Complement depletion accelerates the clearance of immune complexes from the circulation of primates

Molecular recognition and binding between human plasminogen Kringle 5 and α-chain of human complement component C3b by frontal chromatography and dynamics simulation

The binding and molecular recognition between α-chain of human complement C3b (α-chain of C3b) and human plasminogen Kringle 5 (Kringle 5) were studied and explored by frontal chromatography and dynamics simulation in the combination of bio-specific technologies. The specific interaction between the α-chain of C3b and Kringle 5 was initially confirmed by ligand blot and ELISA (Kd = 4.243×10-6 L/mol). Furthermore, the binding determination conducted via frontal chromatography showed that the presence of a single binding site between them, with the binding constant of 2.98 × 105 L/mol. Then the molecular recognition by dynamics simulation and molecular docking showed that there were 9 and 13 amino acid residues respective in the Kringle 5 and α-chain of C3b directly implicated in the binding and the main stabilizing forces were electrostatic force (-55.99 ± 11.82 kcal/mol) and Van der Waals forces (-42.70 ± 3.45 kcal/mol). Additionally, a loop structure (65-71) in Kringle 5 underwent a conformational change from a random structure to an α-helix and a loop structure (417-425) in α-chain of C3b was closer to the molecular center, both of them were more conducive to the binding between them. Meanwhile, the involvement of the lysine binding site of Kringle 5 played an important role in the binding process. In addition, the erythrocyte-antibody complement rosette assay substantiated that the presence of Kringle 5 hindered the transportation of α-chain of C3b to antigen-antibody complex in a dose-dependent manner. These findings collectively indicated that the α-chain of C3b is very likely a receptor protein for Kringle 5, which provides a methodology for other similar investigations and valuable insights into expansion of the pharmacological effects and potential application of Kringle 5 in immune-related diseases.

Acquired decrease of the C3b/C4b receptor (CR1, CD35) and increased C4d deposits on erythrocytes from ICU COVID-19 patients

In order to study the mechanisms of COVID-19 damage following the complement activation phase occurring during the innate immune response to SARS-CoV-2, CR1 (the regulating complement activation factor, CD35, the C3b/C4b receptor), C4d deposits on Erythrocytes (E), and the products of complement activation C3b/C3bi, were assessed in 52 COVID-19 patients undergoing O₂ therapy or assisted ventilation in ICU units in Rheims France. An acquired decrease of CR1 density on E from COVID-19 patients was observed (Mean = 418, SD = 162, N = 52) versus healthy individuals (Mean = 592, SD = 287, N = 400), Student’s t-test p < 10⁻⁶, particularly among fatal cases, and in parallel with several parameters of clinical severity. Large deposits of C4d on E in patients were well above values observed in normal individuals, mostly without concomitant C3 deposits, in more than 80% of the patients. This finding is reminiscent of the increased C4d deposits on E previously observed to correlate with sub endothelial pericapillary deposits in organ transplant rejection, and with clinical SLE flares. Conversely, significant C3 deposits on E were only observed among ¼ of the patients. The decrease of CR1/E density, deposits of C4 fragments on E and previously reported detection of virus spikes or C3 on E among COVID-19 patients, suggest that the handling and clearance of immune complex or complement fragment coated cell debris may play an important role in the pathophysiology of SARS-CoV-2. Measurement of C4d deposits on E might represent a surrogate marker for assessing inflammation and complement activation occurring in organ capillaries and CR1/E decrease might represent a cumulative index of complement activation in COVID-19 patients.

Taken together, these original findings highlight the participation of complement regulatory proteins and indicate that E are important in immune pathophysiology of COVID-19 patients. Besides a potential role for monitoring the course of disease, these observations suggest that novel therapies such as the use of CR1, or CR1-like molecules, in order to down regulate complement activation and inflammation, should be considered.

Revisiting immune complexes: Key to understanding immune-related diseases

Immune complexes (ICs) formed by foreign or self-antigens and antibodies in biological fluids affect various tissues and are thought to cause several diseases. Biological and physical properties of IC, abnormal IC amounts, IC deposition and their relationships with disease pathogenesis had been studied. However, the relationship between ICs and each disease is not well understood and little is known of what determined ICs deposition in particular organ and why different organs are affected in different diseases. Recent technological advance enables identification of ICs in particular its antigens in tissues and body fluids, which may provide a key to discover an important trigger for immunological abnormality occurrence. Further identification of their epitopes, that are the exact origin of antigenicity, is developing and may be useful for diagnosis, elucidation of pathogenesis and treatment against IC-induced diseases. Here, we first make an overview of clearance of ICs, IC-induced pathogenesis and biological properties of ICs. Then, we introduce various methods developed to recover ICs from biological fluids or to identify antigens incorporated into ICs. Furthermore, several methods that can be used in epitope mapping for IC antigens are also documented.

High titres of IgM-bound circulating immune complexes and erythrocytic oxidative damage are indicators of dengue severity

Global incidence of dengue has drastically increased in the last few years. Despite the global morbidity and mortality associated with dengue infection, mechanisms of immune control and viral pathogenesis are poorly explored. Pancytopenias, along with increased oxidative stress, are salient clinical findings in severe dengue patients. Previously, we demonstrated significant differences of circulating immune complexes (CICs) among severe and non-severe dengue patients. Accordingly, here we sought to determine the contributory role of affinity-purified antibody-bound CICs in dengue severity. To characterize intracellular oxidative stress induced by antibody-bound CICs, 5-(and-6)-chloromethyl-2'-7'-dichlorodihydrofluorescein diacetate (DCFDA) was measured by flow cytometry. At the same time, CICs sensitized healthy red blood cells (RBC) and patients' RBC morphology was determined by scanning electron microscopy and flow cytometry analysis. Erythrophagocytosis and ferritin levels were further determined in severe and non-severe dengue patients. Our results showed that the severe patients had high titres of immunoglobulin (Ig)M-bound CICs (P < 0·0001) in their sera, increased intracellular oxidative stress (P < 0·0001), high ferritin levels (P < 0·0001), altered morphology of RBC and finally enhanced erythrophagocytosis. This study shows for the first time that RBC morphology is altered in severe dengue patients. Taken together, this study suggests that the enhanced IgM-bound CICs could contribute to the increased oxidative stress and act directly on RBC destruction of severe dengue patients, and is an important pathophysiological determinant. Hence, IgM-bound CICs can serve as an important laboratory parameter to monitor dengue infection progression.

