The fine specificity and cytokine profile of T-helper cells responsive to the alpha3 chain of type IV collagen in Goodpasture's disease

Research advances on targeted-Treg therapies on immune-mediated kidney diseases

The primary function of regulatory T cells (Tregs) is blocking the pathogenic immunological response mediated by autoreactive cells, establishing and maintaining immune homeostasis in tissues. Kidney diseases are often caused by Immune imbalance, including alloimmune graft damage after renal transplantation, direct immune-mediated kidney diseases like membranous nephropathy (MN) and anti-glomerular basement membrane (anti-GBM) glomerulonephritis, as well as indirect immune-mediated ones like Anti-neutrophil cytoplasmic antibody-associated vasculitis (AAVs), IgA nephropathy (IgAN) and lupus nephritis (LN). Treg cells are deficient numerically and/or functionally in those kidney diseases. Targeted-Treg therapies, including adoptive Tregs transfer therapy and low-dose IL-2 therapy, have begun to thrive in treating autoimmune diseases in recent years. However, the clinical use of targeted Treg-therapies is rarely mentioned in those kidney diseases above except for kidney transplantation. This article mainly discusses the newest progressions of targeted-Treg therapies in those specific examples of immune-mediated kidney diseases. Meanwhile, we also reviewed the main factors that affect Treg development and differentiation, hoping to inspire new strategies to develop target Tregs-therapies. Lastly, we emphasize the significant impediments and prospects to the clinical translation of target-Treg therapy. We advocate for more preclinical and clinical studies on target Tregs-therapies to decipher Tregs in those diseases.

Autoimmunity in Anti-Glomerular Basement Membrane Disease: A Review of Mechanisms and Prospects for Immunotherapy

Anti-glomerular basement membrane (anti-GBM) disease is an organ-specific autoimmune disorder characterized by autoantibodies against the glomerular and alveolar basement membranes, leading to rapidly progressive glomerulonephritis and severe alveolar hemorrhage. The noncollagenous domain of the α3 chain of type IV collagen, α3(IV)NC1, contains the main target autoantigen in this disease. Epitope mapping studies of α3(IV)NC1 have identified several nephritogenic epitopes and critical residues that bind to autoantibodies and trigger anti-GBM disease. The discovery of novel target antigens has revealed the heterogeneous nature of this disease. In addition, both epitope spreading and mimicry have been implicated in the pathogenesis of anti-GBM disease. Epitope spreading refers to the development of autoimmunity to new autoepitopes, thus worsening disease progression, whereas epitope mimicry, which occurs via sharing of critical residues with microbial peptides, can initiate autoimmunity. An understanding of these autoimmune responses may open opportunities to explore potential new therapeutic approaches for this disease. We review how current advances in epitope mapping, identification of novel autoantigens, and the phenomena of epitope spreading and mimicry have heightened the understanding of autoimmunity in the pathogenesis of anti-GBM disease, and we discuss prospects for immunotherapy.

Immune regulation in renal inflammation

Renal inflammation, induced by autoantigen recognition or toxic drugs, leads to renal tissue injury and decline in kidney function. Recent studies have demonstrated the crucial role for regulatory T cells in suppressing pathogenic adaptive but also innate immune responses in the inflamed kidney. However, there is also evidence for other immune cell populations with immunosuppressive function in renal inflammation. This review summarizes mechanisms of immune cell regulation in immune-mediated glomerulonephritis and acute and chronic nephrotoxicity.

Experimental Antiglomerular Basement Membrane GN Induced by a Peptide from Actinomyces

BACKGROUND

Antiglomerular basement membrane (anti-GBM) disease is associated with HLA-DRB1*1501 (the major predisposing genetic factor in the disease), with α3_127-148 as a nephritogenic T and B cell epitope. Although the cause of disease remains unclear, the association of infections with anti-GBM disease has been long suspected.

