MHC class II-bound self peptides from autoimmune MRL/lpr mice reveal potential T cell epitopes for autoantibody production in murine systemic lupus erythematosus

Recent advances and current state of immunotherapy in systemic lupus erythematosus

INTRODUCTION

Systemic lupus erythematosus (SLE) is an autoimmune syndrome that poses significant challenges in diagnosis and treatment. Dysregulated innate and adaptive immune systems are involved in its pathogenesis. A plethora of novel immunotherapies have been developed for the treatment of SLE but many have failed early clinical trials.

AREAS COVERED

This review summarizes immunotherapies under recent development with relevance to the targeted cellular or soluble factors involved in the pathogenesis of SLE.

EXPERT OPINION

SLE is a complicated disease with much heterogeneity. Novel immunotherapies with different mechanisms of action that are currently under development include biologic agents targeting co-stimulatory molecules, cytokines or their receptors and signaling molecules and B cells, cell-based therapy and peptide therapy. Together with good scientific rationale and advanced biological engineering techniques, optimization of clinical trial design, patient selection and disease outcome measures are essential to demonstrate the clinical efficacy and safety of these agents.

Tetramers reveal IL-17-secreting CD4+ T cells that are specific for U1-70 in lupus and mixed connective tissue disease

Antigen-specific CD4(+) T cells are implicated in the autoimmune disease systemic lupus erythematosus (SLE), but little is known about the peptide antigens that they recognize and their precise function in disease. We generated a series of MHC class II tetramers of I-E(k)-containing peptides from the spliceosomal protein U1-70 that specifically stain distinct CD4(+) T-cell populations in MRL/lpr mice. The T-cell populations recognize an epitope differing only by the presence or absence of a single phosphate residue at position serine(140). The frequency of CD4(+) T cells specific for U1-70(131-150):I-E(k) (without phosphorylation) correlates with disease severity and anti-U1-70 autoantibody production. These T cells also express RORγt and produce IL-17A. Furthermore, the U1-70-specific CD4(+) T cells that produce IL-17A are detected in a subset of patients with SLE and are significantly increased in patients with mixed connective tissue disease. These studies provide tools for studying antigen-specific CD4(+) T cells in lupus, and demonstrate an antigen-specific source of IL-17A in autoimmune disease.

Suppression of dsDNA-specific B lymphocytes reduces disease symptoms in SCID model of mouse lupus

Self-specific B cells play a main role in the pathogenesis of lupus. This autoimmune disease is characterized by the generation of autoantibodies against self antigens, and the elimination of B and T cells involved in the pathological immune response is a logical approach for effective therapy. We have previously constructed a chimeric molecule by coupling a DNA-mimotope peptides to an anti-CD32 antibody. Using this protein molecule for the treatment of lupus-prone MRL/lpr mice, we suppressed selectively the autoreactive B-lymphocytes by cross-linking B cell receptors with the inhibitory FcγRIIb receptors. This approach was limited by the development of anti-chimeric antibodies in MRL mice. In order to avoid this problem, we established a murine severe combined immunodeficiency lupus model, allowing a long-term chimera therapy. Elimination of the double-stranded DNA-specific B cells by chimera therapy in MRL-transferred immunodeficient mice resulted in inhibition of T cell proliferation and prevented the appearance of IgG anti-DNA antibodies and of proteinuria.

Stress proteins are used by the immune system for cognate interactions with anti-inflammatory regulatory T cells

Since the initial discovery of the protective role of heat shock protein (HSP) 60 in arthritis, T cell recognition of endogenous HSP was found to be one of the possible underlying mechanisms. Recently we have uncovered potent disease-suppressive Tregs (anti-inflammatory immunosuppressive T cells) recognizing HSP70 self-antigens, and enabling selective targeting of such Tregs to inflamed tissues. HSP70 is a major contributor to the major histocompatibility complex (MHC) Class II ligandome and we have shown that a conserved HSP70-epitope (B29) is abundantly present in murine MHC Class II. Upon transfer, B29-induced CD4+CD25+Foxp3+T cells suppressed established proteoglycan-induced arthritis (PGIA) in mice. These self-antigen specific Tregs were activated in vivo and as little as 4.000 cells sufficed to fully inhibit arthritis. Furthermore, in vivo depletion of transferred Tregs abrogated disease suppression. Given that B29 can be presented by most human MHC class II molecules and that B29 inhibited arthritis in HLA-DQ8 (human MHC) transgenic mice, we feel that therapeutic vaccination with selected HSP peptides can be an effective route for induction of anti-inflammatory Tregs as a novel intervention in chronic inflammatory diseases.

