Low C3b receptor reactivity on erythrocytes from patients with systemic lupus erythematosus detected by immune adherence hemagglutination and radioimmunoassays with monoclonal antibody

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.

The genetics of Alzheimer's disease

Alzheimer's disease is a progressive, neurodegenerative disease that represents a growing global health crisis. Two major forms of the disease exist: early onset (familial) and late onset (sporadic). Early onset Alzheimer's is rare, accounting for less than 5% of disease burden. It is inherited in Mendelian dominant fashion and is caused by mutations in three genes (APP, PSEN1, and PSEN2). Late onset Alzheimer's is common among individuals over 65 years of age. Heritability of this form of the disease is high (79%), but the etiology is driven by a combination of genetic and environmental factors. A large number of genes have been implicated in the development of late onset Alzheimer's. Examples that have been confirmed by multiple studies include ABCA7, APOE, BIN1, CD2AP, CD33, CLU, CR1, EPHA1, MS4A4A/MS4A4E/MS4A6E, PICALM, and SORL1. Despite tremendous progress over the past three decades, roughly half of the heritability for the late onset of the disease remains unidentified. Finding the remaining genetic factors that contribute to the development of late onset Alzheimer's disease holds the potential to provide novel targets for treatment and prevention, leading to the development of effective strategies to combat this devastating disease.

Measurement of erythrocyte C4d and complement receptor 1 in systemic lupus erythematosus

OBJECTIVE

C4-derived activation fragments are the only complement ligands present on the surfaces of normal erythrocytes. The significance of this observation is unknown, and the role of erythrocyte-bound C4 (E-C4) in human disease has not been explored. More than any other human disease, the pathogenesis of systemic lupus erythematosus (SLE) has been characterized by defects in clearance of complement-bearing immune complexes via erythrocytes expressing complement receptor 1 (CR1). This study was undertaken to determine whether these functional defects might be reflected by abnormal patterns of E-C4 and E-CR1 expression on erythrocytes of patients with SLE.

METHODS

We conducted a cross-sectional study of 100 patients with SLE, 133 patients with other diseases, and 84 healthy controls. Erythrocytes were characterized by indirect immunofluorescence and by flow cytometry for determination of levels of C4d and CR1.

RESULTS

Patients with SLE had higher levels of E-C4d and lower levels of E-CR1 than did patients with other diseases (P < or = 0.001) or healthy controls (P < or = 0.001). The test was 81% sensitive and 91% specific for SLE versus healthy controls and 72% sensitive and 79% specific for SLE versus other diseases, and it had an overall negative predictive value of 92%.

CONCLUSION

This is the first report of abnormal levels of E-C4d in human disease. We found that abnormally high levels of E-C4d and low levels of E-CR1 are characteristic of SLE, and combined measurement of the 2 molecules has high diagnostic sensitivity and specificity for lupus. Determination of E-C4d/E-CR1 levels may be a useful addition to current tests and criteria for SLE diagnosis.

Immunoadsorption, current status and future developments

The association of abnormalities in the cellular and humoral immune system with various autoimmune diseases provides the rationale for apheresis technologies. While plasmapheresis or plasma exchange is limited by its non-selective removal of all plasma components, modern apheresis techniques aim to provide more specific elimination according to clinical needs and avoid plasma product replacement. However, the commercialisation has not met the expectations in the early 80's and the number of patients treated by extracorporeal immunoadsorption remains small due to a lack of well-defined controlled trials and limited reimbursement. This review highlights the immunological and technical basis for extracorporeal immunoadsorption, as well as its current status in the treatment of immunologically-mediated diseases.

The C3b/C4b receptor (CR1, CD35) on erythrocytes: methods for study of the polymorphisms

This report is devoted to methodologies used in analyzing the C3b/C4b receptor (CR1, CD35) on erythrocytes (E), its soluble form, the CRI structural or allotype polymorphism, and CR1 density polymorphism. In primates E CR1 serves as the main system for processing and clearance of complement opsonized immune complexes (IC). CR1 copy numbers decrease with aging of E in normal individuals. Erythrocyte CR1 is also decreased in pathological conditions such as systemic lupus erythematosus (SLE), HIV infection, certain hemolytic anemias, and many other conditions featuring immune complexes. Consequently, CRI on E has an important physiological role in immune complex handling and has interesting alterations in disease.

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.

CR1 density polymorphism and expression on erythrocytes of patients with systemic lupus erythematosus

The present study investigated the expressed number of CR1 on erythrocytes (E) in relationship of the CR1 density genotype from 46 patients with systemic lupus erythematosus (SLE) and 47 healthy volunteers. The CR1 genotype was determined by a method based on polymerase chain reaction (PCR) amplification of the genomic DNA fragment of 1.8 kb separated by HindIII endonuclease digestion and agarose gel electrophoresis. Our data supported the earlier results that the number of binding sites/E for monoclonal anti-CR1 decreased among SLE patients compared with normal individuals having the same alleles for the CR1/E density. At the same time the novelty of our recent results was that the decreased expression of CR1 on E correlated significantly with kidney involvement in patients homozygous for the CR1/E high density allele (HH). These data suggest that the deficiency of the detectable number of CR1 on erythrocytes is acquired in this SLE population.