METHODS

To investigate whether microbes might activate autoreactive T and B lymphocytes via molecular mimicry in anti-GBM disease, we used bioinformatic tools, including BLAST, SYFPEITHI, and ABCpred, for peptide searching and epitope prediction. We used sera from patients with anti-GBM disease to assess peptides recognized by antibodies, and immunized WKY rats and a humanized mouse model (HLA-DR15 transgenic mice) with each of the peptide candidates to assess pathogenicity.

RESULTS

On the basis of the critical motif, the bioinformatic approach identified 36 microbial peptides that mimic human α3_127-148. Circulating antibodies in sera from patients with anti-GBM recognized nine of them. One peptide, B7, derived from Actinomyces species, induced proteinuria, linear IgG deposition on the GBM, and crescent formation when injected into WKY rats. The antibodies to B7 also targeted human and rat α3_127-148. B7 induced T cell activation from human α3_127-148-immunized rats. T cell responses to B7 were detected in rats immunized by Actinomyces lysate proteins or recombinant proteins. We confirmed B7's pathogenicity in HLA-DR15 transgenic mice that developed kidney injury similar to that observed in α3_135-145-immunized mice.

CONCLUSIONS

Sera from patients with anti-GBM disease recognized microbial peptides identified through a bioinformatic approach, and a peptide from Actinomyces induced experimental anti-GBM GN by T and B cell crossreactivity. These studies demonstrate that anti-GBM disease may be initiated by immunization with a microbial peptide.

IgA vasculitis and anti-GBM disease: two ends of a spectrum of immune complex vasculitis

Two immune complex vasculitides, IgA vasculitis (IgAV) and anti-GBM disease, represent polar extremes with regard to our understanding of disease pathogenesis, standardized management protocols and outcomes. This report compares our current approach to these uncommon entities in adults. Both diseases demonstrate degrees of small vessel necrosis and glomerular crescent formation. IgAV has an antibody response directed against unknown antigens, is often treated conservatively and has poorly studied long term renal outcomes. By contrast, anti-GBM disease presents with rapidly progressive glomerulonephritis and often results in end stage renal failure, despite intensive immunosuppression. Rarely, some cases of anti-GBM disease may be IgA predominant and bind other α-chains present in the GBM, but their clinical course is as for other anti-GBM disease patients but not IgAV, suggesting that the antigenic target rather than the antibody subclass is the critical factor in determining disease outcome. However, both conditions are associated with increased mortality in adults and result in significant chronic kidney disease and hypertension.

T cell responses to peptides of Goodpasture autoantigen in patients with anti-glomerular basement membrane disease

AIM

Cell-mediated autoimmunity, especially autoreactive T cells, is crucial in the initiation of anti-glomerular membrane (GBM) disease. Epitopes for T cells on Goodpasture autoantigen are not fully defined. This study investigated T cell epitopes in anti-GBM patients, aiming to identify the epitopes and their clinical significance.

METHODS

Peripheral blood mononuclear cells (PBMC) were collected from 13 patients with anti-GBM disease. Twenty-four overlapping linear peptides were synthesized covering the whole sequence of human α3(IV)NC1. PBMC response to each peptide was detected by proliferation assay. Their associations with clinical features were further analyzed.

RESULTS

Peripheral blood mononuclear cells proliferative responses to linear peptides on α3(IV)NC1 could be detected in all patients. Five major epitopes were identified as stimulatory in over half of the patients: α3(IV)NC1_127-148 (P14) (69.2%), α3(IV)NC1_159-178 (77.8%), α3(IV)NC1_179-198 (55.6%), α3(IV)NC1_189-208 (P19) (75.0%) and α3(IV)NC1_141-154 (57.1%). P14 and P19 were highly recognized in patients comparing with healthy controls (69.2% vs. 0.0%, P = 0.011; 75.0% vs. 0.0%, P = 0.021, respectively).

