Regulation of the anti-Sm autoantibody response in systemic lupus erythematosus mice by monoclonal anti-Sm antibodies

Nucleic acid-associated autoantigens: pathogenic involvement and therapeutic potential

Autoimmunity to ubiquitously expressed macromolecular nucleic acid-protein complexes such as the nucleosome or the spliceosome is a characteristic feature of systemic autoimmune diseases. Disease-specificity and/or association with clinical features of some of these autoimmune responses suggest pathogenic involvement which, however, has been proven in only a few cases so far. Although the mechanisms leading to autoimmunity against nucleic acid-containing complexes are still far from being fully understood, there is increasing experimental evidence that the nucleic acid component may act as a co-stimulator or adjuvans via activation of nucleic acid-binding receptor systems such as Toll-like receptors in antigen-presenting cells. Dysregulated apoptosis and inappropriate stimulation of nucleic acid-sensing receptors may lead to loss of tolerance against the protein components of such complexes, activation of autoreactive T cells and formation of autoantibodies. This has been demonstrated to occur in systemic lupus erythematosus and seems to represent a general mechanism that may be crucial for the development of systemic autoimmune diseases. This review provides a comprehensive overview of the most thoroughly-characterized nucleic acid-associated autoantigens, describing their structure and biological function, as well as the nature and pathogenic importance of the reactivities directed against them. Furthermore, recent advances in immunotherapy such as antigen-specific approaches targeted at nucleic acid-binding antigens are discussed.

RNA-associated autoantigens activate B cells by combined B cell antigen receptor/Toll-like receptor 7 engagement

Previous studies (Leadbetter, E.A., I.R. Rifkin, A.H. Hohlbaum, B. Beaudette, M.J. Shlomchik, and A. Marshak-Rothstein. 2002. Nature. 416:603–607; Viglianti, G.A., C.M. Lau, T.M. Hanley, B.A. Miko, M.J. Shlomchik, and A. Marshak-Rothstein. 2003. Immunity. 19:837–847) established the unique capacity of DNA and DNA-associated autoantigens to activate autoreactive B cells via sequential engagement of the B cell antigen receptor (BCR) and Toll-like receptor (TLR) 9. We demonstrate that this two-receptor paradigm can be extended to the BCR/TLR7 activation of autoreactive B cells by RNA and RNA-associated autoantigens. These data implicate TLR recognition of endogenous ligands in the response to both DNA- and RNA-associated autoantigens. Importantly, the response to RNA-associated autoantigens was markedly enhanced by IFN-α, a cytokine strongly linked to disease progression in patients with systemic lupus erythematosus (SLE). As further evidence that TLRs play a key role in autoantibody responses in SLE, we found that autoimmune-prone mice, lacking the TLR adaptor protein MyD88, had markedly reduced chromatin, Sm, and rheumatoid factor autoantibody titers.

Epitope spreading within lupus-associated ribonucleoprotein antigens

Autoantibodies reactive with several cellular antigens are present in the sera of patients with systemic lupus erythematosus. Polypeptides within the Ro-RNP complex and the snRNP complex are often targeted by these autoantibodies. One of the mechanisms responsible for their evolution is that of epitope spreading. Experimental animal model systems provide evidence for this. This review discusses the animal model systems of epitope spreading within these ribonucleoprotein antigens, the mechanisms of epitope spreading, and its relevance for disease pathogenesis.

Mechanisms of autoimmunity

Autoimmunity results from the failure of self-tolerance of the adaptive immune system. The reactivity of antibodies and T cells against endogenous antigens frequently causes organ damage, and consequent autoimmune disease. Systemic lupus erythematosus (SLE) is an extreme example of such a breakdown of tolerance. Our studies with spontaneous genetic mouse models of SLE and an experimentally induced model have elucidated many of the underlying cellular and genetic mechanisms of this immune dysregulation. We find that the B cell plays a key central role in this process and represents an attractive target for therapeutic intervention.

