Selective enhancing effect of the Yaa gene on immune responses against self and foreign antigens

The BXSB Y chromosome-linked mutant gene, Yaa, accelerates the progression of a lupus-like autoimmune syndrome only in mice that are predisposed to autoimmune diseases. Unlike the lpr gene, which causes the defects in the Fas antigen-mediating apoptosis, the autoimmune enhancing activity of the Yaa gene is selective, depending on autoantigens, and varies among lupus-prone mice. To obtain a better definition of the role of the Yaa gene in the acceleration of autoimmune disease, we have investigated immune responses to several foreign antigens to determine whether the Yaa gene is able to potentiate immune responses to foreign antigens in a selective manner. We report here that the Yaa gene potentiated immune responses against foreign antigens only in mice which are genetically (H-2-linked) low responding, but not high or non-responding. Moreover, studies on Yaa⁺-Yaa^? double bone marrow chimeric mice revealed that B cells from Yaa⁺ mice were selectively stimulated to produce antibodies to low-responding antigen, human IgG, while both B cell populations similarly responded to high-responding antigen, ovalbumin. Our results suggest that first, the selective immune enhancing activity of the Yaa gene may be related to differences in the capacity of T helper cells specific for given self or foreign antigens; and second, a specific cognate interaction of T helper cells with Yaa⁺ B cells is apparently responsible for the selective enhancement of immune responses to antigens, to which mice are genetically low responding.     

The Role of the Yaa Gene in Lupus Syndrome

The BXSB/MpJ (BXSB) murine strain (H-2^b) spontaneously develops an autoimmune syndrome with features of systemic lupus erythematosus (SLE) that affects males much earlier than females. A mutant gene located on the BXSB Y chromosome, designated Yaa (Y chromosome-linked autoimmune acceleration), is responsible for the acceleration of the disease observed in male BXSB mice. Studies on H-2 congenic and I-E transgenic mice have clearly demonstrated that the MHC class II genes play a crucial role in the development or protection of SLE. However, the MHC effect can be completely masked by the presence of the Yaa gene in mice with certain genetic backgrounds. It is intriguing that the Yaa gene effect is selective on autoimmune responses, varying in different lupus-prone mice. Studies on immune responses against foreign antigens have shown that the Yaa gene potentiates immune responses only against antigens to which mice are genetically (H-2-linked) low-responding, but not high-responding. Thus, the selective immune enhancing activity of the Yaa gene may be related to differences in the capacity of T helper cells specific for given antigens. Moreover, studies on Yaa⁺Yaa^? bone marrow cell chimeric mice have suggested that a specific cognate interaction of T helper cells with Yaa⁺ B cells is responsible for a selective enhancing effect of immune responses to foreign antigens as well as autoantigens. It is significant that unlike the lpr mutation, whose abnormality is associated with the capacity of the Fas antigen to mediate apoptosis, the Yaa gene by itself is unable to induce significant autoimmune responses in mice without apparent SLE background. This suggests that-the molecular defect of the Yaa gene is likely to differ from that of the lpr gene, and that the Yaa gene effect requires the abnormal autosomal genome present in lupus-prone mice. Based on these findings, a possible molecular nature of the Yaa gene abnormality will be discussed.     

