Role of the major histocompatibility complex class II Ea gene in lupus susceptibility in mice

B cell MHC haplotype affects follicular inclusion, germinal center participation and plasma cell differentiation in a mouse model of lupus

Introduction

MHC class II molecules are essential for appropriate immune responses against pathogens but are also implicated in pathological responses in autoimmune diseases and transplant rejection. Previous studies have shed light on the systemic contributions of MHC haplotypes to the development and severity of autoimmune diseases. In this study, we addressed the B cell intrinsic MHC haplotype impact on follicular inclusion, germinal center (GC) participation and plasma cell (PC) differentiation in the context of systemic lupus erythematosus (SLE).

Methods

We leveraged the 564Igi mouse model which harbors a B cell receptor knock-in from an autoreactive B cell clone recognizing ribonuclear components, including double-stranded DNA (dsDNA). This model recapitulates the central hallmarks of the early stages of SLE. We compared 564Igi heterozygous offspring on either H2b/b, H2b/d, or H2d/d background.

Results

This revealed significantly higher germinal center (GC) B cell levels in the spleens of H2b/b and H2b/d as compared to H2d/d (p<0.0001) mice. In agreement with this, anti-dsDNA-antibody levels were higher in H2b/b and H2b/d than in H2d/d (p<0.0001), with H2b/b also being higher compared to H2b/d (p<0.01). Specifically, these differences held true both for autoantibodies derived from the knock-in clone and from wild-type (WT) derived clones. In mixed chimeras where 564Igi H2b/b, H2b/d and H2d/d cells competed head-to-head in the same environment, we observed a significantly higher inclusion of H2b/b cells in GC and PC compartments relative to their representation in the B cell repertoire, compared to H2b/d and H2d/d cells. Furthermore, in mixed chimeras in which WT H2b/b and WT H2d/d cells competed for inclusion in GCs associated with an epitope spreading process, H2b/b cells participated to a greater extent and contributed more robustly to the PC compartment. Finally, immature WT H2b/b cells had a higher baseline of BCRs with an autoreactive idiotype and were subject to more stringent negative selection at the transitional stage.

Discussion

Taken together, our findings demonstrate that B cell intrinsic MHC haplotype governs their capacity for participation in the autoreactive response at multiple levels: follicular inclusion, GC participation, and PC output. These findings pinpoint B cells as central contributors to precipitation of autoimmunity.

Dual signaling by innate and adaptive immune receptors is required for TLR7-induced B-cell-mediated autoimmunity

Toll-like receptor 7 (Tlr7) has been linked to systemic lupus disease incidence in humans and mice, but how TLR7 potentiates autoimmunity is unclear. We used a Tlr7 transgenic (tg) mouse model to investigate the cellular and molecular events required to induce spontaneous autoimmunity through increased TLR7 activity. We determined that Tlr7 exerts B-cell-intrinsic effects in promoting spontaneous germinal center (GC) and plasmablast B-cell development, and that these B-cell subsets are dependent on T-cell-derived signals through CD40L and SLAM-associated protein (SAP), but not IL-17. Antigen specificity also factored into TLR7-induced disease, as both a restricted T cell receptor (TCR) specificity and MHC haplotype H2(k/k) protected Tlr7tg mice from spontaneous lymphocyte activation and autoantibody production. Inflammatory myeloid cell expansion and autoimmunity did not develop in Tlr7tgIgH(-/-) mice, suggesting either that spontaneous TLR7 activation does not occur in dendritic cells, or, if it does occur, cannot drive these events in the absence of B-cell aid. These data indicate that autoimmune disease in Tlr7tg mice is contingent upon B cells receiving stimulation both through innate pathways and T-cell-derived signals and suggest a codependent relationship between B cells and T cells in the development of autoimmunity.