Hind III genomic polymorphism of the C3b receptor (CR1) in patients with SLE: low erythrocyte CR1 expression is an acquired phenomenon

Expression of the human erythrocyte C3b receptor (CR1-CD35) and its Hind III RFLP was studied in a group of 37 patients with SLE, 15 consanguineous relatives of the patients and 48 healthy normal subjects. The CR1 number on erythrocytes was quantitated by ELISA using a mAb to CR1. Serum levels of complement proteins (C3, C4, C3d) and circulating immune complexes (CIC) were estimated simultaneously in controls and relatives. The patients were followed up during the course of the treatment. The CR1/erythrocyte (CR1/E) in patients were found to be significantly low in comparison to controls. The gene frequencies for the alleles H and L (7.4 and 6.9 kb Hind III restriction fragments) in the patients were 0.75 and 0.25, respectively, which did not differ significantly from the controls (0.77 and 0.23 in normal subjects and 0.79 and 0.21 in consanguineous relatives of the patients). However, patients expressed fewer CR1/E within each genotype than their relatives and healthy subjects. CR/E was found to be stable in consecutive samples in controls. In patients, the numbers varied between low and high during the course of the treatment. The variation in the numbers was significantly correlated with C3d and CIC as well as with the severity of the disease. Our results suggest that low levels of CR1 on erythrocytes in SLE patients are required during the course of the disease and that the 6.9 kb restriction fragment does not play a role in causing susceptibility to the disease.

Complement receptor expression and activation of the complement cascade on B lymphocytes from patients with systemic lupus erythematosus (SLE)

It has previously been reported that the expression of the complement receptors, CR1 on erythrocytes and blood leucocytes and CR2 on B cells, is reduced in patients with SLE, and that the reduced expression of CR1 on erythrocytes is related to disease activity. We have earlier demonstrated that normal B cells are capable of activating the alternative pathway (AP) of complement in a CR2-dependent fashion. In this study we have investigated whether disturbances in this activity may be related to the altered phenotype of SLE B cells. Flow cytometry was used to measure expression of complement receptors and regulatory proteins on B cells from SLE patients, as well as the deposition of C3 fragments occurring in vivo or after in vitro AP activation. We have confirmed, for a proportion of the patients studied, reduced expression of CR1 and CR2 on B cells, and shown a consistency between low CR2 expression and reduced in vitro AP activation in the presence of homologous, normal serum. In addition, the B cells, like erythrocytes, bear raised levels of in vivo-deposited C3dg, but not C3b fragments, compared with normal B cells. The erythrocytes from SLE patients were unable to inhibit in vitro AP activation by B cells in homologous serum. Finally, we demonstrated an inverse relationship between SLE disease activity index (SLEDAI) and the expression of complement receptor 2 (CR2) on SLE B cells. Thus, determination of CR2 on B cells may emerge as an additional laboratory tool in the assessment of SLE activity.

Lowered expression of C3b receptor (CR1) on erythrocytes of rheumatoid arthritis patients

The number of C3b receptors (CR1) on erythrocytes (E) has been quantitated by whole cell enzyme-linked immunosorbent assay (ELISA), using a monoclonal anti-CR1 antibody, in 46 healthy individuals as controls, 58 having rheumatoid arthritis (RA) and 3 hereditary angioedema patients. The mean value of CR1 in RA (381/E) was significantly lower (p < 0.001) when compared to normal controls (646/E). In hereditary angioedema patients CR1 numbers (620/E) were found to be comparable to normal values. No significant difference was found between normal male (708/E) and female (598/E) subjects. Among the patient groups, those on steroid therapy (352/E) showed no change compared to others not receiving such therapy (408/E). The cumulative frequency curve of CR1 in the normal population showed maximum inflection at 32% and 82%. This led us to conclude that there is a trimodal distribution of receptors in the control population. Such a contention is well supported by frequency histogram, when a small group interval (0-50) was chosen. However, if large group intervals (0-100 or 0-150) were considered, a very close approximation to unimodal pattern was obtained. The factors actually contributing to low receptor numbers are yet to be elucidated.

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.

Normal C3b receptor (CR1) genomic polymorphism in patients with insulin-dependent diabetes mellitus (IDDM): is the low erythrocyte CR1 expression an acquired phenomenon?

Expression of the erythrocyte complement receptor (C3bR = CR1 = CD35) and its genomic polymorphism (HindIII RFLP) was studied in a group of 80 patients with IDDM, 31 healthy siblings and 101 healthy blood donors. Defective CR1 expression was found in 26% of the patients with IDDM compared with 9% of the controls (P less than 0.05) and 0% of the siblings. The CR1 gene polymorphism of the IDDM patients did not significantly differ from that of the controls. The presence of a 6.9 kb (L) CR1 gene fragment was associated with a low CR1 expression in the patients (P less than 0.05) and especially in the controls (P less than 0.001). No significant association was found between the presence or absence of the HLA risk antigens for IDDM and CR1 expression. The results confirm that erythrocyte CR1 expression is genetically determined, but the CR1 deficiency associated with IDDM seems to be an acquired rather than a genetic phenomenon.

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).

