The Yaa gene-mediated acceleration of murine lupus: Yaa- T cells from non-autoimmune mice collaborate with Yaa+ B cells to produce lupus autoantibodies in vivo

Cardiac phenotype in mouse models of systemic autoimmunity

Patients suffering from systemic autoimmune diseases are at significant risk of cardiovascular complications. This can be due to systemically increased levels of inflammation leading to accelerated atherosclerosis, or due to direct damage to the tissues and cells of the heart. Cardiac complications include an increased risk of myocardial infarction, myocarditis and dilated cardiomyopathy, valve disease, endothelial dysfunction, excessive fibrosis, and bona fide autoimmune-mediated tissue damage by autoantibodies or auto-reactive cells. There is, however, still a considerable need to better understand how to diagnose and treat cardiac complications in autoimmune patients. A range of inducible and spontaneous mouse models of systemic autoimmune diseases is available for mechanistic and therapeutic studies. For this Review, we systematically collated information on the cardiac phenotype in the most common inducible, spontaneous and engineered mouse models of systemic lupus erythematosus, rheumatoid arthritis and systemic sclerosis. We also highlight selected lesser-known models of interest to provide researchers with a decision framework to choose the most suitable model for their study of heart involvement in systemic autoimmunity.

TLR7 influences germinal center selection in murine SLE

TLR7 enhances germinal center maturation and migration of B cells to the dark zone where proliferation and somatic hypermutation occur. Our goal was to determine how Tlr7 dose influences selection of the autoreactive B cell repertoire in NZW/BXSB. Yaa mice bearing the site-directed heavy chain transgene 3H9 that encodes for the TLR7 regulated anti-CL response. To create a physiologic setting in which autoreactive B cells compete for survival with non-autoreactive B cells, we generated bone marrow chimeras in which disease onset occurred with similar kinetics and the transferred 3H9+ female non-Yaa, male Yaa or male TLR7^-/Yaa cells could be easily identified by positivity for GFP. Deletion of 3H9 B cells occurred in the bone marrow and the remaining 3H9 follicular B cells manifested a decrease in surface IgM. Although there were differences in the naïve repertoire between the chimeras it was not possible to distinguish a clear pattern of selection against lupus related autoreactivity in TLR7^-/Yaa or female chimeras. By contrast, preferential expansion of 3H9+ B cells occurred in the germinal centers of male Yaa chimeras. In addition, although all chimeras preferentially selected 3H9/Vκ5 encoded B cells into the germinal center and plasma cell compartments, 3H9 male Yaa chimeras had a more diverse repertoire and positively selected the 3H9/Vκ5-48/Jκ4 pair that confers high affinity anti-cardiolipin activity. We were unable to demonstrate a consistent effect of Tlr7 dose or Yaa on somatic mutations. Our data show that TLR7 excess influences the selection, expansion and diversification of B cells in the germinal center, independent of other genes in the Yaa locus.

Expansion of an atypical NK cell subset in mouse models of systemic lupus erythematosus

Chronic inflammatory conditions, such as in autoimmune disease, can disturb immune cell homeostasis and induce the expansion of normally rare cell populations. In our analysis of various murine models of lupus, we detect increased frequency of an uncommon subset identified as NK1.1(+)CD11c(+)CD122(+)MHC class II(+). These cells share characteristics with the NK cell lineage and with cells previously described as IFN-producing killer dendritic cells: 1) they depend on IL-15 and express E4BP4; 2) they are cytotoxic and produce type I and type II IFN upon activation; and 3) they are efficient APCs both through MHC class II expression and in cross-presentation to CD8s. These atypical NK cells are responsive to TLR stimulation and thus are most abundant in mice with high copy number of the Tlr7 gene. They are highly proliferative as assessed by in vivo BrdU incorporation. In adoptive transfer experiments they persist in high numbers for months and maintain their surface marker profile, indicating that this population is developmentally stable. Gene expression analyses on both mRNA and microRNAs show a modified cell cycle program in which various miR-15/16 family members are upregulated, presumably as a consequence of the proliferative signal mediated by the increased level of growth factors, Ras and E2F activity. Alternatively, low expression of miR-150, miR-181, and miR-744 in these cells implies a reduction in their differentiation capacity. These results suggest that cells of the NK lineage that undergo TLR stimulation might turn on a proliferative program in detriment of their full differentiation into mature NK cells.

