Nabila Ibnou-Zekri

The Yaa Mutation Promoting Murine Lupus Causes Defective Development of Marginal Zone B Cells

The accelerated development of systemic lupus erythematosus (SLE) in BXSB male mice is associated with the presence of an as yet unidentified mutant gene, Yaa (Y-linked autoimmune acceleration). In view of a possible role of marginal zone (MZ) B cells in murine SLE, we have explored whether the expression of the Yaa mutation affects the differentiation of MZ and follicular B cells, thereby implicating the acceleration of the disease. In this study, we show that both BXSB and C57BL/6 Yaa mice, including two different substrains of BXSB Yaa males that are protected from SLE, displayed an impaired development of MZ B cells early in life. Studies in bone marrow chimeras revealed that the loss of MZ B cells resulted from a defect intrinsic to B cells expressing the Yaa mutation. The lack of selective expansion of MZ B cells in diseased BXSB Yaa males strongly argues against a major role of MZ B cells in the generation of pathogenic autoantibodies in the BXSB model of SLE. Furthermore, a comparative analysis with mice deficient in CD22 or expressing an IgM anti-trinitrophenyl/DNA transgene suggests that the hyperreactive phenotype of Yaa B cells, as judged by a markedly increased spontaneous IgM secretion, is likely to contribute to the enhanced maturation toward follicular B cells and the block in the MZ B cell generation.

The Yaa Gene Model of Systemic Lupus Erythematosus

Discrimination between self and foreign structures operates through an active process that involves several different mechanisms including clonal deletion, clonal anergy and suppression. The failure of these regulatory mechanisms leads to the persistence and activation of potentially self-reactive cells and the development of autoimmune disorders. Such a situation occurs in the case of systemic lupus erythematosus (SLE). a severe chronic autoimmune disease in which antibodies are produced against self components and the resulting immune complexes are involved in the generation of various tissue lesions. Notably, glomerulonephritis, i.e. lupus nephritis, is the major cause of death in patients with SLE. Although the etiology and pathogenesis of SLE are still poorly understood, it is becoming clear that many genetic factors apparently play essential roles in SLE. The discovery of several murine strains that spontaneously develop an autoimmune syndrome resembling human SLE (Theofilopoulus & Dixon 1985) has allowed considerable progress in this field. In particular, these strains have been powerful tools to study the mechanisms by which genetic factors participate in the pathogenesis of SLE. Early genetic studies on New Zealand mice have demonstrated that many individual autoimmune traits segregate independently of each other (Shirai 1982, Izui et al. 1981, Raveche et al. 1981, Bocchieri et al. 1982). This suggests that there is no common genetic defect which causes a predisposition to overall autoimmune responses in the New Zealand strain. However, the clearest examples of the influence of single genes on the development and progression of murine SLE is the efTect of the lpr (lymphoproliferation) and gld (generalized lymphoproliferative disease) genes (Murphy & Roths 1978, Roths et al. 1984). Both mutations not only accelerate the progression of autoimmune

Lessons from BXSB and Related Mouse Models

The BXSB murine strain spontaneously develops an autoimmune syndrome with features of systemic lupus erythematosus (SLE) that affects males much earlier than females, due to the presence of an as yet unidentified mutant gene located on its Y chromosome, designated Yaa (Y-linked autoimmune acceleration). The Yaa gene by itself is unable to induce significant autoimmune responses in mice without an apparent SLE background, while it can induce and accelerate the development of an SLE in combination with autosomal susceptibility alleles present in lupus-prone mice. Although the genes encoded within or closely linked to the MHC locus play an important role in the development or protection of SLE, the MHC effect can be completely masked by the presence of the Yaa gene in mice highly predisposed to SLE. The role of the Yaa gene for the acceleration of SLE is apparently two-fold; it enhances overall autoimmune responses against autoantigens to which mice respond relatively weakly, and promotes Th1 responses against autoantigens to which mice respond relatively well, leading to the production of more pathogenic autoantibodies, i.e., FcγR-fixing IgG2a and cryoglobulin IgG3 autoantibodies. Yaa+-Yaa− double bone marrow chimera experiments revealed that the Yaa defect is expressed in B cells, but not in T cells, and that T cells from non-autoimmune mice are capable of providing help for autoimmune responses by collaborating Yaa+ B cells. We speculate that the Yaa defect may decrease the threshold for antigen receptor-dependent stimulation, leading to the triggering and excessive stimulation of autoreactive T and B cells.

Differential role of three major New Zealand Black-derived loci linked with Yaa-induced murine lupus nephritis

By assessing the development of Y-linked autoimmune acceleration (Yaa) gene-induced systemic lupus erythematosus in C57BL/6 (B6) × (New Zealand Black (NZB) × B6.Yaa)F1 backcross male mice, we mapped three major susceptibility loci derived from the NZB strain. These three quantitative trait loci (QTL) on NZB chromosomes 1, 7, and 13 differentially regulated three different autoimmune traits: anti-nuclear autoantibody production, gp70-anti-gp70 immune complex (gp70 IC) formation, and glomerulonephritis. Contributions to the disease traits were further confirmed by generating and analyzing three different B6.Yaa congenic mice, each carrying one individual NZB QTL. The chromosome 1 locus that overlapped with the previously identified Nba2 (NZB autoimmunity 2) locus regulated all three traits. A newly identified chromosome 7 locus, designated Nba5, selectively promoted anti-gp70 autoantibody production, hence the formation of gp70 IC and glomerulonephritis. B6.Yaa mice bearing the NZB chromosome 13 locus displayed increased serum gp70 production, but not gp70 IC formation and glomerulonephritis. This locus, called Sgp3 (serum gp70 production 3), selectively regulated the production of serum gp70, thereby contributing to the formation of nephritogenic gp70 IC and glomerulonephritis, in combination with Nba2 and Nba5 in NZB mice. Among these three loci, a major role of Nba2 was demonstrated, because B6.Yaa Nba2 congenic male mice developed the most severe disease. Finally, our analysis revealed the presence in B6 mice of an H2-linked QTL, which regulated autoantibody production. This locus had no apparent individual effect, but most likely modulated disease severity through interaction with NZB-derived susceptibility loci.