The MHC haplotype H2b converts two pure nonlupus mouse strains to producers of antinuclear antibodies

Studies of mouse lupus models have linked the MHC H2(b) haplotype with the earlier appearance of antinuclear autoantibodies and the worsening of nephritis. However, it is unknown whether H2(b) by itself, in the context of pure nonlupus strains, is "silent" or sufficient with regard to loss of tolerance to chromatin (nucleosomes). In this study we show that, beginning approximately 6-9 mo of age, H2(b)-congenic BALB/c (denoted BALB.B) mice, unlike BALB/c (H2(d)) and H2(k)-congenic BALB/c (denoted BALB.K) mice, develop strikingly increased serum levels of anti-chromatin Ab dominated by the IgG2a subclass, along with minor increase of Abs to DNA and moderately increased total serum IgG2a. The BALB.B mice did not have glomerulonephritis or an increased mortality rate. H2(b)-congenic C3H/He mice (designated C3.SW mice), unlike C3H/He (H2(k)) mice, showed low but measurable serum levels of chromatin-reactive IgG2a Abs and minor but significant hypergammaglobulinemia. By immunofluorescence, IgG2a of sera from both H2(b)-congenic strains stained HEp-2 cell nuclei, confirming the presence of antinuclear autoantibodies. Thus, in the context of two pure nonlupus genomes, the MHC H2(b) haplotype in homozygous form is sufficient to induce loss of tolerance to chromatin.

Role of MHC-linked genes in autoantigen selection and renal disease in a murine model of systemic lupus erythematosus

We previously described a renal protective effect of factor B deficiency in MRL/lpr mice. Factor B is in the MHC cluster; thus, the deficient mice were H2b, the haplotype on which the knockout was derived, whereas the wild-type littermates were H2k, the H2 of MRL/lpr mice. To determine which protective effects were due to H2 vs factor B deficiency, we derived H2b congenic MRL/lpr mice from the 129/Sv (H2b) strain. Autoantibody profiling using autoantigen microarrays revealed that serum anti-Smith and anti-small nuclear ribonucleoprotein complex autoantibodies, while present in the majority of H2k/k MRL/lpr mice, were absent in the H2b/b MRL/lpr mice. Surprisingly, 70% of MRL/lpr H2b/b mice were found to be serum IgG3 deficient (with few to no IgG3-producing B cells). In addition, H2b/b IgG3-deficient MRL/lpr mice had significantly less proteinuria, decreased glomerular immune complex deposition, and absence of glomerular subepithelial deposits compared with MRL/lpr mice of any H2 type with detectable serum IgG3. Despite these differences, total histopathologic renal scores and survival were similar among the groups. These results indicate that genes encoded within or closely linked to the MHC region regulate autoantigen selection and isotype switching to IgG3 but have minimal effect on end-organ damage or survival in MRL/lpr mice.

Identification of 2 major loci linked to autoimmune hemolytic anemia in NZB mice

Using a cohort of C57BL/6 (B6) x (NZB x B6)F1 backcross male mice bearing the Yaa (Y-linked autoimmune acceleration) mutation, we mapped and characterized the NZB-derived susceptibility loci predisposing to the development of autoimmune hemolytic anemia (AHA). Our analysis identified 2 major loci on NZB chromosome 7 and chromosome 1 linked with Coombs antierythrocyte autoantibody production, and their contributions were confirmed by the analysis of B6.Yaa mice (B6 mice bearing the Yaa mutation) congenic for each NZB-derived susceptibility interval. A newly identified Aia3 (autoimmune anemia 3) locus present on NZB chromosome 7 selectively regulated Coombs antibody responses, while the second locus, directly overlapping with Nba2 (NZB autoimmunity 2) on chromosome 1, promoted the development of AHA, likely as part of its effect on overall production of lupus autoantibodies. A higher incidence of Coombs antibody production in B6.Aia3 congenic mice (B6 mice bearing the NZB-Aia3 locus) than B6.Nba2 mice (B6 mice bearing the NZB-Nba2 locus) indicated a major role for Aia3 in AHA. Notably, lack of expansion of B1 cells in B6.Aia3 congenic mice argued against the involvement of this subset in AHA. Finally, our analysis of BC mice also demonstrated the presence of a B6-derived H2-linked locus on chromosome 17 that apparently regulated the production of Coombs antibodies as a result of its overall autoimmune promoting effect.

