The lpr and gld genes in systemic autoimmunity: life and death in the Fas lane.

The single gene lpr and gld models of spontaneous systemic autoimmunity have attracted much attention in recent years. Here, Philip Cohen and Robert Eisenberg describe the fascinating recent findings that the lpr and gld [corrected] phenotypes result from defects in the Fas gene and, perhaps, in the ligand for fas, respectively.     

Autoantibody production in lpr/lpr gld/gld mice reflects accumulation of CD4+ effector cells that are resistant to regulatory T cell activity

In Fas/FasL-deficient mice anti-chromatin Ab production is T cell dependent and is not apparent until after 10 weeks of age. Early control of anti-chromatin antibodies may be due to the counterbalancing influence of Treg cells. Here we show that Treg cells block lpr/lpr gld/gld Th cells from providing help to anti-chromatin B cells in an in vivo transfer system. Interestingly, the percentage and absolute numbers of Foxp3(+) Treg cells is elevated in BALB/c-lpr/lpr gld/gld mice and increases with age compared to BALB/c mice. The majority of Foxp3 expression is found in the B220(-) CD4(+) T cell population, and Foxp3-expressing cells are localized in the splenic PALS (periarteriolar lymphocyte sheath). Strikingly, although the lack of functional Fas/FasL does not affect the ability of Treg cells to block Th cell proliferation, Treg cells can block the IFN-gamma differentiation of Th cells from BALB/c or young BALB-lpr/lpr gld/gld mice but not of pre-existing Th1 cells from older BALB/c-lpr/lpr gld/gld mice. Thus, we suggest autoantibody production is not caused by the lack of Treg cells but by a defect in activation-induced cell death that leads to the accumulation of T effector cells that are resistant to regulatory T cell activity.     

Fas and Fas ligand: lpr and gld mutations

Fas ligand (FasL) is a death factor that binds to its receptor, Fas, and induces apoptosis. Two mutations that accelerate autoimmune disease, lpr and gld, are known to correspond to mutations within genes encoding Fas and FasL, respectively. Here, Shigekazu Nagata and Takashi Suda summarize current knowledge of Fas and FasL, and discuss the physiological role of the Fas system in T-cell development, cytotoxicity and cytotoxic T lymphocyte (CTL)-mediated autoimmune disease.     

Lymphadenopathy induced by the cooperation between lprcg and gld genes is of lpr but not of gld phenotype

Mice homozygous for the lpr (lymphoproliferation), lpr^cg or gld (generalized lymphoproliferative disease) mutation develop strikingly similar lymphadenopathy with expansion of B220⁺ CD4^? CD8^? double-negative (DN) T cells and autoimmunity. To elucidate the roles of bone marrow (BM) and lymph node (LN) in lymphoproliferation, BM and LN were transplanted simultaneously into normal or +/+ mice in various genotype combinations. In lpr/lpr or lpr^cg/lpr^cg BM recipients grafted lpr/lpr and lpr^cg/lpr^cg LN swelled but +/+ and gld/gld LN atrophied. In gld/gld BM recipients all of LN swelled regardless of genotype. Thus, lpr and lpr^cg are phenotypically different from gld in the interaction of BM-derived DN T cells and +/+ LN. Compared with lpr the lpr^cg gene differs in its ability to complement with gld in induction of lymphadenopathy. To determine whether lymphoproliferation induced by the cooperation between lpr^cg and gld is of lpr or gld phenotype, LN of various genotypes were implanted into double heterozygous lpr^cg/+, gld/+ mice. Grafted lpr/lpr and lpr^cg/lpr^cg LN swelled but +/+ and gld/gld LN atrophied, indicating that it is of lpr phenotype. Moreover, grafted lpr^cg/+ LN swelled but lpr/+ LN atrophied, indicating that, in the heterozygous state, lpr^cg is phenotypically different from lpr as it allows for LN accumulation of DN T cells induced by lpr^cg-gld cooperation.     

Adoptive transfer of the gld syndrome in double congenic nude lpr mice.

