Enumeration of CR1 complement receptors on erythrocytes using a new method for detecting low density cell surface antigens by flow cytometry

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.

Signal amplification in flow cytometry for cell surface antigen analysis

Signal enhancing systems have been introduced to enable detection of cell surface antigens by flow cytometry. Cell surface antigens are important targets that describe the function and lineage of cells. Although flow cytometry is an effective tool for analysing cell surface antigens, this technique has poor sensitivity, which prohibits the detection of many important antigens on cell membranes. Thus, signal amplification is essential for developing practical tools for evaluating cell surface antigens by flow cytometry. Using a bright fluorophore or fluorescent polymer incorporated into antibodies is a straightforward strategy to improve flow cytometry sensitivity but may affect the functional characteristics of the labelled antibody. In contrast, enzymatic signal amplification is a more practical and efficient strategy to improve sensitivity that should not affect antibody activity. Although enzymatic signal amplification still has a number of drawbacks, this approach is a promising strategy to analyse cell surface antigens.

BDNF elevates the axonal levels of hnRNPs Q and R in cultured rat cortical neurons

Local translation plays important roles in the maintenance and various functions of axons, and dysfunctions of local translation in axons are implicated in various neurological diseases. Heterogeneous nuclear ribonucleoproteins (hnRNPs) are RNA binding proteins with multiple functions in RNA metabolism. Here, we identified 20 hnRNPs in the axons of cultured rat cortical neurons by interrogating published axon mass spectrometric databases with rat protein databases. Among those identified in axons are highly related hnRNPs Q and R. RT-PCR analysis indicated that axons also contained low levels of hnRNPs Q and R mRNAs. We further found that BDNF treatments raised the levels of hnRNPs Q and R proteins in whole neurons and axons. BDNF also increased the level of poly(A) RNA as well as the proportion of poly(A) RNA granules containing hnRNPs Q and R in the axon. However, following severing the connection between the cell bodies and axons, BDNF did not affect the levels of hnRNPs Q and R, the content of poly(A) RNA, or the colocalization of poly(A) RNA and hnRNPs Q and R in the axon any more, although BDNF still stimulated the local translation in severed axons as it did in intact axons. The results are consistent with that BDNF enhances the axonal transport of RNA granules. The results further suggest that hnRNPs Q and R play a role in the mechanism underlying the enhancement of axonal RNA transport by BDNF.

Tyramide signal amplification for analysis of kinase activity by intracellular flow cytometry

Intracellular flow cytometry permits quantitation of diverse molecular targets at the single-cell level. However, limitations in detection sensitivity inherently restrict the method, sometimes resulting in the inability to measure proteins of very low abundance or to differentiate cells expressing subtly different protein concentrations. To improve these measurements, an enzymatic amplification approach called tyramide signal amplification (TSA) was optimized for assessment of intracellular kinase cascades. First, Pacific Blue, Pacific Orange, and Alexa Fluor 488 tyramide reporters were shown to exhibit low nonspecific binding in permeabilized cells. Next, the effects of antibody concentration, tyramide concentration, and reaction time on assay resolution were characterized. Use of optimized TSA resulted in a 10-fold or greater improvement in measurement resolution of endogenous Erk and Stat cell signaling pathways relative to standard, nonamplified detection. TSA also enhanced assay sensitivity and, in conjunction with fluorescent cell barcoding, improved assay performance according to a metric used to evaluate high-throughput drug screens. TSA was used to profile Stat1 phosphorylation in primary immune system cells, which revealed heterogeneity in various populations, including CD4+ FoxP3+ regulatory T cells. We anticipate the approach will be broadly applicable to intracellular flow cytometry assays with low signal-to-noise ratios.

