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

Erythrocytes: Member of the Immune System that Should Not Be Ignored

Erythrocytes are the most abundant type of cells in the blood and have a relatively simple structure when mature; they have a long life-span in the circulatory system. The primary function of erythrocytes is as oxygen carriers; however, they also play an important role in the immune system. Erythrocytes recognize and adhere to antigens and promote phagocytosis. The abnormal morphology and function of erythrocytes are also involved in the pathological processes of some diseases. Owing to the large number and immune properties of erythrocytes, their immune functions should not be ignored. Currently, research on immunity is focused on immune cells other than erythrocytes. However, research on the immune function of erythrocytes and the development of erythrocyte-mediated applications is of great significance. Therefore, we aimed to review the relevant literature and summarize the immune functions of erythrocytes.

Risk of Bloodstream Infection in Patients with Systemic Lupus Erythematosus Exposed to Prolonged Medium-to-High-Dose Glucocorticoids

OBJECTIVES

This study aimed to investigate the incidence rate and risk factors of bloodstream infection (BSI) in patients with systemic lupus erythematosus (SLE) exposed to medium to high doses of glucocorticoids.

METHODS

This study included 1109 treatment episodes with prolonged (≥4 weeks) medium-to-high-dose glucocorticoids (≥15 mg/day prednisolone) in 612 patients with SLE for over 14 years. Clinical features regarding systemic lupus erythematosus disease activity index 2000 (SLEDAI-2K), immunosuppressant use, and laboratory results were obtained from the electronic medical database. The primary outcome of this study was the 1-year incidence of BSI. The effect of clinical factors on the outcome was investigated using a generalized estimating equation.

RESULTS

During a total of 1078.64 person-years, 30 cases of BSI occurred, with an incidence rate of 2.78 (95% confidence interval (CI) 1.88-3.97) per 100 person-years. Mortality rate of the treatment episodes with BSI was 16.7%, which was significantly higher than that in the other episodes (incidence rate ratio (IRR) 19.59, 95% CI 7.33-52.44). When the incidence rate of BSI was stratified by baseline glucocorticoid dose and SLEDAI-2K score, a higher incidence rate of BSI occurred as disease activity or baseline glucocorticoid dose increased. In the multivariable analysis, SLEDAI-2K ≥20 (adjusted IRR (aIRR) 4.66, 95% CI 2.17-10.00), initial baseline prednisolone ≥ 60 mg/day (aIRR 2.42, 95% CI 1.11-5.32), and cumulative prednisolone dose ≥15 mg/day during the previous 6 months (aIRR 2.13, 95% CI 1.03-4.40) significantly increased the risk of BSI.

CONCLUSION

In patients with SLE exposed to prolonged medium-to-high-dose glucocorticoids, the 1-year incidence rate of BSI was significantly higher than previously reported in the general patients with SLE. Severe disease activity, and high-dose glucocorticoid treatment previously or at baseline increased the risk of BSI.

Regulatory Architecture of the RCA Gene Cluster Captures an Intragenic TAD Boundary, CTCF-Mediated Chromatin Looping and a Long-Range Intergenic Enhancer

The Regulators of Complement Activation (RCA) gene cluster comprises several tandemly arranged genes with shared functions within the immune system. RCA members, such as complement receptor 2 (CR2), are well-established susceptibility genes in complex autoimmune diseases. Altered expression of RCA genes has been demonstrated at both the functional and genetic level, but the mechanisms underlying their regulation are not fully characterised. We aimed to investigate the structural organisation of the RCA gene cluster to identify key regulatory elements that influence the expression of CR2 and other genes in this immunomodulatory region. Using 4C, we captured extensive CTCF-mediated chromatin looping across the RCA gene cluster in B cells and showed these were organised into two topologically associated domains (TADs). Interestingly, an inter-TAD boundary was located within the CR1 gene at a well-characterised segmental duplication. Additionally, we mapped numerous gene-gene and gene-enhancer interactions across the region, revealing extensive co-regulation. Importantly, we identified an intergenic enhancer and functionally demonstrated this element upregulates two RCA members (CR2 and CD55) in B cells. We have uncovered novel, long-range mechanisms whereby autoimmune disease susceptibility may be influenced by genetic variants, thus highlighting the important contribution of chromatin topology to gene regulation and complex genetic disease.

New aspects in the regulation of human B cell functions by complement receptors CR1, CR2, CR3 and CR4

The involvement of complement in the regulation of antibody responses has been known for long. By now several additional B cell functions - including cytokine production and antigen presentation - have also been shown to be regulated by complement proteins. Most of these important activities are mediated by receptors interacting with activation fragments of the central component of the complement system C3, such as C3b, iC3b and C3d, which are covalently attached to antigens and immune complexes. This review summarizes the role of complement receptors interacting with these ligands, namely CR1 (CD35), CR2 (CD21), CR3 (CD11b/CD18) and CR4 (CD11c/CD18) expressed by B cells in health and disease. Although we focus on human B lymphocytes, we also aim to call the attention to important differences between human and mouse systems.

Atypische Infektionen unter Immunsuppression

Rasches Handeln ist gefragt, wenn der Verdacht auf eine Infektion bei einem immunsupprimierten Patienten besteht. Wenn es sich nicht gerade um einen tropischen Keim von der Kreuzfahrt bzw. den multiresistenten Keim von einem der letzten Klinikaufenthalte handelt – der Erreger ist meist nicht das Problem! Es ist seine Präsentation, die Ausprägung seiner klinischen Manifestation und seine Reaktion auf die Therapie, die für uns zur Herausforderung wird. Ist stets die Veränderung der Immunreaktion durch immer selektiver wirksame Biologika das Thema? Oder müssen wir uns erst mit den Reaktionen des innaten und adaptiven Immunsystems vertraut machen, um zu verstehen, was wir sehen?

Notch signaling induces a transcriptionally permissive state at the Complement C3d Receptor 2 (CR2) promoter in a pre-B cell model

During mammalian lymphoid development, Notch signaling is necessary at multiple stages of T lymphopoiesis, including lineage commitment, and later stages of T cell effector differentiation. In contrast, outside of a defined role in the development of splenic marginal zone B cells, there is conflicting evidence regarding whether Notch signaling plays functional roles in other B cell sub-populations. Complement receptor 2 (CR2) modulates BCR-signaling and is tightly regulated throughout differentiation. During B lymphopoiesis, CR2 is detected on immature and mature B cells with high surface expression on marginal zone B cells. Here, we have explored the possibility that Notch regulates human CR2 transcriptional activity using in vitro models including a co-culture system, co-transfection gene reporters and chromatin accessibility assays. We provide evidence that Notch signaling regulates CR2 promoter activity in a mature B cell line, as well as the induction of endogenous CR2 mRNA in a non-expressing pre-B cell line. The dynamics of endogenous gene activation suggests additional unidentified factors are required to mediate surface CR2 expression on immature and mature B lineage cells.