Immune complexes present in the sera of autoimmune mice activate rheumatoid factor B cells

The fate of an autoreactive B cell is determined in part by the nature of the interaction of the B cell receptor with its autoantigen. In the lpr model of systemic autoimmunity, as well as in certain human diseases, autoreactive B cells expressing rheumatoid factor (RF) binding activity are prominent. A murine B cell transgenic model in which the B cell receptor is a RF that recognizes IgG2a of the j allotype (IgG2aj), but not the b allotype, was used in this study to investigate how the form of the autoantigen influences its ability to activate B cells. We found that sera from autoimmune mice, but not from nonautoimmune mice, were able to induce the proliferation of these RF+ B cells but did not stimulate B cells from RF- littermate controls. The stimulatory factor in serum was found to be IgG2aj, but the IgG2aj was stimulatory only when in the form of immune complexes. Monomeric IgG2aj failed to stimulate. Immune complexes containing lupus-associated nuclear and cytoplasmic autoantigens were particularly potent B cell activators in this system. Appropriate manipulation of such autoantibody/autoantigen complexes may eventually provide a means for therapeutic intervention in patients with certain systemic autoimmune disorders.

Regulation of autoreactive anti-IgG (rheumatoid factor) B cells in normal and autoimmune mice

In systemic autoimmune disease, autoantibodies target specific self-components in patterns that depend on the particular underlying disease. Therefore, in order to understand how tolerance to these self-components breaks down, it is important to study B cells with those specificities, rather than artificial autoantigens. We have been investigating the regulation of autoreactive B cells with specificity for self IgG2a (the rheumatoid factor or RF specificity) in order to understand how normal mice regulate RF autoantibodies and how this fails in autoimmune mice. A transgenic (Tgic) mouse based on an RF isolated from a diseased MRL/lpr/lpr mouse was constructed and studied in both normal and autoimmune-prone genetic backgrounds. Normal mice do not appear to regulate the RF clone negatively, nor do they appear to activate it substantially. Thus, a disease-related RF is "clonally indifferent." However, in a, Fas-deficient autoimmune-prone animal, these RF B cells are activated to divide and secrete in an antigen-specific manner. A high-affinity RF Tgic mouse was also constructed to determine whether RFs could be tolerized in normal mice. These B cells were deleted or edited in the presence of the autoantigen, which originated from maternal IgG in young mice. Interestingly, shortly after weaning, many mice began to produce autoreactive RF. Escape from tolerance could last for months and was most likely perpetuated by a positive feedback mechanism. Such a mechanism could exist in autoimmune animals and could have important implications for chronic autoimmune disease, as discussed.

T Cell Autoimmunity in Ig Transgenic Mice

Autoantibodies directed at a diverse group of proteins of the U1/Sm ribonucleoprotein (snRNP) are characteristic of systemic lupus erythematosus and are found in the MRL murine model of this disease. This study examines the role of transgenic B lymphocytes in the regulation of autoreactive T cells to the snRNP autoantigen. Transgenic mice were developed bearing an Ig heavy chain gene specific for the D protein component of murine snRNP. B lymphocytes in these mice are neither deleted nor anergic and are of an immature (heat-stable Aghigh) phenotype. T lymphocytes from anti-snRNP transgenic mice were examined using a recombinant form of the D protein of the murine snRNP complex. Our results revealed that transgenic anti-snRNP B cell APCs stimulated CD4 T cells from wild-type C57BL/6 and MRL lpr/lpr mice, while nonspecific APCs failed to stimulate CD4 T cells. This study demonstrates that autoreactive T cells are not deleted from wild-type mice, although their activation is facilitated by autoantigen-specific APCs. The snRNP-reactive T cells in C57BL/6 transgenic mice are tolerized, in contrast to those T cells from MRL lpr/lpr transgenic mice. These studies implicate a role for autoreactive B lymphocytes in the in vivo activation and/or diversification of autoreactive T cells.

Further characterization of the autoantibody response of Palmerston North mice

PN mice spontaneously develop, with age, a lupus-like disease. The present study further evaluated autoantibody production in female PN mice. As early as 1 month of age, all PN mice had detectable IgM antibodies to dsDNA and ssDNA and two-thirds produced IgM anticardiolipin antibodies. By 3 months of age, all PN mice exhibited evidence of isotype switch in their autoantibody response; 88-100% had serum IgG antibodies to ssDNA and dsDNA, respectively. By 6-12 months of age, essentially all female PN mice had IgG antibodies to ssDNA, dsDNA, cardiolipin and other phospholipids (PS, PC, PI, and PG), and IgG and 63% produced IgG anti-mouse erythrocyte antibodies. In addition, 50-100% produced IgA antibodies to dsDNA and ssDNA, and one-third produced IgA anti-IgG antibodies. Antibodies to U1RNP and Sm were present in 81% of 6- to 12-month-old PN mice and 39-94% had IgG or IgM antibodies to mouse thymocytes. Although all four IgG isotypes were represented in the anti-dsDNA response, IgG1 antibodies dominated the IgG anticardiolipin response. The presence of IgA autoantibodies and the predominance of IgG1 in the IgG anticardiolipin response suggest that IL-4 and either IL-5 and/or TGF-beta serve as B cell stimulatory cytokines for autoreactive B cells in PN mice.