Homozygosity for either the lymphoproliferation (lpr) or the generalized lymphoproliferative disease (gld) mutation of mice causes the development of strikingly similar autoimmune and lymphoproliferative syndromes. The relationship between the lpr and gld mutations was studied by grafting B6 gld spleen cells (SC) to athymic B6 nude lpr mice (B6 nulpr) or to B6 nude (B6 nu) mice as controls. The injection of B6 gld SC, but not of B6 wild SC, to B6 nulpr mice caused a prolongation of survival of the short living B6 nulpr recipients. This was associated with elevated anti-single stranded DNA antibody titers and a serum hyperglobulinemia, as well as by a splenomegaly which was nearly as high as in genetically B6 gld mice, and by a marked lymphadenopathy (though milder than that of B6 gld mice). In contrast the [gld----nu] chimaeras showed a more attenuated form of gld-induced syndrome. These results suggest that the lpr environment supplied in athymic lpr recipients is compatible with--and may even favour--the development of the gld-induced syndrome.     

A specific intercellular pathway of apoptotic cell death is defective in the mature peripheral T cells of autoimmune lpr and gld mice

Homozygosity for either of the unlinked murine autosomal recessive mutations lpr or gld leads to autoimmunity characterized by peripheral accumulation of CD4^?/CD8^? “double-negative” T cells, autoantibodies and various forms of tissue pathology. Recently, the gene affected by lpr was identified as fas, whose product acts as a trigger for programmed cell death or apoptosis. Data reported here indicate that the Fas receptor and its ligand, the wild-type form of the gld gene product, are essential for antigen-stimulated peripheral T cell apoptosis. Furthermore, the wild-type gld gene product is a non-cell-autonomous protein that is produced by activated T cells. Apoptotic elimination of antigen-receptor-triggered peripheral T cells appears to be abnormal in lpr and gld mice, and this deficiency causes peripheral T cells to accumulate resulting in lymphadenopathy. These findings support the importance of apoptotic regulation of lymphocyte persistence after antigen encounter in vivo.     

Correction of gld autoimmunity by co-infusion of normal bone marrow suggests that gld is a mutation of the Fas ligand gene

lpr and gld mice develop phenotypically indistinguishable systemicautoimmune diseases and marked lymphadenopathy dominated byCD4^–CD8^– In vivo chimera experiments have demonstratedthat both ipr T and ipr B cells are intrinsically defective.Analogous experiments were conducted using gld mice. Lethallyirradiated gld mice were given mixtures of congenic gld andnormal (+/+) bone marrow differentially marked by lg heavy chainallotype. In sharp contrast to ipr-+/+ mixed chimeras, gld-+/+chimeras had little autoantibody production at 5 months andminimal adenopathy at 6 months, indicating that the normal marrow-derivedcells corrected the gld defect. Thus, aberrant autoantibodyproduction is due to a defect extrinsic to the gld B cell andlymphoproliferation is due to a defect extrinsic to the gldT cell. These data support the hypothesis that gld mice lackan apoptosis-inducing ligand. The receptor for this ligand maybe the Fas molecule, which is defective in ipr mice T cells.     

The clearance of apoptotic cells: implications for autoimmunity

Apoptosis has been clearly characterised by the ability to limit the activation of inflammatory responses through the disposal of the apoptotic cell by rapid uptake by phagocytes. The exposure of phosphatidylserine deriving from the loss of plasma lipid asymmetry is the early membrane signal which alerts the phagocyte about the imminent apoptotic death of the cell. Also modifications of membrane carbohydrate groups on apoptotic cells contribute to phagocyte recognition. Soluble proteins such as C1q, mannose-binding lectin, surfactant proteins A and D, C-reactive protein, C3bi, beta2-glycoprotein I and growth arrest specific gene-6 bind to apoptotic cells and act as "opsonins" thus favouring their clearance. A redundant and promiscuous system of receptors including integrins, scavenger receptors, CR3 and CR4, calreticulin, CD14 and Mer receptor ensures an efficient and rapid uptake of apoptotic cells. In animal models and in human pathology, single genetic defects of molecules involved in apoptotic cell clearance seem to be the main determinant in the development of autoimmunity. The uptake of apoptotic cells by phagocytes provides an immunomodulatory effect in that it triggers the release of anti-inflammatory cytokines, inhibits the production of inflammatory cytokines and leads to T cell tolerance. Impaired clearance of apoptotic cells or the presence of 'danger' signals may modify the balance between tolerance induction and activation of T cells leading to an effective autoimmune response.     

Transgenic T cell receptor interactions in the lymphoproliferative and autoimmune syndromes of lpr and gld mutant mice.