Pathogen clearance and immune adherence "revisited": Immuno-regulatory roles for CRIg

Rapid elimination of microbes from the bloodstream, along with the ability to mount an adaptive immune response, are essential for optimal host-defense. Kupffer cells are strategically positioned in the liver sinusoids and efficiently capture circulating microbes from the hepatic artery and portal vein, thus preventing bacterial dissemination. In vivo and in vitro studies have probed how complement receptor of the immunoglobulin superfamily (CRIg), also referred to as Z39Ig and V-set and Ig domain-containing 4 (VSIG4), acts as a critical player in pathogen recognition and clearance. While recent data suggested that CRIg may bind bacterial cell wall components directly, the single transmembrane receptor is best known for its interaction with complement C3 opsonization products on the microbial surface. On Kupffer cells, CRIg must capture opsonized microbes against the shear forces of the blood flow. In vivo work reveals how immune adherence (IA), a process in which blood platelets or erythrocytes associate with circulating bacteria, plays a critical role in regulating pathogen capture by CRIg under flow conditions. In addition to its typical innate immune functions, CRIg was shown to directly and indirectly influence adaptive immune responses. Here, we review our current understanding of the diverse roles of CRIg in pathogen elimination, anti-microbial immunity and autoimmunity. In particular, we will explore how, through selective capturing by CRIg, an important balance is achieved between the immunological and clearance functions of liver and spleen.

Old dogs-new tricks: immunoregulatory properties of C3 and C5 cleavage fragments

The activation of the complement system by canonical and non-canonical mechanisms results in the generation of multiple C3 and C5 cleavage fragments including anaphylatoxins C3a and C5a as well as opsonizing C3b/iC3b. It is now well appreciated that anaphylatoxins not only act as pro-inflammatory mediators but as immunoregulatory molecules that control the activation status of cells and tissue at several levels. Likewise, C3b/iC3b is more than the opsonizing fragment that facilitates engulfment and destruction of targets by phagocytes. In the circulation, it also facilitates the transport and delivery of bacteria and immune complexes to phagocytes, through a process known as immune adherence, with consequences for adaptive immunity. Here, we will discuss non-classical immunoregulatory properties of C3 and C5 cleavage fragments. We highlight the influence of anaphylatoxins on Th2 and Th17 cell development during allergic asthma with a particular emphasis on their role in the modulation of CD11b⁺ conventional dendritic cells and monocyte-derived dendritic cells. Furthermore, we discuss the control of anaphylatoxin-mediated activation of dendritic cells and allergic effector cells by adaptive immune mechanisms that involve allergen-specific IgG1 antibodies and plasma or regulatory T cell-derived IL-10 production. Finally, we take a fresh look at immune adherence with a particular focus on the development of antibacterial cytotoxic T-cell responses.

Targeted Fcγ Receptor (FcγR)-mediated Clearance by a Biparatopic Bispecific Antibody

Soluble ligands have commonly been targeted by antibody therapeutics for cancers and other diseases. Although monoclonal antibodies targeting such ligands can block their interactions with their cognate receptors, they can also significantly increase the half-life of their ligands by FcRn-mediated antibody recycling, thereby evading ligand renal clearance and requiring increasingly high antibody doses to neutralize the increasing pool of target. To overcome this issue, we generated a bispecific/biparatopic antibody (BiSAb) that targets two different epitopes on IL-6 to block IL-6-mediated signaling. The BiSAb formed large immune complexes with IL-6 that can bind Fcγ receptors on phagocytic cells and are rapidly internalized. In addition, rapid clearance of the BiSAb·IL-6 complex was observed in mice while the parental antibodies prolonged the serum half-life of IL-6. Intravital imaging of the liver in mice confirmed that the rapid clearance of these large immune complexes was associated with Fcγ receptor-dependent binding to Kupffer cells in the liver. The approach described here provides a general strategy for therapeutic antibodies with the ability to not only neutralize but also actively drive clearance of their soluble antigens.

Reduced erythrocyte complement receptor type 1 in systemic lupus erythematosus is related to a disease activity index and not to the presence or severity of renal disease

The present study investigated the expression of the complement receptor type 1 (CR1) on the membrane of erythrocytes (CR1/E) of patients with systemic lupus erythematosus (SLE) by flow cytometry. We found a significant reduction in CR1/E numbers in SLE patients (n 1/4 52), compared to controls (512 + 171 and 689 + 146, respectively, P 1/4 0.0001). Reduction was more pronounced in active disease patients. The mean CR1/E number observed in patients with inactive disease was 546 + 163 CR1/E, while active SLE patients presented a mean of 385 + 133 CR1/E(P 1/4 0.001). Patients with SLE with similar activity indexes tend to have similar CR1/E numbers, irrespective of disease severity. We also observed a trend to CR1/E reduction in severe nephritis patients. A small group of SLE patients with chronic renal failure and inactive disease showed CR1/E numbers nearly identical to controls (689 + 146 versus 686 + 123, respectively, P 1/4 0.95). This was the only group of SLE patients with normal CR1/E numbers. These results confirm the CR1/E reduction in SLE patients as previously described, and also suggest that this reduction is related to disease activity and not to disease severity.

Effects of Repeated Complement Activation Associated with Chronic Treatment of Cynomolgus Monkeys with 2'-O-Methoxyethyl Modified Antisense Oligonucleotide

The effects of repeated complement activation in cynomolgus monkeys after chronic antisense oligonucleotide (ASO) treatment were evaluated by using ISIS 104838, a representative 2'-O-methoxyethyl (2'-MOE) modified ASO. The treatment was up to 9 months with a total weekly dose of 30 mg/kg, given either as daily [4.3 mg/kg/day, subcutaneous (s.c.) injection] or once weekly [30 mg/kg, either as s.c. injection or 30-min intravenous (i.v.) infusion]. Acute elevations of complement split products (Bb and C3a) and a transient decrease in C3 occurred after the first dose and were drug plasma concentration dependent. However, with repeated complement activation after chronic ASO treatment, there were progressive increases in basal (predose) levels of Bb and C3a, and a sustained C3 reduction in all treated groups. There was also a progressive increase in C3d-bound circulating immune complex (CIC) that was considered secondary to the C3 depletion. Evidence of vascular inflammation was observed, mostly in the liver, kidney, and heart, and correlated with severe C3 depletion and increases in plasma IgG and IgM. Vascular inflammation was accompanied by increased C3 and IgM immunereactivity in the affected vasculatures and endothelial activation markers in serum. In summary, repeated complement activations in monkeys lead to a sustained decrease in circulating C3 over time. The concomitantly increased inflammatory signals and decreased CIC clearance due to impairment of complement function may lead to vascular inflammation after chronic ASO treatment in monkeys. However, based on the known sensitivity of monkeys to ASO-induced complement activation, these findings have limited relevance to humans.