The lupus-prone BXSB strain: the Yaa gene model of systemic lupus erythematosus

Conclusion Genetic analysis of SLE in the various autoimmune mice has revealed that this disease is multigenic in nature and that several, quite distinct genetic backgrounds are compatible with this disease. Although the nature of these genetic components has not been defined, studies on New Zealand mice have indicated that multiple, unlinked genes are responsible for the expression of various disease manifestations and the production of autoantibodies. However, it is significant that single gene mutations, such as lpr, gld and Yaa, markedly influence the development of the lupus-like autoimmune syndrome. In addition, it is becoming clear, based on the results obtained from H-2 congenic mice, that the MHC class II genes play a crucial role in the development of SLE. However, the absence of severe autoimmune pathology in mice lacking the SLE background, even in the presence of the single autoimmune mutant gene and of the appropriate MHC class II genes, clearly indicates the requirement of supplementary genetic abnormalities for the fullblown manifestations of SLE.Although the abnormality of the lpr and possibly gld mutation is associated with the molecules mediating the apoptosis, the nature of the Yaa gene defect has not yet been identified. The differences in autoimmune accelerating effects between the Yaa gene and the lpr gene on autoimmune-prone and non-autoimmune mice strongly suggest that the molecular defect of the Yaa gene is likely to differ from those of the lpr and gld genes. We propose that the Yaa gene effect may result from the expression of the Yaa gene-related molecule on B cells. This molecule could behave as an intercellular adhesion-like molecule, thereby facilitating the interaction and subsequent activation of autoreactive T and B cells. Alternatively, the Yaa molecule-derived peptide could be efficiently presented in the context of MHC class II antigens by B cells, and its recognition by Yaa-specific T helper cells could activate autoantigen-specific B cells even in the absence of autoantigen-specific T helper cells. Obviously, the molecular identification of the Yaa gene product would be of paramount importance in helping answer this important and interesting question.     

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.     

Phenotype conversion from rheumatoid arthritis to systemic lupus erythematosus by introduction of Yaa mutation into FcγRIIB‐deficient C57BL/6 mice

We previously established an IgG Fc receptor IIB (FcγRIIB)‐deficient C57BL/6 (B6)‐congenic mouse strain (KO1), which spontaneously develops rheumatoid arthritis (RA), but not systemic lupus erythematosus (SLE). Here, we show that when Y chromosome‐linked autoimmune acceleration (Yaa) mutation was introduced in KO1 strain (KO1.Yaa), the majority of KO1.Yaa mice did not develop RA, but instead did develop SLE. This phenotype conversion did not depend on autoantibody specificity, since KO1.Yaa mice, compared with KO1, showed a marked increase in serum levels of both lupus‐related and RA‐related autoantibodies. The increase in frequencies of CD69⁺ activated B cells and T cells, and the spontaneous splenic GC formation with T follicular helper cell generation were manifest early in life of KO1.Yaa, but not KO1 and B6.Yaa, mice. Activated CD4⁺ T cells from KO1.Yaa mice showed upregulated production of IL‐21 and IL‐10, compared with the finding in KO1 mice, indicating the possibility that this aberrant cytokine milieu relates to the disease phenotype conversion. Thus, our model is useful to clarify the shared and the disease‐specific mechanisms underlying the clinically distinct systemic autoimmune diseases RA and SLE.     

Murine lupus in MRL/lpr mice lacking CD4 or CD8 T cells

MRL/lpr mice develop a systemic autoimmune disease similar to systemic lupus erythematosus in humans. The mice show progressive lymphadenopathy due to the accumulation of an unusual population of CD4^?8^?(DN) B220⁺ αβ⁺ T cells. We bred MRL/lpr mice with mice lacking CD4⁺ or CD8⁺ T cells by gene targeting via homologous recombination in embryonal stem cells to determine the roles of these cells in the autoimmune disease. No difference in survival or autoantibody levels was noted between CD8-/- lpr and littermate controls. Interestingly, these CD8-/- lpr mice have a reduced level of B220⁺ DN T cells despite the fact that the degree of lymphadenopathy was unaltered. CD4-/- lpr mice had a diminished autoimmune disease with a reduction in autoantibody production and skin vasculitits, and increased survival compared to littermate controls. However, CD4-/- lpr mice had an enhanced splenomegaly that developed massively by 16–20 weeks of age (5 to 8 greater than lpr control mice) due to the accumulation of DN B220⁺ T cells. In addition, there were no differences in peripheral lymph node enlargement, although the proportion of DN B220⁺ T cells was about twofold higher in the CD4-/- lpr mice. These cells were phenotypically identical to the DN population in control lpr mice, indicating that the accumulating DN T cells can be dissociated from the autoimmune disease in these mice. Collectively, our results reveal that the autoimmune disease is dependent on CD4⁺, but not CD8⁺ T cells, and that many of the B220⁺ DN T cells traverse a CD8 developmental pathway.     