Maternal Ig Mediates Neonatal Tolerance in Rheumatoid Factor Transgenic Mice but Tolerance Breaks Down in Adult Mice

We have recently demonstrated that B cell deletion occurs in the bone marrow of IgHa high affinity anti-IgG2aa (RF) transgenic mice. Here we demonstrate via genetic crosses that the source of IgG2a is the mother, thus establishing a transplacental mechanism that ensures tolerance to developmentally expressed Ags. Since maternal IgG can mediate tolerance in young mice, whether tolerance is maintained or, instead, autoimmunity ensues after weaning was investigated. We find that deletion remits abruptly in these RF transgenic mice beginning at 2 to 3 wk postweaning, and some degree of autoreactivity can be observed thereafter for weeks to months. The mechanism of sustained expression of autoreactive RF B cells in normal mice is unclear as yet, but a plausible mechanism is that once self-reactive cells are present, the antibody they secrete markedly reduces the autoantigen levels, presumably allowing further development, rather than deletion, of newly arising B lineage cells. The phenotype of these RF transgenic mice suggests a positive feedback mechanism that tends to perpetuate autoimmunity once it has been established. If such a mechanism were to exist in autoimmune animals, it could have important implications for the establishment and maintenance of B and T cell tolerance in chronic autoimmune diseases.

Autoreactive B cell regulation: peripheral induction of developmental arrest by lupus-associated autoantigens

Anti-Sm and anti-ssDNA transgenic (Tg) mice were generated using the VH-D-JH rearrangement of an anti-Sm hybridoma of MRL/Mp-lpr/lpr origin. B cells of each specificity account for 15%-35% of the splenic repertoire, but no circulating anti-Sm or anti-ssDNA antibodies are detected. Most autoreactive cells exhibit an immature B cell phenotype and have short half-lives equivalent to those of non-Tg immature B cells. However, at least some anti-Sm B cells are functional, because immunization with murine snRNPs induces anti-Sm secretion. We propose that anti-Sm and anti-ssDNA are eliminated during the transition to mature B cells and that this late stage of tolerance induction is consequential to their spontaneous activation in murine lupus.

Features of autoantigens

The major cellular antigens recognized by autoantibodies in SLE and other systemic autoimmune diseases have been identified and characterized over the past 25 years. The pioneering studies of Eng Tan demonstrate the importance of autoantibodies as diagnostic markers. However, why certain autoantibodies, such as anti-Sm, are pathognomonic of SLE, while others are markers of other autoimmune disease subsets, remains unanswered. This central question continues to drive much current research into the pathogenesis of SLE. Features of the autoantigens recognized by autoantibodies may provide important clues to the causes of lupus. Most autoantigens in systemic autoimmunity are multicomponent nucleoprotein complexes. These particles are encountered by the immune system as units, resulting in the tandem production of autoantibodies recognizing several components of the same complex. However, the intermolecular-intrastructural spreading of autoimmunity is regulated by mechanisms that at present are defined poorly. Also unexplained is the observation that the antigenic determinants recognized by autoantibodies are restricted and frequently correspond to active sites or functional domains. Analysis of experimental models of autoimmunity suggests that altering the structure of autoantigens, due to abnormal protein-protein interactions, hapten binding, altered degradation, or other mechanisms, could help to explain both the restricted patterns of autoantibody spreading and the selective targeting of antigenic sites. This may be a worthwhile area for further investigation of the pathogenesis of systemic autoimmune diseases.