To investigate the role of T cell receptor (TcR) expression and interactions in development of lymphoproliferation and autoimmunity in lpr and gld mutant mice, and to determine whether these autoimmune mutations affect T cell selection and repertoire formation, we generated mice homozygous for either the gld or the lpr mutation and containing TcR alpha/beta transgenes (Von Boehmer, H., Annu. Rev. Immunol. 1990. 8: 531) specific for the male (H-Y) antigen in the context of H-2Db. Four main results emerged from analysis of these mice. First, expression of transgenic TcR had no effect on disease incidence and progression. Second, the accumulating T cells reflected normal processes of positive and negative selection. Third, cells expressing the transgenic TcR participated equally in lymphoproliferation regardless of whether their antigenic peptide and/or presenting major histocompatibility complex molecules were present or not. Fourth, expression of the TcR transgenes markedly altered the phenotype of the major accumulating lymphocyte subset. Thus, in these models of lymphoproliferation and autoimmunity. T cell repertoire formation proceeds normally, specific T cell recognition of antigen has no effect on the participation of individual clones, and the phenotype of the cells accumulating is sensitive to either the timing or the amount of TcR expression. These results are discussed in the context of the primary cause vs. secondary manifestations of autoimmunity in these models.     

Age-dependent, polyclonal hyperactivation of T cells is reduced in TNF-negative gld/gld mice

The generalized lymphoproliferative disorder (gld) mouse strain is characterized by severe splenomegaly/lymphadenopathy, the production of autoimmune antibodies, and the appearance of CD4/CD8-negative T cells. An additional TNF deficiency of gld/gld mice attenuates the course of the disorder through a yet-unknown mechanism. In this study, we could demonstrate that the reduced splenomegaly and lymphadenopathy in B6.gld/gld.TNF-/- mice were correlated with a decreased peripheral T cell proliferation rate and a delayed polyclonal activation. A comparative analysis of na?ve T cells and memory/effector T cells showed an age-dependent difference in the T cell activation pattern in the spleen of B6.gld/gld and B6.gld/gld.TNF-/- mice. T cells from B6.gld/gld.TNF-/- spleens and lymph nodes showed significantly higher levels of CCR7 and CD62 ligand on their surface compared with B6.gld/gld mice when mice of the same age were compared. Additionally, we found an increased titer of the Th1 cytokine IFN-gamma in the serum of B6.gld/gld mice, whereas the concentration of IFN-gamma was markedly reduced in the serum of B6.gld/gld.TNF-/- mice. These findings support the hypothesis that increased T cell activation and proliferation in the presence of TNF contribute to the exacerbation of the gld syndrome.     

The basis of autoimmunity in MRL-lpr/lpr mice: a role for self Ia-reactive T cells

Murine models of systemic lupus eRythematosus (SLE) have significantly contributed to our understanding of human autoimmunity. One such strain, the MRL-lpr/lpr, spontaneously develops an autoimmune disease manifested clinically by arthritis, vasculitis, immune-complex glomerulonephritis and autoantibody production^1–3. In this article Yvonne Rosenberg and her colleagues suggest a theoretical basis for the development of autoimmunity in MRL-lpr/lpr mice.     

The implications of autoimmunity and pregnancy

There are multiple epidemiological studies that document the potential adverse affects of autoimmunity on nearly every aspect of reproduction, even in the absence of clinically manifest autoimmune disease. Two decades ago, it was suggested that women with autoimmune diseases avoid pregnancy due to inordinate risks to the mother and the child. In contrast, newer epidemiological data demonstrated that advances in the treatment of autoimmune diseases and the management of pregnant women with these diseases have similarly improved the prognosis for mother and child. In particular, if pregnancy is planned during periods of inactive or stable disease, the result often is giving birth to healthy full-term babies without increased risks of pregnancy complications. Nonetheless, pregnancies in most autoimmune diseases are still classified as high risk because of the potential for major complications. These complications include disease exacerbations during gestation and increased perinatal mortality and morbidity in most autoimmune diseases, whereas fetal mortality is characteristic of the anti-phospholipid syndrome (APS). In this review, we will discuss these topics, including issues of hormones, along with potential long-term effects of the microchimerism phenomenon. With respect to pregnancy and autoimmune diseases, epidemiological studies have attempted to address the following questions: 1) Is it safe for the mother to become pregnant or are there acute or chronic effects of pregnancy on the course of the disease? 2) Does the disease alter the course and/or the outcome of a pregnancy and thereby represent an inordinate risk for the fetus and infant? And do new therapeutic and management approaches improve the pregnancy outcomes in women with autoimmune diseases? 3) Does passage of maternal autoantibodies represent a risk to the child? 4) Do pregnancy, parity, or other factors influencing hormonal status explain the female predominance of many autoimmune diseases, and is the pregnancy effect related to microchimerism? Answering these questions has taken on additional importance in recent decades as women in western countries now frequently choose to delay pregnancies and have some or all of their pregnancies after disease onset. In this paper, we primarily focus on APS, systemic lupus erythematosus (SLE), multiple sclerosis (MS), rheumatoid arthritis (RA), and type 1 diabetes (T1D).     