Peripheral catabolism of CR1 (the C3b receptor, CD35) on erythrocytes from healthy individuals and patients with systemic lupus erythematosus (SLE)

The present study investigated the rate of catabolism of CR1 (the C3b receptor, CD35) on erythrocytes (E) in vivo, in relationship with the expressed number of CR1/E, the CR1.1 HindIII quantitative CR1 polymorphism, and cell age. The relationship between the number of CR1/E and cell age was analysed by measuring G6PDH activity in E that had been sorted according to high or low expression of CR1 (CD35), by assessing the expression of CR1 (CD35) on E separated according to cell density, and by comparing the number of CR1 (CD35) antigenic sites on reticulocytes and on E. A physiological catabolism of CR1 (CD35) manifested by a reduction in the number of CR1 (CD35) antigenic sites/E with cell ageing was consistently observed in healthy individuals. The number of CR1/E decreased with ageing of E according to a complex pattern that associated an exponential decay and an offset. Calculated half-lives of CR1 (CD35) ranged between 11 and 32 days in healthy individuals. A more rapid loss of CR1 (CD35) with cell ageing occurred on cells from individuals expressing high numbers of CR1/E. In patients with systemic lupus erythematosus (SLE), half-lives of CR1 (CD35) on E were in the same range as those of healthy individuals with a similar quantitative CR1 genotype; the number of CR1 (CD35) on reticulocytes was reduced and linearly related to the number of CR1/E, independently of the patients' quantitative CR1 genotype. Transfusion experiments with E bearing high or low amounts of CR1/E indicated the lack of preferential removal of E bearing high numbers of CR1 (CD35) in patients with SLE. These results indicate that the rate of loss of CR1 (CD35) from E with cell ageing is directly related to the quantitative CR1 phenotype and suggest that enhanced peripheral catabolism is not the sole mechanism of the acquired loss of CR1 (CD35) on E in patients with SLE.

Complement

The complement system mediates a wide range of important biological functions. The use of modern techniques in protein chemistry and molecular biology has greatly facilitated our understanding of the interactions between the fluid phase and cell-bound components of the system. Structural and genetic analysis has shown that while many of these components are polymorphic, there are major similarities between many of the proteins serving enzymatic and regulatory roles in both the alternative and classical pathways. The regulation of complement activation and Class III genes, on chromosomes 1 and 6 respectively, encode nine of the major proteins in the system. The genetic basis of C4 and C3 polymorphisms is now well established, and further study may reveal functional differences between polymorphic variants of other components. The study of individuals with either genetic or acquired deficiencies of complement proteins and receptors has provided insight into the function of these components, leukocyte adherence deficiency (LAD) providing the best example. An appreciation of the genetics, structure and functions of the regulatory proteins decay-accelerating factor (DAF) and homologous restriction factor has enhanced our understanding of the pathogenesis of paroxysmal nocturnal haemoglobinuria. The full importance of CD59 glycoprotein, the newest member of the complement family, remains to be determined.

Distribution of the HindIII restriction fragment length polymorphism among patients with systemic lupus erythematosus with different concentrations of CR1

Sixty six patients with systemic lupus erythematosus (SLE) were genotyped using a HindIII restriction fragment length polymorphism identified by CR1.1 cDNA, then were followed up for an average of 50 months to evaluate the stability of their CR1 activities. The gene frequencies for the two alleles which correlate with the numeric expression of CR1 on the erythrocytes were not significantly different between 66 patients with SLE and 52 normal controls. A discrepancy between homozygosity for a high allele and a negative CR1 activity was found in many patients. These patients, however, had significantly lower concentrations of serum complement than did patients with a positive CR1, and some were in an active state of the disease. Furthermore, there were several patients in whom the CR1 activities changed from negative to positive together with an increase in serum complement. Our results suggest that the decreased expression of CR1 on erythrocytes in patients with SLE is not inherited, rather it is a consequence of the disease processes.

Quantification of complement receptor 1 on erythrocytes: follow-up of HIV-1-infected patients with AIDS-related complex/Walter-Reed 5 under treatment with intravenous immunoglobulin. The ARC-IVIG Study Group

A sensitive flow-cytometric method was established to quantify the number of complement receptor 1 (CR1, C3b/C4b receptor, CD35) on the surface of purified erythrocytes of 12 patients infected by HIV-1 and showing two clinical AIDS-related complex/Walter-Reed 5 criteria. Erythrocytes were incubated with biotinylated monoclonal anti-CR1 antibody followed by phycoerythrin-streptavidin before analysis on a flow cytometer. As few as 50 binding sites/cell could be detected, making this method as sensitive as a radioimmunoassay with 125I anti-CR1. Seven of the patients studied received an immunoglobulin preparation suitable for intravenous use during the 6 months of the study, 5 got an equal amount of placebo preparation consisting of human serum albumin. For a year, erythrocytes were collected and purified every 3 months, frozen and stored at -70 degrees C until the end of the study, when the number of CR1 was determined. No difference between the two groups of patients was found. In 8 patients, small fluctuations of the amount of CR1/erythrocyte were seen during the period of observation, whereas in 4 of the patients a drop of the number of CR1 was observed towards the end of the study. No correlation was found between CR1 numbers on erythrocytes and circulating immune complexes, CH50, C3 and C4 concentrations or CD4-positive lymphocytes.