A novel mouse model expressing human forms for complement receptors CR1 and CR2

BACKGROUND

The complement cascade is increasingly implicated in development of a variety of diseases with strong immune contributions such as Alzheimer's disease and Systemic Lupus Erythematosus. Mouse models have been used to determine function of central components of the complement cascade such as C1q and C3. However, species differences in their gene structures mean that mice do not adequately replicate human complement regulators, including CR1 and CR2. Genetic variation in CR1 and CR2 have been implicated in modifying disease states but the mechanisms are not known.

RESULTS

To decipher the roles of human CR1 and CR2 in health and disease, we engineered C57BL/6J (B6) mice to replace endogenous murine Cr2 with human complement receptors, CR1 and CR2 (B6.CR2CR1). CR1 has an array of allotypes in human populations and using traditional recombination methods (Flp-frt and Cre-loxP) two of the most common alleles (referred to here as CR1long and CR1short) can be replicated within this mouse model, along with a CR1 knockout allele (CR1KO). Transcriptional profiling of spleens and brains identified genes and pathways differentially expressed between mice homozygous for either CR1long, CR1short or CR1KO. Gene set enrichment analysis predicts hematopoietic cell number and cell infiltration are modulated by CR1long, but not CR1short or CR1KO.

CONCLUSION

The B6.CR2CR1 mouse model provides a novel tool for determining the relationship between human-relevant CR1 alleles and disease.

Complement, infection, and autoimmunity

PURPOSE OF REVIEW

Complement system dysfunction in terms of upregulation, downregulation, or dysregulation can create an imbalance of both host defense and inflammatory response leading to autoimmunity. In this review, we aimed at describing the role of complement system in host defense to inflection and in autoimmunity starting from the evidence from primary and secondary complement system deficiencies.

RECENT FINDINGS

Complement system has a determinant role in defense against infections: deficiencies of complement components are associated with increased susceptibility to infections. Primary complement system deficiencies are rare disorders that predispose to both infections and autoimmune diseases. Secondary complement system deficiencies are the result of the complement system activation with consumption. Complement system role in enhancing risk of infective diseases in secondary deficiencies has been demonstrated in patients affected by systemic autoimmune disorders, mainly systemic lupus erythematosus and vasculitis.

SUMMARY

The relationship between the complement system and autoimmunity appears paradoxical as both the deficiency and the activation contribute to inducing autoimmune diseases. In these conditions, the presence of complement deposition in affected tissues, decreased levels of complement proteins, and high levels of complement activation fragments in the blood and vessels have been documented.

Aiolos Overexpression in Systemic Lupus Erythematosus B Cell Subtypes and BAFF-Induced Memory B Cell Differentiation Are Reduced by CC-220 Modulation of Cereblon Activity

BAFF is a B cell survival and maturation factor implicated in the pathogenesis of systemic lupus erythematosus (SLE). In this in vitro study, we describe that soluble BAFF in combination with IL-2 and IL-21 is a T cell contact-independent inducer of human B cell proliferation, plasmablast differentiation, and IgG secretion from circulating CD27⁺ memory and memory-like CD27^-IgD^- double-negative (DN) B cells, but not CD27^-IgD⁺ naive B cells. In contrast, soluble CD40L in combination with IL-2 and IL-21 induces these activities in both memory and naive B cells. Blood from healthy donors and SLE patients have similar circulating levels of IL-2, whereas SLE patients exhibit elevated BAFF and DN B cells and reduced IL-21. B cell differentiation transcription factors in memory, DN, and naive B cells in SLE show elevated levels of Aiolos, whereas Ikaros levels are unchanged. Treatment with CC-220, a modulator of the cullin ring ligase 4-cereblon E3 ubiquitin ligase complex, reduces Aiolos and Ikaros protein levels and BAFF- and CD40L-induced proliferation, plasmablast differentiation, and IgG secretion. The observation that the soluble factors BAFF, IL-2, and IL-21 induce memory and DN B cell activation and differentiation has implications for extrafollicular plasmablast development within inflamed tissue. Inhibition of B cell plasmablast differentiation by reduction of Aiolos and Ikaros may have utility in the treatment of SLE, where elevated levels of BAFF and Aiolos may prime CD27⁺ memory and DN memory-like B cells to become Ab-producing plasmablasts in the presence of BAFF and proinflammatory cytokines.

Infection and Lupus: Which Causes Which?

Infection is a leading cause of morbidity and mortality among patients with systemic lupus erythematous (SLE). Dysfunction of the innate and adaptive immune systems increases the risk of infection in patients with SLE. Infectious agents have also been theorized to play a role in the pathogenesis of SLE. This article summarizes our current knowledge of the infectious risk SLE patients face as a result of their underlying disease including abnormal phagocytes and T cells as well as the increased risk of infection associated with immunosuppressive agents used to treat disease. Pathogens thought to play a role in the pathogenesis of disease including EBV, CMV, human endogenous retroviruses (HERVs), and tuberculosis will also be reviewed, as well as the pathologic potential of microbial amyloids and the microbiome.

Daphnetin inhibits inflammation in the NZB/W F1 systemic lupus erythematosus murine model via inhibition of NF-κB activity

Daphnetin is a compound extracted from Chinese medicinal herbs, which exerts analgesic and anti-inflammatory effects. The present study aimed to investigate the potential therapeutic effect of daphnetin on inflammation in the NZB/W F1 systemic lupus erythematosus (SLE) murine model. Female NZB/WF1 mice (age, 16–18 weeks) were intraperitoneally injected with daphnetin once a day for 12 weeks. It was revealed that daphnetin treatment significantly increased animal survival rates, reduced renal damage and blood urea nitrogen levels, and suppressed serum autoantibody production in the SLE-prone NZB/W F1 mice. In addition, daphnetin treatment significantly decreased the serum levels of tumor necrosis factor-α and interleukin-6, inhibited nuclear factor (NF)-κB activity, suppressed the protein expression of nuclear factor of activated T-cells and promoted A20 protein expression in SLE-prone NZB/W F1 mice. In conclusion, daphnetin inhibited inflammation in the NZB/W F1 murine SLE model via inhibition of NF-κB mediated by upregulation of A20.

Complement Receptor Type 1 Suppresses Human B Cell Functions in SLE Patients

Complement receptors (CRs) play an integral role in innate immunity and also function to initiate and shape the adaptive immune response. Our earlier results showed that complement receptor type 1 (CR1, CD35) is a potent inhibitor of the B cell receptor- (BCR-) induced functions of human B lymphocytes. Here we show that this inhibition occurs already at the initial steps of B cell activation since ligation of CR1 reduces the BCR-induced phosphorylation of key signaling molecules such as Syk and mitogen activated protein kinases (MAPKs). Furthermore, our data give evidence that although B lymphocytes of active systemic lupus erythematosus (SLE) patients express lower level of CR1, the inhibitory capacity of this complement receptor is still maintained and its ligand-induced clustering results in significant inhibition of the main B cell functions, similar to that found in the case of healthy individuals. Since we have found that reduced CR1 expression of SLE patients does not affect the inhibitory capacity of the receptor, our results further support the therapeutical potential of CD35 targeting the decrease of B cell activation and autoantibody production in autoimmune patients.