Protection of murine lupus by the Ead transgene is MHC haplotype-dependent.

A high-level expression of a transgene, Ead, encoding the I-Ed α-chain is very effective in protection against murine lupus. To investigate the specific contribution of select H-2 haplotypes on the Ead transgene-mediated disease-suppressing effect, we generated H-2 congenic (NZB × BXSB)F1 hybrid mice bearing either H-2b/b, H-2d/b, or H-2d/d haplotype, and compared the transgene-mediated protective effect on the clinical development (autoantibody production and glomerulonephritis) of lupus in these F1 hybrids. The level of protection was most remarkable in mice bearing the I-E− H-2b/b haplotype but was only minimal in I-E+ H-2d/d F1 hybrids. Additional analysis demonstrated a marked suppression of lupus in I-E+ H-2k/k (MRL × BXSB)F1 hybrid mice, indicating that the transgene is able to suppress autoimmune responses even in mice already expressing I-E molecules at a homozygous level. Our results indicate that the level of the transgene-mediated protection is dependent on the host H-2 haplotype. This suggests that the autoimmune suppressive activity of the Ead transgene is likely to be determined through the interaction of the transgene product with the host MHC class II molecules, providing new insight into the role of MHC in lupus-like autoimmunity.

MHC-linked Control of Murine SLE

Systemic lupus erythematosus (SLE) is a disorder of generalized autoimmunity characterized by the formation of a variety of autoantibodies and subsequent development of severe glomerulonephritis [1]. It is now well established that SLE is under some form of polygenic control, in which several genetic factors independently contribute to the overall susceptibility and progression of the disease. Studies suggest that heterogeneous combinations of multiple diseaseassociated genes operate in a threshold manner to generate the disease [2–4]. Among such genetic factors, the importance of the genes encoded within or closely linked to the MHC locus has been shown. However, it has not yet been determined whether the development of lupus-like disease is predominantly mediated by MHC class II genes and which other genes within MHC may contribute to this effect.

Epitope-Dependent Inhibition of T Cell Activation by the Ea Transgene: An Explanation for Transgene-Mediated Protection from Murine Lupus

A high level expression of the Ead transgene encoding the I-E α-chain is highly effective in the suppression of lupus autoantibody production in mice. To explore the possible modulation of the Ag-presenting capacity of B cells as a result of the transgene expression, we assessed the ability of the transgenic B cells to activate Ag-specific T cells in vitro. By using four different model Ag-MHC class II combinations, this analysis revealed that a high transgene expression in B cells markedly inhibits the activation of T cells in an epitope-dependent manner, without modulation of the I-E expression. The transgene-mediated suppression of T cell responses is likely to be related to the relative affinity of peptides derived from transgenic I-E α-chains (Eα peptides) vs antigenic peptides to individual class II molecules. Our results support a model of autoimmunity prevention based on competition for Ag presentation, in which the generation of large amounts of Eα peptides with high affinity to I-A molecules decreases the use of I-A for presentation of pathogenic self-peptides by B cells, thereby preventing excessive activation of autoreactive T and B cells.

Enforced Bcl‐2 expression in B lymphocytes induces rheumatoid factor and anti‐DNA production, but the Yaa mutation promotes only anti‐DNA production

The presence of rheumatoid factors (RF) is a characteristic feature of patients with rheumatoid arthritis, but not systemic lupus erythematosus. In this study, we have explored the role of theanti‐apoptotic Bcl‐2 protein and the Y‐linked autoimmune acceleration (Yaa) mutation in the production of IgG RF in comparison with IgG anti‐DNA autoimmune responses. Analysis in C57BL/6 mice, in their F1 hybrids with lupus‐prone NZW mice, and in bone marrow chimeras containing mixtures of C57BL/6 bcl‐2‐transgenic and BXSB non‐transgenic cells demonstrated that an enforced Bcl‐2 expression in B cells promoted the induction of IgG anti‐DNA production in these mice, while significant IgG RF responses were observed only in mice developing high levels of gp70‐anti‐gp70 immune complexes and lethal glomerulonephritis. Moreover, in contrast to a synergistic interaction between the Yaa mutation and Bcl‐2 overexpression on IgG anti‐DNA production, the Yaa mutation failed to enhance the production of IgG RF induced in bcl‐2‐transgenic mice. Our results reveal that defects in the regulation of B cell apoptosis play a critical role in the production of IgG RF, and that the Yaa mutation differentially modulates RF and anti‐DNA autoimmune responses, likely related to the nature of autoantigens involved in each autoimmune response.

Protection of murine systemic lupus erythematosus by an I-E α-chain transgene

SYSTEMIC lupus erythematosus (SLE) is a disorder of generalized autoimmunity characterized by the formation of a variety of autoantibodies and the development of lethal glomerulonephritis. It is now well established that SLE is under some form of polygenic control in which multiple genetic factors independently contribute to the overall susceptibility of individuals to the disease. The gene(s) encoded within the major histocompatibility complex (MHC) acts as one of the major genetic elements contributing to the susceptibility of murine SLE. It has been demonstrated that the lupus susceptibility is more closely linked to the I-E H-2 haplotype than to the I-E H-2 and H-2 haplotypes in lupus-prone BXSB and (NZB 3 BXSB)F1 hybrid mice. 1,2