Immune complex binding to erythrocyte-CR1 (CD 35), CR1 expression and levels of erythrocyte-fixed C3 fragments in SLE outpatients

Erythrocytes (E) from a cross-sectional group of 22 outpatients with systemic lupus erythematosus (SLE) and/or mixed connective tissue disease (MCTD), the majority without active disease (n = 14), were analyzed for CR1 antigen expression and capacity to bind complement opsonized, radiolabelled immune complexes (IC). Furthermore, E-bound C3 fragments and the plasma C3d concentration were determined. E-bound C3b/iC3b fragments were not elevated in patients with SLE, whereas E from 11 out of 22 SLE patients had increased C3d levels which correlated with the plasma C3d concentration (Rs 0.73, p less than 0.001). E-fixed C3d fragments did not affect the binding of Mab or preopsonized IC to E-CR1 and were not correlated with disease activity or medical treatment. Antigen expression of E-CR1 measured by ELISA or agglutination showed positive correlation with the IC binding capacity of E-CR1 (Rs 0.92 and 0.72 respectively, p less than 001). The IC binding capacity of E-CR1 from SLE patients was significantly reduced (p less than 0.005), whereas the antigen expression of CR1 (ELISA) on E from the patients did not differ from that of E from healthy donors (p greater than 0.1). E-CR1 antigen was measured by Mab reacting with an epitope outside the IC-binding site of E-CR1. E-CR1 antigen expression or IC binding showed no correlation either with disease activity or prednisolone treatment. However, 4 og 5 patients with MCTD and 4 of 5 patients receiving Imurel were found to have low E-CR1 expression and capacity to bind IC. Thus, measurement of antigenic E-CR1 in a cross-sectional group of SLE outpatients by use of Mab reacting with an epitope outside the ligand-binding region of CR1 did not reveal a significantly reduced CR1 expression. However, an assay for CR1-mediated IC binding showed a clearly reduced E-CR1 function.

The human C3b receptor: function and role in human diseases

The human C3b receptor (CR1) is a polymorphic glycoprotein which functions regulating the complement system by inhibiting the activation of C3 and C5, through its effect on their convertases, and serving as cofactor for factor I in mediating the degradation of C3b to its inactive fragment C3bi and further to C3d-g. The latter are then ligands for their respective receptors on leukocytes, CR3 and CR2. Additionally, CR1 on erythrocytes endows these cells with the capacity to deliver immune complexes (IC) to the reticuloendothelial system, resulting in their clearance from the circulation. On phagocytes, this receptor participates in the process of endocytosis of foreign particles. There is a wide inherited variation of CR1 expression on erythrocytes (CR1/E) of different individuals. Patients with diseases which feature elevated levels of IC, such as systemic lupus erythematosus, leprosy, and AIDS, have a marked decrease of CR1/E, which may result in an altered clearance. This reduction appears to be related to disease activity, and the most probable site for CR1/E loss is during the transfer of IC to macrophages. Healthy neutrophils increase tenfold their expression of CR1 in response to the effect of chemoattractant peptides. Neutrophils from patients with AIDS display an altered response to stimulation. This defect may be of relevance in the process of endocytosis.

Genetic analysis of CR1 expression on erythrocytes of patients with systemic lupus erythematosus

The role of genetic factors in decreased expression of CR1 in patients with systemic lupus erythematosus (SLE) was investigated by assessing the frequency of genotypes determining the numbers of CR1 on erythrocytes obtained from 52 patients with SLE and from 84 normal individuals. The expression of CR1 was quantitated using flow cytometry. Genotypes were determined by analyzing the CR1 gene restriction fragment length polymorphism using the CR1.1 complementary DNA probe and the Hind III restriction enzyme. In normal subjects, the distribution of the HH, HL, and LL genotypes fit the Hardy-Weinberg law. The frequency for the H allele did not differ between the 2 groups. No individual homozygous for the LL genotypes was found among the SLE patient population. Taken together, data from this and previous studies indicate an under-representation of the LL homozygous genotype in patients with SLE. SLE patients expressed decreased numbers of CR1 per erythrocyte within each genotype compared with these numbers in normal subjects. The results suggest that defective expression of CR1 in SLE patients is acquired and that the presence of the L allele is not linked to a genetic susceptibility for SLE.

Immunogenetics of SLE and primary Sjögren's syndrome

SLE is a syndrome defined by clinical criteria and by the presence of autoantibodies reactive with nucleic acids and proteins concerned with transcription and translation. Breeding experiments in mice have illustrated the enormous genetic heterogeneity of this syndrome, of which the final common pathway is a widespread immune complex disease. The causes of SLE in humans are likely to be equally multifactorial. Family studies have demonstrated that genetic factors exist, but each factor appears to be a relatively weak disease-susceptibility gene. The major exceptions to this are the very rare complete deficiencies of classical pathway complement components, which are almost invariably accompanied by the development of SLE. Observations of these patients have led to the formulation of hypotheses relating complement and its receptor, CR1, to the defective removal of immune complexes from the circulation.

