediate an array of metabolic responses in the kidney in the diabetic setting, including altered fuel utilization, disrupted mitochondrial respiratory function, and reactive oxygen species generation. In diabetes, the lectin pathway is activated via autoreactivity toward altered self-surfaces known as danger-associated molecular patterns, or via sensing altered carbohydrate and acetylation signatures. In addition, endogenous complement inhibitors can be glycated, whereas diet-derived glycated proteins can themselves promote complement activation, worsening DKD, and lending support for environmental influences as an additional avenue for propagating complement-induced inflammation and kidney injury. Critical Issues: Recent evidence indicates that conventional renoprotective agents used in DKD do not target the complement, leaving this web of inflammatory stimuli intact. Future Directions: Future studies should focus on the development of novel pharmacological agents that target the complement pathway to alleviate inflammation, oxidative stress, and kidney fibrosis, thereby reducing the burden of microvascular diseases in diabetes. Antioxid. Redox Signal. 37, 781-801.

Retinal drusen counts are increased in inflammatory bowel disease, and with longer disease duration, more complications and associated IgA glomerulonephritis

Retinal drusen are deposits of inflammatory proteins that are found in macular degeneration and glomerulonephritis and result, in part, from complement activation. This was a cross-sectional observational study of individuals with inflammatory bowel disease (IBD) recruited from a Gastroenterology clinic who underwent non-mydriatic retinal photography. Deidentified images were examined for drusen, and drusen counts and size were compared with matched controls, and examined for clinical associations. The cohort with IBD comprised 19 individuals with ulcerative colitis, 41 with Crohn's disease and three with indeterminate colitis, including 34 males (54%) and an overall median age of 48 (IQR 23) years. Their median IBD duration was 7 (IQR 10) years, median CRP level was 7 (IQR 14) mg/L, and 28 (44%) had complications (fistula, stricture, bowel resection etc.), while 28 with Crohn's disease (68%) had colonic involvement. Drusen counts were higher in IBD than controls (12 ± 34, 3 ± 8 respectively, p = 0.04). Counts ≥ 10 were also more common (14, 22%, and 4, 6%, p = 0.02, OR 4.21, 95%CI 1.30 to 13.63), and associated with longer disease duration (p = 0.01, OR 1.06, 95%CI 1.00 to 1.13), an increased likelihood of complications (p = 0.003, OR 6.90, 95%CI 1.69 to 28.15) and higher CRP levels at recruitment (p = 0.008, OR1.02, 95%CI 1.00 to 1.05). Increased retinal drusen were found in all four individuals with Crohn's disease and IgA glomerulonephritis. IBD and drusen may share pathogenetic mechanisms and underlying risk factors such as complement activation.

Dysregulation of the Enteric Nervous System in the Mid Colon of Complement Component 3 Knockout Mice with Constipation Phenotypes

Complement component 3 (C3) contributes to neurogenesis, neural migration, and synaptic elimination under normal and disease conditions of the brain, even though it has not been studied in the enteric nervous system (ENS). To determine the role of C3 in the regulatory mechanism of ENS during C3 deficiency-induced constipation, the changes in the markers of neuronal and interstitial cells of Cajal (ICCs), the markers for excitatory and inhibitory transmission of ENS, and expression of C3 receptors were analyzed in the mid colon of C3 knockout (KO) mice at 16 weeks of age. Prominent constipation phenotypes, including the decrease in stool parameters, changes in the histological structure, and suppression of mucin secretion, were detected in C3 KO mice compared to wildtype (WT) mice. The expression levels of the neuron specific enolase (NSE), protein gene product 9.5 (PGP9.5), and C-kit markers for myenteric neurons and ICCs were lower in the mid colon of C3 KO mice than WT mice. Excitatory transmission analysis revealed similar suppression of the 5-hydroxytryptamine (5-HT) concentration, expression of 5-HT receptors, acetylcholine (ACh) concentration, ACh esterase (AChE) activity, and expression of muscarinic ACh receptors (mAChRs), despite the mAChRs downstream signaling pathway being activated in the mid colon of C3 KO mice. In inhibitory transmission analysis, C3 KO mice showed an increase in the nitric oxide (NO) concentration and inducible nitric oxide synthase (iNOS) expression, while neuronal NOS (nNOS) expression, cholecystokinin (CCK), and gastrin concentration were decreased in the same mice. Furthermore, the levels of C3a receptor (C3aR) and C3bR expression were enhanced in the mid colon of C3 KO mice compared to the WT mice during C3 deficiency-induced constipation. Overall, these results indicate that a dysregulation of the ENS may play an important role in C3 deficiency-induced constipation in the mid colon of C3 KO mice.

Dysbiosis of Fecal Microbiota From Complement 3 Knockout Mice With Constipation Phenotypes Contributes to Development of Defecation Delay