Two paradoxes and a surprise on the road to an understanding of systemic lupus erythematosus

Whereas systemic lupus erythematosus (SLE) as normally encountered results from the coming together of a complex mix of genetic and environmental factors, SLE also develops in virtually all those rare people who lack a functional gene for the first component of complement (C1q). The pathogenic IgG antibodies against double-stranded DNA characteristic of the disease are made in response to nucleosomes - the package of DNA and histone molecules forming the unit structure of chromatin - which are present in apoptotic cells. Analysis of the C1q phenomenon illuminates the arrangements that are normally in place to ensure tolerance is maintained to nucleosomal antigens. Surprisingly in view of the high level of apoptosis occurring in the thymus, it appears that anti-histone helper T cells, which are likely to be required for IgG anti-DNA production, are not deleted in the thymus. It seems rather that tolerance is maintained by non-availability of antigen brought about by the highly efficient C1q-dependent phagocytosis of apoptotic cells. This 'immunological ignorance' may be backed up by mechanisms of peripheral tolerance if antigen does become available. Idiopathic SLE may arise when apoptotic cell clearance is sub-optimal, making clearance a promising target for therapy.

Ribosomal versus non-ribosomal cellular antigens: factors determining efficiency of indirect presentation to CD4+ T cells

Proteins released from dying cells can be taken up and presented by antigen-presenting cells (APC) to T cells. While the presentation of such self antigens may lead to beneficial anti-tumour responses, in autoimmune disease it leads to pathological immune responses. The sub-set of self proteins targeted in autoimmune disease is circumscribed, and certain cellular components such as ribonucleoprotein (RNP) complexes are often targeted. Although explanations for this antigen selectivity have been proposed, there has been little direct testing of these hypotheses. We and others previously showed that ribosomal proteins, targeted in autoimmune disease, are also targets of anti-tumour T-cell responses. We asked whether particular properties of ribosomal proteins such as incorporation into RNP complexes or sub-cellular localization enhance ribosomal protein presentation by APC to CD4(+) T cells. Ribosomal protein antigens within purified intact ribosomes or free of the ribosomes were equally well taken up and presented by APC, demonstrating that inclusion of ribosomal proteins into an RNP complex does not confer an advantage. However, antigens localized to ribosomes within apoptotic cells were less efficiently taken up and presented by APC than the same antigens localized diffusely throughout the cell. This suggests that presentation of ribosomal proteins is somehow down-regulated, possibly to facilitate presentation of other less-abundant intracellular proteins. Consequently, the explanation for the frequent targeting of ribosomal proteins by both autoimmune and anti-tumour T-cell responses is not at the level of uptake from apoptotic cells and must be sought elsewhere.

The U1-snRNP complex: structural properties relating to autoimmune pathogenesis in rheumatic diseases

The U1 small nuclear ribonucleoprotein particle (snRNP) is a target of autoreactive B cells and T cells in several rheumatic diseases including systemic lupus erythematosus (SLE) and mixed connective tissue disease (MCTD). We propose that inherent structural properties of this autoantigen complex, including common RNA-binding motifs, B and T-cell epitopes, and a unique stimulatory RNA molecule, underlie its susceptibility as a target of the autoimmune response. Immune mechanisms that may contribute to overall U1-snRNP immunogenicity include epitope spreading through B and T-cell interactions, apoptosis-induced modifications, and toll-like receptor (TLR) activation through stimulation by U1-snRNA. We conclude that understanding the interactions between U1-snRNP and the immune system will provide insights into why certain patients develop anti-U1-snRNP autoimmunity, and more importantly how to effectively target therapies against this autoimmune response.

Potential for anti-DNA immunoglobulin peptide therapy in systemic lupus erythematosus

BACKGROUND

Systemic lupus erythematosus (SLE) is a chronic autoimmune disease with elevated morbidity and multi-organ involvement. While many strategies have shown efficacy and improved management of SLE, they have often been associated with adverse effects. Some patients may not respond well to some treatments because of inter-individual variability of the disease. More specific and safer therapies are needed.