Complement C3b receptors on erythrocytes in patients with juvenile rheumatoid arthritis

Forty patients with juvenile rheumatoid arthritis (JRA) were examined for C3b receptor (CR1) levels on erythrocytes (by an enzyme-linked immunosorbent assay), levels of circulating immune complexes (IC) (by a polyethylene glycol precipitation-complement consumption method), C3d split products (by intermediate gel rocket immunoelectrophoresis), and HLA-A, B, and C (by microdroplet lymphocytotoxicity test). Lower CR1 levels were found predominantly among patients with JRA (mean 57%) compared with 40 age-matched controls (mean 68%) (P = 0.008). The CR1 levels differed when the JRA patients were grouped by mode of disease onset and by clinical state at the time of testing (levels in patients with pauciarticular disease were higher than those in patients with polyarticular disease, and levels in patients with polyarticular disease were higher than those in patients with systemic disease) or by sex (girls had higher levels than boys; P = 0.01). The levels of circulating IC and C3d were elevated in 25% and 40% of JRA patients, respectively, and were mutually correlated (P less than 0.05, tau = 0.20). A negative correlation was found between levels of C3d and the numbers of CR1 (P less than 0.01, tau = -0.28), but concentrations of circulating IC did not correlate with CR1 values. CR1 levels were the same in 6 HLA-B27 positive and 25 HLA-B27 negative patients. These findings do not represent conclusive evidence that the number of CR1 on erythrocytes serves as a predictor of disease severity or as an indicator of disease activity in patients with JRA.

Human diseases associated with C3 receptor deficiencies

Genetic deficiencies of complement receptors have recently been described. CR1 expression is reduced on erythrocytes, leucocytes and podocytes of many patients with systemic lupus erythematosus because of both genetic and acquired mechanisms. CR1 deficiency is also found in AIDS and AIDS-related syndromes and correlates with clinical subpopulations of HIV-infected patients. The pathogenic significance of CR1 deficiency relates to the functions of CR1 in clearance of immune complexes, phagocytosis and immune regulation. CR3 deficiency occurs as an autosomal recessive inherited disease characterized by the lack of or severe reduction in expression of the leucocyte antigens CR3, LFA1, p150,95 and their common chain. The disease is associated with severe defects in neutrophil and lymphocyte functions and recurrent bacterial infections. The in vivo effects of C3 receptor deficiencies emphasize the critical role of these membrane molecules in immunity.

Demonstration of human erythrocyte C3b receptors (CR1) by haemadsorption and indirect haemagglutination techniques

A closed chamber haemadsorption (HA) technique with tissue sections and an indirect haemagglutination (IHA) technique with monoclonal anti-C3b receptor (CR1) antibody were used to measure CR1 on human erythrocytes (E) (ECR1). In the HA technique the indicator cells were sheep E coated with rabbit IgM antibodies (A) and various amounts of human C3b (EAC). The ECR1 activity was classified according to the degree of haemadsorption of EAC. High ECR1 activity was found in 24% of healthy blood donors, intermediate activity in 62%, low activity in 12% and no activity in 2% of the blood donors. The results obtained using the IHA technique showed a good correlation with the results obtained using the HA technique. Both techniques are sensitive and the results are highly reproducible. The techniques are easy to perform and do not require expensive reagents or equipment. The HA and IHA techniques therefore offer an alternative to other techniques for measuring ECR1 activity.

Erythrocyte CR1 (C3b/C4b receptor) levels and disease activity in patients with SLE

Fifty-four patients with systemic lupus erythematosus (SLE) were examined for (1) CR1 (C3b/C4b receptor) levels on erythrocytes by an enzyme-linked immunosorbent assay, (2) levels of circulating immune complexes (IC) by a polyethylene glycol precipitation complement consumption method, and (3) concentrations of C3d split products in plasma by intermediate gel rocket immunoelectrophoresis. A preponderance of low CR1 levels was found among patients with SLE (mean 40%, range 13-106) as compared with normal controls (mean 70%, range 24-130) (p less than 0.001). The concentrations of circulating IC were elevated in 38% of 58 samples. The concentrations of C3d were elevated in 72%, and were positively correlated with the levels of IC (tau = 0.28; p less than 0.005) and with disease activity as assessed by a modification of the UCH/Middlesex criteria. Negative correlations were seen between the CR1 numbers and disease activity (p = 0.01), concentrations of circulating IC (tau = -0.14; p less than 0.005), and C3d (tau = -0.21; p less than 0.005). The changes found in CR1 levels on repeated study were, however, relatively small (less than or equal to 27%; median 5), even during periods of changing disease activity.

Quantitative variations of the C3b/C4b receptor (CR1) in human erythrocytes are controlled by genes within the regulator of complement activation (RCA) gene cluster

The genetic relationships of quantitative and structural variations of the C3b/C4b receptor (CR1) in human erythrocytes have been analyzed in informative families. Our results demonstrate the existence of multiple discrete quantitative variations of CR1 controlled by a locus, C3bRQ, closely linked to the CR1 structural locus, C3bR. Since the amounts of CR1 produced by each C3bR allele are shown to be independently regulated, we propose that a cis-acting genetic mechanism controls the level of expression of the C3bR alleles, and that this quantitative control plays a major, if not the sole, role in determining the total amounts of CR1 on normal human erythrocytes.