Effect of cyclophosphamide on lymphokine production in MRL/lpr.Yaa mice

The Y chromosome (Yaa gene) from autoimmune BXSB mice has been shown to be responsible for the acceleration of autoimmune symptoms when transferred to MRL/lpr mice. We examined the pathological, serological and immunological characteristics of MRL/lpr.Yaa mice and the suppressive effect of cyclophosphamide (CP) on the mice. MRL/lpr.Yaa mice spontaneously developed a massive lymphadenopathy characterized by hypergammaglobulinemia, the presence of several autoantibodies, and autoimmune disease. In MRL/lpr.Yaa mice, IL-2, IL-4 and IL-5 production in concanavalin A (Con A)-stimulated splenocytes were about 10-fold lower than in BALB/c mice at 5 weeks of age.The concentrations of these lymphokines remained low until the mice were 16 weeks of age. The production of IFN- and IL-6 in 16 week old MRL/lpr.Yaa mice was about 4- and 2-fold lower, respectively, though these levels were similar in both strains at 8 weeks of age. It was found that this dysregulation of T cell function was almost identical to that in MRL/lpr mice. Administration of CP to MRL/lpr.Yaa mice ameliorated nephritis, and suppressed production of autoantibodies and the accumulation of abnormal T cells. CP also significantly elevated the production of lymphokines. These findings suggest that an abnormality of T cell function may contribute to the autoimmune pathogenesis of MRL/lpr.Yaa mice and that CP probably ameliorates autoimmune disease by improving the T cell functions.     

Reduced development of CD4−8−B220+ T cells but normal autoantibody production in lpr/lpr mice lacking major histocompatibility complex class I molecules

The lpr gene has recently been shown to encode a functional mutation in the Fas receptor, a molecule involved in transducing apoptotic signals. Mice homozygous for the lpr gene develop an autoimmune syndrome accompanied by massive accumulation of double-negative (DN) CD4^?8^?B220⁺ T cell receptor-α/β⁺ cells. In order to investigate the origin of these DN T cells, we derived lpr/lpr mice lacking major histocompatibility complex (MHC) class I molecules by intercrossing them with β2-microglobulin (β2m)-deficient mice. Interestingly, these lprβ2m–/– mice develop 13-fold fewer DN T cells in lymph nodes as compared to lpr/lpr wild-type (lprWT) mice. Analysis of anti-DNA antibodies and rheumatoid factor in serum demonstrates that lprβ2m–/– mice produce comparable levels of autoantibodies to lprWT mice. Collectively our data indicate that MHC class I molecules control the development of DN T cells but not autoantibody production in Ipr/lpr mice and support the hypothesis that the majority of DN T cells may be derived from cells of the CD8 lineage.     

Lack of association of Vβ8+ T cells with lupus-like syndrome in MRL-lpr/lpr mice

To evaluate the role of Vβ8⁺ T cells in the development of lupus-like autoimmune syndrome in MRL-lpr/lpr mice, we treated them with the F23.1 anti-Vβ8 monoclonal antibody (mAb) from birth to 4 months of age. Here we report that almost complete depletion of Vβ8⁺ T cells by the F23.1 mAb treatment neither inhibited nor delayed the development of hypergammaglobulinemia, autoantibody production and autoimmune glomerulonephritis in MRL-lpr/lpr mice. In addition, the F23.1 mAb treatment did not prevent the development of lymphadenopathy and the generation of a CD4^?CD8^? double-negative T cell subset, characteristically accumulating in lpr lymph nodes. Our results strongly argue against the idea that the Vβ8⁺ T cells play a critical role in the development of lupus-like autoimmune syndrome in MRL-lpr/lpr mice.     

