Delineating the genetic basis of systemic lupus erythematosus

I-PLA(2) activation during apoptosis promotes the exposure of membrane lysophosphatidylcholine leading to binding by natural immunoglobulin M antibodies and complement activation

Deficiency of serum immunoglobulin (Ig)M is associated with the development of a lupus-like disease in mice. Recent studies suggest that classical complement components facilitate the clearance of apoptotic cells and that failure to do so predisposes mice to lupus. Since IgM is a potent activator of the classical complement pathway, we examined IgM binding to dying cells. IgM, but not IgG, bound to apoptotic T cells through the Fab' portion of the antibody. Exposure of apoptotic cell membranes to phospholipase (PL) A2 increased, whereas PLD reduced, IgM binding and complement activation. Absorption studies combined with direct plate binding assays, revealed that IgM antibodies failed to bind to phosphatidyl lipids, but did recognize lysophosphatidylcholine and the phosphorylcholine head group. Both iPLA(2) and cPLA(2) are activated during apoptosis. Since inhibition of iPLA2, but not cPLA2, attenuated IgM binding to apoptotic cells, these results strongly suggest that the endogenous calcium independent PLA(2), iPLA(2), is involved in the hydrolysis of plasma membrane phospholipids and exposure of the epitope(s) recognized by IgM. We propose that recognition of dying cells by natural IgM antibodies is, in part, responsible for complement activation on dying cells leading to their safe clearance.

Update on complement in the pathogenesis of systemic lupus erythematosus

Complement is involved in inflammation and in the optimization of adaptive responses. Abnormalities in the complement system have been associated with autoimmunity, especially systemic lupus erythematosus. A paradoxic relation exits between complement and lupus. Complement-mediated tissue damage is accepted as a mechanism in disease pathophysiology. Conversely, complement exerts a protective effect in disease development. The theoretic framework explaining this protective influence involves the adequate disposal of apoptotic material by classic pathway components. Inadequate clearance of apoptotic material may evoke a proinflammatory autoimmune response. This conceptual model is substantiated by studies indicating that complement receptor genes are within major susceptibility loci of systemic lupus erythematosus, that functional and structural abnormalities in these receptors are found in lupus mouse models, and that genetic polymorphism of lectin pathway genes correlates with increased risk of disease development. Finally, new diagnostic and therapeutic modalities based on complement regulation have been described in the past year.

Visualizing human leukocyte antigen class II risk haplotypes in human systemic lupus erythematosus

Human leukocyte antigen (HLA) class I and class II alleles are implicated as genetic risk factors for many autoimmune diseases. However, the role of the HLA loci in human systemic lupus erythematosus (SLE) remains unclear. Using a dense map of polymorphic microsatellites across the HLA region in a large collection of families with SLE, we identified three distinct haplotypes that encompassed the class II region and exhibited transmission distortion. DRB1 and DQB1 typing of founders showed that the three haplotypes contained DRB1*1501/ DQB1*0602, DRB1*0801/ DQB1*0402, and DRB1*0301/DQB1*0201 alleles, respectively. By visualizing ancestral recombinants, we narrowed the disease-associated haplotypes containing DRB1*1501 and DRB1*0801 to an approximately 500-kb region. We conclude that HLA class II haplotypes containing DRB1 and DQB1 alleles are strong risk factors for human SLE.

Genetic dissection of SLE: SLE1 and FAS impact alternate pathways leading to lymphoproliferative autoimmunity

Genetic dissection of lupus pathogenesis in the NZM2410 strain has recently revealed that Sle1 is a potent locus that triggers the formation of IgG anti-histone/DNA antibodies, when expressed on the B6 background as a congenic interval. B6.lpr mice, in contrast, exhibit distinctly different cellular and serological phenotypes. Both strains, however, do not usually exhibit pathogenic autoantibodies, or succumb to lupus nephritis. In this study, we show that the epistatic interaction of Sle1 (in particular, Sle1/Sle1) with FAS(lpr) leads to massive lymphosplenomegaly (with elevated numbers of activated CD4 T cells, CD4(-)CD8(-) double negative (DN) T cells, and B1a cells), high levels of IgG and IgM antinuclear (including anti-ssDNA, anti-dsDNA, and anti-histone/DNA), and antiglomerular autoantibodies, histological, and clinical evidence of glomerulonephritis, and >80% mortality by 5-6 mo of age. Whereas FAS(lpr) functions as a recessive gene, Sle1 exhibits a gene dosage effect. These studies indicate that Sle1 and FAS(lpr) must be impacting alternate pathways leading to lymphoproliferative autoimmunity.