Differential complement activation pathways promote C3b deposition on native and acetylated LDL thereby inducing lipoprotein binding to the complement receptor 1

Lipoproteins can induce complement activation resulting in opsonization and binding of these complexes to complement receptors. We investigated the binding of opsonized native LDL and acetylated LDL (acLDL) to the complement receptor 1 (CR1). Binding of complement factors C3b, IgM, C1q, mannose-binding lectin (MBL), and properdin to LDL and acLDL were investigated by ELISA. Subsequent binding of opsonized LDL and acLDL to CR1 on CR1-transfected Chinese Hamster Ovarian cells (CHO-CR1) was tested by flow cytometry. Both native LDL and acLDL induced complement activation with subsequent C3b opsonization upon incubation with normal human serum. Opsonized LDL and acLDL bound to CR1. Binding to CHO-CR1 was reduced by EDTA, whereas MgEGTA only reduced the binding of opsonized LDL, but not of acLDL suggesting involvement of the alternative pathway in the binding of acLDL to CR1. In vitro incubations showed that LDL bound C1q, whereas acLDL bound to C1q, IgM, and properdin. MBL did neither bind to LDL nor to acLDL. The relevance of these findings was demonstrated by the fact that ex vivo up-regulation of CR1 on leukocytes was accompanied by a concomitant increased binding of apolipoprotein B-containing lipoproteins to leukocytes without changes in LDL-receptor expression. In conclusion, CR1 is able to bind opsonized native LDL and acLDL. Binding of LDL to CR1 is mediated via the classical pathway, whereas binding of acLDL is mediated via both the classical and alternative pathways. Binding of lipoproteins to CR1 may be of clinical relevance due to the ubiquitous cellular distribution of CR1.

Formation, Clearance, Deposition, Pathogenicity, and Identification of Biopharmaceutical-related Immune Complexes

Vascular inflammation, infusion reactions, glomerulopathies, and other potentially adverse effects may be observed in laboratory animals, including monkeys, on toxicity studies of therapeutic monoclonal antibodies and recombinant human protein drugs. Histopathologic and immunohistochemical (IHC) evaluation suggests these effects may be mediated by deposition of immune complexes (ICs) containing the drug, endogenous immunoglobulin, and/or complement components in the affected tissues. ICs may be observed in glomerulus, blood vessels, synovium, lung, liver, skin, eye, choroid plexus, or other tissues or bound to neutrophils, monocytes/macrophages, or platelets. IC deposition may activate complement, kinin, and/or coagulation/fibrinolytic pathways and result in a systemic proinflammatory response. IC clearance is biphasic in humans and monkeys (first from plasma to liver and/or spleen, second from liver or spleen). IC deposition/clearance is affected by IC composition, immunomodulation, and/or complement activation. Case studies are presented from toxicity study monkeys or rats and indicate IHC-IC deposition patterns similar to those predicted by experimental studies of IC-mediated reactions to heterologous protein administration to monkeys and other species. The IHC-staining patterns are consistent with findings associated with generalized and localized IC-associated pathology in humans. However, manifestations of immunogenicity in preclinical species are generally not considered predictive to humans.

Reduced immune complex binding capacity and increased complement susceptibility of red cells from children with severe malaria-associated anemia

Plasmodium falciparum malaria causes 1-2 million deaths per year. Most deaths occur as a result of complications such as severe anemia and cerebral malaria (CM) (coma). Red cells of children with severe malaria-associated anemia (SMA) have acquired deficiencies in the complement regulatory proteins complement receptor 1 (CR1, CD35) and decay accelerating factor (DAF, CD55). We investigated whether these deficiencies affect the ability of erythrocytes to bind immune complexes (ICs) and regulate complement activation. We recruited 75 children with SMA (Hb < or = 6 g/dL) from the holoendemic malaria region of the Lake Victoria basin, western Kenya, and 74 age- and gender-matched uncomplicated malaria controls. In addition, we recruited 32 children with CM and 52 age- and gender-matched controls. Deficiencies in red cell CR1 and CD55 in children with SMA were accompanied by a marked decline in IC binding capacity and increased C3b deposition in vivo and ex vivo. Importantly, these changes were specific because they were not seen in red cells of children with CM or their controls. These data suggest that the declines in red cell CR1 and CD55 seen in children with SMA are of physiologic significance and may predispose erythrocytes to complement-mediated damage and phagocytosis in vivo.

Deficiency of the effector mechanisms of the immune response and autoimmunity

It has long been the conventional wisdom that most autoimmune responses represent a pathological aberration of the immune system and a great deal of effort has been devoted to investigating how such abnormal responses may be induced. It seems likely, however, particularly in the form of autoimmunity seen in diseases like systemic lupus erythematosus (LE) and associated with immune complex disease, that it is not the induction of the autoimmune response that is primarily abnormal, but its persistence, and that this abnormal persistence is a consequence of a failure of the proper functioning of the effector mechanisms of the immune response. The strongest reason for so believing is the striking incidence of these diseases in subjects with deficiencies of the early components of the classical complement pathway. Total, homozygous deficiencies are rare and account for only a small proportion of patients with systemic LE. However, partial, heterozygous deficiency of these components is much commoner and also carries an increased susceptibility to these diseases. An explanation for this association is given. In the presence of an adequately functioning complement system immune complexes remain soluble and of relatively small size. It is proposed that this is a result of the incorporation of C4 and C3 into the antigen-antibody lattice leading to a reduction in the effective valency of antigen and antibody. The Goldberg theory of immune precipitation predicts that a reduction in valency would inhibit precipitation and the formation of large complexes. In the absence of adequate complement function this mechanism will fail and large, potentially insoluble complexes with little C4 and C3 on them will be formed. These large immune complexes without sufficient C4 and C3 bound on them will also not be bound normally to erythrocyte CR1 and will therefore be transported in the (peripheral) plasma stream rather than in the (central) erythrocyte stream. It is proposed that this will result in the deposition of immune complexes in peripheral small blood vessels rather than in the sinusoids of the liver and spleen; and that this peripheral deposition gives rise to inflammation, with the release of autoantigens and the formation of further autoantibodies. The importance of CR1 in relation to these diseases is emphasized by the reduction in CR1 numbers that accompanies their active phase. This appears to be due to proteolysis of the receptor while the immune complex-bearing erythrocyte is sequestered in the reticuloendothelial system.

Consumption of erythrocyte CR1 (CD35) is associated with protection against systemic lupus erythematosus renal flare

Erythrocyte complement receptor type one (E-CR1) is thought to protect against immune complex (IC) disease through interactions that lead to E-CR1 consumption, and low E-CR1 levels are characteristic of systemic lupus erythematosus (SLE). The purpose of this study was to test the hypothesis that E-CR1 consumption can predict or mark SLE flare. Recurrently active SLE patients [n = 43; 28 with past or present major renal manifestations (SLER) and 15 without (SLENR)], were evaluated every 2 months by detailed protocol testing (mean follow-up 22 months), including direct measurements of E-CR1 levels using a radioimmunoassay. In all patients, detectable E-CR1 levels fluctuated widely through acute periods of consumption and regeneration, preventing the use of any single value as a baseline. However, when individual chronic baseline values were used, determined as the mean of all E-CR1 values 4 months or more from a flare, a clear trend was observed. In 16 of 16 instances of non-renal flare in SLER patients, E-CR1 levels decreased at flare (mean decrease 34%, P < 0.0001). In contrast, no consistent difference was observed for flare in SLENR patients or for renal flare in SLER patients. Changes in E-CR1 levels did not correlate with plasma CR1 levels. In conclusion, single occurrences of E-CR1 consumption did not generally predict or mark SLE flare. However, compared to the average E-CR1 levels measured during no-flare intervals, E-CR1 consumption in SLER patients at flare was strongly associated with freedom from signs of renal involvement. We postulate that E-CR1 consumption reflects E-CR1 function that includes protecting against SLE nephritis.