Cutting Edge: Marginal Zone Macrophages Regulate Antigen Transport by B Cells to the Follicle in the Spleen via CD21

Marginal zone macrophages (MZM) are strategically located in the spleen, lining the marginal sinus where they sense inflammation and capture Ag from the circulation. One of the receptors expressed by MZM is scavenger receptor macrophage receptor with collagenous structure (MARCO), which has affinity for modified self-antigens. In this article, we show that engagement of MARCO on murine macrophages induces extracellular ATP and loss of CD21 and CD62L on marginal zone B cells. Engagement of MARCO also leads to reduction of Ag transport by marginal zone B cells and affects the subsequent immune response. This study highlights a novel function for MZM in regulating Ag transport and activation, and we suggest that MARCO-dependent ATP release regulates this through shedding of CD21 and CD62L. Because systemic lupus erythematosus patients were shown to acquire autoantibodies against MARCO, this highlights a mechanism that could affect a patient's ability to combat infections.

Infection Risk and Safety of Corticosteroid Use

Corticosteroids are frequently used to treat rheumatic diseases. Their use comes with several well-established risks, including osteoporosis, avascular necrosis, glaucoma, and diabetes. The risk of infection is of utmost concern and is well documented, although randomized controlled trials of short-term and lower-dose steroids have generally shown little or no increased risk. Observational studies from the real world, however, have consistently shown dose-dependent increases in risk for serious infections as well as certain opportunistic infections. In patients who begin chronic steroid therapy, vaccination and screening strategies should be used in an attempt to mitigate this risk.

Focused transcription from the human CR2/CD21 core promoter is regulated by synergistic activity of TATA and Initiator elements in mature B cells

Complement receptor 2 (CR2/CD21) is predominantly expressed on the surface of mature B cells where it forms part of a coreceptor complex that functions, in part, to modulate B-cell receptor signal strength. CR2/CD21 expression is tightly regulated throughout B-cell development such that CR2/CD21 cannot be detected on pre-B or terminally differentiated plasma cells. CR2/CD21 expression is upregulated at B-cell maturation and can be induced by IL-4 and CD40 signaling pathways. We have previously characterized elements in the proximal promoter and first intron of CR2/CD21 that are involved in regulating basal and tissue-specific expression. We now extend these analyses to the CR2/CD21 core promoter. We show that in mature B cells, CR2/CD21 transcription proceeds from a focused TSS regulated by a non-consensus TATA box, an initiator element and a downstream promoter element. Furthermore, occupancy of the general transcriptional machinery in pre-B versus mature B-cell lines correlate with CR2/CD21 expression level and indicate that promoter accessibility must switch from inactive to active during the transitional B-cell window.

Infection risk in systemic lupus erythematosus patients: susceptibility factors and preventive strategies

Infection is one of the leading causes of morbidity and mortality in systemic lupus erythematosus (SLE). Bacterial infections are most frequent, followed by viral and fungal infections. The impaired cellular and humoral immune functions seen in patients with SLE are predisposing conditions, whilst disease activity, prednisone doses over 7.5–10 mg/day, high doses of methylprednisolone or cyclophosphamide are well-recognised risk factors for infection. The first six months after rituximab treatment and the use of more than three courses are also associated with an increased susceptibility for infection. It has not been established whether belimumab, azathioprine and mycophenolate mofetil increase the risk of serious infections. Most vaccines are effective and safe in SLE patients, although vaccination should be avoided during periods of active disease. Live virus vaccines are contraindicated for immunosuppressed patients. Influenza and pneumococcal vaccines are universally recommended. Tuberculosis prophylaxis should be considered in selected cases. Therefore, it is advisable not to exceed doses of 5 mg/day of prednisone in chronic treatment. Methylprednisolone and cyclophosphamide should be used in low-dose regimens. Antimalarials have a well-known protective role against infection, in addition to other beneficial properties, thus, hydroxychloroquine is recommended for all SLE patients where no contraindication exists.

Membrane-bound complement regulatory proteins as biomarkers and potential therapeutic targets for SLE

For the last two decades, there had been remarkable advancement in understanding the role of complement regulatory proteins in autoimmune disorders and importance of complement inhibitors as therapeutics. Systemic lupus erythematosus is a prototype of systemic autoimmune disorders. The disease, though rare, is potentially fatal and afflicts women at their reproductive age. It is a complex disease with multiorgan involvement, and each patient presents with a different set of symptoms. The diagnosis is often difficult and is based on the diagnostic criteria set by the American Rheumatology Association. Presence of antinuclear antibodies and more specifically antidouble-stranded DNA indicates SLE. Since the disease is multifactorial and its phenotypes are highly heterogeneous, there is a need to identify multiple noninvasive biomarkers for SLE. Lack of validated biomarkers for SLE disease activity or response to treatment is a barrier to the efficient management of the disease, drug discovery, as well as development of new therapeutics. Recent studies with gene knockout mice have suggested that membrane-bound complement regulatory proteins (CRPs) may critically determine the sensitivity of host tissues to complement injury in autoimmune and inflammatory disorders. Case-controlled and followup studies carried out in our laboratory suggest an intimate relation between the level of DAF, MCP, CR1, and CD59 transcripts and the disease activity in SLE. Based on comparative evaluation of our data on these four membrane-bound complement regulatory proteins, we envisaged CR1 and MCP transcripts as putative noninvasive disease activity markers and the respective proteins as therapeutic targets for SLE. Following is a brief appraisal on membrane-bound complement regulatory proteins DAF, MCP, CR1, and CD59 as biomarkers and therapeutic targets for SLE.

Transcriptional effects of a lupus-associated polymorphism in the 5' untranslated region (UTR) of human complement receptor 2 (CR2/CD21)

Systemic lupus erythematosus (SLE) is a complex autoimmune disease with a strong genetic component that determines risk. A common three single-nucleotide polymorphism (SNP) haplotype of the complement receptor 2 (CR2) gene has been associated with increased risk of SLE (Wu et al., 2007; Douglas et al., 2009), and a less common haplotype consisting of the major allele at SNP1 and minor alleles at SNP2 and 3 confers protection (Douglas et al., 2009). SNP1 (rs3813946), which is located in the 5' untranslated region (UTR) of the CR2 gene, altered transcriptional activity of a CR2 promoter-luciferase reporter gene construct transiently transfected into a B cell line (Wu et al., 2007) and had an independent effect in the protective haplotype (Douglas et al., 2009). In this study, we show that this SNP alters transcriptional activity in a transiently transfected non B-cell line as well as in stably transfected cell lines, supporting its relevance in vivo. Furthermore, the allele at this SNP affects chromatin accessibility of the surrounding sequence and transcription factor binding. These data confirm the effects of rs3813946 on CR2 transcription, identifying the 5' UTR to be a novel regulatory element for the CR2 gene in which variation may alter gene function and modify the development of lupus.