The Yaa locus and IFN-α fine-tune germinal center B cell selection in murine systemic lupus erythematosus

Male NZW/BXSB.Yaa (W/B) mice express two copies of TLR7 and develop pathogenic autoantibodies, whereas females with only one copy of TLR7 have attenuated disease. Our goal was to analyze the regulation of the autoantibody response in male and female W/B mice bearing the autoreactive site-directed H chain transgene 3H9. Serum anti-dsDNA Abs appeared in males at 12 wk, and most had high-titer IgG anti-dsDNA and anti-cardiolipin Abs and developed >300 mg/dl proteinuria by 8 mo. Females had only low-titer IgG anti-cardiolipin Abs, and none developed proteinuria by 1 y. Males had a smaller marginal zone than females with a repertoire that was distinct from the follicular repertoire, indicating that the loss of marginal zone B cells was not due to diversion to the follicular compartment. Vk5-43 and Vk5-48, which were rare in the naive repertoire, were markedly overrepresented in the germinal center repertoire of both males and females, but the VJ junctions differed between males and females with higher-affinity autoreactive B cells being selected into the germinal centers of males. Administration of IFN-α to females induced anti-cardiolipin and anti-DNA autoantibodies and proteinuria and was associated with a male pattern of junctional diversity in Vk5-43 and Vk5-48. Our studies are consistent with the hypothesis that presence of the Yaa locus, which includes an extra copy of Tlr7, or administration of exogenous IFN-α relaxes the stringency for selection in the germinal centers resulting in increased autoreactivity of the Ag-driven B cell repertoire.

Taming lupus-a new understanding of pathogenesis is leading to clinical advances

Systemic lupus erythematosus (SLE) is an autoimmune disease that is characterized by the loss of tolerance to nuclear self antigens, the production of pathogenic autoantibodies and damage to multiple organ systems. Over the years, patients with SLE have been managed largely with empiric immunosuppressive therapies, which are associated with substantial toxicities and do not always provide adequate control of the disease. The development of targeted therapies that specifically address disease pathogenesis or progression has lagged, largely because of the complex and heterogeneous nature of the disease, as well as difficulties in designing uniform outcome measures for clinical trials. Recent advances that could improve the treatment of SLE include the identification of genetic variations that influence the risk of developing the disease, an enhanced understanding of innate and adaptive immune activation and regulation of tolerance, dissection of immune cell activation and inflammatory pathways and elucidation of mechanisms and markers of tissue damage. These discoveries, together with improvements in clinical trial design, form a platform from which to launch the development of a new generation of lupus therapies.

Cognate interaction plays a key role in the surveillance of autoreactive B cells in induced mixed bone marrow chimerism in BXSB lupus mice

The effects of bone marrow transplantation (BMT) as a treatment for and/or preventive measure against autoimmune diseases in mice were investigated extensively. The reconstitution of the hematopoietic system with a mixture of autologous and heterologous bone marrow cells was reported to suppress the development of autoimmune diseases. However, the pathological mechanism through which mixed chimerism results in the suppression of disease development is still unknown. We have previously reported that the induction of fully major histocompatibility complex (MHC)-mismatched allogeneic mixed chimerism can prevent the disease development in BXSB mice. Interestingly, serum anti-dsDNA IgM antibody (anti-DNA IgM) levels were not significantly decreased in these chimeric mice, though other symptoms of autoimmune disease were ameliorated. In this study, we showed that self-reactive anti-DNA IgM production was mainly attributable to genetically normal B cells from the donor rather than genetically deficient B cells from the host. Host-type B cells responded normally to foreign antigens and produced the appropriate antibodies. BMT from fully MHC-matched or haplo-identical donors could suppress the production of anti-DNA antibodies. Our present study suggests the existence of a surveillance system dependent on the recognition of MHC molecules on B cells.