Enhancement of deoxyribonucleic acid microarray performance using post-hybridization signal amplification

Microarray performance depends upon the ability to screen samples against a vast array of probes with the appropriate sensitivity and selectivity. While these factors are significantly influenced by probe design, they are also subject to the particular detection methodology and reagents employed. Herein we describe the incorporation of super avidin-biotin system (SABS) and secondary enzymatic enhancement (SEE) as post-hybridization signal amplification techniques to improve the sensitivity of oligonucleotide microarrays. To these ends, we tested these methods on electrochemically interrogated arrays using both purified influenza A PCR products and randomly amplified genomic Francisella tularensis DNA as targets. While SABS treatment did not improve sensitivity for CombiMatrix ElectraSense(®) arrays using purified influenza A cDNA, chip sensitivity was improved 10-fold for randomly amplified targets. SEE improved performance to a greater degree and was able to lower the detection limits 10-fold for influenza A and 100-fold for F. tularensis DNA. These results indicate the promising capability of post-hybridization amplification techniques for enhancing microarray performance.

Retrovirus-mediated over-expression of decay-accelerating factor rescues Crry-deficient erythrocytes from acute alternative pathway complement attack

Decay-accelerating factor (DAF) and complement receptor 1-related gene/protein y (Crry) are two membrane-bound complement regulators on murine erythrocytes that inhibit C3/C5 convertases. Previously, we found that Crry- but not DAF-deficient erythrocytes were susceptible to alternative pathway complement-mediated elimination in vivo. To determine whether it is a unique activity or a higher level expression of Crry makes it indispensable on murine erythrocytes, we over-expressed DAF on Crry-deficient (Crry(-/-)) erythrocytes by retroviral vector-mediated DAF gene transduction of bone marrow stem cells. DAF retrovirus-transduced erythrocytes expressed 846 +/- 127 DAF molecules/cell (DAF(high)) compared with 249 +/- 94 DAF molecules/cell (DAF(low)) and 774 +/- 135 Crry molecules/cell on control mouse erythrocytes. DAF(high)-Crry(-/-) erythrocytes were significantly more resistant than either DAF(low)-Crry(-/-), DAF(-/-) -Crry(+/+) or wild-type erythrocytes to classical pathway complement-mediated C3 deposition in vitro. Furthermore, increased DAF expression rescued Crry(-/-) erythrocytes from acute alternative pathway complement attack in vivo. Notably, long term monitoring revealed that DAF(high)-Crry(-/-) erythrocytes were still more susceptible than wild-type erythrocytes to complement-mediated elimination as they had a shorter half-life in complement-sufficient mice but survived equally well in complement-deficient mice. These results suggest that both a high level expression and a more potent anti-alternative pathway complement activity of Crry contributed to its indispensable role on murine erythrocytes. Additionally, they demonstrate the feasibility of using stem cell gene therapy to correct membrane complement regulator deficiency on blood cells in vivo.

Acquired but reversible loss of erythrocyte complement receptor 1 (CR1, CD35) and its longitudinal alteration in patients with severe acute respiratory syndrome

This longitudinal study investigates the change of erythrocyte complement receptor (E-CR1) expression in patients with severe acute respiratory syndrome (SARS). Circulating E-CR1 expression was semiquantified by flow cytometric analyses in 54 SARS patients and in 212 healthy individuals as a control. Since E-CR1 expression is influenced by the genetic polymorphisms in the CR1 gene, a major genetic polymorphism located within intron 27 of the CR1 gene was simultaneously analysed by polymerase chain reaction (PCR) and restriction fragment length polymorphism (RFLP). The results showed that the expression level of E-CR1 (referred to as net fluorescence intensity values, NFI) was statistically correlated with the relevant genetic genotypes among the Chinese population including the healthy individuals (NFI: 5·14 ± 0·82, 3·57 ± 0·66 and 2·67 ± 0·32 for HH, HL and LL genotypes, respectively) and SARS patients (NFI: 3·52 ± 0·91 and 2·63 ± 0·70 for HH and HL genotypes, respectively). Interestingly, the expression density of E-CR1 was found to fall significantly during the initiation and progressive phases (weeks 1 and 2 after the disease onset) and gradually returned close to normal through their whole convalescent phase (beginning from weeks 2 or 3 to weeks 7 or 8) in SARS patients irrespective CR1 genotype. In conclusion, our findings, at least, suggest that E-CR1 is likely involved in immune pathogenesis of SARS disease.