Significant phenotypes for constipation were detected in complement 3 (C3) knockout (KO) mice, although no research has been conducted on an association with alteration of gut microbiota. To investigate the effects of dysbiosis on fecal microbiota from C3 KO mice with constipation, the composition of fecal microbiota was characterized in mid-colons of 16-week-old C3 KO mice, and their function for defecation delay development was examined after fecal microbiota transplantation (FMT) of C3 KO mice. Some significant alterations in constipation phenotypes, including stool parameters and histopathological structure, were detected in 16-week-old C3 KO mice compared to those of wild-type (WT) mice. Fecal microbiota of C3 KO mice exhibited decreases in Anaerocolumna, Caecibacterium, Christensenella, Kineothrix, and Oscillibacter populations and increases in Prevotellamassilia, Reuthenibacterium, Prevotella, Eubacterium, Culturomica, Bacteroides, and Muribaculum populations. In FMT study, key stool parameters, including weight and water content, were remarkably declined in a transplanted KO (KFMT) group of antibiotics-induced depletion of microbiota (AiDM)-WT and AiDM-KO mice, and a similar change was observed in fecal morphology. However, intestine length decreased in only the KFMT group of AiDM-WT mice compared with that of AiDM-KO mice. The mucosal layer and muscle thickness were commonly decreased in the KFMT group of AiDM-WT and AiDM-KO mice, and significant alterations in the crypt structure of Lieberkuhn and molecular regulators, including AQP8, C-kit, and 5-HT, were observed in the same group. Taken together, results of the present study indicate that dysbiosis of fecal microbiota from C3 KO mice with constipation phenotypes has a key role in the induction and regulation of defecation delay.

Deficiency of complement component 3 may be linked to the development of constipation in FVB/N-C3em1Hlee /Korl mice

Alterations in complement component 3 (C3) expression has been reported to be linked to several bowel diseases including Crohn's disease, inflammatory bowel disease, and ulcerative colitis; however, the association with constipation has never been investigated. In this study, we aimed to investigate the correlation between C3 regulation and constipation development using a C3 deficiency model. To achieve these, alterations in stool excretion, transverse colon histological structure, and mucin secretion were analyzed in FVB/N-C3em1Hlee /Korl (C3 knockout, C3 KO) mice with the deletion of 11 nucleotides in exon 2 of the C3 gene. The stool excretion parameters, gastrointestinal transit, and intestine length were remarkably decreased in C3 KO mice compared with wild-type (WT) mice, although there was no specific change in feeding behavior. Furthermore, C3 KO mice showed a decrease in mucosal and muscle layer thickness, alterations in crypt structure, irregular distribution of goblet cells, and an increase of mucin droplets in the transverse colon. Mucin secretion was suppressed, and they accumulated in the crypts of C3 KO mice. In addition, the constipation phenotypes detected during C3 deficiency were confirmed in FVB/N mice treated with C3 convertase inhibitor (rosmarinic acid (RA)). Similar phenotypes were observed with respect to stool excretion parameters, gastrointestinal transit, intestine length, alterations in crypt structure, and mucin secretion in RA-treated FVB/N mice. Therefore, the results of the present study provide the first scientific evidence that C3 deficiency may play an important role in the development of constipation phenotypes in C3 KO mice.

Shiga Toxin 2a Binds to Complement Components C3b and C5 and Upregulates Their Gene Expression in Human Cell Lines

Enterohemorrhagic Escherichia coli (EHEC) infections can cause EHEC-associated hemolytic uremic syndrome (eHUS) via its main virulent factor, Shiga toxins (Stxs). Complement has been reported to be involved in the progression of eHUS. The aim of this study was to investigate the interactions of the most effective subtype of the toxin, Stx2a, with pivotal complement proteins C3b and C5. The study further examined the effect of Stx2a stimulation on the transcription and synthesis of these complement proteins in human target cell lines. Binding of Stx2a to C3b and C5 was evaluated by ELISA. Kidney and gut cell lines (HK-2 and HCT-8) were stimulated with varied concentrations of Stx2a. Subsequent evaluation of complement gene transcription was studied by real-time PCR (qPCR), and ELISAs and Western blots were performed to examine protein synthesis of C3 and C5 in supernatants and lysates of stimulated HK-2 cells. Stx2a showed a specific binding to C3b and C5. Gene transcription of C3 and C5 was upregulated with increasing concentrations of Stx2a in both cell lines, but protein synthesis was not. This study demonstrates the binding of Stx2a to complement proteins C3b and C5, which could potentially be involved in regulating complement during eHUS infection, supporting further investigations into elucidating the role of complement in eHUS pathogenesis.

From immunohistological to anatomical alterations of human pancreas in type 1 diabetes: New concepts on the stage

The histological analysis of human pancreatic samples in type 1 diabetes (T1D) has been proven essential to move forward in the evaluation of in situ events characterizing T1D. Increasing availability of pancreatic tissues collected from diabetic multiorgan donors by centralized biorepositories, which have shared tissues among researchers in the field, has allowed a deeper understanding of T1D pathophysiology, using novel immunohistological and high-throughput methods. In this review, we provide a comprehensive update of the main recent advancements in the characterization of cellular and molecular events involving endocrine and exocrine pancreas as well as the immune system in the onset and progression of T1D. Additionally, we underline novel elements, which provide evidence that T1D pathological changes affect not only islet β-cells but also the entire pancreas.

High-level integration of murine intestinal transcriptomics data highlights the importance of the complement system in mucosal homeostasis

Background

The mammalian intestine is a complex biological system that exhibits functional plasticity in its response to diverse stimuli to maintain homeostasis. To improve our understanding of this plasticity, we performed a high-level data integration of 14 whole-genome transcriptomics datasets from samples of intestinal mouse mucosa. We used the tool Centrality based Pathway Analysis (CePa), along with information from the Reactome database.

Results

The results show an integrated response of the mouse intestinal mucosa to challenges with agents introduced orally that were expected to perturb homeostasis. We observed that a common set of pathways respond to different stimuli, of which the most reactive was the Regulation of Complement Cascade pathway. Altered expression of the Regulation of Complement Cascade pathway was verified in mouse organoids challenged with different stimuli in vitro.

Conclusions

Results of the integrated transcriptomics analysis and data driven experiment suggest an important role of epithelial production of complement and host complement defence factors in the maintenance of homeostasis.