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) x (New Zealand Black (NZB) x B6.Yaa)F(1) 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.

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 Ea(d) transgene encoding the I-E alpha-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 alpha-chains (Ealpha 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 Ealpha 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.

A Critical Role for FcγRIIB in the Induction of Rheumatoid Factors

Rheumatoid factors (RF) are autoantibodies with specificity for the Fc portion of IgG, and IgG-containing immune complexes are likely to be the major source of RF autoantigens. Therefore, the activation of RF-producing B cells could be controlled specifically through recognition of IgG immune complexes by the low-affinity IgG FcR, FcgammaRIIB, a potent negative regulator of the BCR. To test this possibility, we determined the development of RF in C57BL/6 (B6) mice lacking FcgammaRIIB, in relation to the H2 haplotype, complement C3, and the Y-linked autoimmune acceleration (Yaa) mutation. FcgammaRIIB-null B6 mice displayed substantial anti-IgG2a RF activities in their sera, in addition to anti-DNA autoantibodies. Their RF and anti-DNA responses were linked to the H2(b) haplotype, but were suppressed almost completely by the H2(d) haplotype. Strikingly, the absence of C3 failed to modulate RF production, but strongly inhibited anti-DNA production. Furthermore, we observed that partial FcgammaRIIB deficiency (i.e., heterozygous level of FcgammaRIIB expression) was sufficient to induce the production of RF and anti-DNA autoantibodies in the presence of the Yaa mutation. In contrast to FcgammaRIIB, the deficiency in another BCR negative regulator, CD22, was unable to promote RF and anti-DNA autoimmune responses in B6 mice. Our results indicate that RF autoimmune responses are critically controlled by FcgammaRIIB, together with the H2(b) and Yaa gene, while C3 regulates positively and specifically anti-DNA, but not RF autoimmune responses.

Transgenic overexpression of GATA-3 in T lymphocytes improves autoimmune glomerulonephritis in mice with a BXSB/MpJ-Yaa genetic background

A T helper 1 (Th1)/Th2 imbalance is thought to contribute to the pathogenesis of autoimmune diseases. The differentiation of T cells into Th1 or Th2 subtypes is under the regulation of several transcription factors. Among these, transcription factor GATA-3 is thought to play an indispensable role in the development of T cells and the differentiation of Th2 cells. To examine how a Th1/Th2 imbalance affects the development of autoimmune disease, GATA-3 was overexpressed in the T lymphocytes of C57BL/6 x BXSB/MpJ-Yaa F(1) (Yaa) mice. Yaa mice developed autoimmune nephritis similarly to BXSB/MpJ-Yaa mice, which are commonly used as a model for Th1-dominant murine lupus. GATA-3 overexpression in T cells improved the 50% mortality incidence time for GATA-3-transgenic Yaa mice (41.6 wk), compared with Yaa mice (30.9 wk), and reduced proteinuria, serum creatinine levels, and the severity of glomerulonephritis in GATA-3-transgenic Yaa mice. GATA-3 overexpression in Yaa mice led to simultaneously elevated Th2 Ig (IgG1) and decreased Th1 Ig (IgG2a and IgG3) production and serum IFN-gamma levels. Although IL-4 production remained unchanged, intracellular cytokine analyses demonstrated that IL-5 was induced and IFN-gamma was suppressed in stimulated T cells from the GATA-3-transgenic Yaa mice. These results indicated that abundant GATA-3 was unable to stimulate complete differentiation of Th2 cells but did counteract the dominance of Th1 cells and alleviated the disease severity in Yaa mice. These data suggest that transcriptional regulation therapy may have potential as an effective strategy for treating autoimmune glomerulonephritis.