A transgenic mouse model for studying the clearance of blood-borne pathogens via human complement receptor 1 (CR1)

Complement receptor 1 (CR1) on the surface of human erythrocytes facilitates intravascular clearance of complement-opsonized pathogens. The need for complement activation can be circumvented by directly coupling the organism to CR1 using a bispecific monoclonal antibody heteropolymer (HP). Lack of a functional homologue to CR1 on mouse erythrocytes has made it difficult to study HP-dependent clearance of pathogens in small animals. We have developed a transgenic mouse that expresses human CR1 on erythrocytes. CR1 antigen is of appropriate size and in a clustered distribution as confirmed by immunoblotting and fluorescence microscopy, respectively. HP that immobilized bacteriophage PhiX174 prototype pathogen to erythrocyte CR1 of the transgenic mice increased the rate of clearance of the virus compared with HP that bound bacteriophage, but not CR1. This transgenic mouse model will allow evaluation of different HPs for their in vivo efficacy and potential as human therapeutics.

A CR1 polymorphism associated with constitutive erythrocyte CR1 levels affects binding to C4b but not C3b

The erythrocyte type one complement receptor (E-CR1) mediates erythrocyte binding of complement-opsonized immune complexes (IC), and helps protect against random deposition of circulating IC. Two linked CR1 polymorphisms occur in binding domains, at I643T and Q981H. In Caucasians, the variant alleles (643T, 981H) are associated with low constitutive E-CR1 expression levels. This study was conducted to determine if these polymorphisms affect ligand binding, and if so, represent risk factors for the autoimmune IC disease, systemic lupus erythematosus (SLE). In an ELISA comparing relative ligand binding differences, E-CR1 from individuals homozygous for the variant residues (643TT/981HH) exhibited greater binding to C4b, but not C3b, than homozygous wild-type E-CR1. Analysis of single-binding domain CR1 constructs demonstrated that the 981H residue imparted this enhanced C4b binding. No differences were observed in the 981H allele frequency between Caucasian controls (0.170, n = 100) and SLE patients (0.130, n = 150, P = 0.133), or between African American controls (0.169, n = 71) and SLE patients (0.157, n = 67). In a subset of individuals assessed for CR1 size, excluding from this analysis those expressing at least one B allele revealed a trend for over-representation of the 981H allele in Caucasian controls (0.231 frequency, n = 26) versus SLE patients (0.139, n = 83, P = 0.089), but again no difference between African American controls (0.188, n = 24) and SLE patients (0.191, n = 34). These data suggest that the 981H residue compensates for low constitutive expression of E-CR1 in Caucasians by enhancing C4b binding. This may contribute protection against SLE.

Pneumococcal immune adherence to human erythrocytes

BACKGROUND

Human red blood cells bind various C3b-coated microorganisms via their C3b/CR1 receptor, a phenomenon referred to as immune adherence. The aim of the present study was to measure pneumococcal adherence to human red blood cells by flow cytometry and to study kinetic aspects of this binding.

MATERIAL AND METHODS

We quantified pneumococcal adherence to human erythrocytes by FACS analysis and tested the involvement of antibodies and complement activation in this process.

RESULTS

Pneumococci are able to bind to human red blood cells in the presence of human serum. Coating with C3b/C4b appeared obligatory for pneumococcal adherence to red blood cells. The ligand on erythrocytes was confirmed to be complement receptor 1. Kinetic studies showed that innate (mannose-binding lectin) and specific immune factors (IgG antibodies) contributed to the binding of C3b-coated pneumococci to human erythrocytes. After initial binding, serum-derived factor I was found to induce bacterial detachment from the erythrocyte.

CONCLUSIONS

Pneumococci are able to adhere to red blood cells. Both the classical and lectin complement pathways are important for optimal C3b-coating of pneumococci for immune adherence. Bound pneumococci are detached from red blood cells by factor I. These findings are in line with the hypothesis of immune adherence in which human erythrocytes are able to bind pneumococci and target the bacteria to the reticulo-endothelial system in the spleen.

Microglia and inflammatory mechanisms in the clearance of amyloid beta peptide

There is now abundant evidence that brain microglia, when activated, have the lineage, receptors, and synthetic capacity to participate in both potentially neurotoxic inflammatory responses and potentially beneficial phagocytic responses. Amyloid beta peptide (Abeta) forms highly insoluble, beta-pleated aggregates that are widely deposited in the Alzheimer's disease (AD) cortex and limbic system. Aggregated Abeta also activates the classical and alternative complement cascades. These properties make Abeta an excellent target for microglial phagocytosis, a view supported by multiple reports, through well established mechanisms of phagocyte clearance.

Of mice and men: the relevance of the mouse to the study of human SLE

A number of mouse models have been utilized to study the pathophysiology of immune complex (IC) disease, and the hallmark IC disease systemic lupus erythematosus (SLE). Many of these studies have provided exciting new insights into IC-mediated inflammation and autoimmunity. However, numerous differences exist between mice and humans that suggest that mouse studies are not always applicable to human disease. These differences can be found in the biological systems that interact with circulating IC, in the specifics of disease presentation, and in the general physiology of the two species. Furthermore, although the mechanisms of SLE-like autoimmune disease in the mouse are being defined through analyses of the murine models of SLE, it remains to be proven that these mechanisms are relevant to human SLE. Thus, generalizing the results of the mouse studies to human SLE and other human IC diseases must be done with caution.

Relationship between complement activation and renal deposition of immune complexes made with IgG2a monoclonal antibodies

Previous studies identified a single murine monoclonal IgG2a anti-dinitrophenyl antibody that, when combined with the antigen, formed immune complexes (IC) that were preferentially deposited in glomeruli. The present study examined the clearance and organ localization in Balb/c mice of expanded panels of radiolabeled IC containing murine monoclonal antibodies. The results identified a second IgG2a antibody that formed IC with a predilection for renal deposition. IC made with the two IgG2a antibodies that were preferentially deposited in the kidney were the least efficient binders of human C1q or homologous murine C3b and C4b within the IgG2a panel. These observations suggest a new model of IC-mediated renal disease initiation in which relatively weak complement activation leads to inefficient IC clearance by complement receptor-bearing circulating cells and consequent IC deposition in tissues susceptible to IC-mediated injury.

Immune complex processing in C1q-deficient mice

Complement and Fcgamma receptors are known to mediate the processing of immune complexes (IC), and abnormalities in these mechanisms may predispose to the development of lupus. We explored the processing of IC in mice deficient in complement component C1q. 125I-labelled IC comprising Hepatitis B surface antigen (HBsAg)/human anti-HBsAg (HBsAg/Ab) were injected intravenously and the sites of IC clearance determined by direct counting of organ uptake at various time points. The liver and spleen were the main sites of IC uptake in all mice. The splenic uptake of IC was significantly reduced in the C1q-deficient mice compared with the control mice. C1q-deficient mice also exhibited an initial accelerated hepatic uptake of IC similar to that seen in human subjects with hypocomplementaemia. The hepatic localization of IC at later time points was similar in both groups of mice. These data in mice are consistent with previous observations in humans that confirm that the classical pathway of complement plays an important role in the appropriate processing of IC in vivo.