Complement component C4-like protein in Atlantic cod (Gadus morhua L.) - Detection in ontogeny and identification of post-translational deimination in serum and extracellular vesicles

The complement system is a critical part of teleost immune defences, with complement component C4 forming part of the classical and lectin pathways. Cod C4-like protein was isolated from plasma, specific antibodies generated and C4-like protein was assessed in cod sera, mucus and in extracellular vesicles (EVs) from serum and mucus. Higher levels of C4-like protein were detected in serum- than mucus-derived EVs. Post-translational deimination, caused by conversion of arginine into citrulline, can affect protein structure and function. Here we detected deiminated forms of C4-like protein in cod serum and at lower levels in mucus. C4-like protein was also found in deiminated form at low levels in EVs from both serum and mucus. C4-like protein was assessed by immunohistochemistry in cod larvae and detected in a range of organs including in liver, kidney, gut, muscle, skin and mucus, as well as in neuronal tissues of the brain, spinal cord and eye. This abundance of C4-like protein during early development may indicate roles in tissue remodelling, in addition to immune defences. The presence of deiminated C4-like protein in serum and mucosa, as well as in EVs, may suggest C4 protein moonlighting via post-translational deimination.

Metaproteomics of fecal samples of Crohn's disease and Ulcerative Colitis

Crohn's Disease (CD) and Ulcerative Colitis (UC) are chronic inflammatory bowel diseases (IBD) of the gastrointestinal tract. This study used non-invasive LC-MS/MS to find disease specific microbial and human proteins which might be used later for an easier diagnosis. Therefore, 17 healthy controls, 11 CD patients and 14 UC patients but also 13 Irritable Bowel Disease (IBS) patients, 8 Colon Adenoma (CA) patients, and 8 Gastric Carcinoma (GCA) patients were investigated. The proteins were extracted from the fecal samples with liquid phenol in a ball mill. Subsequently, the proteins were digested tryptically to peptides and analyzed by an Orbitrap LC-MS/MS. For protein identification and interpretation of taxonomic and functional results, the MetaProteomeAnalyzer software was used. Cluster analysis and non-parametric test (analysis of similarities) separated healthy controls from patients with CD and UC as well as from patients with GCA. Among others, CD and UC correlated with an increase of neutrophil extracellular traps and immune globulins G (IgG). In addition, a decrease of human IgA and the transcriptional regulatory protein RprY from Bacillus fragilis was found for CD and UC. A specific marker in feces for CD was an increased amount of the human enzyme sucrose-isomaltase. SIGNIFICANCE: Crohn's Disease and Ulcerative Colitis are chronic inflammatory diseases of the gastrointestinal tract, whose diagnosis required comprehensive medical examinations including colonoscopy. The impact of the microbial communities in the gut on the pathogenesis of these diseases is poorly understood. Therefore, this study investigated the impact of gut microbiome on these diseases by a metaproteome approach, revealing several disease specific marker proteins. Overall, this indicated that fecal metaproteomics has the potential to be useful as non-invasive tool for a better and easier diagnosis of both diseases.

Serum immunoinflammation-related protein complexes discriminate between inflammatory bowel disease and colorectal cancer

PURPOSE

Inflammatory bowel disease (IBD) is an important risk factor for colon cancer. Novel serum immunoinflammation-related protein complexes (IIRPCs) have shown associations with early cancer detection. Herein, we investigated the potential of serum IIRPCs for discriminating between IBD and colorectal cancer (CRC) patients.

METHODS

Serum protein complexes of 65 healthy controls, 57 CRC, 69 (ulcerative colitis) UC, and 67 (Crohn's disease) CD patients were isolated by native-PAGE. The gray values of serum IIRPCs bands in the gel were quantified using Quantity One software. The receiver-operating characteristic (ROC) curves were constructed to assess the discriminating ability by calculating the area under the ROC curve.

RESULTS

The serum IIRPCs levels in IBD and CRC patients were significantly elevated compared to healthy controls. ROC analysis indicated certain diagnostic ability of serum IIRPCs in differentiating IBD from CRC. Specifically, "a3" complex discriminated UC from CRC, with an AUC value of 0.722, sensitivity of 69.4% and specificity of 63.8%. Similarly, "b4" complex discriminated UC from CRC, with an AUC value of 0.709, sensitivity of 70.4%, and specificity of 60.0%. In addition, the "a3" complex also discriminated CD from CRC, with an AUC value of 0.785, sensitivity of 73.1%, and specificity of 74.1%, while the "b4" complex showed a tendency to discriminate CD from CRC, with an AUC value of 0.663, sensitivity of 67.9% and specificity of 50.0%. Thus, an equation based on multiple IIRPCs was built to further improve the discriminating power.

CONCLUSIONS

Serum IIRPCs can be used to discriminate IBD from CRC and may also be associated with early screening of colitis-associated cancer.