A simple method for accurate quantification of complement receptor 1 on erythrocytes preserved by fixing or freezing

The mean number of complement receptor 1 (CR1) molecules on erythrocytes differs between normal individuals within the range of 100-1000 molecules per cell. In some disease states such as systemic lupus erythematosus (SLE), acquired immune deficiency syndrome (AIDS), insulin-dependent diabetes mellitus and malaria, erythrocyte CR1 levels are reduced and CR1 function may be impaired. Current methods for determining erythrocyte CR1 levels by flow cytometry require the use of freshly drawn blood samples because CR1 is lost from erythrocytes during storage. In order to facilitate field studies of associations between erythrocyte CR1 levels and disease, we have developed and validated an assay to quantify CR1 on both healthy and diseased erythrocytes that have been fixed in 5% formaldehyde or frozen in glycerol. These methods enable blood samples to be collected in areas lacking the facilities for flow cytometry and stored for later accurate quantification of CR1. Such procedures will be of particular benefit for future investigations of erythrocyte CR1 expression level and malaria susceptibility.

Activated STAT4 has an essential role in Th1 differentiation and proliferation that is independent of its role in the maintenance of IL-12R beta 2 chain expression and signaling

In this study we demonstrated that CD4(+) T cells from STAT4(-/-) mice exhibit reduced IL-12R expression and poor IL-12R signaling function. This raised the question of whether activated STAT4 participates in Th1 cell development mainly through its effects on IL-12 signaling. In a first approach to this question we determined the capacity of CD4(+) T cells from STAT4(-/-) bearing an IL-12Rbeta2 chain transgene (and thus capable of normal IL-12R expression and signaling) to undergo Th1 differentiation when stimulated by Con A and APCs. We found that such cells were still unable to exhibit IL-12-mediated IFN-gamma production. In a second approach to this question, we created Th2 cell lines (D10 cells) transfected with STAT4-expressing plasmids with various tyrosine-->phenylalanine mutations and CD4(+) T cell lines from IL-12beta2(-/-) mice infected with retroviruses expressing similarly STAT4 mutations that nevertheless express surface IL-12Rbeta2 chains. We then showed that constructs that were unable to support STAT4 tyrosine phosphorylation (in D10 cells) as a result of mutation were also incapable of supporting IL-12-induced IFN-gamma production (in IL-12Rbeta2(-/-) cells). Thus, by two complementary approaches we demonstrated that activated STAT4 has an essential downstream role in Th1 cell differentiation that is independent of its role in the support of IL-12Rbeta2 chain signaling. This implies that STAT4 is an essential element in the early events of Th1 differentiation.

BALB/c mice bearing a transgenic IL-12 receptor beta 2 gene exhibit a nonhealing phenotype to Leishmania major infection despite intact IL-12 signaling

In BALB/c mice infected with Leishmania major, early secretion of IL-4 leads to a Th2-type response and nonhealing. We explored the role of IL-4-induced down-regulation of the IL-12Rbeta2 chain in the establishment of this Th2 response. First, we showed that the draining lymph nodes of resistant C57BL/6 mice infected with L. major were enriched in CD4+/IL-12Rbeta2 chain+ cells producing IFN-gamma. Next, we demonstrated that BALB/c background mice bearing an IL-12Rbeta2-chain transgene manifested a nonhealing phenotype similar to wild-type littermates despite the persistence of their ability to undergo STAT4 activation. Finally, we found that such transgenic mice display more severe infection than wild-type littermates when treated with IL-12 7 days after infection, and under this condition, the mice display increased Leishmania Ag-induced IL-4 secretion. These studies indicate that although CD4+/IL-12Rbeta2 chain+ T cells are important components of the Th1 response, maintenance of IL-12Rbeta2 chain expression is not sufficient to change a Th2 response to a Th1 response in vivo and thus to allow BALB/c mice to heal L. major infection.