Evidence that Yaa-induced loss of marginal zone B cells is a result of dendritic cell-mediated enhanced activation

The development of systemic lupus is accelerated by the Yaa (Y-linked autoimmune acceleration) mutation, which is the consequence of a translocation of the telomeric end containing the Tlr7 gene from the X chromosome onto the Y chromosome. However, the loss of marginal zone (MZ) B cells, one of the Yaa-linked cellular abnormalities, has previously been shown to be unrelated to the Tlr7 gene duplication, and the present study therefore aimed to investigate the mechanism responsible for MZ B-cell loss. Analyses of Yaa and non-Yaa C57BL/6 male mice expressing an MD4 anti-HEL IgM transgene or those deficient in fms-like tyrosine kinase 3 ligand (FL) revealed that the proportion of MZ B cells in these Yaa mice was comparable to that of the respective non-Yaa control mice. Notably, the activation of MZ B cells was compromised in both of these transgenic model systems, due to the absence of cognate antigens or the impaired development of dendritic cells, respectively. These results contrasted with the loss of MZ B cells in non-Yaa mice treated with FL and the lack of accumulation of MZ B cells in Yaa mice treated with a B-cell survival factor, BAFF. Taken together, our results suggest that the persistent and enhanced activation of Yaa-bearing hyperactive MZ B cells by dendritic cells is responsible for the loss of this B-cell subset in Yaa mice.

Emerging roles of TLR7 and TLR9 in murine SLE

Systemic lupus erythematosus (SLE) is an autoimmune disorder characterized by B cell hyperactivity leading to the production of various autoantibodies and subsequent development of glomerulonephritis, i.e. lupus nephritis. Among the principal targets of autoantibodies produced in murine SLE are nucleic acid-protein complexes, such as chromatin and ribonucleoproteins, and the envelope glycoprotein gp70 of endogenous retroviruses. The preferential production of these autoantibodies is apparently promoted by the presence of genetic abnormalities leading to defects in the elimination of apoptotic cells and to an enhanced expression of endogenous retroviruses. Moreover, recent studies revealed that the innate receptors TLR7 and TLR9 are critically involved in the activation of dendritic cells and autoreactive B cells through the recognition of endogenous DNA- or RNA-containing antigens and subsequent development of autoimmune responses against nuclear autoantigens. Furthermore, the regulation of autoimmune responses against endogenous retroviral gp70 by TLR7 suggested the implication of endogenous retroviruses in this autoimmune response. Clearly, further elucidation of the precise molecular role of TLR7 and TLR9 in the development of autoimmune responses will help to develop novel therapeutic strategies and targets for SLE.

Inhibitory short synthetic oligodeoxynucleotides and lupus

B cells and antigen-presenting cells express a group of intracellular Toll-like receptors (TLRs) that recognize nucleic acids and can be accessed only when apoptotic debris or immune complexes are internalized by B-cell receptors or Fc receptors. This results in rapid cell activation and release of inflammatory mediators that perpetuate the autoantibody response. TLR-7 and TLR-9 are required to generate autoantibodies to RNA and DNA, respectively. Synthetic oligodeoxynucleotides that inhibit the activity of these intracellular TLRs attenuate systemic lupus erythematosus in mouse models and may be of therapeutic benefit in human systemic lupus erythematosus.