Transfer of immune complexes from erythrocyte CR1 to mouse macrophages

We are developing a potential therapeutic approach for removing pathogens from the circulation of primates in which the pathogen is bound to the complement receptor (CR1) on E using a bispecific mAb complex, a heteropolymer (HP). We have used mAb this approach to demonstrate that cleared prototype pathogens are localized to, phagocytosed in, and destroyed in the liver. Extension of this work to a clinical setting will require a detailed understanding of the mechanism by which the E-bound immune complex substrates are transferred to fixed tissue macrophages in the liver, the transfer reaction. Therefore, we examined an in vitro system to study this process using bacteriophage phiX174 as a model pathogen. E containing phiX174 (bound via an anti-CR1/anti-phiX174 HP) were incubated with P388D1 murine macrophages, and the two cell types were separated by centrifugation through Ficoll. Both E and macrophages were then probed and analyzed by RIA or flow cytometry. The results indicate that all three components of the E-bound IC (phiX174, HP, and CR1) were removed from the E and internalized by the macrophages. We found that transfer requires the Fc portion of IgG, because little transfer of phiX174 occurs when it is bound to E CR1 using a HP containing only Fab fragments. These findings, taken in the context of other studies, suggest a general mechanism for the transfer reaction in which Fc receptors facilitate close juxtaposition of the macrophage to the E-bound IC which then allows a macrophage-associated protease to cleave CR1. The released IC are then internalized and processed by the macrophages.

Glomerular deposition of immune complexes made with IgG2a monoclonal antibodies

The factors that determine whether immune complexes (IC) are cleared safely from the circulation or are deposited in vulnerable tissues such as glomeruli are not well defined. To better understand how IC are handled, the present study examined the fate in vivo of three model IC preparations with different immunochemical characteristics. Radiolabeled IC were constructed with murine IgG1, IgG2a, or IgG3 anti-DNP mAbs bound to DNP-BSA, designated IgG1 IC, IgG2a IC, and IgG3 IC, respectively. The IC were infused i.v. into BALB/c mice, and clearance and tissue localization of the three IC probes were compared. The results indicate that the major portion of each IC preparation was cleared from the circulation by the liver. However, compared with the other two probes, IgG2a IC were preferentially deposited in the kidney. Histologic examination revealed the presence of IgG2a IC in glomeruli. The enhanced renal uptake of IgG2a IC could not be attributed solely to such characteristics as IC size, Ag/Ab ratio, Ab charge, or affinity. However, the preferential renal deposition of IgG2a IC was abrogated by complement depletion. Thus, enhanced renal uptake in normal mice was complement dependent. These data suggest that interactions between IC and the complement system can influence the propensity of IC to deposit in tissues susceptible to IC-mediated injury.

Histidine-rich glycoprotein regulates the binding of monomeric IgG and immune complexes to monocytes

Histidine-rich glycoprotein (HRG) is a relatively abundant plasma protein which we have shown previously inhibits the formation of insoluble immune complexes (IC). In this study we examined the ability of HRG to regulate the binding of monomeric IgG and IC to monocytes. Initial studies demonstrated that HRG interacts with FcgammaRI on the monocytic cell line THP1 and blocks the binding of monomeric IgG to these cells. However, despite totally blocking the binding of monomeric IgG to FcgammaRI, pre-incubation of THP1 cells with HRG had no effect on the binding of IC to these cells. In contrast, depending on the HRG:IgG molar ratio, pre-incubation of monomeric IgG with HRG resulted in either enhanced or reduced IgG binding to FcgammaRI. Similarly, under certain highly defined conditions, incorporation of HRG in IgG-containing IC potentiated the binding of IC to THP1 cells. The key conditions involved incorporating approximately equimolar concentrations of HRG and IgG in the IC, the IC being formed at a near equivalence antigen:antibody ratio and usually physiological concentration (20 microM) of Zn(2+) being present. Collectively these observations indicate that HRG is an important regulator of IC uptake by monocytes. Thus HRG can interact with FcgammaRI on monocytes and block monomeric IgG binding, whereas when incorporated in IgG containing IC, HRG can enhance the uptake of IC by monocytes, probably via its heparan sulfate binding domain.

Functional characterization of soluble and membrane-bound forms of vaccinia virus complement control protein (VCP)

Vaccinia virus secretes a 35 kD protein, vaccinia virus complement control protein (VCP), that inhibits the classical and alternative pathways of complement at several points, indicating that it may be a viral analogue of human complement receptor type 1 (CR1; CD35). Structurally, however, CR1 is composed of 30 short consensus repeats (SCRs), whereas VCP consists entirely of four SCRs. We have begun a structure-function analysis of VCP to define the minimum number of SCRs necessary for function, the functional differences between VCP and CR1, and the potential therapeutic roles for VCP. We addressed these questions by creating and characterizing recombinant soluble and membrane-bound forms of VCP. We have determined that (1) VCP requires all four SCRs to bind C3b, (2) whereas CR1 binds C3b and iC3b, VCP binds C3b but not iC3b, and (3) although normally secreted, if expressed on the membrane of mammalian cells, VCP effectively protects the cells from complement-mediated lysis. Thus, VCP appears to be a compact and unique complement regulatory protein with the ability to inhibit both arms of the complement cascade, but lacking affinity for iC3b. By releasing rather than capturing iC3b-bearing complexes following inactivation of C3b, VCP may 'recycle' its active site locally among infected cells, and thereby enable the virus to evade more efficiently host immune and inflammatory responses. The unique function, compact structure, and capacity of VCP to protect mammalian cells from complement-mediated attack, suggests that it could be used both to better understand the structure-function relationship of complement regulatory proteins, in general, and also to rationally design and develop novel therapeutic agents.

Mechanism of regulation of complement receptor type 1 transcription by cytosine arabinoside in a pre-erythroid model

Binding to erythrocyte complement receptor type 1 (CR1) clears immune complexes from blood and tissues, preventing complement-mediated pathological inflammation in disease. Previous work has demonstrated that Ara-C, a cytosine analogue, induces an 11-fold increase in CR1 mRNA expression in K-562 erythroleukaemia cells. In this work we therefore investigated whether the Ara-C/K-562 system could be used as a model for studying the pre-erythroid regulation of CR1. We demonstrated that increased CR1 expression could be induced independently of increased haemoglobin expression. Increases in CR1 mRNA levels produced by Ara-C treatment were not a function of increased stability of the message. However, Ara-C induced a protein synthesis-dependent increase in transcription initiation rate as early as 12h after treatment. Further data suggest that the effect of Ara-C on transcription is not a result of its direct DNA-damaging or DNA polymerase-inhibition activities. Induction of receptor transcription was inhibited by tyrosine kinase (TK) and protein kinase C (PKC) inhibitors. These data suggest that TK, PKC and dCTP-adducted phospholipid signalling pathways may all play a role in the mechanism of Ara-C-induced CR1 transcription.