Altered erythrocyte C3b receptor expression, immune complexes, and complement activation in homosexual men in varying risk groups for acquired immune deficiency syndrome

We studied levels of erythrocyte C3b receptors (E-CR1) and correlated them to the level of circulating immune complexes (CIC) and complement activation in patients with or at risk for acquired immunodeficiency syndrome (AIDS). A significant reduction was found in patients with AIDS (185 +/- 93 CR1/cell), AIDS-related complex, and generalized lymphadenopathy, whereas healthy male homosexuals or normal controls had 434 +/- 193 and 509 +/- 140 CR1/cell, respectively (P less than 0.001). Family studies indicate that this defect is acquired. Reduction in E-CR1 was associated with increased levels of CIC when assayed by binding to Raji cells, but not when tested by C1q binding. Complement activation was assessed by levels of C3bi/C3d-g in plasma, measured with a monoclonal antibody specific for a neoantigen in C3d. AIDS patients had increased C3 activation (2.68 +/- 1.67%) when compared with normal controls (0.9 +/- 0.22%) (P less than 0.01). The decreased E-CR1, the presence of CIC, and C3 activation suggest that complement activation by immune complexes may play a role in the clinical expression of the disease.

Erythrocyte CR1 determination using monoclonal antibody in a microtiter plate ELISA; receptors are not masked by immune complexes

The microtiter plate ELISA using monoclonal antibody is a specific, sensitive and quantitative technique for measuring CR1 on human erythrocytes. The present investigations established that receptor occupancy by immune complexes did not affect the measurements. The monoclonal anti-CR1 antibody To5 bound unimpeded to receptors that had reacted with an excess of complement-opsonized tetanus toxoid anti-tetanus toxoid complexes prepared at antigen:antibody ratios between 32:1 and 1:8. The CR1 levels on erythrocytes from 11 patients with systemic lupus erythematosus (SLE) were not increased (P greater than 0.30) after release of CR1-bound immune complexes by incubation with factor I. Neither did the serum from these patients contain blocking anti-CR1 activity (P greater than 0.10). Additionally, the number of antigenic CR1 sites in 10 normals and in the 11 patients with SLE was well correlated with the number of functional receptor sites as assessed by binding of soluble complexes (P less than 0.001). These data establish that the true CR1 levels are determined using the microtiter plate ELISA for quantitation of CR1 in patients with diseases involving immune complexes and/or autoantibodies.

Identification of a restriction fragment length polymorphism by a CR1 cDNA that correlates with the number of CR1 on erythrocytes

A genetic basis for the regulation of the number of CR1 on E of different normal individuals was investigated by probing Southern blots of their genomic DNA with a 0.75-kb fragment of CR1 cDNA. Using Hind III, we observed a RFLP involving fragments of 7.4 kb and 6.9 kb that correlated with the number of CR1 on E. 32 individuals having only the 7.4-kb restriction fragment had a mean of 661 +/- 33 (SEM) CR1/E, 11 donors having both restriction fragments had a mean of 455 +/- 52 CR1/E, and 7 individuals having only the 6.9-kb fragment had a mean of 156 +/- 13 CR1/E, all means being significantly different (p less than 0.005). Cosegregation in a normal family of the Hind III restriction fragments with the S, F, and F' structural allotypes of CR1 confirmed that the regulatory element identified by these fragments is linked to the CR1 gene. Moreover, an analysis of the relative expression on E of these structural allotypes in association with either the 7.4-kb Hind III fragment or the 6.9-kb fragment showed that this regulatory element is cis-acting. In contrast, quantitation of CR1 of B lymphocytes and neutrophils revealed no differences in total CR1 expression between individuals homozygous for the 7.4-kb and 6.9-kb Hind III fragments. Thus, we have identified a genomic polymorphism that is linked to the CR1 gene and is associated with a cis-acting regulatory element for the expression of CR1 on E.

Decreased expression of the C3b/C4b receptor (CR1) and the C3d receptor (CR2) on B lymphocytes and of CR1 on neutrophils of patients with systemic lupus erythematosus

Decreased numbers of complement receptor type 1 (CR1) have been observed on erythrocytes of patients with systemic lupus erythematosus (SLE) and on glomerular podocytes of patients having proliferative nephritis of SLE. In the present study, the analysis of the cellular expression of CR1 has been extended to include leukocytes. In addition, expression by B lymphocytes of the C3d receptor (CR2), which also serves as the receptor for the Epstein-Barr virus, was assessed. Receptor expression was measured by 2-color fluorescent flow cytometry of peripheral blood B cells, identified by the presence of the B1 antigen, that had also been stained with anti-CR1 or anti-CR2. B cells from 17 patients with SLE exhibited a mean relative fluorescence for CR1 that was 61% of that found in 17 normal individuals (P less than 0.001). The expression of CR2 by the patients' B cells (n = 14) was 62% of that of the B cells from normal subjects (n = 17) (P less than 0.001). The expression of CR1 correlated with that of CR2 among patients (r = 0.63; P less than 0.01) but not with the expression of CR2 among normal individuals (r = 0.36; P greater than 0.1). The mean CR1 content of the patients' neutrophils was only 59% of the normal mean (P less than 0.001). Thus, abnormalities of complement receptor expression occur on the leukocytes of patients with SLE. These deficiencies may be secondary to interaction of the cells with the products of complement activation, or, in some individuals, the deficiencies may be familial.