A soluble recombinant multimeric anti-Rh(D) single-chain Fv/CR1 molecule restores the immune complex binding ability of CR1-deficient erythrocytes

CR1 (CD35, the C3b/C4b receptor) is a widely distributed membrane glycoprotein with a unique cluster conformation on the surface of erythrocytes (E). CR1 on E is responsible for the transport of immune complexes (IC) to liver and spleen. As a cofactor of the C3b cleavage by factor I, CR1 is also a potent inhibitor of C activation and inflammation. In some diseases (systemic lupus erythematosus, hemolytic anemia, AIDS, etc.) an acquired low level of CR1 on E has been observed, leading to an impaired clearance of IC. The aim of this study was to design a heterofunctional molecule that will bind to E and restore a normal or a supranormal CR1 density on E that could mimic the unique distribution pattern of CR1 on normal E. For that purpose a new multimerizing system based on the properties of the C-terminal part of the alpha-chain of the C4 binding protein (C4bp) was used. We first produced a multimeric soluble CR1 that proved to be a better inhibitor of in vitro C activation than the monomeric form of CR1, then a heteromultimeric molecule made of CR1 and single-chain Fv anti-Rh(D) valences able to attach E and providing E with as much as a 10-fold increase in CR1 density with the same CR1 distribution pattern as native E. CR1/single-chain Fv anti-Rh(D)-treated E were able in vitro to attach as many opsonized IC as native E. These data open the way for future use of multimeric and heteromultimeric forms of soluble recombinant CR1 as therapy of IC 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.

No quantitative relationship between CR1 and Lutheran expression on erythrocytes: In(Lu) gene product is not a common regulator of CR1 expression on erythrocytes

The density of CR1, the C3b/C4b receptor (CD35), on erythrocytes (E) (CR1/E) is genetically determined. However, the broad distribution of CR1/E within a given genotype suggests that other genetic elements might contribute to the regulation of CR1/E. In some pathological conditions, including systemic lupus erythematosus (SLE), AIDS and hemolytic anemia, CR1 deficiency parallels the severity of the disease. When compared to healthy individuals, an accelerated decrease in CR1/E in these patients has been demonstrated, but other mechanisms interfering with CR1 density regulation during erythropoiesis might also contribute. In exceptional circumstances, CR1/E can be dramatically decreased in healthy individuals by the effect of a regulatory gene, In(Lu), that switches off various surface molecules on E, the structure genes of which are located on four different chromosomes, suggesting a transcription regulatory role for In(Lu) gene products. The hypothesis that products of this gene could physiologically regulate the surface density of all these molecules has been tested by determining Lub density on E (Lub/E) using quantitative flow cytometry. Lub antigenic sites were then compared to CR1/E among healthy individuals of the different CR1 density phenotypes, SLE patients with and without CR1 deficiency, and an exceptional SLE patient totally lacking CR1/E and reticulocytes. No quantitative relationship was found between CR1 and Lub expression in either normal or pathological conditions. These data establish that In(Lu) products are not involved in normal or pathological CR1 density regulation.

The Presence and Clearance of Immune Complexes in the Untreated Generalised Psoriasis

Psoriasis is a hyperproliferative inflammatory disease and 70% of patients develop a chronic plaque form of the disease. The pathogenesis of psoriasis is not known but evidence exists that changes in micro vascular occur. There are micro vascular abnormalities in the capillaries which display a multilayer basement membrane with fenestration. Study of involved synovium in psoriatic arthritis reveals endothelial cell swelling, thickening of the vessel walls and inflammatory cell infiltration. Investigation on expression of CD35 molecules that clear the immune complexes were carried out in this study. CD35 is single chain glycoprotein (MW160–240 kD) and is located on the long arm of the chromosome 1. FACScan was used as laser flow cytometer. Initially 40 blood samples from normal individuals, 35 untreated Lupus Erythematosus Systemic and 35 Rheumatoid arthritis patients were studied as controls for reference values of CD35, Circulating immune Complexes (CIC), Complement C3 & C4. Next 34 patients suffering from psoriasis were studied for, Circulating immune Complexes (CIC), Complement C3 & C4. Comparison of these results with those of reference values normal ranges showed significant increase of CIC (P<0.05) and decrease of CD35 (P<0.001). Where as, the complement component levels C3 (P>0.05) and C4(P>0.05) indicates werenot significantly altered. This study shows that the decrease in expression of CD35 on Red blood cells in psoriasis patients is more crucial than C3 and C4 levels. Thus decrease of CD35 molecule in psoriasis may cause increased levels of CIC in patients' sera and promotion of inflammatory responses.