Analysis of complement receptor type 1 (CR1) expression on erythrocytes and of CR1 allelic markers in Caucasian and African American populations

CR1 expression on erythrocytes (E) is regulated by an element that is tightly linked in Caucasians to the site of an RFLP of the CR1 gene. Genomic HindIII fragments of 7.4 and 6.9 kb identify alleles that are expressed in high (H allele) or low (L allele) amounts, respectively. When age-fractionated E of donors heterozygous for both the H and L alleles and for CR1 allotypes of differing molecular weights were analyzed in Western blots, the product of the L allele appeared to have an increased rate of loss during cell aging. A coding sequence polymorphism of CR1 predicted to cause a Pro-->Arg substitution in its proximal extramembranous region was tightly linked in Caucasians to the site of the HindIII RFLP. However, neither this polymorphism nor the HindIII RFLP correlated with CR1 expression among African Americans. Relative instability of CR1 encoded by the L allele thus may derive from another coding sequence polymorphism, or may require both the Pro-->Arg substitution and epistatic effects of another polymorphic gene.

Diminished ability of erythrocytes from patients with systemic lupus erythematosus to limit opsonized immune complex deposition on leukocytes and activation of granulocytes

OBJECTIVE

To compare the ability of normal erythrocytes and erythrocytes from systemic lupus erythematosus (SLE) patients to bind immune complexes (IC), thereby inhibiting IC deposition on polymorphonuclear leukocytes (PMN) and the consequent induction of a PMN respiratory burst (RB).

METHODS

The binding of fluorescein isothiocyanate-labeled IC in 75% autologous serum to whole blood cells or isolated leukocytes from 17 SLE patients and 10 controls was assessed by flow cytometry. Reactive oxygen metabolite (ROM) production by PMN was measured as the intracellular oxidation of dihydrorhodamine 123, on stimulation with unlabeled IC.

RESULTS

Erythrocyte-mediated inhibition of IC uptake by PMN reached a mean +/- SD maximum of 68 +/- 18% in controls and 29 +/- 51% in SLE patients (P < 0.05) and, in the patients, correlated inversely with disease activity. In the presence of erythrocytes from various donors, IC binding to a standard preparation of PMN and their ROM production were inversely proportional to the number of type 1 complement receptors (CR1) per donor erythrocyte. Thus, the ROM production was higher in the presence of SLE patients' erythrocytes (125 +/- 67 CR1/erythrocyte) than with erythrocytes from controls (235 +/- 118 CR1/erythrocyte).

CONCLUSION

Erythrocytes from SLE patients are defective in protecting their PMN against IC deposition and induction of the RB.

A novel ELISA for the assessment of classical pathway of complement activation in vivo by measurement of C4-C3 complexes

Measurements of complement split products by enzyme-linked immunosorbent assays (ELISA) are well established for the assessment of in vivo complement activation. We have combined two monoclonal antibodies (mAb) with specificities for C3b/iC3b/C3dg (mAb I3/15) and C4/C4b/C4d (mAb M4d2), respectively, in a sandwich ELISA to quantitate C4-C3 complexes as an indicator of complement activation. Serum incubated with heat aggregated IgG (HAG) was used as a standard and the C4-C3 levels expressed as microgram equivalent HAG/ml (microgram HAG-equ/ml). Normal values of C4-C3 complexes in plasma (EDTA) of healthy probands (n = 11) were 6.3 micrograms HAG-equ/ml +/- 1.5 (mean +/- 1 standard deviation (SD), with a range from 3.6 to 9.1). In patients with systemic lupus erythematosus (SLE, n = 23) C4-C3 values were clearly elevated (48.8 micrograms HAG-equ/ml +/- 52.9, range 7.5-184.7) as compared to samples from patients with idiopathic hypertension (IDH, n = 10) (6.5 micrograms HAG-equ/ml +/- 1.7, range 4.1-9.4). For SLE patients C4-C3 levels significantly correlated with values for C3b/iC3b/C3d (r = 0.69, p < 0.001) and C3 containing immune complexes (r = 0.68, p < 0.001), but not with the C4d fragment (r = 0.26). C4-C3 levels of 96% of the studied SLE patients were increased more than 2 SD above the normal mean as compared to 74% of C4d and activated C3 values, respectively. Serum treated with zymosan as an activator of the alternative pathway of complement did not exhibit higher C4-C3 values. These results demonstrate that the quantitation of in vivo generated C4-C3 complexes by ELISA provide a novel, sensitive parameter for classical pathway of complement activation.

Interactions of opsonized immune complexes with whole blood cells: binding to erythrocytes restricts complex uptake by leucocyte populations

The binding of opsonized, fluorescein-labelled bovine serum albumin (BSA)/rabbit anti-BSA complexes (IC) to washed human whole blood cells and isolated leucocytes in the presence of autologous serum was investigated by flow cytometry. In the presence of erythrocytes (E), the IC-binding to granulocytes (PMN), monocytes and lymphocytes was inhibited by up to 46%, 61% and 48%, respectively, depending on the incubation time and the IC-concentration tested. The E-mediated inhibition of the binding to PMN was found to correlate with the average numbers of CR1 per E during the initial 15 min of incubation. Thereafter, the difference between IC binding to PMN in absence and presence of E, decreased in accordance with decreasing binding to E. IC-uptake by PMN induced a drop in side-scatter characteristics, attributable to degranulation, which could be prevented by the presence of E. In contrast to the findings for PMN, the difference between IC-binding to monocytes in the absence and presence of E increased progressively over the 90 min observation period, suggesting that different mechanisms are involved in the late-phase IC uptake by monocytes and PMN. Lymphocytes were heterogeneous with respect to IC binding, the main contributors being B cells. E initially inhibited and then later enhanced the IC binding to lymphocytes, suggesting that E promote B cell uptake of C3d,g-covered IC via CR2. Our findings, that E can restrict the IC uptake by circulating leucocytes, and that an IC-induced degranulation of PMN may be prevented by E, indicate that E may act as a high capacity buffer limiting inappropriate activation of phagocytes by circulating IC.