The regulation and activation of lupus-associated B cells

Summary: Anti‐double‐stranded DNA (anti‐dsDNA) B cells are regulated in non‐autoimmune mice. While some are deleted or undergo receptor editing, a population of anti‐dsDNA (VH3H9/Vλ1) B cells that emigrate into the periphery has also been identified. These cells have an altered phenotype relative to normal B cells in that they have a reduced lifespan, appear developmentally arrested, and localize primarily to the T/B‐cell interface in the spleen. This phenotype may be the consequence of immature B cells encountering antigen in the absence of T‐cell help. When provided with T‐cell help, the anti‐dsDNA B cells differentiate into antibody‐forming cells. In the context of the autoimmune‐prone lpr/lpr or gld/gld mutations, the VH3H9/Vλ1 anti‐dsDNA B cells populate the B‐cell follicle and by 12 weeks of age produce serum autoantibodies. The early event of anti‐dsDNA B‐cell follicular entry, in the absence of autoantibody production, is dependent upon CD4⁺ T cells. We hypothesize that control of autoantibody production in young autoimmune‐prone mice may be regulated by the counterbalancing effect of T‐regulatory (T_reg) cells. Consistent with this model, we have demonstrated that T_reg cells are able to prevent autoantibody production induced by T‐cell help. Additional studies are aimed at investigating the mechanisms of this suppression as well as probing the impact of distinct forms of T‐cell‐dependent and ‐independent activation on anti‐dsDNA B cells.     

Phenotypic Abnormalities of Splenic and Bone Marrow B Cells inlprandgldMice

Mice homozygous for the mutantFasgenelprdevelop generalized lymphoproliferation and produce autoantibodies resembling those found in human SLE. We have previously shown that these autoantibodies are produced by B2 cells rather than B1 cells and that the autoantibody-producing B cells are intrinsically abnormal. We investigated further thelprB cell with a large panel of antibodies to B-cell surface markers to identify phenotypic abnormalities. B cells from spleen and bone marrow of age-matched congenic mice differing only at thelprlocus were examined by flow cytometry. Two consistent phenotypic differences were identified. First, spleen cells from olderlprmice had an increase in the number and percentage of IgM⁺B cells expressing low levels of CD23. Second,lprbone marrow had decreased numbers of B220^h1IgM⁺-syndecan-1⁺CD23⁺B cells. All other markers tested, except the previously identified modest increase of Ia onlprspleen cells, showed no consistent differences. B cells fromgldmice showed the same phenotypic abnormalities as those fromlpr.Compared to T cells, the relative paucity of cell surface marker differences betweenlprand +/+ B cells suggests that B cells may have fewer regulatory mechanisms to silence autoreactive specificities. The phenotypic differences identified may provide clues to the mechanism of autoantibody production inlprmice, while the overwhelming phenotypic similarity betweenlprand +/+ B cells suggests that the major abnormality oflprB cells may lie in their specificity, that is, in their inability to delete autoreactive subsets.     

Suppression and Reversal ofgldDisease by Parabiosis with Normal Mice

The disruption of the Fas receptor or Fas ligand by thelprorgldmutations, respectively, results in severe autoimmune and lymphoproliferative disease due to the failure of Fas-mediated deletion of self-reactive lymphocytes. Recently, we have shown in mixed chimeras thatgld-induced autoimmunity could be corrected by normal bone marrow, in particular by normal T cells. In contrast,lpr-mediated autoimmunity could not be influenced by normal bone marrow-derived cells. In the present report, we have studied the role of normal lymphocytes in suppressing or reversinggld-induced autoimmunity by parabiosis with normal mice. Our results show a suppression of lymphadenopathy, fewer CD4⁻CD8⁻T cells, and lower levels of autoantibody production ingldmice parabiosed with normal mice at 4–6 weeks of age. Thegldmice parabiosed with normal mice at 4 months of age also exhibited a substantial reduction of both total and CD4⁻CD8⁻T cells in the periphery 2 months after surgery. However, they showed little reduction of autoantibodies compared togldmice parabiosed withgldmice. In contrast, olderlprmice did not exhibit any reduction in lymphadenopathy or autoantibody production after parabiosis with normal mice. The prevention or reversal of lymphadenopathy in parabiosedgldmice suggests that ongoing Fas-mediated deletion in the periphery may play an important role in maintaining self-tolerance. The relative irreversibility of autoantibody synthesis in older parabiosedgldmice suggests that autoantibody-producing B cells or their committed precursors are long lived and do not express functional Fas receptor.     