Deciphering microbiome and neuroactive immune gene interactions in schizophrenia

The body's microbiome represents an actively regulated network of novel mechanisms that potentially underlie the etiology and pathophysiology of a wide range of diseases. For complex brain disorders such as schizophrenia, understanding the cellular and molecular pathways that intersect the bidirectional gut-brain axis is anticipated to lead to new methods of treatment. The means by which the microbiome might differ across neuropsychiatric and neurological disorders are not known. Brain disorders as diverse as schizophrenia, major depression, Parkinson's disease and multiple sclerosis appear to share a common pathology of an imbalanced community of commensal microbiota, often measured in terms of a leaky gut phenotype accompanied by low level systemic inflammation. While environmental factors associated with these disease states might contribute to intestinal pathologies, products from a perturbed microbiome may also directly promote specific signs, symptoms and etiologies of individual disorders. We hypothesize that in schizophrenia, it is the putatively unique susceptibility related to genes that modulate the immune system and the gut-brain pleiotropy of these genes which leads to a particularly neuropathological response when challenged by a microbiome in dysbiosis. Consequences from exposure to this dysbiosis may occur during pre- or post-natal time periods and thus may interfere with normal neurodevelopment in those who are genetically predisposed. Here, we review the evidence from the literature which supports the idea that the intersection of the microbiome and immune gene susceptibility in schizophrenia is relevant etiologically and for disease progression. Figuring prominently at both ends of the gut-brain axis and at points in between are proteins encoded by genes found in the major histocompatibility complex (MHC), including select MHC as well as non-MHC complement pathway genes.

The intestinal complement system in inflammatory bowel disease: Shaping intestinal barrier function

The complement system is part of innate sensor and effector systems such as the Toll-like receptors (TLRs). It recognizes and quickly systemically and/or locally respond to microbial-associated molecular patterns (MAMPs) with a tailored defense reaction. MAMP recognition by intestinal epithelial cells (IECs) and appropriate immune responses are of major importance for the maintenance of intestinal barrier function. Enterocytes highly express various complement components that are suggested to be pivotal for proper IEC function. Appropriate activation of the intestinal complement system seems to play an important role in the resolution of chronic intestinal inflammation, while over-activation and/or dysregulation may worsen intestinal inflammation. Mice deficient for single complement components suffer from enhanced intestinal inflammation mimicking the phenotype of patients with chronic inflammatory bowel disease (IBD) such as Crohn's disease (CD) or ulcerative colitis (UC). However, the mechanisms leading to complement expression in IECs seem to differ markedly between UC and CD patients. Hence, how IECs, intestinal bacteria and epithelial cell expressed complement components interact in the course of IBD still remains to be mostly elucidated to define potential unique patterns contributing to the distinct subtypes of intestinal inflammation observed in CD and UC.

Regulation of epithelial cell expressed C3 in the intestine - Relevance for the pathophysiology of inflammatory bowel disease?

The complement system not only plays a critical role in efficient detection and clearance of bacteria, but also in intestinal immune homeostasis as mice deficient for key complement components display enhanced intestinal inflammation upon experimental colitis. Because underlying molecular mechanisms for this observation are unclear, we investigated the crosstalk between intestinal epithelial cells (IEC), bacteria and the complement system in the course of chronic colitis. Surprisingly, mouse intestinal epithelial cell lines constitutively express high mRNA levels of complement component 3 (C3), Toll-like receptor 2 (Tlr2) and Tlr4. Stimulation of these cells with lipopolysaccharide (LPS), but not with flagellin, LD-muramyldipeptide or peptidoglycan, triggered increased C3 expression, secretion and activation. Stimulation of the C3aR on these cell lines with C3a resulted in an increase of LPS-triggered pro-inflammatory response. Tissue biopsies from C57BL/6J mice revealed higher expression of C3, Tlr1, Tlr2 and Tlr4 in colonic primary IECs (pIECs) compared to ileal pIECs, while in germ-free mice no differences in C3 protein expression was observed. In DSS-induced chronic colitis mouse models, C3 mRNA expression was upregulated in colonic biopsies and ileal pIECs with elevated C3 protein in the lamina propria, IECs and the mucus. Notably, increased C3b opsonization of mucosa-attached bacteria and decreased fecal full-length C3 protein was observed in DSS-treated compared to untreated mice. Of significant interest, non-inflamed and inflamed colonic biopsy samples from CD but not UC patients displayed exacerbated C3 expression compared to controls. These findings suggest that a novel TLR4-C3 axis could control the intestinal immune response during chronic colitis.

Production of complement components by cells of the immune system

The complement system is an important part of the innate immune defence. It contributes not only to local inflammation, removal and killing of pathogens, but it also assists in shaping of the adaptive immune response. Besides a role in inflammation, complement is also involved in physiological processes such as waste disposal and developmental programmes. The complement system comprises several soluble and membrane-bound proteins. The bulk of the soluble proteins is produced mainly by the liver. While several complement proteins are produced by a wide variety of cell types, other complement proteins are produced by only a few related cell types. As these data suggest that local production by specific cell types may have specific functions, more detailed studies have been employed recently analysing the local and even intracellular role of these complement proteins. Here we review the current knowledge about extrahepatic production and/or secretion of complement components. More specifically, we address what is known about complement synthesis by cells of the human immune system.

Intracellular Activation of Complement 3 Is Responsible for Intestinal Tissue Damage during Mesenteric Ischemia

Intestinal ischemia followed by reperfusion leads to local and remote organ injury attributed to inflammatory response during the reperfusion phase. The extent to which ischemia contributes to ischemia/reperfusion injury has not been thoroughly studied. After careful evaluation of intestinal tissue following 30 min of ischemia, we noticed significant local mucosal injury in wild-type mice. This injury was drastically reduced in C3-deficient mice, suggesting C3 involvement. Depletion of circulating complement with cobra venom factor eliminated, as expected, injury recorded at the end of the reperfusion phase but failed to eliminate injury that occurred during the ischemic phase. Immunohistochemical studies showed that tissue damage during ischemia was associated with increased expression of C3/C3 fragments primarily in the intestinal epithelial cells, suggesting local involvement of complement. In vitro studies using Caco2 intestinal epithelial cells showed that in the presence of LPS or exposure to hypoxic conditions the cells produce higher C3 mRNA as well as C3a fragment. Caco2 cells were also noted to produce cathepsins B and L, and inhibition of cathepsins suppressed the release of C3a. Finally, we found that mice treated with a cathepsin inhibitor and cathepsin B-deficient mice suffer limited intestinal injury during the ischemic phase. To our knowledge, our findings demonstrate for the first time that significant intestinal injury occurs during ischemia prior to reperfusion and that this is due to activation of C3 within the intestinal epithelial cells in a cathepsin-dependent manner. Modulation of cathepsin activity may prevent injury of organs exposed to ischemia.