Deficiency of erythrocyte C3b receptor (CR1) in AIDS and AIDS-related syndromes

The expression of C3b receptors (CR1) on erythrocytes of gay men at various levels of risk for AIDS was studied. Fourty-nine heterosexual male controls had a mean (X) +/- standard deviation of 516 +/- 136 CR1 per erythrocyte (CR1-3); 17 asymptomatic gay men had X = 423 +/- 156, 16 gay men with one AIDS-related complex (ARC) symptom or sign had X = 342 +/- 154, 9 patients with ARC had X = 252 +/- 76, and 14 gay men with AIDS had X = 173 +/- 76 CR1-E. The patients with ARC and AIDS had a highly significant decrease in CR1-E when compared with normal individuals (p = less than 0.001) and studies of families of 4 AIDS patients suggest that this defect is acquired.

Complement in the pathophysiology and diagnosis of human diseases

Complement is a humoral effector system composed of 21 plasma proteins that was identified initially because of its cytolytic effects. In addition to cytolysis, complement has a number of different functions related to inflammatory and other host defense processes. The description of the reaction mechanism includes: (1) activation of the classical pathway through recognition of IgG and IgM antibodies by C1q, (2) activation of the alternative pathway which is usually achieved without participation of immunoglobulins, (3) generation of proteolytic enzymes composed of heteropolymers that cleave certain precursor proteins, (4) formation of the membrane attack complex (MAC), and (5) participation of control mechanisms. Methodologies for studying protein concentration and functional activities of complement components include not only the classical hemolytic techniques but also the extremely sensitive new radioimmunoassays and enzyme immunoassays for measuring the products of complement activation that are generated in vivo. Examples of genetically controlled complement deficiencies have been published for most complement components. The symptomatology of some of these patients serves to emphasize the protective role of complement. Acquired deficiencies are significant not only as laboratory aids in diagnosis and to evaluate the course of certain diseases, but also to indicate possible pathogenic disease mechanisms. Recently, it has been recognized that the complement proteins with genes located in the HLA region are polymorphic. Certain variants of proteins C2, C4, and factor B occur with higher frequencies in certain diseases than in the general population, which appears to be of great practical importance in laboratory medicine.

Hematologic correlates and the role of erythrocyte CR1 (C3b receptor) in the development of AIDS

Circulating immune complexes (CICs) are common in patients with the acquired immunodeficiency syndrome (AIDS) as in anemia. In our previous reports, we observed that the deposition of CICs on erythrocytes via C3b receptors (CR1) resulted in a defective CIC clearing system of erythrocytes and in high membrane osmotic fragility of such erythrocytes. We investigated the functional activity of erythrocyte CR1 in 89 patients with AIDS, 41 with AIDS related complex (ARC), 102 healthy homosexual volunteers, and 37 heterosexual males, in relation to the presence of CICs, antibody to lymphadenopathy associated virus/human T lymphotropic virus-III (LAV/HTLV-III), anemia, and the direct and indirect Coombs' tests. CICs were frequently found in all groups except heterosexual males. Absence of CR1 activity was observed in 85% of patients with AIDS, and in 59% with ARC. Impaired CR1 activity also occurred in the homosexual volunteer group. Positive direct Coombs' test and the presence of CICs correlated inversely with CR1 activity while a lowered hematocrit and the presence of antibody to LAV/HTLV-III correlated directly. Neither the sera nor the eluates from erythrocytes with a positive IgG Coombs' test contained IgG antibody against erythrocytes. This suggests decremental loss of CR1 activity progressing from asymptomatic LAV/HTLV-III antibody positive homosexual volunteers to the prodromal spectrum of ARC and finally progressing to a total disappearance in overt AIDS. Of 8 homosexuals volunteers demonstrating the composite of impaired CR1 activity, positive antibody to LAV/HTLV-III, and polyvalent positive direct Coombs' test (with gamma, mu, and C3b), all developed ARC or overt AIDS within 2 years of these observations.

Correlative expression of C3b receptors in the glomerulus and on erythrocytes

The reactivity of glomerular C3b receptors (CR1) measured by the number of sheep erythrocytes bearing C3b (EAC) that adhered to glomeruli in frozen sections, was compared with the reactivity of erythrocyte CR1, which was determined by immune adherence hemagglutination (IAHA), in 22 patients with renal and non-renal diseases. Among seven patients with primary glomerulonephritis whose erythrocytes were positive for IAHA, the reactivity of glomerular CR1 was high in five. In the remaining two, the reactivity of glomerular CR1 was low, accompanied by severe sclerotic glomerular changes. Erythrocytes from five of six patients with systemic lupus erythematosus were IAHA negative, and their glomeruli failed to produce adherence of EAC, even in three cases in which there were no detectable C3 deposits or histopathological changes. In the other nine patients without appreciable glomerular changes, the reactivity of glomerular CR1 was low in three along with negative erythrocyte IAHA, whereas the remaining six exhibited high CR1 reactivity both in glomeruli and on erythrocytes. The results indicate a close correlation between the expression of CR1 in the glomerulus and on erythrocytes.