Development of grafted gld cells in athymic and euthymic recipients.

Mice homozygous for the gld (generalized lymphoproliferative disease) and lpr (lymphoproliferation) mutations display similar autoimmune and lymphoproliferative diseases. Both result from defective apoptosis, the targets of the lpr and gld mutations being the genes for, respectively, an apoptosis-signalling receptor [the Fas antigen receptor (FasR)] and its counter-receptor [the Fas ligand (FasL)]. Though this definitely causes the development and accumulation of large numbers of unusual Thy-1+ B220+ cells in peripheral lymphoid organs, details on how this actually occurs are still lacking. Whether differentiation of gld T cells into Thy-1+ B220+ cells might depend on the environment was analysed by phenotyping the cells which expanded in four different immunodeficient environments (nubg, nulpr, scid and scidbg). Though all four types of congenic chimeras developed hyperglobulinaemia, autoimmunity and a lymphoproliferative disease, substantial differences were found for the athymic and euthymic chimeras. In the athymic gld chimeras, the lymphoproliferation concerned all cell subsets, whereas in the euthymic gld chimeras it was, as in gld mice, due to the accumulation of cells of the Thy-1+ B220+ subset. Thus, the gld T cells could proliferate without differentiating into the Thy-1+ B220+ subset, but this depended on the nature of the environment. Furthermore, emergence of a gld syndrome in these four environments would suggest that B cells and stromal cells do not express FasL, at least in levels sufficient to compensate for the deficiency of the grafted gld cells.     

Autoantibody production is associated with polyclonal B cell activation in autoimmune mice which express the lpr or gld genes

Systemic autoimmune disease in humans and mice is characterized by hypergammaglobulinemia and abnormally high levels of serum autoantibodies. The presence of certain genes, such as the lpr and gld genes, induces otherwise normal mice to spontaneously develop systemic autoimmunity. To better understand the effect of these genes on the development of hypergammaglobulinemia, we quantitated the absolute number of splenic B cells producing antibodies reactive with each of four autoantigens and two conventional antigens and compared this to the total number of Ig-secreting spleen cells present in these mice. Whereas autoimmune mice had significantly greater numbers of autoantibody-secreting spleen cells than normal mice, they also had significantly greater numbers producing antibodies of conventional specificity. When expressed as a proportion of the total repertoire, no bias towards autoantibody production was present when autoimmune lpr and gld animals were compared to their congenic nonautoimmune C57BL/6 and C3H/HeJ counterparts. We also examined the B cell repertoires of recombinant inbred mice derived by mating autoimmune NZB with normal NFS mice. Some recombinant inbred (RI) lines developed hypergammaglobulinemia and produced large quantities of autoantibody. While evidence for specific (auto)antigenic stimulation was present in some RI lines, hypergammaglobulinemia was commonly associated with polyclonal B cell activation in these autoimmune mice as well.     

Self-antigen presentation by dendritic cells and lymphoid stroma and its implications for autoimmunity

The induction and maintenance of T cell tolerance is essential to prevent autoimmunity. A combination of central and peripheral mechanisms acts to control autoreactive T cells. In secondary lymphoid organs, dendritic cells (DCs) presenting self-antigen were thought to play a major role in the induction of peripheral T cell tolerance. Multiple recent studies have demonstrated that DCs are not absolutely essential to induce and maintain tolerance. Furthermore, it has also been recently shown that non-hematopoietic stromal cells expressing peripheral tissue-restricted antigens can induce T cell tolerance, independently of DCs. Together these studies imply that peripheral tolerance is more complex than previously thought and a consequence of the tolerogenic functions of the hematopoietic and non-hematopoietic compartments within secondary lymphoid organs.