A Phase-Variable Surface Layer from the Gut Symbiont Bacteroides thetaiotaomicron

The capsule from Bacteroides, a common gut symbiont, has long been a model system for studying the molecular mechanisms of host-symbiont interactions. The Bacteroides capsule is thought to consist of an array of phase-variable polysaccharides that give rise to subpopulations with distinct cell surface structures. Here, we report the serendipitous discovery of a previously unknown surface structure in Bacteroides thetaiotaomicron: a surface layer composed of a protein of unknown function, BT1927. BT1927, which is expressed in a phase-variable manner by ~1:1,000 cells in a wild-type culture, forms a hexagonally tessellated surface layer. The BT1927-expressing subpopulation is profoundly resistant to complement-mediated killing, due in part to the BT1927-mediated blockade of C3b deposition. Our results show that the Bacteroides surface structure is capable of a far greater degree of structural variation than previously known, and they suggest that structural variation within a Bacteroides species is important for productive gut colonization.

Many bacterial species elaborate a capsule, a structure that resides outside the cell wall and mediates microbe-microbe and microbe-host interactions. Species of Bacteroides, the most abundant genus in the human gut, produce a capsule that consists of an array of polysaccharides, some of which are known to mediate interactions with the host immune system. Here, we report the discovery of a previously unknown surface structure in Bacteroides thetaiotaomicron. We show that this protein-based structure is expressed by a subset of cells in a population and protects Bacteroides from killing by complement, a component of the innate immune system. This novel surface layer protein is conserved across many species of the genus Bacteroides, suggesting an important role in colonization and host immune modulation.

Do antimicrobial peptides and complement collaborate in the intestinal mucosa?

It is well understood that multiple antimicrobial peptides (AMPs) are constitutively deployed by the epithelium to bolster the innate defenses along the entire length of the intestines. In addition to this constitutive/homeostatic production, AMPs may be inducible and levels changed during disease. In contrast to this level of knowledge on AMP sources and roles in the intestines, our understanding of the complement cascade in the healthy and diseased intestines is rudimentary. Epithelial cells make many complement proteins and there is compelling evidence that complement becomes activated in the lumen. With the common goal of defending the host against microbes, the opportunities for cross-talk between these two processes is great, both in terms of actions on the target microbes but also on regulating the synthesis and secretion of the alternate family of molecules. This possibility is beginning to become apparent with the finding that colonic epithelial cells possess anaphylatoxin receptors. There still remains much to be learned about the possible points of collaboration between AMPs and complement, for example, whether there is reciprocal control over expression in the intestinal mucosa in homeostasis and restoring the balance following infection and inflammation.

Exposure to zearalenone mycotoxin alters in vitro porcine intestinal epithelial cells by differential gene expression

The gut represents the main route of intoxication with mycotoxins. To evaluate the effect and the underlying molecular changes that occurred when the intestine is exposed to zearalenone, a Fusarium sp mycotoxin, porcine epithelial cells (IPEC-1) were treated with 10μM of ZEA for 24h and analysed by microarray using Gene Spring GX v.11.5. Our results showed that 10μM of ZEA did not affect cell viability, but can increase the expression of toll like receptors (TLR1-10) and of certain cytokines involved in inflammation (TNF-α, IL-1β, IL-6, IL-8, MCP-1, IL-12p40, CCL20) or responsible for the recruitment of immune cells (IL-10, IL-18). Microarray results identified 190 genes significantly and differentially expressed, of which 70% were up-regulated. ZEA determined the over expression of ITGB5 gene, essential against the attachment and adhesion of ETEC to porcine jejunal cells and of TFF2 implicated in mucosal protection. An up-regulation of glutathione peroxidase enzymes (GPx6, GPx2, GPx1) was also observed. Upon ZEA challenge, genes like GTF3C4 responsible for the recruitment of polymerase III and initiation of tRNA transcription in eukaryotes and STAT5B were significantly higher induced. The up-regulation of CD97 gene and the down-regulation of tumour suppressor genes (DKK-1, PCDH11X and TC531386) demonstrates the carcinogenic potential of ZEA.

The complement system in inflammatory bowel disease

Complement is well appreciated to be a potent innate immune defense against microbes and is important in the housekeeping act of removal of apoptotic and effete cells. It is also understood that hyperactivation of complement, or the lack of regulators, may underlie chronic inflammatory diseases. A pipeline of products to intervene in complement activation, some already in clinical use, is being studied in various chronic inflammatory diseases. To date, the role of complement in inflammatory bowel disease has not received a lot of research interest. Novel genetically modified laboratory animals and experiments using antagonists to complement effector molecules have kindled important research observations implicating the complement system in inflammatory bowel disease pathogenesis. We review the evidence base for the role and potential therapeutic manipulation of the complement cascade in inflammatory bowel disease.