CONCLUSION

T cell proliferation to linear epitopes was detected in human anti-GBM disease. α3_127-148 was a mutual T and B cell epitope, implying its initial role in epitope spreading process.

Goodpasture's autoimmune disease - A collagen IV disorder

Goodpasture's (GP) disease is an autoimmune disorder characterized by the deposition of pathogenic autoantibodies in basement membranes of kidney and lung eliciting rapidly progressive glomerulonephritis and pulmonary hemorrhage. The principal autoantigen is the α345 network of collagen IV, which expression is restricted to target tissues. Recent discoveries include a key role of chloride and bromide for network assembly, a novel posttranslational modification of the antigen, a sulfilimine bond that crosslinks the antigen, and the mechanistic role of HLA in genetic susceptibility and resistance to GP disease. These advances provide further insights into molecular mechanisms of initiation and progression of GP disease and serve as a basis for developing of novel diagnostic tools and therapies for treatment of Goodpasture's disease.

Effector CD4+ T cells recognize intravascular antigen presented by patrolling monocytes

Although effector CD4⁺ T cells readily respond to antigen outside the vasculature, how they respond to intravascular antigens is unknown. Here we show the process of intravascular antigen recognition using intravital multiphoton microscopy of glomeruli. CD4⁺ T cells undergo intravascular migration within uninflamed glomeruli. Similarly, while MHCII is not expressed by intrinsic glomerular cells, intravascular MHCII-expressing immune cells patrol glomerular capillaries, interacting with CD4⁺ T cells. Following intravascular deposition of antigen in glomeruli, effector CD4⁺ T-cell responses, including NFAT1 nuclear translocation and decreased migration, are consistent with antigen recognition. Of the MHCII⁺ immune cells adherent in glomerular capillaries, only monocytes are retained for prolonged durations. These cells can also induce T-cell proliferation in vitro. Moreover, monocyte depletion reduces CD4⁺ T-cell-dependent glomerular inflammation. These findings indicate that MHCII⁺ monocytes patrolling the glomerular microvasculature can present intravascular antigen to CD4⁺ T cells within glomerular capillaries, leading to antigen-dependent inflammation.

Monocytes constitutively adhere and crawl along the glomerular endothelium and are thought to contribute to glomerulonephritis. Here the authors use multiphoton microscopy to show local antigen presentation by MHCII⁺ monocytes to T cells in glomerular capillaries of mice.

Regulatory T cells in renal disease

The kidney is vulnerable to injury, both acute and chronic from a variety of immune and metabolic insults, all of which at least to some degree involve inflammation. Regulatory T cells modulate systemic autoimmune and allogenic responses in glomerulonephritis and transplantation. Intrarenal regulatory T cells (Tregs), including those recruited to the kidney, have suppressive effects on both adaptive and innate immune cells, and probably also intrinsic kidney cells. Evidence from autoimmune glomerulonephritis implicates antigen-specific Tregs in HLA-mediated dominant protection, while in several human renal diseases Tregs are abnormal in number or phenotype. Experimentally, Tregs can protect the kidney from injury in a variety of renal diseases. Mechanisms of Treg recruitment to the kidney include via the chemokine receptors CCR6 and CXCR3 and potentially, at least in innate injury TLR9. The effects of Tregs may be context dependent, with evidence for roles for immunoregulatory roles both for endogenous Tbet-expressing Tregs and STAT-3-expressing Tregs in experimental glomerulonephritis. Most experimental work and some of the ongoing human trials in renal transplantation have focussed on unfractionated thymically derived Tregs (tTregs). However, induced Tregs (iTregs), type 1 regulatory T (Tr1) cells and in particular antigen-specific Tregs also have therapeutic potential not only in renal transplantation, but also in other kidney diseases.