A critical role for IL-21 receptor signaling in the pathogenesis of systemic lupus erythematosus in BXSB-Yaa mice

Interleukin 21 (IL-21) is a pleiotropic cytokine produced by CD4 T cells that affects the differentiation and function of T, B, and NK cells by binding to a receptor consisting of the common cytokine receptor gamma chain and the IL-21 receptor (IL-21R). IL-21, a product associated with IL-17-producing CD4 T cells (T(H)17) and follicular CD4 T helper cells (T(FH)), has been implicated in autoimmune disorders including the severe systemic lupus erythematosus (SLE)-like disease characteristic of BXSB-Yaa mice. To determine whether IL-21 plays a significant role in this disease, we compared IL-21R-deficient and -competent BXSB-Yaa mice for multiple parameters of SLE. The deficient mice showed none of the abnormalities characteristic of SLE in IL-21R-competent Yaa mice, including hypergammaglobulinemia, autoantibody production, reduced frequencies of marginal zone B cells and monocytosis, renal disease, and premature morbidity. IL-21 production associated with this autoimmune disease was not a product of T(H)17 cells and was not limited to conventional CXCR5(+) T(FH) but instead was produced broadly by ICOS(+) CD4(+) splenic T cells. IL-21 arising from an abnormal population of CD4 T cells is thus central to the development of this lethal disease, and, more generally, could play an important role in human SLE and related autoimmune disorders.

p21CIP1/WAF1 Controls Proliferation of Activated/Memory T Cells and Affects Homeostasis and Memory T Cell Responses

Development of autoantibodies and lupus-like autoimmunity by 129/Sv x C57BL/6 p21(-/-) mice has established that cell cycle deregulation is one the defective pathways leading to break of tolerance. Memory T cell accumulation is thought to be related to tolerance loss in murine lupus models. We studied T cell memory responses in C57BL/6 p21(-/-) mice that develop lupus-like disease manifestations. p21 did not affect primary proliferation of naive T cells, and was required for cycling control, but not for apoptosis of activated/memory T cells. When we induced apoptosis by secondary TCR challenge, surviving memory T cells depended on p21 for proliferation control. Under conditions of secondary T cell stimulation that did not cause apoptosis, p21 was also needed for regulation of activated/memory T cell expansion. The requirement for p21 in the control of T cell proliferation of activated/memory T cells suggests that in addition to apoptosis, cycling regulation by p21 constitutes a new pathway for T cell homeostasis. Concurring with this view, we found accumulation in p21(-/-) mice of memory CD4(+) T cells that showed increased proliferative potential after TCR stimulation. Furthermore, OVA immunization of p21(-/-) mice generated hyperresponsive OVA-specific T cells. Overall, the data show that p21 controls the proliferation of only activated/memory T cells, and suggest that p21 forms part of the memory T cell homeostasis mechanism, contributing to maintenance of tolerance.

Systemic lupus erythematosus: multiple immunological phenotypes in a complex genetic disease

Systemic lupus erythematosus (SLE) is a complex polygenic autoimmune disease characterized by the presence of anti-nuclear autoantibodies (ANAs) that are often detectable years prior to the onset of clinical disease. The disease is associated with a chronic activation of the immune system, with the most severe forms progressing to inflammatory damage that can impact multiple organ systems in afflicted individuals. Current therapeutic strategies poorly control disease manifestations and are generally immunosuppressive. Recent studies in human patient populations and animal models have associated elements of the innate immune system and abnormalities in the immature B lymphocyte receptor repertoires with disease initiation. A variety of cytokines, most notably type I interferons, play important roles in disease pathogenesis and effector mechanisms. The genetic basis for disease susceptibility is complex, and analyses in humans and mice have identified multiple susceptibility loci, several of which are located in genomic regions that are syntenic between humans and mice. The complexities of the genetic interactions that mediate lupus have been investigated in murine model systems by characterizing the progressive development of disease in strains expressing various combinations of susceptibility alleles. These analyses indicate that genetic epistasis dramatically impact disease development and support the feasibility of identifying molecular pathways that can suppress disease progression without completely impairing normal immune function.