OBJECTIVE/METHODS

To review literature on peptide-based therapy of SLE.

RESULTS/CONCLUSIONS

Recently, emphasis has been placed on targeting molecules and pathways involved in the inflammatory response in SLE, including the use of immunogenic peptides derived from anti-DNA antibodies. Encouraging data from murine models of SLE have led to tests in initial clinical trials in humans--which have unfortunately not met the primary endpoints. The current challenge is to design improved strategies for immunotherapeutic use of anti-DNA peptides in SLE.

IK cytokine ameliorates the progression of lupus nephritis in MRL/lpr mice

OBJECTIVE

IK cytokine has been isolated as a factor that inhibits interferon-gamma (IFNgamma)-induced expression of class II major histocompatibility complex (MHC) antigens. Aberrant expression of class II MHC antigens has reportedly been recognized in the target organs of autoimmune diseases and been associated with disease activity. In this study, we investigated whether IK cytokine can ameliorate the progression of lupus nephritis in MRL/lpr mice.

METHODS

A truncated IK analog was prepared and transfected into a nonmetastatic fibroblastoid cell line, and then injected subcutaneously into MRL/lpr mice at ages 8 weeks (before the onset of lupus nephritis) and 12 weeks (at the early stage of the disease).

RESULTS

An IK cytokine, when it was translated from methionine at position 316, acted as a secretory protein. This truncated IK cytokine (tIK) reduced IFNgamma-induced class II MHC expression in various cells through decreased expression of class II MHC transcription activator. Treatment of MRL/lpr mice with tIK significantly reduced renal damage as compared with control mice. A significant decrease in macrophage and T cell infiltration was found in the kidneys of tIK-treated mice, resulting in decreased production of IFNgamma and interleukin-2. Mice treated with tIK also showed significant reduction of anti-DNA antibodies and circulating immune complexes. A specific reduction of class II MHC expression was observed on B cells and monocytes as well as in the kidney.

CONCLUSION

We prepared a potent IK analog and demonstrated its ability to ameliorate the progression of lupus nephritis. This agent may therefore provide a new therapeutic approach for lupus nephritis.

B cells and dendritic cells from V kappa 8 light chain transgenic mice activate MRL-lpr/gld CD4+ T cells

Autoreactive CD4+ T cells are required for full expression of disease in human systemic lupus erythematosus and in spontaneous murine lupus. However, the Ag specificity of these CD4+ T cells remains largely unknown. Rheumatoid factor (RF) B cells function as highly efficient APCs by taking up immune complexes (IC) and presenting IC constituents to T cells. We hypothesized that Ag-specific CD4+ T cells in lupus-prone mice could be identified by stimulating the CD4+ T cells with RF B cells from AM14 RF BCR transgenic mice pulsed with IC containing lupus-associated autoantibodies and autoantigens. This approach identified several independent T cell lines that proliferated robustly in response to IC-pulsed spleen cells from the AM14 RF BCR transgenic mice. However, these T cells did not recognize an IC constituent. Instead, these T cells recognized a determinant dependent on the inheritance of the transgene-encoded Vkappa8 L chain, most likely a neoantigen created by the insertion of the transgene into the genome. Additionally, although the precise nature of the neoantigen is not known, the T cells described in this report may provide a useful tool for examining the role of T cells in the RF autoantibody response.

Most nuclear systemic autoantigens are extremely disordered proteins: implications for the etiology of systemic autoimmunity

Patients with systemic autoimmune diseases usually produce high levels of antibodies to self-antigens (autoantigens). The repertoire of common autoantigens is remarkably limited, yet no readily understandable shared thread links these apparently diverse proteins. Using computer prediction algorithms, we have found that most nuclear systemic autoantigens are predicted to contain long regions of extreme structural disorder. Such disordered regions would generally make poor B cell epitopes and are predicted to be under-represented as potential T cell epitopes. Consideration of the potential role of protein disorder may give novel insights into the possible role of molecular mimicry in the pathogenesis of autoimmunity. The recognition of extreme autoantigen protein disorder has led us to an explicit model of epitope spreading that explains many of the paradoxical aspects of autoimmunity - in particular, the difficulty in identifying autoantigen-specific helper T cells that might collaborate with the B cells activated in systemic autoimmunity. The model also explains the experimentally observed breakdown of major histocompatibility complex (MHC) class specificity in peptides associated with the MHC II proteins of activated autoimmune B cells, and sheds light on the selection of particular T cell epitopes in autoimmunity. Finally, the model helps to rationalize the relative rarity of clinically significant autoimmunity despite the prevalence of low specificity/low avidity autoantibodies in normal individuals.