Catabolism of the human erythrocyte C3b/C4b receptor (CR1, CD35): vesiculation and/or proteolysis?

Human erythrocytes (E) react by exocytosis of membrane vesicles to various stresses including the fixation of the membrane attack complex of Complement. E from normal individuals loose a notable proportion of their initial number of surface CR1 molecules during the ageing process. An acquired decrease of CR1 on E also occurs in pathological conditions such as Systemic Lupus Erythematosus or AIDS. The present study investigated whether calcium ionophore A23187 (Ca-ion) induced vesicle formation of human E in vitro is responsible for a preferential loss of CR1 as well as whether CR1 molecules at the surface of Ca-ion treated E or vesicles are: (i) functional, (ii) native or protease degraded, or (iii) more clustered than CR1 on native E. A study of E from 137 normal individuals showed that a one-hour Ca-ion induced vesicle formation preferentially removed one third of E surface CR1. Kinetic experiments suggested that all surface CR1 could be removed from E upon longer incubation times. CR1 molecules on vesicles were still able to inhibit Complement activation, and were found in larger clusters than on native E. These data suggest that a significant part of surface CR1 molecules may be removed from E by vesicle formation during the life of E in normal individuals. This phenomenon could be exacerbated in pathological conditions.

A Recombinant Human scFv Anti-Rh(D) Antibody With Multiple Valences Using a C-Terminal Fragment of C4-Binding Protein

Monomeric recombinant molecules prove generally unsatisfactory for in vivo use. Most biological systems are indeed multivalent either structurally, associating different chains, or functionally, when cross-linked by their ligands. Mimicking natural molecules for immune intervention implies the need for multimerizing systems to create multivalent molecules capable of interfering with physiological processing. A multivalent anti-Rh(D) recombinant protein has been designed by reconstructing the antibody binding site of a human monoclonal anti-Rh(D) antibody as a single chain Fv mini antibody, then multimerizing it by inserting at its C-terminal end the C-terminal part of the C4 binding protein (C4bp) alpha chain, which is responsible for the octamer multimerization of that molecule. This soluble multivalent recombinant molecule was functional, bound red blood cells (RBCs), agglutinated them, and did not activate complement. This demonstration model opens the way for future in vivo use of multivalent molecules associating antibody valences and other functional molecules for cell targeting, imaging, or removal of cells such as Rh(D)-positive RBCs for preventing Rh alloimmunization.

Low expression of erythrocyte complement receptor type 1 in chronic hepatitis C patients

Primate erythrocyte complement receptor type 1 (CR1) plays an essential role in complement-associated immune complex clearance by transporting complexes to macrophages in the liver and/or spleen. Antibody-bound hepatitis C virus, which consists of immune complexes, is observed in patients with chronic hepatitis C. The aim of this study was to clarify the pathophysiological roles of erythrocyte CR1 in hepatitis C virus-infected individuals. We quantified the expression of erythrocyte CR1 with a fluorescence-activated cell sorter system in 57 chronic hepatitis C and 37 chronic hepatitis B cases and 20 normal volunteers. Complement-bound immune complexes were quantified by means of an enzyme-linked immunosorbent assay using anti-C1q and anti-C3d antibodies. Hepatitis C virus-infected patients showed lower erythrocyte CR1 and higher C3d immune complex levels than volunteers (P < 0.01 and P < 0.05, respectively). An inverse correlation was observed between the erythrocyte CR1 and C3d immune complex levels in hepatitis C virus infection (r = -0.300, P = 0.032). The erythrocyte CR1 levels in hepatitis C virus infection were lower in patients with severe liver inflammation, cirrhosis, or hepatocellular carcinoma than in those with mild inflammation, whereas the levels did not differ regardless of the disease stage in hepatitis B virus infection. These findings demonstrate that the expression of erythrocyte CR1 is related to immune complex quantity and the severity of liver disease in hepatitis C virus infection.