Complement regulates TLR4-mediated inflammatory responses during intestinal ischemia reperfusion

Innate immune responses including TLR4 and complement activation are required for mesenteric ischemia/reperfusion (IR)-induced tissue damage. We examined the regulation of TLR4 and complement activation in a mouse model of intestinal IR. Intestinal IR-induced C3 deposition in a TLR4 dependent manner. In addition, in wild-type but not TLR4 deficient mice, IR significantly increased C3 and Factor B (FB) mRNA expression within the intestine. To further examine the role of TLR4 and complement, we administered the complement inhibitor, CR2-Crry, to target local complement activation in wild-type C57Bl/10, and TLR4 deficient B10/ScN mice. TLR4 deficient mice sustained less damage and inflammation after IR than wild-type mice, but administration of CR2-Crry did not further reduce tissue damage. In contrast, CR2-Crry treatment of wild-type mice was accompanied by a reduction in complement activation and in C3 and FB transcription in response to IR. CR2-Crry also significantly decreased intestinal IL-6 and IL-12p40 production in both the wild-type and TLR4 deficient mice. These data indicate that TLR4 regulates extrahepatic complement production while complement regulates TLR4-mediated cytokine production during intestinal IR.

The role of the complement and contact systems in the dextran sulfate sodium-induced colitis model: the effect of C1 inhibitor in inflammatory bowel disease

The complement and contact systems may be involved in the pathophysiological process of inflammatory bowel disease (IBD). C1 inhibitor (C1INH) is the most important inhibitor of both the complement and contact systems. We evaluated the role of these systems and the effect of both active and inactive forms of C1INH (iC1INH) in dextran sulfate sodium (DSS)-induced colitis mouse model. Three percent DSS was used in drinking water to induce colitis in complement C3-deficient (C3(-/-)) mice, bradykinin type 2 receptor deficient (Bk(2)R(-/-)) mice, and C57BL/6 mice. After ten days DSS exposure, C3(-/-) mice exhibited markedly less weight loss than wild-type (WT) mice (12 +/- 3.3% vs. 30 +/- 1.2%, P < 0.05) and developed a milder disease-activity index (DAI), histological score, colon shortening, and myeloperoxidase (MPO) elevation (P < 0.05, respectively). The Bk(2)R(-/-) mice were not protected from the disease. Seven-day treatment with either native C1INH or iC1INH reduced the severity of the disease in WT mice, as indicated by decreased weight loss (15 +/- 1.8%, 14 +/- 2.1% vs. 30 +/- 1.2%, P < 0.05, respectively), DAI, intestinal tissue damage, and MPO elevation compared with untreated WT DSS control mice (P < 0.05, respectively). These findings suggest that complement plays a role in the development of DSS-induced colitis and that blockade of the complement system might be useful for the acute phase of IBD treatment. C1INH, however, leads to an amelioration of DSS-induced colitis via a mechanism that does not involve the inhibition of complement or contact system activation but does result in significant suppression of leukocyte infiltration.

Enterocyte shedding and epithelial lining repair following ischemia of the human small intestine attenuate inflammation

Background

Recently, we observed that small-intestinal ischemia and reperfusion was found to entail a rapid loss of apoptotic and necrotic cells. This study was conducted to investigate whether the observed shedding of ischemically damaged epithelial cells affects IR induced inflammation in the human small gut.

Methods and Findings

Using a newly developed IR model of the human small intestine, the inflammatory response was studied on cellular, protein and mRNA level. Thirty patients were consecutively included. Part of the jejunum was subjected to 30 minutes of ischemia and variable reperfusion periods (mean reperfusion time 120 (±11) minutes). Ethical approval and informed consent were obtained. Increased plasma intestinal fatty acid binding protein (I-FABP) levels indicated loss in epithelial cell integrity in response to ischemia and reperfusion (p<0.001 vs healthy). HIF-1α gene expression doubled (p = 0.02) and C3 gene expression increased 4-fold (p = 0.01) over the course of IR. Gut barrier failure, assessed as LPS concentration in small bowel venous effluent blood, was not observed (p = 0.18). Additionally, mRNA expression of HO-1, IL-6, IL-8 did not alter. No increased expression of endothelial adhesion molecules, TNFα release, increased numbers of inflammatory cells (p = 0.71) or complement activation, assessed as activated C3 (p = 0.14), were detected in the reperfused tissue.

Conclusions

In the human small intestine, thirty minutes of ischemia followed by up to 4 hours of reperfusion, does not seem to lead to an explicit inflammatory response. This may be explained by a unique mechanism of shedding of damaged enterocytes, reported for the first time by our group.

The enterocyte microvillus is a vesicle-generating organelle

For decades, enterocyte brush border microvilli have been viewed as passive cytoskeletal scaffolds that serve to increase apical membrane surface area. However, recent studies revealed that in the in vitro context of isolated brush borders, myosin-1a (myo1a) powers the sliding of microvillar membrane along core actin bundles. This activity also leads to the shedding of small vesicles from microvillar tips, suggesting that microvilli may function as vesicle-generating organelles in vivo. In this study, we present data in support of this hypothesis, showing that enterocyte microvilli release unilamellar vesicles into the intestinal lumen; these vesicles retain the right side out orientation of microvillar membrane, contain catalytically active brush border enzymes, and are specifically enriched in intestinal alkaline phosphatase. Moreover, myo1a knockout mice demonstrate striking perturbations in vesicle production, clearly implicating this motor in the in vivo regulation of this novel activity. In combination, these data show that microvilli function as vesicle-generating organelles, which enable enterocytes to deploy catalytic activities into the intestinal lumen.

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.