Plasticity and heterogeneity of Th17 in immune-mediated kidney diseases

Anti-neutrophil cytoplasmatic antibody (ANCA)-associated glomerulonephritis, anti-glomerular basement membrane (GBM) glomerulonephritis and lupus nephritis are the most common causes of rapid progressive glomerulonephritis (RPGN) in the Western world. These aggressive forms of autoimmune kidney diseases significantly contribute to end-stage renal disease and are associated with high morbidity and mortality. Moreover, patients show significant heterogeneity with respect to clinical outcome and response to therapy. T cell infiltration is a morphological hallmark of RPGN and it is a critical driver of kidney injury. Different CD4⁺ T cell subsets that are endowed with distinct regulatory and effector functions are involved in this detrimental inflammatory process. In particular, the identification and functional characterization of IL-17-expressing CD4⁺ Th17 cells have substantially advanced our understanding of organ-specific autoimmunity. In experimental models of crescentic and proliferative GN, including ANCA-associated GN, anti-GBM-GN and lupus nephritis, the Th17/IL-17 axis significantly contributes to renal tissue damage. In patients with ANCA-associated GN or lupus nephritis, IL-17 serum levels correlated with disease activity. Moreover, Th17 cells are present in the kidneys of these patients and represents a topic of intense ongoing clinical and basic research. Importantly, recent studies have challenged the view of CD4⁺ T cells subsets as terminally differentiated homogenous cells, showing that T cells, in particular Th17 cells, are much more flexible and heterogeneous than previously thought. However, analysis of Th17 cell fate in mouse models of autoimmune kidney disease revealed a high degree of stability within these cells, an observation that is in contrast to Th17 cells in other models of autoimmune diseases including experimental autoimmune encephalomyelitis. Interestingly, anti-CD3 treatment interferes with stable Th17 cells and induces a potential regulatory phenotype in Th17 cells, highlighting the therapeutic potential of targeting pathogenic Th17 cells in autoimmunity. In this review, we discuss the current knowledge of Th17 cell plasticity and heterogeneity in autoimmune kidney diseases with a special focus on the underlying mechanisms of this process and debate if Th17 cell plasticity is beneficial or harmful to renal inflammation.

The susceptible HLA class II alleles and their presenting epitope(s) in Goodpasture's disease

Goodpasture's disease is closely associated with HLA, particularly DRB1*1501. Other susceptible or protective HLA alleles are not clearly elucidated. The presentation models of epitopes by susceptible HLA alleles are also unclear. We genotyped 140 Chinese patients and 599 controls for four-digit HLA II genes, and extracted the encoding sequences from the IMGT/HLA database. T-cell epitopes of α3(IV)NC1 were predicted and the structures of DR molecule-peptide-T-cell receptor were constructed. We confirmed DRB1*1501 (OR = 4·6, P = 5·7 × 10^-28 ) to be a risk allele for Goodpasture's disease. Arginine at position 13 (ARG13) (OR = 4·0, P = 1·0 × 10^-17 ) and proline at position 11 (PRO11) (OR = 4·0, P = 2·0 × 10^-17 ) on DRβ1, encoded by DRB1*1501, were associated with disease susceptibility. α_134-148 (HGWISLWKGFSFIMF) was predicted as a T-cell epitope presented by DRB1*1501. Isoleucine₁₃₇ , tryptophan₁₄₀ , glycine₁₄₂ , phenylalanine₁₄₃ and phenylalanine₁₄₅ , were presented in peptide-binding pockets 1, 4, 6, 7 and 9 of DR2b, respectively. ARG13 in pocket 4 interacts with tryptophan₁₄₀ and forms a hydrogen bond. In conclusion, we propose a mechanism for DRB1*1501 susceptibility for Goodpasture's disease through encoding ARG13 and PRO11 on MHC-DRβ1 chain and presenting T-cell epitope, α_134-148 , with five critical residues.