Expression and significance of leukocyte membrane cofactor protein transcript in systemic lupus erythematosus

Membrane cofactor protein (MCP) is a complement regulatory protein ubiquitously expressed on most nucleated cells. Since MCP protects autologous cells from complement-mediated injury, it is suggested to have a protective role against the self-tissue damage in inflammatory conditions such as systemic lupus erythematosus (SLE). However, the relevance of MCP in human SLE is not well explored. To assess the significance of MCP in SLE, we studied expression of leukocyte MCP transcript in 60 healthy individuals (controls) and 60 patients with SLE and correlated that with the levels of circulating immune complex (CIC), C3, C3d and SLEDAI scores. The levels of leukocyte MCP transcript were significantly higher ( p < 0.001) in patients with SLE than the controls. Furthermore, MCP transcript levels exhibited significant positive correlations with SLEDAI scores and CIC level and a negative correlation with C3d level in patients. Twelve patients were followed-up until remission. The levels of MCP transcripts decreased significantly during remission as compared with the state of active disease. These findings suggest that in SLE, the expression of leukocyte MCP at the mRNA level is closely related to disease activity. A protective role of MCP in response to increased disease burden may be speculated. The follow-up study suggested MCP as a potential disease marker.

Relationship of leukocyte CR1 transcript and protein with the pathophysiology and prognosis of systemic lupus erythematosus: a follow-up study

Complement Receptor 1 (CR1) is a key complement regulatory protein (CRP) involved in the clearance of immune complexes. Earlier, we reported a marked decline of leukocyte CR1 (L-CR1) transcript and protein in patients with active systemic lupus erythematosus (SLE) and suggested L-CR1 transcript as a putative non-invasive disease marker for SLE. This follow-up study involving 18 patients with active SLE was conducted for further confirmation of the relationship between L-CR1 and SLE. Blood samples from the patients were collected on day 1 of the diagnosis (0 month) and at different time intervals (3 and 6 months) for analysis of L-CR1 transcript and L-CR1 protein by semi-quantitative reverse-transcriptase-polymerase chain reaction (RT-PCR) and western blotting respectively. Within 6 months, 15 patients entered remission. On day 1, the mean values of L-CR1 transcript (8.42 ± 3.53) and L-CR1 protein (4683 ± 1094) in the SLE patients were 6 times and 12 times lower than the normal controls (n = 103). At the end of month 6, these values increased by 4.5 and 6.5 times respectively for CR1 transcript (37.86 ± 8.52) and protein (30,265 ± 8614). Simultaneously, the SLE Disease Activity Index (SLEDAI) scores decreased by 4.8 times (4.47 ± 3.32) as compared with the scores obtained on day 1 (21.45 ± 5.67). Moreover, CR1 values correlated negatively with the SLEDAI scores. Levels of L-CR1 protein and transcript remained low in the three patients who did not enter remission. All of the above results suggested that an increase in the levels of L-CR1 related to good prognosis. Since the levels of L-CR1 protein is influenced by variables like proteolytic cleavage and secretion from leukocytes, the values of L-CR1 transcript on day 1 and subsequent follow-up points may bring a better insight into the state of the disease activity. An extended follow-up study is needed to confirm the significance of L-CR1 as a prognostic marker for SLE.

Complement receptor 2 polymorphisms associated with systemic lupus erythematosus modulate alternative splicing

Genetic factors influence susceptibility to systemic lupus erythematosus (SLE). A recent family-based analysis in Caucasian and Chinese populations provided evidence for association of single-nucleotide polymorphisms (SNPs) in the complement receptor 2 (CR2/CD21) gene with SLE. Here we confirmed this result in a case-control analysis of an independent European-derived population including 2084 patients with SLE and 2853 healthy controls. A haplotype formed by the minor alleles of three CR2 SNPs (rs1048971, rs17615, rs4308977) showed significant association with decreased risk of SLE (30.4% in cases vs 32.6% in controls, P=0.016, OR=0.90 (0.82-0.98)). Two of these SNPs are in exon 10, directly 5' of an alternatively spliced exon preferentially expressed in follicular dendritic cells (FDC), and the third is in the alternatively spliced exon. Effects of these SNPs and a fourth SNP in exon 11 (rs17616) on alternative splicing were evaluated. We found that the minor alleles of these SNPs decreased splicing efficiency of exon 11 both in vitro and ex vivo. These findings further implicate CR2 in the pathogenesis of SLE and suggest that CR2 variants alter the maintenance of tolerance and autoantibody production in the secondary lymphoid tissues where B cells and FDCs interact.

Expression and role of CR1 and CR2 on B and T lymphocytes under physiological and autoimmune conditions

The involvement of complement in the development and regulation of antibody responses under both healthy and pathological conditions is known for long. Unravelling the molecular mechanisms underlying the events however is still in progress. This review focuses on the role of complement receptors CR1 (CD35) and CR2 (CD21) expressed on T and B cells. Alteration in the expression and function of these receptors may contribute to the initiation and maintenance of immune complex mediated autoimmune diseases such as systemic lupus erythematosus and rheumatoid arthritis. Recent data regarding complement receptor expression on T lymphocytes and on memory B cells are also discussed.

Recent advances in understanding the pathophysiology of Wiskott-Aldrich syndrome

Wiskott-Aldrich syndrome (WAS) is a severe X-linked immunodeficiency caused by mutations in the gene encoding for WASP, a key regulator of signaling and cytoskeletal reorganization in hematopoietic cells. Mutations in WASP result in a wide spectrum of clinical manifestations ranging from the relatively mild X-linked thrombocytopenia to the classic full-blown WAS phenotype characterized by thrombocytopenia, immunodeficiency, eczema, and high susceptibility to developing tumors and autoimmune manifestations. The life expectancy of patients affected by severe WAS is reduced, unless they are successfully cured by bone marrow transplantation from related identical or matched unrelated donors. Because many patients lack a compatible bone marrow donor, the administration of WAS gene–corrected autologous hematopoietic stem cells could represent an alternative therapeutic approach. In the present review, we focus on recent progress in understanding the molecular and cellular mechanisms contributing to the pathophysiology of WAS. Although molecular and cellular studies have extensively analyzed the mechanisms leading to defects in T, B, and dendritic cells, the basis of autoimmunity and thrombocytopenia still remains poorly understood. A full understanding of these mechanisms is still needed to further implement new therapeutic strategies for this peculiar immunodeficiency.