A Tlr7 translocation accelerates systemic autoimmunity in murine lupus

The y-linked autoimmune accelerating (yaa) locus is a potent autoimmune disease allele. Transcription profiling of yaa-bearing B cells revealed the overexpression of a cluster of X-linked genes that included Tlr7. FISH analysis demonstrated the translocation of this segment onto the yaa chromosome. The resulting overexpression of Tlr7 increased in vitro responses to Toll-like receptor (TLR) 7 signaling in all yaa-bearing males. B6.yaa mice are not overtly autoimmune, but the addition of Sle1, which contains the autoimmune-predisposing Slam/Cd2 haplotype, causes the development of fatal lupus with numerous immunological aberrations. B6.Sle1yaa CD4 T cells develop the molecular signature for T(FH) cells and also show expression changes in numerous cytokines and chemokines. Disease development and all component autoimmune phenotypes were inhibited by Sles1, a potent suppressor locus. Sles1 had no effect on yaa-enhanced TLR7 signaling in vitro, and these data place Sles1 downstream from the lesion in innate immune responses mediated by TLR7, suggesting that Sles1 modulates the activation of adaptive immunity in response to innate immune signaling.

Contribution of NZB Autoimmunity 2 to Y-Linked Autoimmune Acceleration-Induced Monocytosis in Association with Murine Systemic Lupus

The accelerated development of systemic lupus erythematosus (SLE) in BXSB male mice is associated with the presence of the Y-linked autoimmune acceleration (Yaa) mutation, which induces an age-dependent monocytosis. Using a cohort of C57BL/6 (B6) x (NZB x B6)F1 backcross male mice bearing the Yaa mutation, we defined the pathogenic role and genetic basis for Yaa-associated monocytosis. We observed a remarkable correlation of monocytosis with autoantibody production and subsequent development of lethal lupus nephritis, indicating that monocytosis is an additional useful indicator for severe SLE. In addition, we identified an NZB-derived locus on chromosome 1 predisposing to the development of monocytosis, which peaked at Fcgr2b encoding FcgammaRIIB and directly overlapped with the previously identified NZB autoimmunity 2 (Nba2) locus. The contribution of Nba2 to monocytosis was confirmed by the analysis of Yaa-bearing B6 mice congenic for the NZB-Nba2 locus. Finally, we observed a very low-level expression of FcgammaRIIB on macrophages bearing the NZB-type Fcgr2b allele, compared with those bearing the B6-type allele, and the development of monocytosis in FcgammaRIIB haploinsufficient B6 mice carrying the Yaa mutation. These data suggest that the Nba2 locus may play a supplementary role in the pathogenesis of SLE by promoting the development of monocytosis and the activation of effector cells bearing stimulatory FcgammaR, in addition to its implication in the dysregulated activation of autoreactive B cells.

Differential activation of anti-erythrocyte and anti-DNA autoreactive B lymphocytes by the Yaa mutation

An as-yet-unidentified mutation, Y-linked autoimmune acceleration (Yaa), is responsible for the accelerated development of lupus-like autoimmune syndrome in mice. In view of a possible role for Yaa as a positive regulator of BCR signaling, we have explored whether the expression of the Yaa mutation affects the development and activation of transgenic autoreactive B cells expressing either 4C8 IgM anti-RBC or Sp6 IgM anti-DNA. In this study, we show that the expression of the Yaa mutation induced a lethal form of autoimmune hemolytic anemia in 4C8 transgenic C57BL/6 mice, likely as a result of activation of 4C8 anti-RBC autoreactive B cells early in life. This was further supported, although indirectly, by increased T cell-independent IgM production in spleens of nontransgenic C57BL/6 mice bearing the Yaa mutation. In contrast, Yaa failed to induce activation of Sp6 anti-DNA autoreactive B cells, consistent with a lack of increased IgM anti-DNA production in nontransgenic C57BL/6 Yaa mice. Our results suggest that Yaa can activate autoreactive B cells in a BCR-dependent manner, related to differences in the form and nature of autoantigens.