Gene expression profiling of jejunal Peyer’s patches in juvenile and adult pigs

Peyer's patches are organized lymphoid tissues of the small intestine that play a critical role in disease resistance and oral tolerance. Peyer's patches in the jejunum contain lymphocytes, dendritic cells, macrophages, villous epithelium, and specialized follicle-associated epithelium. Little is known about the mechanisms and processes by which cells of the Peyer's patches discriminate food nutrients and commensal microflora from pathogenic microbiota. We hypothesize that the jejunal Peyer's patches express genes that mediate and regulate its essential functions. Expression patterns of approximately 2600 cDNAs from a porcine Peyer's patch subtracted library were examined by microarray profiling. Individual mRNAs of interest were further examined by quantitative RT-PCR. Innate immunity-associated genes, including complement 3 and lysozyme, and the genes for epithelial chloride channel and trappin 1 were highly expressed by jejunal Peyer's patch in both juvenile and adult pigs. The growth- and apoptosis-associated genes CIDE-B, GW112, and PSP/Reg I (pancreatic stone protein or regenerating gene) were differentially expressed in juvenile pig Peyer's patches. Many sequences which were highly expressed in jejunal Peyer's patches have previously been described with functions in epithelial cells. Animal-to-animal variation in basal jejunal Peyer's patch gene expression was considerable and reflects the dynamic physiological environment of the gut in addition to genetic, epigenetic, and microbiological variation in the small intestine.

Neutrophils migrate across intestinal epithelium using beta2 integrin (CD11b/CD18)-independent mechanisms

Recruitment of polymorphonuclear leucocytes (PMN) across the intestinal epithelium is dependent on specific adhesion molecules and chemoattractants diffusing from the intestinal lumen. The present understanding is that in response to fMLP, PMN migration across a T84 colon carcinoma monolayer is dependent on the beta(2) integrin, Mac-1 (CD11b/CD18). To further understand PMN transepithelial migration, we sought to determine whether migration to C5a, IL-8 and LTB(4) was similarly Mac-1-, or even CD18-dependent. T84 epithelial cell monolayers growing on Transwell filters were used in combination with radiolabelled peripheral blood PMN. The number of migrated PMN was established by the amount of radioactivity recovered from the well after the migration period. Monoclonal antibodies were used to block integrin function. Whereas essentially all migration to fMLP across T84 monolayers was prevented by anti-CD18 antibody, significant migration to C5a, IL-8 or LTB(4) persisted despite anti-CD18 antibody, indicating PMN are capable of beta(2) integrin-independent transepithelial migration. An antibody to CD11b but not CD11a blocked migration to an extent similar as with anti-CD18. CD18-independent PMN migration to C5a occurred only in the basolateral-to-apical direction across epithelial cells. Co-stimulation of PMN with C5a and fMLP or IL-8 plus LTB(4) and fMLP still resulted in CD18-independent migration. Thus CD18 use during PMN migration across this model epithelium is a function of the chemoattractant inducing migration. The finding of CD18-independent migration mechanisms needs to be considered when developing antiadhesion molecule strategies to reduce or reverse intestinal inflammation.

CD55 expression patterns on intestinal neuronal tissue are divergent from the brain

BACKGROUND

and objectives: In this study, we investigated how enteric plexuses protect themselves from complement mediated attack. For this purpose, the expression patterns of membrane bound complement regulatory proteins (mCRP) and their association with C3 deposition was determined. In addition, mCRP expression patterns of enteric plexuses were compared with those in the central nervous system (CNS).

METHODS

Immunohistochemical stainings were performed to discriminate neuronal cells from glial cells and to detect the presence of CD46, CD55, CD59, and C3d. RNA in situ hybridisation (RISH) was used to determine the cell types that produce CD55 mRNA.

RESULTS

Enteric plexuses minimally expressed CD46 whereas CD55 and CD59 were highly expressed. CD55 expression was also observed in a ring around Auerbach's plexuses which was not observed for CD46 and CD59. C3d was deposited around the plexuses but plexus cells themselves did not stain for C3d. In contrast with CNS neurones, enteric neurones were shown to express CD55 whereas enteric glial cells did not. This was confirmed with CD55 RISH. Phospholipase C mediated cleavage of CD55 demonstrated that CD55 was most likely attached to elastic fibres surrounding the plexus. Attached CD55 might protect CD55 negative glial cells from complement mediated injury during inflammatory reactions. CD55 on elastic fibres surrounding the plexuses most likely originated from enteric neuronal cells.

CONCLUSION

In contrast with the CNS, enteric neurones express CD55 and enteric glial cells lack CD55 expression. CD55, produced by neuronal cells, attached to elastic fibres surrounding the plexuses is proposed to protect the CD55 negative glial cells within plexuses.

Dancing with complement C4 and the RP-C4-CYP21-TNX (RCCX) modules of the major histocompatibility complex

The number of the complement component C4 genes varies from 2 to 8 in a diploid genome among different human individuals. Three quarters of the C4 genes in Caucasian populations have the endogenous retrovirus, HERV-K(C4), in the ninth intron. The remainder does not. The C4 serum proteins are highly polymorphic and their concentrations vary from 100 to approximately 1000 microg/ml. There are two distinct classes of C4 protein, C4A and C4B, which have diversified to fulfill (a) the opsonization/immunoclearance purposes and (b) the well-known complement function in the killing of microbes by lysis and neutralization, respectively. Many infectious and autoimmune diseases are associated with complete or partial deficiency of C4A and/or C4B. The adverse effects of high C4 gene dosages, however, are just emerging, as the concepts of human C4 genetics are revised and accurate techniques are applied to distinguish partial deficiencies from differential expression caused by unequal C4A and C4B gene dosages and gene sizes. This review attempts to dissect the sophisticated genetics of complement C4A and C4B. The emphases are on the qualitative and quantitative diversities of C4 genotypes and phenotypes. The many allotypic variants and the processed products of human and mouse C4 proteins are described. The modular variation of C4 genes together with the serine/threonine nuclear kinase gene RP, the steroid 21-hydroxylase CYP21, and extracellular matrix protein TNX (RCCX modules) are investigated for the effects on homogenization of C4 protein polymorphisms, and on the unequal genetic crossovers that knocked out the functions of CYP21 and/or TNX. Furthermore, the influence of the endogenous retrovirus HERV-K(C4) on C4 gene expression and the dispersal of HERV-K(C4) family members in the human genome are discussed.