Increased B cell deletion and significantly reduced auto-antibody titre due to premature expression of human complement receptor 2 (CR2, CD21)

The involvement of complement receptor 2 (CR2) in B cell tolerance and autoimmune disease has been revealed over the past decade or so. Our previous studies have established that mice prematurely expressing human CR2 under the control of a lambda light chain promoter (in particular the hCR2^high line) have a marked deficit in their immune response to various antigens and fail to develop collagen-induced arthritis. This phenotype appears to be the result of irreversible changes in B cell signalling pathways and suggested that hCR2 expressing mice are protected from developing autoimmune disease. To test this hypothesis, we examined the ability of the hCR2 to block the development of spontaneous autoimmune disease on the C57BL/6j-Fas^lpr/Fas^lpr (B6^lpr) background. We found that expression of hCR2 on the B6^lpr background resulted in a significant reduction in levels of anti-nuclear antibodies (ANA) generated as mice aged but the levels of ANA were still higher than those found in age matched C57BL/6j (B6) mice. B cells from hCR2^high mice were found to display a higher baseline level of apoptosis, whether analysed ex vivo or after in vitro culture, than their B6 counterparts and this was apparently linked to both surface IgM expression by the B cells and C3 levels in the mice. Our data also provides evidence that B cell survival in the presence of hCR2 is heavily modified by the background strain of the mouse. Overall, we have demonstrated that mice expressing hCR2 on their B cells during bone marrow development display a higher degree of apoptosis which may lead to a deletion of autoreactive B cells and be protective against the development of autoimmune disease.

Association of a common complement receptor 2 haplotype with increased risk of systemic lupus erythematosus

A genomic region on distal mouse chromosome 1 and its syntenic human counterpart 1q23-42 show strong evidence of harboring lupus susceptibility genes. We found evidence of linkage at 1q32.2 in a targeted genome scan of 1q21-43 in 126 lupus multiplex families containing 151 affected sibpairs (nonparametric linkage score 2.52, P = 0.006). A positional candidate gene at 1q32.2, complement receptor 2 (CR2), is also a candidate in the murine Sle1c lupus susceptibility locus. To explore its role in human disease, we analyzed 1,416 individuals from 258 Caucasian and 142 Chinese lupus simplex families and demonstrated that a common three-single-nucleotide polymorphism CR2 haplotype (rs3813946, rs1048971, rs17615) was associated with lupus susceptibility (P = 0.00001) with a 1.54-fold increased risk for the development of disease. Single-nucleotide polymorphism 1 (rs3813946), located in the 5' untranslated region of the CR2 gene, altered transcriptional activity, suggesting a potential mechanism by which CR2 could contribute to the development of lupus. Our findings reveal that CR2 is a likely susceptibility gene for human lupus at 1q32.2, extending previous studies suggesting that CR2 participates in the pathogenesis of systemic lupus erythematosus.

The role of CR2 in autoimmunity

Complement activation is one of the most powerful mechanisms taking place during inflammation and immune responses. Over the last 30 years increasing evidence has proven the role of C3 and receptors for its activation fragments in the initiation and regulation of immune responses. Since complement also has a basic importance in the maintenance of immune homeostasis, abnormalities affecting complement proteins and their receptors may lead to pathological conditions. Autoimmune conditions develop as a result of a range of genetic and environmental factors. Findings obtained from animal models support the notion that malfunctioning of complement receptors, particularly CR2, might be involved in the breakdown of tolerance and excessive antibody production by auto reactive B-cell clones. In addition to B cells, activated, CR2-bearing T cells may also contribute to the pathogenesis of autoimmunity as they can receive activating/survival signals in the inflamed tissue. Results obtained from mouse experiments however, should be extended to the human system with great care, since there are basic differences between the structure and function of human and murine CR1 and CR2.

CR1 on erythrocytes of Brazilian systemic lupus erythematosus patients: The influence of disease activity on expression and ability of this receptor to bind immune complexes opsonized with complement from normal human serum

Hypocomplementaemia and low expression of CR1 on erythrocytes (E) of patients with systemic lupus erythematosus (SLE) are associated with defective clearance of circulating immune complexes (IC) and so they may have pathogenic significance. Here, we investigated whether the reduced CR1/E in SLE patients per se might affect the binding of IC to CR1/E. First, we analysed the expression of CR1 on E of active (n=30) and inactive (n=34) SLE patients using a FITC-conjugated mouse anti-CR1 monoclonal antibody E11 and flow cytometry. Both groups of patients had a significantly reduced CR1/E expression compared with healthy controls (n=40). It was also observed that the number of E bearing CR1 was reduced in both groups of SLE patients studied. Second, we determined the functional activity of CR1/E by measuring the binding to E of FITC-bovine serum albumin (BSA)/rabbit anti-BSA complexes, formed at equivalence, which were opsonized with complement from normal human serum (NHS). On the other hand, we did not find differences between the patient and control groups in the ability of E to bind IC/NHS. There was also a positive correlation between the CR1/E expression and the number of E bearing CR1 in control and inactive SLE groups, which was not observed in the group of active SLE patients. Considering the involvement of low levels of complement and CR1/E expression on complex processing, in this in vitro model the results show that an effective coating of the complexes with complement is sufficient to bind them preferentially to CR1 over normal levels of receptor expression.

Association of leukocyte CR1 gene transcription with the disease severity and renal involvement in systemic lupus erythematosus

The reduced level of complement receptor 1 (CR1) on erythrocytes is speculated as a key mechanism contributing to immune complex (IC) overload and exaggerated complement (C) activation in systemic lupus erythematosus (SLE). Comparatively, fewer studies documented lower levels of CR1 on leukocytes and glomerular podocytes in this disease. The decline in E-CR1 is largely believed as an acquired phenomenon caused due to the proteolytic cleavage of CR1 from erythrocyte membrane. The mechanism underlying reduced CR1 expression on nucleated cells is under constant investigation. Recently, reduced leukocytes CR1 gene transcription had been demonstrated in SLE and was suggested as the main cause of decline in leukocyte CR1 (L-CR1). The relationship of L-CR1 gene transcription with severity and pathophysiology of disease needs to be elucidated. We determined the levels of L-CR1 in 30 active SLE patients and compared with normal healthy controls (n = 30). Patients were categorized into two groups i.e., with nephritis (n = 14) or without nephritis (n = 16). The expression of L-CR1 at transcriptional level was correlated with the levels of serum CIC, C3 and anti dsDNA antibodies. The levels of L-CR1 transcription were significantly reduced in all SLE patients as compared to controls (P < 0.001). This decline in L-CR1 however, was more marked in patients with nephritis than those without nephritis. In addition, the serum levels of CIC, anti dsDNA antibodies were higher and the levels of serum C3 were lower than the normal range in the patients. The difference was much more marked in SLE patients with nephritis than those without nephritis. The levels of L-CR1 transcription correlated negatively with the levels of CIC and anti dsDNA antibodies and positively with serum C3 levels. Thus, between SLE patients with and without nephritis, we found significant difference in the levels of L-CR1 transcription (P < 0.01), CIC (P < 0.05), anti dsDNA antibodies (P < 0.01) and C3 (P < 0.01). Our findings suggest that L-CR1 is drastically reduced in patients with severe form of SLE, i.e., lupus nephritis. Determination of L-CR1 expression at transcriptional level in addition to disease hallmarks like C3, CIC and anti-dsDNA antibodies may facilitate the assessment of severity of SLE and discrimination between patients with or without renal involvement.