CD47 deficiency ameliorates autoimmune nephritis in Faslpr mice by suppressing IgG autoantibody production

CD47, a self‐recognition marker, plays an important role in both innate and adaptive immune responses. To explore the potential role of CD47 in activation of autoreactive T and B cells and the production of autoantibodies in autoimmune disease, especially systemic lupus erythematosus (SLE), we have generated CD47 knockout Fas^lpr (CD47^?/??Fas^lpr) mice and examined histopathological changes in the kidneys, cumulative survival rates, proteinuria, extent of splenomegaly and autoantibodies, serum chemistry and immunological parameters. In comparison with Fas^lpr mice, CD47^?/??Fas^lpr mice exhibit a prolonged lifespan and delayed autoimmune nephritis, including glomerular cell proliferation, basement membrane thickening, acute tubular atrophy and vacuolization. CD47^?/??Fas^lpr mice have lower levels of proteinuria, associated with reduced deposition of complement C3 and C1q, and IgG but not IgM in the glomeruli, compared to age‐matched Fas^lpr mice. Serum levels of antinuclear antibodies and anti‐double‐stranded DNA antibodies are significantly lower in CD47^?/??Fas^lpr than in Fas^lpr mice. CD47^?/?–Fas^lpr mice also display less pronounced splenomegaly than Fas^lpr mice. The mechanistic studies further suggest that CD47 deficiency impairs the antigenic challenge‐induced production of IgG but not IgM, and that this effect is associated with reduction of T follicular cells and impairment of germinal centre development in lymphoid tissues. In conclusion, our results demonstrate that CD47 deficiency ameliorates lupus nephritis in Fas^lpr mice via suppression of IgG autoantibody production. Copyright © 2015 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.     

T cells of staphylococcal enterotoxin B-tolerized autoimmune MRL-lpr/lpr mice require co-stimulation through the B7-CD28/CTLA-4 pathway for activation and can be reanergized in vivo by stimulation of the T cell receptor in the absence of this co-stimulatory signal

The CD28/CTLA-4 receptors on T cells interact with the B7 molecule on antigen-presenting cells (APC) to produce a co-stimulatory signal that determines the outcome of activation. The role of this co-stimulatory signal in T cell activation and loss of tolerance in autoimmune MRL-lpr/lpr mice has not been investigated previously. The present study examines the contribution of the CD28/CTLA-4 co-stimulatory pathway to the loss of T cell tolerance in Vβ8 transgenic MRL-lpr/lpr and -+/+ mice in which neonatal tolerance has been induced by the superantigen staphylococcal enterotoxin B (SEB). An artificial APC transfected with the murine B7 gene, and a CTLA-4-Ig fusion protein were used to analyze the significance of the CD28/CTLA-4 pathway in vitro. The CTLA-4-Ig fusion protein was also used to inhibit the pathway in vivo. Our results demonstrate that CD28 and CTLA-4 mRNA was overexpressed in the lymph nodes of lpr/lpr mice (MRL, C57BL/6, C3H and AKR), but not in +/+ mice of the same background strain. Lymph node T cells and thymocytes from SEB neonatally tolerized MRL-lpr/lpr mice that had undergone tolerance loss, proliferated when cultured with SEB and B7⁺ fibroblasts in vitro, but did not proliferate when the SEB was presented in the context of B7^? fibroblasts. This in vitro tolerance loss could be prevented by blocking of B7 signaling by CTLA-4-Ig. This loss of tolerance did not occur in lymph node T cells from thymectomized MRL-lpr/lpr mice. SEB challenge of tolerized MRL-lpr/lpr mice in vivo led to weight loss, increased serum cytokine levels and depletion of Vβ8⁺ T cells. These effects were blocked by blocking of the co-stimulatory pathway by treatment with the CTLA-4-Ig fusion protein prior to and during challenge with SEB. T cells from thymus and lymph nodes of these mice did not proliferate later in response to stimulation in vitro with SEB even in the presence of B7⁺ APC. Nonresponsiveness was not due to deletion of Vβ8⁺ CD28⁺ T cells, as the number of these cells was increased after treatment with SEB and the CTLA-4-Ig fusion protein. These results suggest that the response of autoreactive T cells in the thymus and lymph nodes depends on signaling by B7 in vivo and in vitro and that SEB-reactive T cells can be reanergized in vivo by stimulation of the T cell receptor in the absence of signaling through the CD28/CTLA-4 co-stimulatory pathway.     