Dominant protection from HLA-linked autoimmunity by antigen-specific regulatory T cells

Susceptibility and protection against human autoimmune diseases, including type I diabetes, multiple sclerosis, and Goodpasture disease, is associated with particular human leukocyte antigen (HLA) alleles. However, the mechanisms underpinning such HLA-mediated effects on self-tolerance remain unclear. Here we investigate the molecular mechanism of Goodpasture disease, an HLA-linked autoimmune renal disorder characterized by an immunodominant CD4⁺ T-cell self-epitope derived from the α3 chain of type IV collagen (α3_135-145). While HLA-DR15 confers a markedly increased disease risk, the protective HLA-DR1 allele is dominantly protective in trans with HLA-DR15 (ref. 2). We show that autoreactive α3_135-145-specific T cells expand in patients with Goodpasture disease and, in α3_135-145-immunized HLA-DR15 transgenic mice, α3_135-145-specific T cells infiltrate the kidney and mice develop Goodpasture disease. HLA-DR15 and HLA-DR1 exhibit distinct peptide repertoires and binding preferences and present the α3_135-145 epitope in different binding registers. HLA-DR15-α3_135-145 tetramer⁺ T cells in HLA-DR15 transgenic mice exhibit a conventional T-cell phenotype (T_conv) that secretes pro-inflammatory cytokines. In contrast, HLA-DR1-α3_135-145 tetramer⁺ T cells in HLA-DR1 and HLA-DR15/DR1 transgenic mice are predominantly CD4⁺Foxp3⁺ regulatory T cells (T_reg cells) expressing tolerogenic cytokines. HLA-DR1-induced T_reg cells confer resistance to disease in HLA-DR15/DR1 transgenic mice. HLA-DR15⁺ and HLA-DR1⁺ healthy human donors display altered α3_135-145-specific T-cell antigen receptor usage, HLA-DR15-α3_135-145 tetramer⁺ Foxp3^- T_conv and HLA-DR1-α3_135-145 tetramer⁺ Foxp3⁺CD25^hiCD127^lo T_reg dominant phenotypes. Moreover, patients with Goodpasture disease display a clonally expanded α3_135-145-specific CD4⁺ T-cell repertoire. Accordingly, we provide a mechanistic basis for the dominantly protective effect of HLA in autoimmune disease, whereby HLA polymorphism shapes the relative abundance of self-epitope specific T_reg cells that leads to protection or causation of autoimmunity.

Plasma from patients with anti-glomerular basement membrane disease could recognize microbial peptides

Infection has long been suspected as a trigger of autoimmune diseases, and molecular mimicry mechanism was hypothesized in this study. Microbe originated peptides were searched from the Uniprot database based on a previous defined critical amino acid motif within α3_129−150, isoleucine137, tryptophan140, glycine142, phenylalanine 143 and phenylalanine 145. 23826 microbial peptides were identified using our searching strategy, among which seven were related with human infections. Circulating IgG and IgM antibodies against the seven microbial peptides were detected using ELISA in 76 patients with anti-GBM disease. Four peptides were recognized by both IgG and IgM antibodies, and one peptide was recognized by IgG antibodies only. Peptides from Bacteroides, Saccharomyces cerevisiae, and Bifidobacterium thermophilum possessed the highest recognition frequency with the prevalence of 73.7%, 61.8% and 67.1% for IgG, 56.6%, 44.7% and 67.1% for IgM in anti-GBM patients. Patients with antibodies against these microbial peptides showed more severe kidney injury, including higher serum creatinine and higher percentage of crescent formation. In conclusion, antibodies against microbial peptides were identified in the circulation of anti-GBM patients, implying its etiological role in eliciting autoimmune response against α3(IV)NC1 through molecular mimicry.