High normal serum levels of C3 and C1 inhibitor, two acute-phase proteins belonging to the complement system, occur more frequently in patients with Crohn's disease than ulcerative colitis

Few data are available on measurements of serum concentrations of complement proteins in inflammatory bowel disease (IBD). Therefore we measured serum levels of C3, C4, and C1-esterase inhibitor (C1-INH) as well as C-reactive protein (CRP) in 167 patients with Crohn's disease (CD) and 111 patients with ulcerative colitis (UC). Median serum concentrations of C3 and C1-INH were significantly higher in CD than in UC. According to multiple logistic regression analysis adjusted to age, sex, activity of disease, and presence of extraintestinal manifestations, IBD patients with high-normal (> or = 128%, > or = 75th percentile ) C1-INH concentrations had significantly (0.0275) higher odds ratio to have a diagnosis of CD than UC. Patients with high-normal C3 (> or = 1.40 g/liter) and high (> or =20 mg/liter) CRP concentrations had an even higher odds ratio of a CD diagnosis (P = 0.0132). Our findings indicate that measurement of C3, C1-INH, and CRP can be used as an additional marker to pANCA/ASCA for distinguishing patients with CD and UC.

Genetic, structural and functional diversities of human complement components C4A and C4B and their mouse homologues, Slp and C4

The complement protein C4 is a non-enzymatic component of the C3 and C5 convertases and thus essential for the propagation of the classical complement pathway. The covalent binding of C4 to immunoglobulins and immune complexes (IC) also enhances the solubilization of immune aggregates, and the clearance of IC through complement receptor one (CR1) on erythrocytes. Human C4 is the most polymorphic protein of the complement system. In this review, we summarize the current concepts on the 1-2-3 loci model of C4A and C4B genes in the population, factors affecting the expression levels of C4 transcripts and proteins, and the structural, functional and serological diversities of the C4A and C4B proteins. The diversities and polymorphisms of the mouse homologues Slp and C4 proteins are described and contrasted with their human homologues. The human C4 genes are located in the MHC class III region on chromosome 6. Each human C4 gene consists of 41 exons coding for a 5.4-kb transcript. The long gene is 20.6 kb and the short gene is 14.2 kb. In the Caucasian population 55% of the MHC haplotypes have the 2-locus, C4A-C4B configurations and 45% have an unequal number of C4A and C4B genes. Moreover, three-quarters of C4 genes harbor the 6.4 kb endogenous retrovirus HERV-K(C4) in the intron 9 of the long genes. Duplication of a C4 gene always concurs with its adjacent genes RP, CYP21 and TNX, which together form a genetic unit termed an RCCX module. Monomodular, bimodular and trimodular RCCX structures with 1, 2 and 3 complement C4 genes have frequencies of 17%, 69% and 14%, respectively. Partial deficiencies of C4A and C4B, primarily due to the presence of monomodular haplotypes and homo-expression of C4A proteins from bimodular structures, have a combined frequency of 31.6%. Multiple structural isoforms of each C4A and C4B allotype exist in the circulation because of the imperfect and incomplete proteolytic processing of the precursor protein to form the beta-alpha-gamma structures. Immunofixation experiments of C4A and C4B demonstrate > 41 allotypes in the two classes of proteins. A compilation of polymorphic sites from limited C4 sequences revealed the presence of 24 polymophic residues, mostly clustered C-terminal to the thioester bond within the C4d region of the alpha-chain. The covalent binding affinities of the thioester carbonyl group of C4A and C4B appear to be modulated by four isotypic residues at positions 1101, 1102, 1105 and 1106. Site directed mutagenesis experiments revealed that D1106 is responsible for the effective binding of C4A to form amide bonds with immune aggregates or protein antigens, and H1106 of C4B catalyzes the transacylation of the thioester carbonyl group to form ester bonds with carbohydrate antigens. The expression of C4 is inducible or enhanced by gamma-interferon. The liver is the main organ that synthesizes and secretes C4A and C4B to the circulation but there are many extra-hepatic sites producing moderate quantities of C4 for local defense. The plasma protein levels of C4A and C4B are mainly determined by the corresponding gene dosage. However, C4B proteins encoded by monomodular short genes may have relatively higher concentrations than those from long C4A genes. The 5' regulatory sequence of a C4 gene contains a Spl site, three E-boxes but no TATA box. The sequences beyond--1524 nt may be completely different as the C4 genes at RCCX module I have RPI-specific sequences, while those at Modules II, III and IV have TNXA-specific sequences. The remarkable genetic diversity of human C4A and C4B probably promotes the exchange of genetic information to create and maintain the quantitative and qualitative variations of C4A and C4B proteins in the population, as driven by the selection pressure against a great variety of microbes. An undesirable accompanying byproduct of this phenomenon is the inherent deleterious recombinations among the RCCX constituents leading to autoimmune and genetic disorders.