Ly108 expression distinguishes subsets of invariant NKT cells that help autoantibody production and secrete IL-21 from those that secrete IL-17 in lupus prone NZB/W mice

Lupus is a systemic autoimmune disease characterized by anti-nuclear antibodies in humans and genetically susceptible NZB/W mice that can cause immune complex glomerulonephritis. T cells contribute to lupus pathogenesis by secreting pro-inflammatory cytokines such as IL-17, and by interacting with B cells and secreting helper factors such as IL-21 that promote production of IgG autoantibodies. In the current study, we determined whether purified NKT cells or far more numerous conventional non-NKT cells in the spleen of NZB/W female mice secrete IL-17 and/or IL-21 after TCR activation in vitro, and provide help for spontaneous IgG autoantibody production by purified splenic CD19⁺ B cells. Whereas invariant NKT cells secreted large amounts of IL-17 and IL-21, and helped B cells, non-NKT cells did not. The subset of IL-17 secreting NZB/W NKT cells expressed the Ly108^loCD4⁻NK1.1⁻ phenotype, whereas the IL-21 secreting subset expressed the Ly108^hiCD4⁺NK1.1⁻ phenotype and helped B cells secrete a variety of IgG anti-nuclear antibodies. α-galactocylceramide enhanced the helper activity of NZB/W and B6.Sle1b NKT cells for IgG autoantibody secretion by syngeneic B cells. In conclusion, different subsets of iNKT cells from mice with genetic susceptibility to lupus can contribute to pathogenesis by secreting pro-inflammatory cytokines and helping autoantibody production.     

Urtica dioica agglutinin,a Vβ8.3-specific superantigen,prevents the development of the systemic lupus erythematosus-like pathology of MRL lpr/lpr mice

The Vβ8.3-specific superantigenic lectin Urtica dioica agglutinin (UDA) was used to delete the Vβ8.3⁺ T cells in MRL lpr/lpr mice. In contrast to the systemic lupus erythematosus-like pathology which progresses with age in the phosphate-buffered saline-injected MRL lpr/lpr controls, UDA-treated animals did not develop overt clinical signs of lupus and nephritis. The pathogenic T cell clones thus reside within the Vβ8.3⁺ T cell population, which includes an expanded T cell clone described previously. Finally, UDA alters the production of autoantibodies in a sex-dependent manner.     

Hypogammaglobulinaemia occurs in Fas-deficient MRL-lpr mice following deletion of MHC class II molecules

Fas (CD95)-mediated apoptosis in B and T cells is deficient in both human autoimmune lymphoproliferative syndrome and in MRL-lpr mice, a model for systemic lupus erythematosis (SLE). Autoimmune disease in these mice is associated with polyclonal B cell activation, increased serum immunoglobulin and autoantibodies. In non-autoimmune mice MHC class II is not required for normal serum immunoglobulin expression, and previously we have shown using MHC class II-deficient MRL-lpr mice (MRL-lpr Ab−/−) that generation of specific antibodies to DNA requires MHC class II-directed T cell help. In contrast, in the present study we demonstrate that MRL-lpr Ab−/− mice also have a profound reduction of total serum immunoglobulin levels, suggesting abnormal polyclonal regulation of B cells by MHC class II-directed T cells occurs in the autoimmune MRL-lpr strain. This abrogation of immunoglobulin production does not occur in MHC class II-deficient non-obese diabetic (NOD) mice, nor in MHC class I-deficient NOD or MRL-lpr mice. Reduced immunoglobulin levels in MRL-lpr Ab−/− mice were not due to a lack of B cells or to an increased loss of circulating immunoglobulin, but were associated with reduced numbers of surface IgG-positive B cells. These results define a general abnormal regulation of B cells in MRL-lpr mice through a process requiring MHC class II, and suggest that Fas deficiency may allow expansion of totally T-dependent B cells.     