Genetic linkage and association of Fcgamma receptor IIIA (CD16A) on chromosome 1q23 with human systemic lupus erythematosus

OBJECTIVE

Although low-affinity alleles of human Fcgamma receptor types IIA and IIIA (FcgammaRIIA and FcgammaRIIIA, respectively) polymorphisms have been associated with systemic lupus erythematosus (SLE) in case-control studies, the relative contribution of these genes to SLE susceptibility has not been resolved.

METHODS

We analyzed the distribution of alleles of FcgammaRIIA, FcgammaRIIIA, and FcgammaRIIIB in 126 multiplex-SLE pedigrees and FcgammaRIIA and FcgammaRIIIA in a case-control replication study, using allele-specific polymerase chain reaction and direct sequencing of genomic DNA. Statistical tests of association were performed to detect evidence of linkage between the single nucleotide polymorphisms and SLE.

RESULTS

We found evidence for linkage at both the FcgammaRIIIA (single-point nonparametric linkage [NPL] 1.8, P = 0.038; multipoint NPL 2.7, P = 0.004) and the FcgammaRIIA (single-point NPL 2.0, P = 0.021; multipoint NPL 2.6, P = 0.006) loci, but not the FcgammaRIIIB locus. Family-based tests of association demonstrated increased transmission of the low-affinity F176 allele at the FcgammaRIIIA locus (odds ratio [OR] 2.18, P = 0.0005 by transmission disequilibrium test and P = 0.002, by pedigree disequilibrium test [PDT]), but little evidence of preferential transmission of alleles at FcgammaRIIA (P = 0.089 by PDT). Stratification by ethnicity showed preferential transmission of the associated FcgammaRIIIA allele both in families of African American ancestry and in those of European American ancestry. Despite significant linkage disequilibrium between these genes, 2- and 3-locus haplotype analysis of the extended Fcgamma receptor cluster did not reveal any significant association beyond that observed with FcgammaRIIIA alone. In a large case-control replication study of 438 patients with SLE and 219 controls, FcgammaRIIIA provided the strongest evidence of an FcgammaR-SLE association (additive model: V/V 176 versus V/F 176 OR 1.51, V/V 176 versus F/F 176 OR 1.98, P = 0.007).

CONCLUSION

To our knowledge, these data are the first to demonstrate linkage and both family-based and case-control-based association of FcgammaRIIIA with SLE. These data provide genetic evidence supporting a role for the physiologically relevant single nucleotide polymorphism of the FcgammaRIIIA gene in the pathophysiology of this complex genetic disease.

An update on genetic studies of systemic lupus erythematosus

Systemic lupus erythematosus (SLE) is a complex, multifactorial autoimmune disease. Genetic factors are thought to contribute to its pathogenesis. There have been numerous recent advances in the study of murine and human lupus genetics. In well-defined experimental transgenic or gene-knockout mouse models, the development of lupus-like disease has implicated specific genes and pathways in the disease pathogenesis. Linkage analyses have mapped multiple susceptibility loci and disease suppressive loci using inbred strains of mice that spontaneously develop lupus-like disease. Elegant genetic dissection and function studies have led to the recent identification of two murine candidate susceptibility genes, Ifi202 (encoding an interferon-inducible protein) and Cr2 (encoding complement receptors 1 and 2). In human lupus, case- control studies have established associations of SLE with certain major histocompatibility class II alleles, complement deficiencies, and polymorphisms of Fc gamma receptor genes, a complement-related gene, and cytokine genes. During the past several years, linkage analyses using SLE multiplex families have provided many chromosomal regions for further exploration of susceptibility genes. Six regions exhibiting significant linkage to SLE are promising. Studies are underway to fine map these linked regions and to identify the genes in the susceptibility regions. An understanding of the genes involved in the development of lupus should provide targets for more focused therapy in lupus.