Phenotypic perturbation of B cells in the Wiskott-Aldrich syndrome

Wiskott-Aldrich syndrome (WAS) is an X-linked immunodeficiency/platelet disease due to mutations of WASP, a cytoskeletal regulatory protein of blood cells. Patients exhibit a range of immune defects generally attributed to defective T-cell function, including poor response to immunization, skewed immunoglobulin isotypes, eczema, recurrent infections, autoimmune disease and increased frequency of malignancies. Here we show a deficit of total B-cells in WAS patients of various ages and identify phenotypic perturbations involving complement receptors and CD27. Whereas B-cells of normal healthy donors are overwhelmingly CD21/CD35-positive, B-cells expressing these receptors are significantly reduced in number in WAS patients, and their paucity may cause suboptimal antigen capture and presentation. The frequencies of IgD(-) and IgG(+) patient B-cells were not different from healthy donors (although absolute numbers were decreased), indicating that isotype switching is occurring. In contrast, the frequency of cells positive for CD27, the marker of post germinal centre B-cells, was significantly decreased even among isotype-switched cells, and B-cells resembling germinal centre progenitors (CD10(+)CD27(-)CD38(bright)) were more frequent in adult patients, suggesting impaired germinal centre maturation/differentiation. The documentation of these phenotypic perturbations and deficit of total cells suggest that defects intrinsic to B-cells contribute to the impaired humoral immunity that characterizes this disease.

Reduction of soluble complement receptor 2/CD21 in systemic lupus erythomatosus and Sjogren's syndrome but not juvenile arthritis

A soluble form of the complement receptor CD21 (sCD21) is shed from the lymphocyte surface. The amount of sCD21 in serum may modulate immunity as sCD21 levels are correlated with several clinical conditions. We report here the serum levels of sCD21 in juvenile arthritis (JA), systemic lupus erythematosus (SLE) and Sjogren's syndrome (SS). Using enzyme-linked immunosorbent assay, we determined sCD21 levels in SLE, SS and JA patients. Mann-Whitney test for nonparametric two-tail P value was performed to obtain statistical significance. Cytometrical analysis of synovial fluid leucocytes of JA patients was done on a FACSsort. While sCD21 levels in SLE and SS are reduced to levels previously found in rheumatoid arthritis (RA), JA sCD21 levels were normal. sCD21 levels did not correlate with clinical parameters and immunophenotype of synovial cells. CD4 T cells in the synovium were almost all of the CD45RO memory type and 13 of 40 patients displayed synovial expansion of gammadeltaT cells. CD21 shedding in JA differs from RA/SS/SLE. JA sCD21 levels in synovial fluid are always lower compared to blood levels of the same patients. Analysis of JA synovial T cells indicates a T-cell driven response.

CR1/CR2 deficiency alters IgG3 autoantibody production and IgA glomerular deposition in the MRL/lpr model of SLE

CR1 and CR2 expression is decreased by approximately 50% on B cells of patients with systemic lupus erythematosus (SLE). Expression is also decreased in the MRL/lpr murine model of SLE prior to the development of clinical disease, suggesting that this alteration may play a role in pathogenesis. To determine whether the decrease in receptor levels affects the development of SLE, we analyzed MRL/lpr mice in which CR1/CR2 expression was altered by gene targeting. Mice from each cohort (Cr2+/+, Cr2+/-, and Cr2-/-) were analyzed biweekly for the development of proteinuria and autoantibodies. Kidneys were examined at 12 and 16 weeks for evidence of immune complex deposition and renal disease. Deficiency of CR1/CR2 did not affect survival or development of renal disease as measured by proteinuria. Mice deficient in CR1/CR2 had significantly lower levels of IgG3 rheumatoid factor (RF) and total serum IgG3, suggesting a specific defect in production of IgG3 in response to endogenous autoantigens. Since IgG3 RF has been associated with the development of vasculitis in this model, we examined the mice for alterations in development of this clinical manifestation. Although there was no difference in the development of ear necrosis among the three groups, renal arteritis was not identified in any of the Cr2+/- mice, whereas it was present in 20% of the Cr2+/- and 40% of the Cr2+/+ mice. Finally, significantly higher levels of IgA were seen in the glomeruli of Cr2+/- mice compared to Cr2+/- or Cr2+/+ mice, suggesting that CR1/CR2 are involved in either the regulation of IgA production or the clearance of IgA immune complexes. Together these data support the concept that alterations in CR1/CR2 expression or function affect the regulation of autoantibody production and/or clearance and may have clinical consequences.

A new CD21low B cell population in the peripheral blood of patients with SLE

A hallmark of systemic lupus erythematosus (SLE) is the production of autoantibodies. Recent reports suggest an abnormal peripheral blood B cell homeostasis in SLE patients without being conclusive. We analyzed by four color flow-cytometry peripheral blood B cell subpopulations of SLE patients, healthy donors, and patients with other systemic autoimmune diseases. IgM memory but not switched memory B cells of SLE patients were significantly decreased compared to healthy donors, whereas transitional B cells, characterized by CD19+IgMhiIgD+CD24hiCD38hi, were significantly expanded in SLE patients but also found in other autoimmune disorders. The population of plasmablasts (CD19loCD21loCD27++CD38++) was increased in active disease. Most interestingly, B cells in autoimmune disorders contain a so far uncharacterized subpopulation with an activated phenotype (CD19hiCD21loCD38loCD86int). None of the identified subpopulations was associated with current or previous therapy and therefore may represent different aspects of the disturbed B cell homeostasis in patients with SLE.

Decreased survival of B cells of HIV-viremic patients mediated by altered expression of receptors of the TNF superfamily

Human immunodeficiency virus (HIV) infection leads to numerous perturbations of B cells through mechanisms that remain elusive. We performed DNA microarray, phenotypic, and functional analyses in an effort to elucidate mechanisms of B cell perturbation associated with ongoing HIV replication. 42 genes were up-regulated in B cells of HIV-viremic patients when compared with HIV-aviremic and HIV-negative patients, the majority of which were interferon (IFN)-stimulated or associated with terminal differentiation. Flow cytometry confirmed these increases and indicated that CD21^low B cells, enhanced in HIV-viremic patients, were largely responsible for the changes. Increased expression of the tumor necrosis factor (TNF) superfamily (TNFSF) receptor CD95 correlated with increased susceptibility to CD95-mediated apoptosis of CD21^low B cells, which, in turn, correlated with HIV plasma viremia. Increased expression of BCMA, a weak TNFSF receptor for B lymphocyte stimulator (BLyS), on CD21^low B cells was associated with a concomitant reduction in the expression of the more potent BLyS receptor, BAFF-R, that resulted in reduced BLyS binding and BLyS-mediated survival. These findings demonstrate that altered expression of genes associated with IFN stimulation and terminal differentiation in B cells of HIV-viremic patients lead to an increased propensity to cell death, which may have substantial deleterious effects on B cell responsiveness to antigenic stimulation.