B-lymphocytes, innate immunity, and autoimmunity

Having evolved to generate a huge Ag-specific repertoire and to mount T cell-dependent responses and long-term memory, the B lymphocyte is a central player in the adaptive branch of immune defense. However, accumulating evidence indicates that B-1 cells of the peritoneal cavity and marginal zone (MZ) B cells of the spleen also can play innate-like immune functions. Their anatomical locations allow frequent Ag encounter. Secreting essentially germline-encoded, polyreactive Abs, and responding rapidly and vigorously to stimulation, these two B cell subsets have evolved to impart potentially protective responses. With their additional capacities to secrete factors that can directly mediate microbial destruction and to express Toll-like receptors (TLR), B cells provide an important link between the innate and adaptive branches of the immune system. Currently, the relevance of these innate-like B cells to the pathogenesis of autoimmune disease is the focus of investigation. In experimental models of autoimmunity, the sequestration of autoreactive B cells in the MZ has been proposed to be essential for the maintenance of self-tolerance. The low activation threshold of MZ B cells makes them particularly reactive to high loads and/or altered self-Ags, potentially exacerbating autoimmune disease. Their expansion in autoimmune models and their association with autoantibody secretion indicate that they may participate in tissue damage. The demonstration that B cell depletion therapies may represent a highly beneficial therapeutic goal in autoimmune disorders suggests that specific elimination of B-1 and MZ B cells may represent a more efficient immunointervention strategy in systemic autoimmunity.

Selective expansion of a monocyte subset expressing the CD11c dendritic cell marker in theYaa model of systemic lupus erythematosus

OBJECTIVE

Monocytosis is a unique cellular abnormality associated with the Yaa (Y-linked autoimmune acceleration) mutation. The present study was designed to define the cellular mechanism responsible for the development of monocytosis and to characterize the effect of the Yaa mutation on the development of monocyte subsets.

METHODS

We produced bone marrow chimeras reconstituted with a mixture of Yaa and non-Yaa bone marrow cells bearing distinct Ly-17 alloantigens, and determined whether monocytes of Yaa origin became dominant. Moreover, we defined the 2 major inflammatory (Gr-1+,CD62 ligand [CD62L]+) and resident (Gr-1-,CD62L-) subsets of blood monocytes in aged BXSB Yaa male mice, as compared with BXSB male mice lacking the Yaa mutation.

RESULTS

Analysis of the Ly17 allotype of blood monocytes in chimeric mice revealed that monocytes of both Yaa and non-Yaa origin were similarly involved in monocytosis. Significantly, the development of monocytosis paralleled a selective expansion of the resident monocyte subset compared with the inflammatory subset, and the former expressed CD11c, a marker of dendritic cells. Neither monocytosis nor the change in monocyte subpopulations, including CD11c expression, was observed in Yaa-bearing C57BL/6 mice, in which systemic lupus erythematosus (SLE) fails to develop.

CONCLUSION

Our results suggest that Yaa-associated monocytosis is not attributable to an intrinsic abnormality in the growth potential of monocyte lineage cells bearing the Yaa mutation and that the Yaa mutation could lead to the expansion of dendritic cells, thereby contributing to the accelerated development of SLE.

Genetic interactions between susceptibility loci reveal epistatic pathogenic networks in murine lupus

Interactions between Sle1 and other susceptibility loci were required for disease development in the NZM2410 model of lupus. Sle1 corresponds to at least three subloci, Sle1a, Sle1b, and Sle1c, each of which independently causes loss of tolerance to chromatin, but displays a distinctive immune profile. We have used congenic strains to analyze the interactions between the Sle1 subloci and other lupus susceptibility loci using Y autoimmunity accelerator (Yaa) and Faslpr as sensitizing mutations. Sle1 coexpressed with either one of these single susceptibility alleles resulted in a highly penetrant nephritis, splenomegaly, production of nephrophilic antibodies, and increased expression of B- and T-cell activation markers. Here, we show that only Sle1b interacted with Yaa to produce these phenotypes, suggesting that Sle1b and Yaa belong to the same functional pathway. Interactions between the three Sle1 loci and lpr resulted in lymphocyte activation and lupus nephritis, but a significant mortality was observed only for the Sle1a.lpr combination. This suggests a major role for the FAS pathway in keeping in check the loss of tolerance mediated by the Sle1 loci, especially for Sle1a. Our results illustrate the complexity of interactions between susceptibility loci in polygenic diseases such as lupus and may explain the clinical heterogeneity of the disease.