The pathogenicity of T cell epitopes on human Goodpasture antigen and its critical amino acid motif

Goodpasture antigen, the non‐collagenous domain of α3 chain of type IV collagen [α3(IV)NC1], is the target antigen of anti‐glomerular basement membrane (GBM) antibodies. The pathogenicity of T cell epitopes is not elucidated clearly. In this study, we aim to define the nephritogenic T cell epitopes and its critical amino acid residues. Twenty‐four overlapping linear peptides were synthesized covering the whole sequence of human α3(IV)NC1. Wistar–Kyoto rats were immunized with linear peptides, and experimental autoimmune glomerulonephritis was evaluated. Critical amino acid was identified by the loss of nephritogenic function after each amino acid substitution by alanine. Of the 24 peptides, P14 (α3_127‐148) could induce 90.5% (19/21) of WKY rats developing anti‐GBM glomerulonephritis with proteinuria, elevated serum urea and creatinine, IgG linear deposit on GBM and substantial (in average 82.4 ± 5.6%) crescent formation in glomeruli. Lymphocytes of immunized rats proliferated in response to α3_127‐148 and α3(IV)NC1 in vitro. Sera of these rats recognized α3_127‐148 and later on together with intact human α3(IV)NC1. Antibodies towards α3_127‐148 and intact α3(IV)NC1 could also be detected from the kidney elutes. These antibodies showed no cross‐reaction with each other, which implies intramolecular epitope spreading during disease progress. After sequential amino acid substitution, the α3_127‐148 with substitution of tryptophan₁₃₆, isoleucine₁₃₇, leucine₁₃₉ or tryptophan₁₄₀ lost its nephritogenicity. Human α3_127‐148 is a nephritogenic T cell epitope in WKY rats, with the critical amino acids as W₁₃₆I₁₃₇xL₁₃₉W₁₄₀. These findings might facilitate future investigation on microbial aetiology and potential specific immunotherapy of anti‐GBM disease.

Regulatory T cells in immune-mediated renal disease

Regulatory T cells (Tregs) are CD4+ T cells that can suppress immune responses by effector T cells, B cells and innate immune cells. This review discusses the role that Tregs play in murine models of immune-mediated renal diseases and acute kidney injury and in human autoimmune kidney disease (such as systemic lupus erythematosus, anti-glomerular basement membrane disease, anti-neutrophil cytoplasmic antibody-associated vasculitis). Current research suggests that Tregs may be reduced in number and/or have impaired regulatory function in these diseases. Tregs possess several mechanisms by which they can limit renal and systemic inflammatory immune responses. Potential therapeutic applications involving Tregs include in vivo induction of Tregs or inducing Tregs from naïve CD4+ T cells or expanding natural Tregs ex vivo, to use as a cellular therapy. At present, the optimal method of generating a phenotypically stable pool of Tregs with long-lasting suppressive effects is not established, but human studies in renal transplantation are underway exploring the therapeutic potential of Tregs as a cellular therapy, and if successful may have a role as a novel therapy in immune-mediated renal diseases.

Induced regulatory T cells are phenotypically unstable and do not protect mice from rapidly progressive glomerulonephritis

Regulatory T (Treg) cells are a suppressive CD4⁺ T-cell subset. We generated induced Treg (iTreg) cells and explored their therapeutic potential in a murine model of rapidly progressive glomerulonephritis. Polyclonal naive CD4⁺ T cells were cultured in vitro with interleukin-2 (IL-2), transforming growth factor-β1, all-trans-retinoic acid and monoclonal antibodies against interferon-γ and IL-4, generating Foxp3⁺ iTreg cells. To enhance their suppressive phenotype, iTreg cultures were modified with the addition of a monoclonal antibody against IL-12p40 or by using RORγt^-/- CD4⁺ T cells. Induced Treg cells were transferred into models of delayed-type hypersensitivity and experimental glomerulonephritis. The iTreg cells exhibited comparable surface receptor expression and in vitro suppressive ability to natural Treg cells, but did not regulate antigen-specific delayed-type hypersensitivity or systemic inflammatory immune responses, losing Foxp3 expression in vivo. In glomerulonephritis, transferred iTreg cells did not prevent renal injury or modulate systemic T helper type 1 immune responses. Induced Treg cells cultured with anti-IL-12p40 had an enhanced suppressive phenotype in vitro and regulated dermal delayed-type hypersensitivity in vivo, but were not protective against renal injury, losing Foxp3 expression, especially in the transferred cells recruited to the kidney. Use of RORγt^-/- CD4⁺ T cells or iTreg cells generated from sensitized CD4⁺ Foxp3^- cells did not regulate renal or systemic inflammatory responses in vivo. In conclusion, iTreg cells suppress T-cell proliferation in vitro, but do not regulate experimental glomerulonephritis, being unstable in this inflammatory milieu in vivo.