Association of complement receptor 1 (CR1, CD35, C3b/C4b receptor) density polymorphism with glomerulonephritis in Indian subjects

Complement receptor 1 (CR1, CD35, C3b/C4b receptor), a polymorphic membrane bound glycoprotein is important both as a complement regulatory protein, and as a vehicle for immune complex clearance. It is differentially expressed on erythrocytes, eosinophils, monocytes, B and T-lymphocytes, dendritic cells and kidney podocytes. It also occurs in the plasma as soluble CR1 (sCR1) and in urine as urinary CR1 (uCR1). Different population studies have either suggested or refuted the functional and physiological significance of genomic (HH, high erythrocyte CR1 expression; HL, intermediate and LL, low expression) polymorphism of CR1 in health and disease. Prevalence of autoimmune disorders like RA, GN and SLE is higher in Asian-Indians compared to the western world. Although several studies from India emphasize the modulation of E-CR1 levels as a key factor in the pathophysiology of glomerulonephritis (GN), none of them, however, provide much information on the role of CR1 gene variance in this context. We, therefore, carried out the study of CR1 polymorphism in 117 normal Indian subjects and 65 patients suffering from glomerulonephritis in order to study its possible association with the disease and E-CR1 levels. This is the first study of its kind in the Indian population, in which, the direct effect of a particular genotype on the E-CR1 levels and its possible association with the disease has been studied simultaneously.

BAFF/BLyS can potentiate B-cell selection with the B-cell coreceptor complex

The tumor necrosis factor (TNF)–like ligand BAFF/BLyS (B-cell activating factor of the TNF family/B-lymphocyte stimulator) is a potent B-cell survival factor, yet its functional relationship with other B-cell surface molecules such as CD19 and CD40 is poorly understood. We found that follicular dendritic cells (FDCs) in human lymph nodes expressed BAFF abundantly. BAFF up-regulated a B cell–specific transcription factor Pax5/BSAP (Pax5/B cell–specific activator protein) activity and its target CD19, a major component of the B-cell coreceptor complex, and synergistically enhanced CD19 phosphorylation by B-cell antigen receptor (BCR). BAFF further enhanced B-cell proliferation, immunoglobulin G (IgG) production, and reactivity to CD154 by BCR/CD19 coligation and interleukin-15 (IL-15). Our results suggest that BAFF may play an important role in FDC–B-cell interactions through the B-cell coreceptor complex and a possibly sequential link between the T cell–independent and –dependent B-cell responses in the germinal centers.

Complement and autoimmunity

The complement system is comprised of a number of serum and membrane-bound proteins that play an important role in the elimination of foreign microorganisms while protecting the host organism from complement-related damage. Complement has also been shown to participate in the generation of normal humoral immune responses to foreign antigens. Recent studies suggest that the functions of complement may be extended to include the maintenance of B cell tolerance. Complement receptor 2 (CR2/CD21) has been implicated in lupus susceptibility in both humans and animal models of disease. Located primarily on B cells and follicular dendritic cells, CR2 binds C3 degradation products that have become covalently bound to antigen or immune complexes in the process of complement activation. The mechanism by which CR2 might regulate B cell reactivity to autoantigens has not been elucidated, but may involve direct effects on B cell tolerance or indirect effects on T cell tolerance.

Cr2, a candidate gene in the murine Sle1c lupus susceptibility locus, encodes a dysfunctional protein

The major murine systemic lupus erythematosus (SLE) susceptibility locus, Sle1, corresponds to three loci independently affecting loss of tolerance to chromatin in the NZM2410 mouse. The congenic interval corresponding to Sle1c contains Cr2, which encodes complement receptors 1 and 2 (CR1/CR2, CD35/CD21). NZM2410/NZW Cr2 exhibits a single nucleotide polymorphism that introduces a novel glycosylation site, resulting in higher molecular weight proteins. This polymorphism, located in the C3d binding domain, reduces ligand binding and receptor-mediated cell signaling. Molecular modeling based on the recently solved CR2 structure in complex with C3d reveals that this glycosylation interferes with receptor dimerization. These data demonstrate a functionally significant phenotype for the NZM2410 Cr2 allele and strongly support its role as a lupus susceptibility gene.

B-cell kinase lyn deficiency in patients with systemic lupus erythematosus

BACKGROUND

To better understand the molecular background of B-cell overactivity characterizing systemic lupus erythematosus (SLE), we examined the expression of the CD22 co-receptor and of kinase Lyn, which are involved in signaling inhibitory pathways, in B cells from patients with SLE.

METHODS

Two-color flow cytometry was used to study the expression of surface antigens on freshly isolated peripheral B cells from patients with SLE, disease-control patients, and healthy volunteers. Intracellular kinases Lyn and Syk were analyzed using Western immunoblots, and differences at the messenger RNA (mRNA) level were evaluated using semiquantitative polymerase chain reaction (PCR).

RESULTS

Expression of B-cell surface CD22 was intact in patients with SLE, but expression of the B-cell kinase Lyn was significantly decreased in resting, as well as in anti-sIgM-stimulated B-cell-enriched cell lysates obtained from 66% of patients with SLE. Lyn deficiency was disease-specific and unrelated to disease activity. Expression of B-cell kinase Syk was similar in all study groups. Semiquantitative PCR revealed that Lyn mRNA was significantly decreased in lupus patients with decreased Lyn protein expression, suggesting that Lyn deficiency may be caused at least in part by defects at the transcription level.

CONCLUSIONS

Decreased expression of Lyn in some patients with SLE represents a B-cell defect that may enhance our understanding of SLE molecular pathogenesis by providing rational therapeutic targets.

The role of complement in the acquired immune response

Studies over the past three decades have clearly established a central role for complement in the promotion of a humoral immune response. The primary function of complement, in this regard, is to opsonize antigen or immune complexes for uptake by complement receptor type 2 (CR2, CD21) expressed on B cells, follicular dendritic cells (FDC) and some T cells. A variety of mechanisms appear to be involved in complement-mediated promotion of the humoral response. These include: enhancement of antigen (Ag) uptake and processing by both Ag-specific and non-specific B cells for presentation to specific T cells; the activation of a CD21/CD19 complex-mediated signalling pathway in B cells, which provides a stimulus synergistic to that induced by antigen interaction with the B-cell receptor (BCR); and promotion of the interaction between B cells and FDC, where C3d-bearing immune complexes participate in intercellular bridging. Finally, current studies suggest that CR2 may also play a role in the determination of B-cell tolerance towards self-antigens and thereby hold the key to the previously observed correlation between deficiencies of the early complement components and autoimmune disease.