Transgenic models of tolerance and autoimmunity: with special reference to systemic lupus erythematosus

Transgenic and knockout mouse carrying rearranged antigen-receptor genes have been invaluable for the elucidation of basic mechanisms in autoimmunity and have contributed new models of human autoimmune diseases. Several examples of transgenic models expressing rearranged immunoglobulin genes have been described. These models have provided a window into the events involved in this process, allowing the development and fate of self-reactive lymphocytes to be followed in vivo. In the B cell lineage, as in T cells, self-reactive cells have been found to undergo several distinct fates in vivo: they can be physically eliminated, functionally inactivated, or they can persist unchanged or become activated. Nevertheless the precise understanding of the molecular events leading to lymphocyte deletion, anergy or activation remains a challenge.

A genetic analysis of lupus

Systemic lupus erythematosus (SLE) is a complex multigenic inherited disease with susceptibility determined by a combination of genetic, environmental and stochastic factors. Although not yet defined, recent technical advances have provided the means to dissect the component genetic contributions of polygenic traits. We have applied such approaches to mouse models of spontaneous SLE and, in this report, summarize our genome wide mapping studies that identified loci predisposing to several major lupus-related traits. Through the generation and study of interval congenic lines, precise mapping, and screening of candidate genes, identification of the specific genes and mechanisms associated with some of the major loci is currently being pursued.

Genetics of systemic autoimmunity in mouse models of lupus

Systemic lupus erythematosus (SLE) is inherited as a complex polygenic trait, involving genetic, environmental and stochastic factors. Although definition of these etiologic processes has been elusive, solid progress has been made toward elucidating the genetic basis for susceptibility. Herein, we summarize our genome wide mapping effort that has defined loci for component phenotypes for lupus-prone NZB, NZW, MRL-Fas(lpr) and BXSB strains. With this framework in place, identification of the specific genetic alterations and mechanisms is now proceeding through the generation of interval congenic lines, precise mapping and screening of candidate genes. In addition to this approach, transgenic and gene knockout studies have begun to identify genes that can induce or modify autoimmunity in nonautoimmune and lupus-prone background mice, including studies by us and others on Th1 and Th2 cytokine genes in lupus. It is apparent that a diversity of genes and mechanisms can independently or in combination promote systemic autoimmunity in mice. This complexity, which is also observed in human lupus, emphasizes the importance of using experimental and less complex mouse models to define these processes, a tactic that has already yielded new insights. With current technologies and the anticipated definition of mammalian genomes, identification of genes predisposing to lupus and elucidation of processes critical for disease pathogenesis appear within grasp.

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 Th 1 responses against autoantigens to which mice respond relatively well, leading to the production of more pathogenic autoantibodies, i.e., FcgammaR-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.

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

A high-level expression of a transgene, Ead, encoding the I-Ed alpha-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 x 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 x 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.

Effector and predisposing genes in murine lupus

This Publication is No. 11789-IMM from the Department of Immunology, The Scripps Research Institute, 10550 North Torrey Pines Road/IMM3, La Jolla, CA 92037. The work of the author reported herein was supported, in part, by NIH grants AR39555, AR31203, and AG15061.

Linkage of a major quantitative trait locus to Yaa gene-induced lupus-like nephritis in (NZW x C57BL/6)F1 mice

In the present study, we mapped the major quantitative trait loci (QTL) differing between the NZW and C57BL/6 inbred strains of mice by making use of (NZW x C57BL/6.Yaa)F1 mice, a model in which the lupus-like autoimmune syndrome observed in male mice is associated with the presence of an as yet unidentified Y chromosome-linked autoimmune acceleration gene, Yaa. Linkage analysis of 126 C57BL/6 x (NZW x C57BL/6.Yaa)F1 backcross males provided evidence for a major QTL on chromosome 7 controlling both the severity of glomerulonephritis and the production of IgG anti-DNA autoantibody and retroviral gp70-anti-gp70 immune complexes. Two additional QTL of C57BL/6 origin on chromosome 17 had no apparent individual effects, but showed strong epistatic interaction with chromosome 7 QTL for disease severity and anti-DNA autoantibody production. Our data also identified on chromosome 13 a QTL of NZW origin with a major effect on the level of gp70, and showing an additive effect with the chromosome 7 QTL on the level of gp70 immune complexes. Our study thus provides a model to dissect the complex genetic interactions that result in manifestations of murine lupus-like disease.