Linkage of a major quantitative trait locus to Yaa gene-induced lupus-like nephritis in (NZW × 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 × 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 × (NZW × 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.     

Mycobacteria-induced autoantibody production is associated with susceptibility to infection but not with host propensity to develop autoimmune disease

Mycobacteria cause increase in autoantibody production in the host during the first weeks of infection. The level of the autoantibody enhancement varies widely in different hosts, suggesting that it depends on features of the host make-up. We have investigated the participation of two characteristics of the host in the modulation of mycobacteria-induced autoantibody production: (i) the host being genetically determined to later develop spontaneous autoimmune disease; (ii) the host being susceptible/resistant to mycobacterial infection. Mycobacterium avium infection was studied in 3-month-old mice that are prone (NZB and C57B1/6-lpr/lpr strains) or not (NZW and C.D2 strains) to develop, when older, autoimmune disease; these murine strains are either naturally susceptible (C57B1/6-lpr/lpr and NZW) or resistant (NZB and C.D2) to mycobacteria. Mycobacterium avium infection was produced by i.p. injection of 3 ± 10⁷ viable bacilli. At days 15 and 30 of the infection, we determined the following parameters; (i) number of cells producing natural autoantibodies (splenic cells showing surface antibodies against bromelain-treated mouse (BrM) erythrocytes); (ii) suppression of the primary response to T cell-dependent antigen (i.e. to sheep erythrocytes); (iii) immunoglobulin classes and IgG isotypes; (iv) titres of anti-dsDNA antibodies; and (v) serum concentrations of interferon-gamma (IFN-γ). We found that the highest elevations in natural autoantibodies were associated with hosts being naturally susceptible to mycobacteria, but not with the host being genetically determined to later develop autoimmune disease. The rise in autoantibodies was predominantly of the IgM type, being associated with suppression of the T cell response and accompanied by increase in serum IFN-γ. Mycobacteria failed to induce any significant enhancement in pathogenic anti-dsDNA antibodies. Our data suggest that the finding of a high level of autoantibodies during the early phase of mycobacterial infection reflects host susceptibility to the infectious agent, and that it is not related with its propensity to later develop autoimmune disorders.     

Toll-like receptor 8 deletion accelerates autoimmunity in a mouse model of lupus through a Toll-like receptor 7-dependent mechanism

Systemic lupus erythematosus is an autoimmune disorder characterized by increased levels of lymphocyte activation, antigen presentation by dendritic cells, and the formation of autoantibodies. This leads to immune complex-mediated glomerulonephritis. Toll-like receptor 7 (T7) and TLR9 localize to the endosomal compartment and play important roles in the generation of autoantibodies against nuclear components, as they recognize RNA and DNA, respectively. In contrast, very little is known about endogenous TLR8 activation in mice. We therefore tested whether TLR8 could affect autoimmune responses in a murine model of lupus. We introduced a Tlr8 null mutation into C57BL/6 mice congenic for the Nba2 (NZB autoimmunity 2) locus and bearing the Yaa (Y-linked autoimmune acceleration) mutation containing a tlr8 duplicated gene, and monitored disease development, autoantibody production, and glomerulonephritis-associated mortality. Cellular responses were investigated in female Nba2.TLR8^−/− mice bearing no copy of tlr8. The TLR8 deficiency accelerated disease progression and mortality, increased the number of circulating antibodies and activated monocytes, and heightened cellular responses to TLR7 ligation. TLR8-deficient antigen-presenting cells exhibited increased levels of MHC class II expression. The ability of dendritic cells to present antigens to allogeneic T cells after TLR7 ligation was also improved by TLR8 deficiency. TLR8 deletion accelerated autoimmunity in lupus-prone mice in response to TLR7 activation. Antigen-presenting cell function seemed to play a key role in mediating the effects of TLR8 deficiency.