Induction of Autoimmunity in a Transgenic Model of B Cell Receptor Peripheral Tolerance: Changes in Coreceptors and B Cell Receptor-Induced Tyrosine-Phosphoproteins

Abrogation of peripheral tolerance in transgenic mice that express a uniform B-cell receptor may create a powerful tool to examine the molecular mechanisms that underlie the autoimmune response in B cells. Here we report that processes that induce a systemic lupus erythematosus-like syndrome in normal mice, namely chronic graft vs host reaction, trigger systemic autoimmunity in a well-established transgenic mice model of B cell receptor peripheral tolerance. The induction of graft vs host reaction in mice that carry both a rearranged B cell Ag receptors specific for hen egg lysozyme and expressing chronically circulating hen egg lysozyme Ag resulted in induction of high and sustained levels of circulating anti-hen egg lysoyme autoantibodies and glomerulonephritis with proteinuria. This was associated with marked changes in expression of cell-surface proteins, such as CD23 and complement receptor 2. B cells from the graft vs host-induced mice could proliferate in vitro in response to self-Ag, and upon stimulation with anti-IgD demonstrated rapid phosphotyrosine phosphorylation of specific proteins, which could not be induced in the anergic double transgenic B cells. Conversely, loss of tolerance was not associated with a higher induction in the level of Syk kinase phosphorylation following stimulation with anti-IgD. Taken collectively, these data establish that 1) processes that induce a systemic lupus erythematosus-like syndrome in normal mice can abrogate peripheral tolerance in transgenic mice expressing self-tolerized B cells, and that 2) loss of tolerance in this model is associated with marked changes in surface expression of B cell coreceptors as well as with selective changes in IgD-induced signaling by discrete tyrosine-phosphoproteins, but not Syk kinase.

Complement Receptor 1 (CD35) on Human Reticulocytes: Normal Expression in Systemic Lupus Erythematosus and HIV-Infected Patients

The low levels of complement receptor 1 (CR1) on erythrocytes in autoimmune diseases and AIDS may be due to accelerated loss in the circulation, or to a diminished expression of CR1 on the red cell lineage. Therefore, we analyzed the expression of CR1 on reticulocytes (R) vs erythrocytes (E). Healthy subjects had a significant higher CR1 number per cell on R (919 ± 99 CR1/cell) than on E (279 ± 30 CR1/cell, n = 23), which corresponded to a 3.5- ± 1.3-fold loss of CR1. This intravascular loss was confirmed by FACS analysis, which showed that all R expressed CR1, whereas a large fraction of E was negative. The systemic lupus erythematosus (SLE), HIV-infected, and cold hemolytic Ab disease (CHAD) patients had a CR1 number on R identical to the healthy subjects, contrasting with a lower CR1 on their E. The data indicated a significantly higher loss of CR1 in the three diseases, i.e., 7.0- ± 3.8-, 6.1- ± 2.9-, and 9.6- ± 5.6-fold, respectively. The intravascular loss was best exemplified in a patient with factor I deficiency whose CR1 dropped from 520 CR1/R to 28 CR1/E, i.e., 18.6-fold loss. In one SLE patient and in the factor I-deficient patient, the FACS data were consistent with a loss of CR1 already on some R. In conclusion, CR1 is lost progressively from normal E during in vivo aging so that old E are almost devoid of CR1. The low CR1 of RBC in autoimmune diseases and HIV-infection is due to a loss occurring in the circulation by an active process that remains to be defined.

MRL-lpr/lpr mice exhibit a defect in maintaining developmental arrest and follicular exclusion of anti-double-stranded DNA B cells

A hallmark of systemic lupus erythematosus and the MRL murine model for lupus is the presence of anti-double-stranded (ds)DNA antibodies (Abs). To identify the steps leading to the production of these Abs in autoimmune mice, we have compared the phenotype and localization of anti-dsDNA B cells in autoimmune (MRL+/+ and lpr/lpr) mice with that in nonautoimmune (BALB/c) mice. Anti-dsDNA B cells are actively regulated in BALB/c mice as indicated by their developmental arrest and accumulation at the T-B interface of the splenic follicle. In the MRL genetic background, anti-dsDNA B cells are no longer developmentally arrested, suggesting an intrinsic B cell defect conferred by MRL background genes. With intact Fas, they continue to exhibit follicular exclusion; however, in the presence of the lpr/lpr mutation, anti-dsDNA B cells are now present in the follicle. Coincident with the altered localization of anti-dsDNA B cells is a follicular infiltration of CD4 T cells. Together, these data suggest that MRL mice are defective in maintaining the developmental arrest of autoreactive B cells and indicate a role for Fas in restricting entry into the follicle.

Complement Opsonization Is Required for Presentation of Immune Complexes by Resting Peripheral Blood B Cells

Complement receptor 2 (CD21, CR2) is a B cell receptor for complement degradation products bound to Ag or immune complexes. The role of CD21 in mediating Ag presentation of soluble immune complexes by resting B cells was studied. Complement-coated immune complexes were formed by the incubation of influenza virus with serum from immune donors. These complexes bound to peripheral blood B cells in a complement-dependent manner. The binding required CD21 or, to a lesser extent, complement receptor 1 (CR1, CD35). B cells pulsed with immune complexes containing complement elicited a response from a panel of influenza-specific T cell clones, while those pulsed with immune complexes formed in the absence of complement did not. The expression of the early activation marker CD69 and the costimulatory molecule CD86 were not induced by CD21 ligation alone, suggesting that CD21-mediated Ag presentation occurs independently of B cell activation. Up-regulation of these markers required exposure to T cell factors elicited by the recognition of Ag derived from complement-containing immune complexes. These findings suggest that binding of Ag to CD21 enables Ag-nonspecific B cells to participate in the activation of Ag-specific T cells in a process that occurs independently of well-characterized B cell activation events.

Immune cell signaling aberrations in human lupus

A large array of heterogeneous aberrations of the immune system have been described in systemic lupus erythematosus (SLE). Since the function and the fate of the immune system cells are governed principally by the biochemical events that follow ligation of specialized cell-surface receptors, we will review in this article recent developments in our understanding of abnormalities in the biochemistry of signals generated either by the antigen-receptor complex or by systems of costimulatory cell-surface molecules, like the CD28/CTLA4:CD80/CD86 and the CD40:CD40L pairs found on the surface membrane of lupus immune cells.

Complement receptor 2 in the regulation of the immune response

Antigens coated with split products of C3, the result of complement activation, are capable of crosslinking the complement receptor 2 (CR2, CD21) and the antigen receptor on the surface of B cells simultaneously. This dual recognition leads to increased cell proliferation and differentiation and enhanced antibody production. CR2 is also considered to be a regulator of the B cell response to antigen. In this review we summarize the biology of the CR2 and focus on its essential role in generating an effective B cell response to antigenic stimuli. The involvement of CR2 in the pathophysiology of infectious and autoimmune diseases is also discussed.

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.