C3b receptor (CR1) genomic polymorphism in rheumatoid arthritis. Low receptor levels on erythrocytes are an acquired phenomenon

The number of complement receptor 1 (CR1, CD35) molecules on erythrocytes is genetically determined by two codominant alleles. The numerical expression of CR1 on erythrocytes correlates with a HindIII-RFLP or CR1 gene using CR1-1, a complementary DNA probe. We have found low CR1 on erythrocytes in patients with rheumatoid arthritis (RA) in an Indian population. Low levels in RA patients may be acquired or genetically determined. Fifty-two patients with RA, 48 nonrelated healthy subjects and 19 consanguineous relatives of patients were genotyped. CR1 numbers on erythrocytes were quantitated by the enzyme-linked immunosorbent assay using monoclonal anti-CR1 antibody. Normal subjects and patients were followed up for a period of 6 months to evaluate the stability of their CR1 expression. The gene frequency for allele H and L (7.4- and 6.9-kb HindIII restriction fragment, respectively), which correlated with high and low expression of CR1 on erythrocytes was 0.77 and 0.23 in the normal controls. Gene frequency in RA patients was 0.78 and 0.22 for H and L allele, which did not differ significantly from either controls or relatives (0.80 and 0.20 for H and L allele, respectively). However, RA patients expressed fewer CR1 on erythrocytes within each genotype than their relatives and controls. CR1 on erythrocytes were found to be stable in consecutive samples in controls. In RA patients, the number varied between low and high during the course of the disease. The variation in number was significantly correlated (p < 0.05, r = -0.85 to -0.98) with disease activity as monitored by erythrocyte sedimentation rate. Our results suggest that low levels of CR1 on erythrocytes in patients with RA are not inherited, rather they are acquired during the course of the disease.

Differential susceptibility of immune complexes to release from the erythrocyte CR1 receptor by factor I

This study was designed to explore the role of Factor I in the release of immune complexes (IC) from human erythrocytes (E). The interactions between E and IC constructed with murine monoclonal antibodies were examined using, as a complement source, autologous plasma, plasma depleted of Factor I by > 90%, or Factor I-depleted plasma reconstituted with purified Factor I. Striking differences were observed in the interactions between E and different types of IC in Factor I-depleted plasma. The release of IC constructed with IgG1, IgG3, IgM or IgA antibodies was abolished by Factor I depletion whereas IC containing IgG2a or IgG2b antibodies were still released from E in Factor I-depleted plasma. Moreover, when IC containing IgG2a antibodies were incubated briefly in Factor I-depleted plasma, under conditions in which the IC were bound but not released, and then resuspended in the presence or absence of Factor I, as little as 5% of the normal physiologic level of Factor I released the IC from E. Thus, IC containing IgG2a antibodies appear to be exquisitely susceptible to release from E by Factor I. Additional differences in the susceptibility of IC containing IgG1, IgG3, IgM or IgA antibodies to release from E were revealed when Factor I-depleted plasma was reconstituted with Factor I. Under these conditions, the relative susceptibility of IC to release was: IC containing IgG1 or IgA antibodies > IC containing IgM antibodies > IC constructed with IgG3 antibodies. While isotype was critical in determining susceptibility to release, some clonotypic differences between isotype-matched pairs of IC were also evident. Differences in IC release from E by Factor I may reflect antibody matrix-mediated differential susceptibility of IC-bound C3b and/or C4b to cleavage by Factor I and may have implications for immunoregulation, host effector cell mechanisms and the pathophysiology of IC diseases.

Clonal variations in complement activation and deposition of C3b and C4b on model immune complexes

This study examined the relationship between complement activation and the deposition of C3b and C4b on a panel of model immune complexes (IC). IC were constructed by combining murine monoclonal IgM, IgA, IgG1, IgG2a or IgG3 anti-dinitrophenyl (DNP) antibodies with DNP-bovine serum albumin (DNP-BSA). The IC were incubated with human plasma as a complement source and the formation of C4a and C3a, as well as the deposition of C4b and C3b on the IC, measured by radioimmunoassay. The results indicate that there were isotype-independent variations in the capacity of different types of IC to activate the classical pathway, especially for isotype-matched pairs of IC containing IgG1, IgG2a and IgG3 antibodies. In most cases, there was a direct relationship between classical pathway activation and the cleavage of C3. There was, for most of the IC, a direct correlation between cleavage of C4 and C3 and the subsequent deposition of C4b and C3b on the IC. However, a pair of IC constructed with independently derived IgG1 antibodies was virtually identical with respect to C3 cleavage and yet differed in the number of C3b molecules deposited on the IC. Collectively, these data suggest that the immunoglobulin variable region can play a significant role in both complement activation and the deposition of C3b and C4b on IC.

Complement enhances the elimination of soluble aggregates of IgG by rat liver endothelial cells in vivo

In the present study, we have investigated the role of complement (C) and possible C receptors present on rat liver endothelial cells (EC) in the clearance and tissue distribution of soluble aggregates of IgG (AIgG). To study the effect of elimination of AIgG by EC in vivo, Kupffer cell (KC)-depleted rats were used, with or without an intact C system (These rats will be referred to throughout this report as EC-rats.) In EC-rats with an intact C system, clearance of AIgG (2000-3000 kDa, 20-27 IgG molecules/aggregate) occurred in a biphasic manner with a first T 1/2 (T1) of 9.4 +/- 2.3 min and a second T 1/2 (T2) of 44.7 +/- 16.0 min. In EC-rats without an intact C system [cobra venom factor (COVF)-treated group], clearance of AIgG was significantly delayed with a T1 of 25.3 +/- 9.9 min (p < 0.005) and a T2 of 124.5 +/- 18.4 min (p < 0.001). There were less degradation products of AIgG in the circulation in EC-rats treated with COVF as compared to EC-rats with an intact C system. Eight minutes after injection, 27.5 +/- 11.6% of the injected AIgG was found in the livers of EC-rats while 15.1 +/- 3.2% was found in the livers of the COVF-treated group. Double immunofluorescence studies indicated that AIgG in the liver was associated with EC in the rats with an intact C system. Clear deposits of C3 and lesser amounts of C1q accompanied the deposition of AIgG. In COVF-treated EC-rats, AIgG together with C1q was also associated with EC but no detectable C3 was seen. These data suggest clearance of AIgG via Fc and C receptors present on EC in vivo.

Deficiency of complement component C3 is associated with accelerated removal of soluble 123I-labelled aggregates of IgG from the circulation

Complement and erythrocyte complement receptors CR1 (CD35) play an important role in the clearance of immune complexes. We studied the elimination of soluble 123I-labelled aggregates of human immunoglobulin G (123I-AIgG), used as a model for immune complexes, in two patients with a congenital and two patients with an acquired deficiency of complement component C3, and compared these with 10 healthy controls. The first disappearance halflife of 123I-AIgG was shorter (3.3 +/- 0.4 versus 7.0 +/- 0.4 min in the controls, P = 0.005) and maximal hepatic uptake of aggregates was increased in the C3 deficient patients (maximal liver/background ratio 3.6 +/- 0.4 versus 2.7 +/- 0.2 in controls, P = 0.04). Apparently, in the absence of C3, removal of circulating immune complexes by the liver is accelerated, probably through Fc receptor-dependent mechanisms.