Antiphospholipid syndrome and the idiotypic network

To study whether monoclonal anticardiolipin antibodies (aCL), derived from patients with antiphospholipid syndrome (APS), have similar pathogenic potential, we have employed an experimental model of antiphospholipid syndrome. Monoclonal aCL were produced by the combined method of EBV transformation and somatic cell hybridization of lymphocytes, derived from patients with APS. The monoclonal aCL were used to immunize mice at the footpads and the mice were followed for serological and clinical manifestations of APS. The monoclonal antibody EY2C9, was found to bind weakly to cardiolipin and other phospholipids (i.e. phosphatidyl-serine, phosphatidyl-ethanolamine and phosphatidyl-inositol). The antibody TM1B9, although derived from a patient with SLE and with secondary APS, did not react with phospholipids. Immunization of naive BALB/c mice with EY2C9 was followed by production of sustained high titers of antiphospholipid antibodies associated with prolonged activated partial thromboplastin time (APTT) (46.8 +/- 5.0 s vs. 22.4 +/- 1.7 s, in the non-immunized mice). Mice immunized with TM1B9 had a more moderate titer of antiphospholipid antibodies and did not show prolonged APTT. The pregnant mice, that were immunized with EY2C9, had increased fetal resorption rate (the equivalent of fetal loss in the human) of 36.8 +/- 10% (vs. 2 +/- 4% in mice immunized with TM1B9). Our results confirm that monoclonal aCL, derived from a patient with APS, can have a pathogenic potential, dysregulating the idiotypic network and leading to the development of characteristic signs of APS.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of interleukin-1-induced effects in synoviocytes transduced with the human IL-1 receptor antagonist cDNA using an adenoviral vector

In this report, we present data showing that a recombinant adenoviral vector (Ad.RSVIL-1ra) containing the cDNA for human interleukin-1 receptor antagonist protein (IL-1ra) can genetically modify synoviocytes both in vitro and in vivo. Human synoviocytes infected with Ad.RSVIL-1ra in vitro expressed and secreted high levels of human IL-1ra that were detected by ELISA of tissue culture supernatants. New Zealand White rabbits that received intra-articular injections of Ad.RSVIL-1ra expressed transgenic IL-1ra in synoviocytes, and secretion was detected for at least 4 weeks post-infection. Further, biological activity of the transgenic IL-1ra was demonstrated by its ability to inhibit IL-1-induced prostaglandin E2 (PGE2) synthesis in vitro and IL-1-induced glycosaminoglycan (GAG) degradation in vivo. These data demonstrate that recombinant adenoviral vectors can mediate the intra-articular expression of anti-inflammatory proteins and may be a reasonable method to deliver therapeutically relevant proteins for the regional treatment of synovial inflammation.

Pathogenic natural anti-cardiolipin antibodies: the experience from monoclonal gammopathy

Anti-cardiolipin antibodies (ACA) were detected in 19% of sera from patients with monoclonal gammopathies (MG). ACA were purified from the sera of patients with MG. One of the IgG-ACA was found to be monospecific with high affinity for cardiolipin, and to carry a pathogenic ACA Id (1.10). Active immunization of naive BALB/c mice with the purified IgG-ACA was followed by production in the mice of sustained high titres of ACA, associated with prolonged activated partial thromboplastin time (APTT) (61 +/- 14s versus 31 +/- 2s in control mice; P < 0.001) and thrombocytopenia (468,000 +/- 224,000/mm3 versus 994,000 +/- 92,000/mm3 in controls; P < 0.001). The titres of other autoantibodies (e.g. anti-DNA, anti-histones), although being high after immunization, decreased rapidly and were undetected after 1 month following the boost injection. The mice immunized with the IgG-ACA exhibited low fecundity (36% of mice became pregnant versus 62% observed in the group immunized with control IgG). The pregnant mice had increased resorption rate (the equivalent of fetal loss in the human) of 52 +/- 8% (versus 5 +/- 4% in the control group). The mean (+/- s.d.) embryo and placental weights in mice with anti-phospholipid syndrome (APLS) were significantly lower compared with the mice injected with control IgG (682 +/- 304 mg and 102 +/- 12 mg versus 1303 +/- 105 mg and 145 +/- 8 mg, respectively; P < 0.001). Serum monoclonal immunoglobulins having autoantibody activity may be regarded as an expansion of clones producing natural autoantibodies. Our results confirm the pathogenic role of natural ACA in the pathogenesis of the anti-phospholipid syndrome.

Pathogenic serum IgG anticardiolipin antibodies and the idiotypic network

OBJECTIVES

To determine whether active immunisation of mice with pathogenic anticardiolipin antibodies (IgG and IgM), derived from the serum of a patient with the antiphospholipid syndrome, could dysregulate the idiotypic cascade and induce the production of anti-anti-anti-cardiolipin (Ab3) with anticardiolipin activity by the mice with the association of overt antiphospholipid syndrome.

METHODS

Anticardiolipin antibodies were purified from the serum of a patient with the antiphospholipid syndrome. The purified anticardiolipin antibodies were used to immunise mice at the footpads and the mice were then followed up for serological and clinical manifestations of the antiphospholipid syndrome.