Anti-C3b-receptor (CR1) antibodies in patients with systemic lupus erythematosus

Using an enzyme-linked immunoadsorbent assay, IgG in the plasma and purified IgG from 2 patients with systemic lupus erythematosus (SLE) were found to strongly react with purified C3b receptor (CR1) insolubilized on microtiter plates. The amount of IgG that bound to CR1 in 201 plasma samples from 179 other patients with SLE did not significantly differ from that which bound in 72 control samples from normal individuals. Purified IgG from the patients with anti-CR1 reactivity did not inhibit CR1 function in vitro. The number of CR1 antigenic sites expressed on erythrocytes from both patients was much lower than that observed in a normal population and in the lowest range of the decreased numbers found in patients with SLE. The occurrence of anti-CR1 antibodies in patients with SLE could provide an acquired mechanism for decreased expression of CR1 through antigenic modulation of the receptor on precursor cells and/or alter the function of cells of the immune system bearing C3b receptors.

Human complement receptors for C3b (CR1) and C3d (CR2)

The human C3b receptor (CR1) is a polymorphic glycoprotein comprised of a single polypeptide chain. Of the 4 allotype forms of CR1 that have been described, the 2 most common have Mr's of 250,000 and 260,000, and are regulated by alleles having frequencies in a Caucasian population of 81.5% and 18.5%, respectively. CR1 is present on erythrocytes, neutrophils, eosinophils, monocytes, macrophages, B lymphocytes, some T lymphocytes, mast cells, and glomerular podocytes. CR1 number on erythrocytes is genetically regulated, and ranges from less than 100 sites per cell to greater than 1000 sites per cell, the average in the normal population being 500-600 sites per cell. A model accounting for this wide distribution proposes the existence of 2 codominant alleles determining low and high receptor number respectively; CR1 number is not affected by the structural polymorphism, so that the loci for these two phenotypic characteristics are distinct. The function of CR1 on erythrocytes may be to promote the clearance of immune complexes from the circulation. CR1 number on myelomonocytic cells is regulated by chemotactic factors which can rapidly transfer CR1 sites from a latent, presumably intracellular, site to the plasma membrane of these cells, thereby enhancing their ability to interact with opsonized foreign material. The receptor is involved in the endocytic reactions of these cells, and recent findings have demonstrated that this function can be modulated by T cell-derived factors, fibronectin, and phorbol esters. The role of CR1 on lymphocytes remains to be fully explored, although the receptor may enhance the differentiation of B cells into antibody-secreting cells.

Autoantibody to the C3b/C4b receptor and absence of this receptor from erythrocytes of a patient with systemic lupus erythematosus

A 29-yr-old woman with systemic lupus erythematosus (SLE) was found to have no detectable C3b/C4b receptors (CR1) on her erythrocytes (E) when they were assayed by the binding of rabbit polyclonal and murine monoclonal (Yz-1) anti-CR1. Analysis by two-color fluorescent flow cytometry of CR1 expression on the patient's B lymphocytes that had been stained indirectly with monoclonal anti-B1 and rabbit F(ab')2 anti-CR1 also revealed a marked deficiency of CR1. Total cellular CR1 of neutrophils, assessed by a sandwich radioimmunoassay, was about half that of neutrophils from normal individuals. Because her E had expressed 173 sites/cell 2 yr before, the CR1 deficiency was considered to be acquired and a possible mechanism was sought. Autoantibody to CR1 was measured by a radioimmunoassay in which serum or its fractions were incubated in microtiter wells that had been coated with purified CR1, and binding of immunoglobulin to the wells was quantitated with 125I-labeled goat IgG antihuman F(ab')2. The CR1-specific binding of immunoglobulin from the patient's serum was 19.1 ng/well of the detecting antibody when her E had eight CR1 sites per cell; that of 28 healthy donors was 1.3 +/- 0.5 ng/well (mean +/- SEM), and that of 34 additional patients with SLE was 0.5 +/- 0.3 ng/well. The activity was present also in purified IgG and its F(ab')2 fragment, indicating that the binding of serum immunoglobulin to CR1 was not mediated by C3 fragments. The specificity of the patient's IgG for CR1 was confirmed when pretreatment of the CR1-coated wells with affinity-purified rabbit F(ab')2 anti-CR1 was shown to inhibit by 68% the binding of the IgG. The autoantibody also interacted with CR1 in cell membranes, as assessed by its capacity to inhibit the binding of indirectly fluoresceinated Yz-1 to neutrophils, and, when combined with goat IgG antihuman F(ab')2, to diminish the binding of dimeric C3b to normal E. During the period of the marked deficiency of CR1 the patient experienced an exacerbation of disease activity which was treated with prednisone. Clinical improvement was accompanied by a decrease in the serum concentration of anti-CR1 to levels present 2 yr earlier, and an increase of CR1 to 170 sites/E. The temporal association between high titers of an autoantibody to CR1, absence of CR1 from E, and heightened activity of SLE suggest that the former may have had a role in the other manifestations of the patient's disease.