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.

Time-resolved flow cytometry for the measurement of lanthanide chelate fluorescence: I. Concept and theoretical evaluation

The concept of a flow cytometer suited for the time-resolved measurement of lanthanide chelate luminescence with a decay time on the order of 10 microseconds to 2 ms is presented and evaluated. The instrument proposed encompasses a continuous-wave laser for fluorescence excitation and an optical switch for the elimination of cellular autofluorescence decaying within 1 ns to 1 microseconds during the luminescence detection period. The slowly decaying fluorescence is to be quantified by a photon-counting system, whereas light scatter and prompt fluorescence parameters are acquired by a conventional detection system. The detection limit of the method, in terms of the smallest detectable number of fluorescing chelates per cell, is examined. It was found to be nearly 30,000 complexes of a europium chelate with a decay time of 1.6 ms and a quantum efficiency of 17%, independent of fast decaying cellular autofluorescence or prompt dye emission intensity. The probability of cells passing through the instrument without being detected while the laser beam is turned off was estimated, and the implications for cell throughput and sorting performance of the instrument were assessed. At typical fluorescence detection intervals of 500 microseconds to 1 ms, cell flow rates of 100-200 particles per second lead to detection probabilities of more than 90% and sorting purities comparable to those found in conventional fluorescence-activated cell sorting.

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.

Streptavidin-based quantitative staining of intracellular antigens for flow cytometric analysis

A streptavidin-biotin-based three-step immunolabeling protocol for quantitative staining of intracellular antigens for flow cytometric analysis was evaluated using simian virus 40 (SV40) large T antigen. The concentration as well as the quantity of antibody used required optimization. The optimum labeling conditions varied moderately with cell lines that express T antigen levels over a 40-50-fold range. The procedure resulted in specific fluorescence 2.4 times higher than that using a comparable two-step indirect immunofluorescence technique. The gain in resolution was shown to be greater when staining cells with lower antigen levels. In the analysis of background fluorescence, the principal components were, as for the two-step technique, autofluorescence and propidium spectral overlap. While streptavidin does add to the background, the increase is relatively small. Decreasing the propidium concentration from 50 micrograms/ml to 5 micrograms/ml was found to reduce significantly the level of background from this source. Theoretical aspects of quantitative staining and of resolution versus quantification are discussed.

A flow cytometric assay for measuring complement receptor 1 (CR1) and the complement fragments C3d and C4d on erythrocytes

A flow cytometric assay (FCA) was developed to measure complement receptor 1 (CR1) and the complement fragments C3d and C4d on erythrocytes. It was possible to measure these parameters accurately with intra- and interassay coefficients of variation of 2.0% and 6.5% respectively. The method was able to discriminate between low and high levels of erythrocyte CR1, C3d and C4d. Comparison with a previously described RIA method gave excellent correlation coefficients with r2 values of 0.94, 0.93 and 0.91 for CR1, C3d and C4d respectively. The flow cytometric assay was used to measure CR1, C3d and C4d on the erythrocytes of 98 healthy individuals and the 95% upper limits for C3d and C4d were established. There was a wide distribution of CR1 levels amongst these individuals but their C3d and C4d levels were low and often not above background. The possible application of this method in clinical medicine is discussed.