Effective antigen presentation to helper T cells by human eosinophils

Although eosinophils are inflammatory cells, there is increasing attention on their immunomodulatory roles. For example, murine eosinophils can present antigen to CD4⁺ T helper (Th) cells, but it remains unclear whether human eosinophils also have this ability. This study determined whether human eosinophils present a range of antigens, including allergens, to activate Th cells, and characterized their expression of MHC class II and co-stimulatory molecules required for effective presentation. Human peripheral blood eosinophils purified from non-allergic donors were pulsed with the antigens house dust mite extract (HDM), Timothy Grass extract (TG) or Mycobacterium tuberculosis purified protein derivative (PPD), before co-culture with autologous CD4⁺ Th cells. Proliferative and cytokine responses were measured, with eosinophil expression of HLA-DR/DP/DQ and the co-stimulatory molecules CD40, CD80 and CD86 determined by flow cytometry. Eosinophils pulsed with HDM, TG or PPD drove Th cell proliferation, with the response strength dependent on antigen concentration. The cytokine responses varied with donor and antigen, and were not biased towards any particular Th subset, often including combinations of pro- and anti-inflammatory cytokines. Eosinophils up-regulated surface expression of HLA-DR/DP/DQ, CD80, CD86 and CD40 in culture, increases that were sustained over 5 days when incubated with antigens, including HDM, or the major allergens it contains, Der p I or Der p II. Human eosinophils can, therefore, act as effective antigen-presenting cells to stimulate varied Th cell responses against a panel of antigens including HDM, TG or PPD, an ability that may help to determine the development of allergic disease.

Basement membranes and autoimmune diseases

Basement membrane components are targets of autoimmune attack in diverse diseases that destroy kidneys, lungs, skin, mucous membranes, joints, and other organs in man. Epitopes on collagen and laminin, in particular, are targeted by autoantibodies and T cells in anti-glomerular basement membrane glomerulonephritis, Goodpasture's disease, rheumatoid arthritis, post-lung transplant bronchiolitis obliterans syndrome, and multiple autoimmune dermatoses. This review examines major diseases linked to basement membrane autoreactivity, with a focus on investigations in patients and animal models that advance our understanding of disease pathogenesis. Autoimmunity to glomerular basement membrane type IV is discussed in depth as a prototypic organ-specific autoimmune disease yielding novel insights into the complexity of anti-basement membrane immunity and the roles of genetic and environmental susceptibility.

Optimizing the translational value of animal models of glomerulonephritis: insights from recent murine prototypes

Animal models are indispensable for the study of glomerulonephritis, a group of diseases that destroy kidneys but for which specific therapies do not yet exist. Novel interventions are urgently needed, but their rational design requires suitable in vivo platforms to identify and test new candidates. Animal models can recreate the complex immunologic microenvironments that foster human autoimmunity and nephritis and provide access to tissue compartments not readily examined in patients. Study of rat Heymann nephritis identified fundamental disease mechanisms that ultimately revolutionized our understanding of human membranous nephropathy. Significant species differences in expression of a major target antigen, however, and lack of spontaneous autoimmunity in animals remain roadblocks to full exploitation of preclinical models in this disease. For several glomerulonephritides, humanized models have been developed to circumvent cross-species barriers and to study the effects of human genetic risk variants. Herein we review humanized mouse prototypes that provide fresh insight into mediators of IgA nephropathy and origins of antiglomerular basement membrane nephritis and Goodpasture's disease, as well as a means to test novel therapies for ANCA vasculitis. Additional and refined model systems are needed to mirror the full spectrum of human disease in a genetically diverse population, to facilitate development of patient-specific interventions, to determine the origin of nephritogenic autoimmunity, and to define the role of environmental exposures in disease initiation and relapse.