Increased frequency of the long (S) allotype of CR1 (the C3b/C4b receptor, CD35) in patients with systemic lupus erythematosus

The allotypic forms of the C3b/C4b receptor (CR1, CD35) differ in length, in the number of expressed C3b binding sites and thus in their ability to mediate the processing of circulating C3- and C4-bearing immune complexes. We have used a combination of three informative restriction fragment length polymorphisms (RFLPs) to assess the frequencies of the F (most frequent allele comprised of four long homologous repeats (LHR)), S (five LHR) and F' (three LHR) alleles of the C3b/C4b receptor (CR1, CD35) in a French population of patients with systemic lupus erythematosus (SLE) (n = 63) and healthy controls (n = 158). A significantly higher frequency of the S phenotype was observed among patients (51%) as compared with controls (26%). The F' allele was found in 2/61 patients and 1/85 healthy controls, indicating the rare occurrence of the short CR1 allele in SLE. This allele is also extremely rare in the normal population. The overrepresentation of the S long allele among patients is indicative of a genetic linkage between CR1 and susceptibility to SLE.

Mechanism of transfer of immune complexes from red blood cell CR1 to monocytes

Complement receptor 1 (CR1) on primate red blood cells (RBC) binds most complement-fixing immune complexes in the circulation. It has been postulated that by binding them, RBC keep immune complexes in the intravascular space and deliver them to the tissue macrophages of the mononuclear phagocyte system. We have developed an in vitro model to study the transfer of RBC-bound immune complexes (heat-aggregated IgG and DNA-anti-DNA) to phagocytic cells (human monocytes). Transfer of immune complexes from RBC to monocytes occurred significantly more rapidly than monocyte uptake of the same immune complexes from solution. In the transfer process, complex-bearing RBC were not bound or sequestered by the monocytes. To define the monocyte receptors involved in binding immune complexes from the RBC surface, monocyte receptors were blocked with MoAbs (anti-CR1, anti-FcRII) or EDTA (to block CR3). Monocyte binding of immune complexes primarily used CR1 with a small contribution from FcRII, and with little or no contribution from CR3 and FcRI. Uptake of immune complexes from solution employed the same monocyte receptors as binding of complexes from the RBC surface. Immune complexes in solution bound to RBC and to monocytes with equally high avidity (approximately 1 x 10(11) l/M), but monocytes expressed a 15-20-fold greater number of immune complex binding sites. We propose that immune complexes distribute between RBC and monocytes according to the binding capacity of these cells, such that at equal or high RBC/monocyte ratios as would be seen in the circulation immune complexes bind to RBC, but at low RBC/monocyte ratios (as would be seen in the sinusoidal circulation of the liver and spleen), most immune complexes bind to monocytes. To define the pathway by which immune complexes move from RBC to monocytes, their release from RBC CR1 was examined. Under various conditions, the dissociation rate was extremely slow, and did not increase with the addition of monocyte supernatants. To examine whether factor I-mediated processing of immune complexes enhances binding of immune complexes to monocytes, RBC-bound complexes were released with factor I, and binding of these 'processed' immune complexes to monocytes was examined. Monocyte binding of these processed immune complexes was slower than of control ones; furthermore, performance of transfer experiments at 4 degrees C, which significantly shows enzymatic processes, did not decrease the rate of immune complex transfer from RBC to monocytes.(ABSTRACT TRUNCATED AT 400 WORDS)

Isotypic and clonal variations in the interactions between model monoclonal immune complexes and the human erythrocyte CR1 receptor

Erythrocytes (E) play a central role in handling circulating immune complexes (IC) in primates. E capture IC via complement receptors, type 1 (CR1) which can bind to C3b and C4b ligand sites generated on IC during activation of the complement cascade. The present study was designed to explore how the immunochemical properties of IC affected their interactions with human E. Model IC were constructed by combining murine monoclonal anti-dinitrophenyl (DNP) antibodies with DNP-bovine serum albumin. A panel of 10 independently-derived monoclonal IgG1, IgG2a, IgG2b, IgG3, IgM and IgA antibodies were used to construct IC and their interactions with human E were examined in vitro. The data reveal that IC constructed with the different monoclonal antibodies differed with respect to their rate of binding to E, the peak magnitude of IC binding to E, and the rate and extent of IC release from E. IC containing IgG1 antibodies (IgG1 IC), IgG2a IC, IgG2b IC, and IgA IC all bound rapidly to E, whereas IgG3 IC and IgM IC were bound relatively slowly to E. The peak magnitude of IC binding to E correlated directly with their binding rate. There was an inverse correlation between the antigen/antibody ratio of the IC and the magnitude of IC binding to E. The rate of release of the various types of IC from E also differed. IgG2a IC and IgG2b IC displayed the most rapid maximum release rates while IgG3 IC had the slowest peak release rate. IgM IC and IgA IC were also released relatively slowly from E. IgG1 IC had an intermediate release rate. There was no direct correlation between the maximum release rate and either the maximum binding rate or the peak magnitude of IC binding to E. While there were some clonotypic differences in binding and release rates between IC made with different IgG2a, IgG3 and IgM antibodies, antibody isotype appears to be of fundamental importance with respect to both the binding of IC to E and the release of IC from E. These data indicate that the immunochemical properties of IC can profoundly affect their interactions with human E and that the panel of IC constructed with monoclonal antibodies can serve as a useful model to explore these interactions.

Distribution and quantitative expression of the complement receptor type 1 (CR1) on human peripheral blood T lymphocytes

The complement receptor, type 1 (CR1) is expressed on a variety of cell types including primate erythrocytes, phagocytic cells, and B lymphocytes. On these cells, CR1 plays a role in a diverse spectrum of biological activities including the clearance of immune complexes from the circulation, down-regulation of the complement system, recognition of complement-coated microorganisms, and cellular activation. CR1 is also expressed by some, but not all, T lymphocytes. The present study was undertaken in order to examine the distribution of CR1 on normal human T cell subsets by flow cytometry and to quantify the expression of T cell CR1 by radioimmunoassay. Data presented here indicate that, in a panel of 19 normal individuals, a mean of 9.7% of the overall peripheral blood lymphocyte population expressed CR1 and that, as assessed by two-color flow cytometry, 12.0% of CD3+, 13.0% of CD4+, and 20.0% of CD8+ cells expressed CR1. While single peaks of CR1 staining were observed within the CD3 and CD4 subsets, a biphasic pattern of staining was evident within the CD8 subset in which relatively high-intensity CR1 staining was detected within the subpopulation of "dull" CD8+ cells, whereas a lower intensity of CR1 staining was observed within the subpopulation of "bright" CD8+ cells. Duplicate analyses performed over a relatively short time frame suggested that, while the overall percentage of cells that expressed CR1 varied considerably among normal individuals, in at least some individuals the percentage of cells expressing CR1 was relatively stable, especially within the CD4 subset. In cell suspensions enriched for T lymphocytes by rosetting with sheep erythrocytes, 10.0% of the cells were CR1+ and a mean of approximately 3700 CR1 were expressed per CR1+ cell. There was no apparent correlation between the number of CR1 per T cell and the number of CR1 expressed per erythrocyte in the same blood sample. The expression of CR1 on subpopulations within the CD3+, CD4+, and CD8+ lymphocyte subsets may play a role in both normal cell function and in the pathophysiology of disease states including the acquired immune deficiency syndrome (AIDS).