RESULTS

The IgG anticardiolipin antibody was found to be monospecific and to bind cardiolipin with high affinity. Immunisation of naive BALB/c mice with the purified IgG anticardiolipin antibody was followed by production in the mice of sustained high titres of IgG anticardiolipin antibody, associated with a prolonged activated partial thromboplastin time (64.5 (9.7) v 30.1 (1.7) seconds in control mice) and thrombocytopenia (0.4 (0.06) x 10(9) v 1.0 (0.09) x 10(9)/l platelets in controls). The titres of other autoantibodies (for example, antibodies to DNA, histone), though high after the immunisation, decreased rapidly and were almost undetected one month after the boost injection. The mice immunised with the IgG anticardiolipin antibody showed low fecundity (36% of mice became pregnant v 62% in the group immunised with control IgG). The pregnant mice had an increased resorption rate (the equivalent of fetal loss in the human) of 61 (9)% v 5 (4)% in the control group. The mean (SD) embryo and placental weights in mice with the antiphospholipid syndrome were significantly lower than in the mice injected with control IgG (641 (210) and 103 (14) mg v 1303 (105) and 145 (8) mg respectively. The IgM anticardiolipin antibodies purified from the same patient were found to be polyspecific, binding with low affinity to anticardiolipin antibodies and double stranded DNA, and carried the anti-DNA idiotype 16/6. Mice immunised with the purified IgM anticardiolipin antibodies, though showing reduced fecundity (30%), had only a slightly increased resorption rate (12 (9) v 3 (5)% in controls) and only a slight and statistically non-significant decrease in mean (SD) embryo and placental weights (1134 (188) and 136 (11) mg respectively).

CONCLUSIONS

The results confirm the induction of pathogenic anticardiolipin antibodies by immunisation with serum anticardiolipin, dysregulating the idiotypic network, and point to the higher pathogenic potential of serum IgG v IgM anticardiolipin antibodies.

The murine Sm-D autoantigen: multiple genes, genetic polymorphism, evolutionary conservation and lack of intervening sequences in the coding region

Antibodies to the Sm nuclear antigen are diagnostic of systemic lupus erythematosus (SLE). MRL/Mp-lpr/lpr mice develop a similar illness, and a proportion also develop anti-Sm. To understand better anti-Sm reactivity in this murine model, we have cloned the murine Sm-D autoantigen. One cDNA clone was 517 bp long with an open reading frame of 357 nucleotides, encoding a 13.3 kDa protein of 119 amino acids. At the nucleotide level, the murine Sm-D cDNA was 89.8% homologous with human Sm-D (94% in the coding region), yet there was identity at the protein level, including a Gly-Arg nine-fold repeated C-terminus motif. Southern blot analysis of PstI-digested genomic DNA from seven mouse strains demonstrated a 7.8 kb band in every strain; in addition, a 2.8 kb band was seen in AKR/J, LG/J and MRL/Mp-lpr/lpr. PCR amplification of genomic DNA showed a single Sm-D gene product of 360 bp, which indicated a lack of intervening sequences. The Sm-D protein is thus highly conserved in evolution, probably owing to its essential role in the physiology of the cell.

Spontaneous production of anti-mouse red blood cell autoantibodies is independent of the polyclonal activation in NZB mice

New Zealand Black (NZB) mice spontaneously develop an autoimmune hemolytic anemia together with a markedly increased production of polyclonal antibodies. The spontaneous generation of anti-mouse red blood cells (MRBC), anti-bromelain-treated MRBC (BrMRBC) and anti-DNA autoantibodies was compared to the polyclonal antibody formation in irradiated (800 rad) 2-month-old NZB mice reconstituted with bone marrow cells (BMC) from 2- or 10-month-old NZB mice. The injection of 10-month-old NZB BMC markedly accelerated the mortality rate in parallel with the progressive increase of anti-MRBC and anti-BrMRBC autoantibody production, but the spontaneous production of polyclonal IgM antibodies and anti-DNA autoantibodies was completely abolished down to the levels of non-autoimmune mice. In contrast, mice reconstituted with 2-month-old NZB BMC exhibited neither the acceleration of anemia nor the lack of polyclonal antibody production. These results strongly suggest that the spontaneous production of anti-MRBC autoantibodies, including anti-BrMRBC autoantibodies, in the NZB mouse occurs independently of the polyclonal B cell activation, and that they result from a specific immune stimulation, while the anti-DNA autoantibody production is a consequence of polyclonal antibody formation.