Studies of murine systemic autoimmunity

Systemic autoimmunity is an important clinical problem, offering a window into fundamental questions of self-nonself tolerance. We have used cellular immunology, serology, and immunopathology to approach several spontaneous mouse models. Although much remains to be done, a picture is emerging of pathological antigen-driven immune responses to self nuclear antigens, highly dependent on multiple genes, and susceptible to abnormalities of apoptosis.

Peptide-based immunotherapy of systemic lupus erythematosus

Current drug-based therapy for systemic lupus erythematosus (SLE) are non-specific and often counterbalanced by adverse effects. Current research aims at developing specific treatments that target deleterious cells only and not the whole immune system. This strategy requires the identification of sequences derived from major lupus autoantigens, responsible for the activation of autoreactive B and T cells. This review summarizes the identification and characterization of peptides, which are able to modulate T cells ex vivo, and describes the promising results obtained after administration of some of these peptides in lupus mice. Although these therapeutic trials are encouraging, the precise mode of action of peptide-based immunotherapy is still elusive. Here, we discuss the possible mechanisms leading to T-cell tolerance induction and the feasibility of extending the success of peptide-based therapy from animal models to human.

Interferon and granulopoiesis signatures in systemic lupus erythematosus blood

Systemic lupus erythematosus (SLE) is a prototype systemic autoimmune disease characterized by flares of high morbidity. Using oligonucleotide microarrays, we now show that active SLE can be distinguished by a remarkably homogeneous gene expression pattern with overexpression of granulopoiesis-related and interferon (IFN)-induced genes. Using the most stringent statistical analysis (Bonferroni correction), 15 genes were found highly up-regulated in SLE patients, 14 of which are targets of IFN and one, defensin DEFA-3, a major product of immature granulocytes. A more liberal correction (Benjamini and Hochberg correction) yielded 18 additional genes, 12 of which are IFN-regulated and 4 granulocyte-specific. Indeed immature neutrophils were identified in a large fraction of SLE patients white blood cells. High dose glucocorticoids, a standard treatment of disease flares, shuts down the interferon signature, further supporting the role of this cytokine in SLE. The expression of 10 genes correlated with disease activity according to the SLEDAI. The most striking correlation (P < 0.001, r = 0.55) was found with the formyl peptide receptor-like 1 protein that mediates chemotactic activities of defensins. Therefore, while the IFN signature confirms the central role of this cytokine in SLE, microarray analysis of blood cells reveals that immature granulocytes may be involved in SLE pathogenesis.

T cell reactivity to MHC class II-bound self peptides in systemic lupus erythematosus-prone MRL/lpr mice

The epitopes recognized by pathogenic T cells in systemic autoimmune disease remain poorly defined. Certain MHC class II-bound self peptides from autoimmune MRL/lpr mice are not found in eluates from class II molecules of MHC-identical C3H mice. Eleven of 16 such peptides elicited lymph node cell and spleen cell T cell proliferation in both MRL/lpr (stimulation index = 2.03-5.01) and C3H mice (stimulation index = 2.03-3.75). IL-2 and IFN-gamma production were detected, but not IL-4. In contrast to what was seen after immunization, four self peptides induced spleen cell proliferation of T cells from naive MRL/lpr, but not from C3H and C57BL/6.H2(k), mice. These peptides were derived from RNA splicing factor SRp20, histone H2A, beta(2)-microglobulin, and MHC class II I-A(k)beta. The first three peptides were isolated from I-E(k) molecules and the last peptide was bound to I-A(k). T cell responses, evident as early as 1 mo of age, depended on MHC class II binding motifs and were inhibited by anti-MHC class II Abs. Thus, although immunization can evoke peripheral self-reactive T cells in normal mice, the presence in MRL/lpr mice of spontaneous T cells reactive to certain MHC-bound self peptides suggests that these T cells actively participate in systemic autoimmunity. Peptides eluted from self MHC class II molecules may yield important clues to T cell epitopes in systemic autoimmunity.