Separation of the New Zealand Black genetic contribution to lupus from New Zealand Black determined expansions of marginal zone B and B1a cells

The F(1) hybrid of New Zealand Black (NZB) and New Zealand White (NZW) mice develop an autoimmune disease similar to human systemic lupus erythematosus. Because NZB and (NZB x NZW)F(1) mice manifest expansions of marginal zone (MZ) B and B1a cells, it has been postulated that these B cell abnormalities are central to the NZB genetic contribution to lupus. Our previous studies have shown that a major NZB contribution comes from the Nba2 locus on chromosome 1. C57BL/6 (B6) mice congenic for Nba2 produce antinuclear Abs, and (B6.Nba2 x NZW)F(1) mice develop elevated autoantibodies and nephritis similar to (NZB x NZW)F(1) mice. We studied B cell populations of B6.Nba2 mice to better understand the mechanism by which Nba2 leads to disease. The results showed evidence of B cell activation early in life, including increased levels of serum IgM, CD69(+) B cells, and spontaneous IgM production in culture. However, B6.Nba2 compared with B6 mice had a decreased percentage of MZ B cells in spleen, and no increase of B1a cells in the spleen or peritoneum. Expansions of these B cell subsets were also absent in (B6.Nba2 x NZW)F(1) mice. Among the strains studied, B cell expression of beta(1) integrin correlated with differences in MZ B cell development. These results show that expansions of MZ B and B1a cells are not necessary for the NZB contribution to lupus and argue against a major role for these subsets in disease pathogenesis. The data also provide additional insight into how Nba2 contributes to lupus.

Genetic Complementation in Female (BXSB × NZW)F2 Mice

F(1) hybrids among New Zealand Black (NZB), New Zealand White (NZW), and BXSB lupus-prone strains develop accelerated autoimmunity in both sexes regardless of the specific combination. To identify BXSB susceptibility loci in the absence of the Y chromosome accelerator of autoimmunity (Yaa) and to study the genetics of this complementation, genome-wide quantitative trait locus (QTL) mapping was performed on female (BXSB x NZW)F(2) mice. Six QTL were identified on chromosomes 1, 4, 5, 6, 7, and 17. Survival mapped to chromosomes 5 and 17, anti-chromatin Ab to chromosomes 4 and 17, glomerulonephritis to chromosomes 6 and 17, and splenomegaly to chromosomes 1, 7, and 17. QTL on chromosomes 4 and 6 were new and designated as Lxw1 and -2, respectively. Two non-MHC QTL (chromosomes 1 and 4) were inherited from the BXSB and the rest were NZW-derived, including two similar to previously defined loci. Only two of 11 previously defined non-MHC BXSB QTL using male (Yaa(+)) crosses were implicated, suggesting that some male-defined BXSB QTL may require coexpression of the Yaa. Findings from this and other studies indicate that BXSB and NZB backgrounds contribute completely different sets of genes to complement NZW mice. Identification of susceptibility genes and complementing genes in several lupus-prone strain combinations will be important for defining the epistatic effects and background influences on the heterogeneous genetic factors responsible for lupus induction.

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.