Covalent binding of non-proteolysed C3 to Jurkat T cells

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

Calibration of fluorescence intensities to quantify antibody binding surface determinants of cell subpopulations by flow cytometry

Quantitative indirect immunofluorescence analysis by flow cytometry was used to determine the mean number of antibody binding sites per cell in a small subpopulation of rat brain cells expressing low levels of a cell surface differentiation antigen recognized by monoclonal antibody (Mab) RB13-6 (Kindler-Röhrborn et al.: Differentiation 30:53-60, 1985). For these non-disjunct distributions of fluorescence intensities, the cut-off border between antigen-positive and antigen-negative cells was defined by a statistical test. To eliminate the influence of accidental disturbances leading to incorrect statistical decisions, the curves for antigen-negative cells were fitted according to cell number and shape. The flow cytometer was calibrated with the use of a clonal cell line for which the average number of Mab RB13-6 binding sites per cell had previously been determined by radioimmunoassay and Scatchard-plot analysis. Using this analytical procedure, both the proportion of Mab binding brain cells and the mean number of Mab binding sites per Mab binding cell could be determined as a function of developmental stage.

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.

Human T lymphocytes expressing the C3b/C4b complement receptor type one (CR1, CD35) belong to Fc gamma receptor-positive CD4-positive T cells

The phenotypic characteristics of human T lymphocytes expressing the C3b/C4b complement receptor type one (CR1, CD35) were investigated using dual-color surface immunofluorescence and cytofluorometric analysis of stained peripheral blood mononuclear cells (PBMC) from normal individuals. Two to ten percent of PBMC coexpressed CR1 and the CD5, CD2, or CD3 antigen. CR1 was detected on a subset of CD4+ T lymphocytes but not on CD8+ or on Leu-7+ lymphocytes. Costaining for CR1 and for the CD4 subpopulation markers anti-Leu-8, TQ1, OKT17, 2H4, and 4B4 indicated that CR1 on lymphocytes may be coexpressed with any of these phenotypic determinants. All CR1+ lymphocytes expressed Fc gamma receptors (Fc gamma Rs) as assessed by their ability to bind biotinylated dimeric human IgG. The expression of CR1 was increased in mixed lymphocyte reaction with kinetics similar to those of HLA-DR antigen expression. Coexpression of CR1 and Fc gamma R+ may provide a subset of CD4+ lymphocytes with an enhanced ability to bind and respond to C3-bearing complexes of IgG and antigen.

Physiological and pathological aspects of circulating immune complexes

Complement participates in the elimination of IC in many circumstances. When antigen/antibody IC first form in the circulation, complement inhibits their aggregation because the covalent binding of C3b to the IC modifies their biophysical properties and they remain soluble. Such opsonized (C3b coated) IC attach to cells bearing C3b receptors (CR1) in the circulation, in particular to erythrocytes, since in humans 85 to 90% of CR1 in the blood is located on these cells. This immune adherence binding reaction appears to be a physiological system that allows IC to be transported through the circulation to the fixed macrophages of the MPS where they are safely eliminated. The deposition of circulating complement-fixing IC in various organs such as the kidney may be considered as a failure of this transport system. This is apparent in complement deficient and depleted states, and also for non-complement-fixing IC (IgA IC). The formation of insoluble IC (by definition immune deposits found in human pathology are insoluble) produces complement activation and inflammation at the site of the immune aggregate.

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.

Correction of cellular autofluorescence in flow cytometry by mathematical modeling of cellular fluorescence

A method for the correction of background fluorescence in flow cytometry with special relevance to the quantitation of low levels of cellular surface membrane antigens is presented. The method is based on the mathematical modeling of cellular fluorescence distributions of background fluorescence (autofluorescence control or irrelevant antibody control) and total fluorescence (positively stained cells). Algorithms based on two models and utilizing only the routinely available background and total fluorescence histograms are developed and implemented in computer programs. These allow estimation of the fluorescence histogram corresponding exclusively to immunofluorescence staining of the cell surface antigen of interest. Thus, the correction of background fluorescence is effected solely with software processing of routinely available data; no additional hardware or parameter determinations are necessary. Two models were chosen to be physically plausible and to represent extremes in correlation between background and probe fluorescence. Extremes were chosen to assess the solution dependence on model and to provide bounds to the actual solution when no information on correlation is available. Results are presented for both computer simulations and for an actual assay of the CR1 complement receptor on human erythrocytes to test and illustrate the technique. Alternatively, data can be tested assuming a particular model to explore the relationship, if any, between specific and nonspecific fluorescence.