Naturally processed chromatin peptides reveal a major autoepitope that primes pathogenic T and B cells of lupus

Major autoepitopes for pathogenic Th cells of lupus were previously found in core histones of nucleosomes by testing overlapping synthetic peptides. To detect other dominant epitopes, we eluted peptides from MHC class II molecules of a murine lupus APC line that was fed with crude chromatin. The eluted peptides were purified by reverse-phase HPLC and tested for their ability to stimulate autoimmune Th clones, and then analyzed by mass spectrometry. Amino acid sequences of stimulatory fractions revealed three new autoepitopes. Two of the epitopes were homologous to brain transcription factor BRN-3, whereas the third sequence was homologous to histone H1'(22-42). H1'(22-42) stimulated autoimmune Th cells to augment the production of pathogenic antinuclear Abs, and was much more potent than other nucleosomal epitopes in accelerating glomerulonephritis in lupus-prone (SWR x NZB)F(1) (SNF(1)) mice. Remarkably, a marked expansion of Th1 cells recognizing the H1'(22-42) epitope occurred spontaneously in SNF(1) mice very early in life. A significant proportion of H1'(22-42)-specific T cell clones cross-reacted with one or more core histone epitopes, but not with epitopes in other lupus autoantigens. The H1'(22-42) epitope was also recognized by autoimmune B cells, and with the onset of lupus nephritis, serum autoantibodies to the H1'(22-42) epitope become increasingly cross-reactive with nuclear autoantigens. Convergence of T and B cell epitopes in H1'(22-42) and its ability to elicit a cross-reactive response make it a highly dominant epitope that could be targeted for therapy and for tracking autoimmune T and B cells.

Ly-6A.2 expression regulates antigen-specific CD4+ T cell proliferation and cytokine production

Ly-6 proteins appear to serve cell adhesion and cell signaling function, but the precise role of Ly-6A.2 in CD4+ T lymphocytes is still unclear. Overexpression of Ly-6A.2 in T lymphocytes has allowed us to analyze the influence of elevated Ly-6A.2 expression on T cell function. In this study we report reduced proliferation of CD4+ T cells overexpressing Ly-6A.2 in response to a peptide Ag. Moreover, the Ly-6A.2-overexpressing CD4+ cells generated elevated levels of IL-4, a key factor that propels the differentiation of naive CD4+ T cells into Th2 subset. The hyporesponsiveness of Ly-6A.2 transgenic CD4+ T cells is dependent on the interaction of Ly-6A.2 T cells with the APCs and can be reversed by blocking the interaction between Ly-6A.2 and a recently reported candidate ligand. Overexpression of Ly-6A.2 in CD4+ T cells reduced their Ca(2+) responses to TCR stimulation, therefore suggesting effects of Ly-6A.2 signaling on membrane proximal activation events. In contrast to the observed Ag-specific hyporesponsiveness, the Ly-6A.2 transgenic CD4+ T cells produced IL-4 independent of the interactions between Ly-6A.2 and the candidate Ly-6A.2 ligand. Our results suggest that 1) interaction of Ly-6A.2 with a candidate ligand regulates clonal expansion of CD4+ Th cells in response to an Ag (these results also provide further functional evidence for presence of Ly-6A.2 ligand on APC); and 2) Ly-6A.2 expression on CD4+ T cells promotes production of IL-4, a Th2 differentiation factor.

The potential use of peptides and vaccination to treat systemic lupus erythematosus

Studies in humans and mice with systemic lupus erythematosus (SLE) suggest that the development of autoantibodies and disease is dependent on T helper (Th) cells. This review highlights recent efforts to identify the antigens that activate such autoreactive Th cells. Various laboratories are using different approaches to identify the autoantigenic epitopes, which appear to be derived from diverse sources such as nucleosome core histones, ribonucleoproteins, and immunoglobulin variable regions. Identification of the putative autoantigenic epitopes has raised the possibility of peptide-specific vaccination as therapy for SLE. Indeed, vaccination of prenephritic lupus-susceptible mice with such peptides delays the development of autoantibodies and nephritis, and prolongs survival. Recent data suggest that peptide treatment can also influence established disease in older lupus mice. These studies offer new hope for a similar treatment approach in patients with SLE. Studies have begun to identify T cell epitopes in human disease.