Orderly Pattern of Development of the Autoantibody Response in (New Zealand White × BXSB)F1 Lupus Mice: Characterization of Target Antigens and Antigen Spreading by Two-Dimensional Gel Electrophoresis and Mass Spectrometry

Immunoblots of a two-dimensional PAGE-separated HL-60 cell proteomic map and mass spectrometry were combined to characterize proteins targeted by autoantibodies produced by male (New Zealand White x BXSB)F(1) (WB) mice that develop lupus and anti-phospholipid syndrome. Analysis of sera sequentially obtained from seven individual mice at different ages showed that six proteins, vimentin, heat shock protein 60, UV excision-repair protein RAD23, alpha-enolase, heterogeneous nuclear ribonucleoprotein L, and nucleophosmin, were the targets of the B cell autoimmune response, and that autoantibodies to them were synthesized sequentially in an orderly pattern that recurred in all the male WB mice analyzed: anti-vimentin first and anti-nucleophosmin last, with anti-RAD23 and anti-heat shock protein 60, then anti-alpha-enolase and anti-heterogeneous nuclear ribonucleoprotein L Abs occuring concomitantly. Anti-vimentin reactivity always appeared before anti-cardiolipin and anti-DNA Abs, suggesting that vimentin is the immunogen initiating the autoimmune process. The pattern of HL-60 proteins recognized by female WB sera differed from that of male sera, indicating that the Y chromosome-linked autoimmune acceleration gene is not an accelerator but a strong modifier of the autoimmune response. Thus, 1) combining two-dimensional PAGE and mass spectrometry constitutes a powerful tool to identify the set of Ags bound by autoantibodies present in a single serum and the whole autoantibody pattern of an autoimmune disease; 2) the diversification of the autoimmune response in male WB mice occurs in a predetermined pattern consistent with Ag spreading, and thus provides a useful model to further our understanding of the development of the autoantibody response in lupus.

The role of alpha beta+ T cells and homeostatic T cell proliferation in Y-chromosome-associated murine lupus

Male BXSB mice develop an early life, severe lupus-like disease largely attributed to an undefined Y-chromosome-associated autoimmunity accelerator, termed YAA: Although the exact disease pathogenesis is uncertain, indirect evidence suggests that T cells play an important role in the male BXSB disease. We have developed TCR alpha-chain gene-deleted BXSB mice to directly examine the role of alphabeta+ T cells and the mode by which Yaa promotes disease in this strain. All disease parameters, including hypergammaglobulinemia, autoantibody production, glomerulonephritis, and the unique monocytosis of BXSB males, were severely reduced or absent in the alphabeta+ T cell-deficient mice. Adoptively transferred CD4+ T cells of either male or female BXSB origin showed equal homeostatic proliferation in alphabeta+ T cell-deficient male recipients. Moreover, deficient male mice eventually developed equally severe lupus-like disease after adoptive transfer and homeostatic expansion of T cells from wild-type BXSB males or females. The results directly demonstrate that the Yaa-mediated disease requires alphabeta+ T cells that are not, in themselves, abnormal in either composition or properties, but are engaged by a Yaa-encoded abnormality in a non-T cell component. In addition, homeostatic anti-self proliferation of mature T cells derived from a small number of precursors can induce systemic autoimmunity in an appropriate background.

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.

Mechanisms of systemic autoimmunity in murine models of SLE

Our laboratory has utilized spontaneous and experimentally induced models of systemic autoimmunity in mice in order to elucidate the cellular deficiencies in immunoregulation that are essential to this process. In the spontaneously autoimmune mouse strains, genetic defects in T and B cell tolerance are the primary abnormalities that drive the syndrome. The induced chronic graft-vs-host model depends on abnormal T-B interactions resulting from allogeneic recognition of major histocompatibility complex (MHC) class II. Future investigations will target the biochemistry of the loss of tolerance and the specificity of autoreactive T cells that provide help for autoantibody production.

B cells in systemic autoimmune disease: recent insights from Fas-deficient mice and men

In mice functionally deficient for either Fas or Fas ligand expression, the failure of Fas-ligand-expressing cytotoxic T cells to eliminate autoreactive B cells can result in excessive autoantibody production. Recent in vitro studies have shown that B cells activated by CD40 ligand become extremely sensitive to Fas-mediated apoptosis while IL-4 and/or surface IgM receptor engagement protects B cells from Fas ligand cytolysis. Potential in vivo sites for Fas ligand regulation of self-reactive B cells have been suggested and implications for human disease have been investigated.