T-cell signalling and autoimmunity: molecular mechanisms of disease

The genetic manipulation of mice has led to insights into the molecular mechanisms of autoimmune disease. Recent studies have begun to identify ways in which signalling cascades can be disrupted that preclude the development of autoimmunity. This review outlines a new model for the induction of T-cell-mediated autoimmune diseases. I highlight recent data that illustrate the ways in which the altered survival of T cells and defects in the inhibitory signalling pathways of T cells can contribute to autoimmunity.

Genetic modifiers of systemic lupus erythematosus in FcgammaRIIB(-/-) mice

FcgammaRIIB is a potent lupus susceptibility gene as demonstrated by the observation that mice deficient in this molecule develop spontaneous antinuclear antibodies (ANA) and fatal glomerulonephritis when on the C57BL/6 background. To determine the mechanisms underlying the epistasis displayed by this gene we have constructed hybrids between FcgammaRIIB(-/-) and the systemic lupus erythematosus (SLE) modifiers yaa and lpr and the susceptibility locus Sle1. Sle1 and B6.RIIB(-/-) are both physically and functionally coupled; compound heterozygotes of Sle1 and B6.RIIB(-/-) develop significant disease, while single heterozygotes display no evidence of autoimmunity or disease, indicating that these genes lie on the same genetic pathway resulting in the loss of tolerance to nuclear antigens. However, the generation of ANA in itself is insufficient to account for the severity of autoimmune disease in this model, as demonstrated by analysis of yaa and lpr hybrids. Thus, B6.RIIB(-/-)/lpr mice are protected from disease progression, despite equivalent titers of ANA. In contrast, B6.RIIB(-/-)/yaa mice have significantly enhanced disease despite reduced ANA titers. Yaa modifies the specificity and thus the pathogenicity of the B6. RIIB(-/-) ANA, by converting them to antinucleolar antibodies. In addition to these known modifier pathways, we have discovered two novel, recessive loci contributed by the C57BL/6 genome that are required for the ANA phenotype, further indicating the epistatic properties of this SLE model.

Susceptibility genes in the pathogenesis of murine lupus

Systemic lupus erythematosus (SLE) is the paradigm of a multisystem autoimmune disease in which genetic factors strongly influence susceptibility. Through genome scans and congenic dissection, numerous loci associated with lupus susceptibility have been defined and the complexity of the inheritance of this disease has been revealed. In this review, we provide a brief description of animal models of SLE, both spontaneous models and synthetic models, with an emphasis on the B6 congenic model derived from analyses of the NZM2410 strain. A hypothetical model of disease progression that organizes many of the identified SLE susceptibility loci in three distinct biological pathways that interact to mediate disease pathogenesis is also described. We finally discuss our recent fine mapping analysis, which revealed a cluster of loci that actually comprise the Sle1 locus.

Complement and systemic lupus erythematosus

Complement is implicated in the pathogenesis of systemic lupus erythematosus (SLE) in several ways and may act as both friend and foe. Homozygous deficiency of any of the proteins of the classical pathway is causally associated with susceptibility to the development of SLE, especially deficiency of the earliest proteins of the activation pathway. However, complement is also implicated in the effector inflammatory phase of the autoimmune response that characterizes the disease. Complement proteins are deposited in inflamed tissues and, in experimental models, inhibition of C5 ameliorates disease in a murine model. As a further twist to the associations between the complement system and SLE, autoantibodies to some complement proteins, especially to C1q, develop as part of the autoantibody response. The presence of anti-C1q autoantibodies is associated with severe illness, including glomerulonephritis. In this chapter the role of the complement system in SLE is reviewed and hypotheses are advanced to explain the complex relationships between complement and lupus.

Induction of dendritic cell differentiation by IFN-alpha in systemic lupus erythematosus

Dendritic cells (DCs) are important in regulating both immunity and tolerance. Hence, we hypothesized that systemic lupus erythematosus (SLE), an autoimmune disease characterized by autoreactive B and T cells, may be caused by alterations in the functions of DCs. Consistent with this, monocytes from SLE patients' blood were found to function as antigen-presenting cells, in vitro. Furthermore, serum from SLE patients induced normal monocytes to differentiate into DCs. These DCs could capture antigens from dying cells and present them to CD4-positive T cells. The capacity of SLE patients' serum to induce DC differentiation correlated with disease activity and depended on the actions of interferon-alpha (IFN-alpha). Thus, unabated induction of DCs by IFN-alpha may drive the autoimmune response in SLE.