Biologics for the treatment of autoimmune renal diseases

Biological therapeutics (biologics) that target autoimmune responses and inflammatory injury pathways have a marked beneficial impact on the management of many chronic diseases, including rheumatoid arthritis, psoriasis, inflammatory bowel disease, and ankylosing spondylitis. Accumulating data suggest that a growing number of renal diseases result from autoimmune injury - including lupus nephritis, IgA nephropathy, anti-neutrophil cytoplasmic antibody-associated glomerulonephritis, autoimmune (formerly idiopathic) membranous nephropathy, anti-glomerular basement membrane glomerulonephritis, and C3 nephropathy - and one can speculate that biologics might also be applicable to these diseases. As many autoimmune renal diseases are relatively uncommon, with long natural histories and diverse outcomes, clinical trials that aim to validate potentially useful biologics are difficult to design and/or perform. Some excellent consortia are undertaking cohort studies and clinical trials, but more multicentre international collaborations are needed to advance the introduction of new biologics to patients with autoimmune renal disorders. This Review discusses the key molecules that direct injurious inflammation and the biologics that are available to modulate them. The opportunities and challenges for the introduction of relevant biologics into treatment protocols for autoimmune renal diseases are also discussed.

TH1 and TH17 cells promote crescent formation in experimental autoimmune glomerulonephritis

Autoimmunity against the Goodpasture antigen α3IV-NC1 results in crescentic glomerulonephritis (GN). Both antibodies and T cells directed against α3IV-NC1 have been implicated in disease development and progression. Using the model of experimental autoimmune glomerulonephritis (EAG) in DBA/1 mice, we aimed to characterize the frequency and function of α3IV-NC1-specific CD4(+) T cells in the kidneys. DBA/1 mice repeatedly immunized with human α3IV-NC1 developed necrotizing/crescentic GN. Kidneys with crescentic GN contained CD4(+) cells responding to α3IV-NC1 with the production of IFN-γ or IL-17A, demonstrating the accumulation of both α3IV-NC1-specific TH1 and TH17 cells. To test the functional relevance of TH1 and TH17 cells, EAG was induced in DBA/1 mice deficient in IFN-γR, IL-17A or IL-23p19. Mice of all knockout groups mounted α3IV-NC1 IgG, developed nephrotic range proteinuria, and IgG deposition to the glomerular basement membranes at levels similar to immunized wild-type mice. However, all knockout groups showed significantly fewer glomerular crescents and attenuated tubulointerstitial damage. Our results suggest that both α3IV-NC1-specific TH1 and TH17 cells accumulate in the kidneys and are crucial for the development of necrotizing/crescentic GN.

Identification of critical residues of linear B cell epitope on Goodpasture autoantigen

Background

The autoantigen of anti-glomerular basement membrane (GBM) disease has been identified as the non-collagenous domain 1 of α3 chain of type IV collagen, α3(IV)NC1. Our previous study revealed a peptide on α3(IV)NC1 as a major linear epitope for B cells and potentially nephrogenic, designated as P14 (α3129-150). This peptide has also been proven to be the epitope of auto-reactive T cells in anti-GBM patients. This study was aimed to further characterize the critical motif of P14.

Methods

16 patients with anti-GBM disease and positive anti-P14 antibodies were enrolled. A set of truncated and alanine substituted peptides derived from P14 were synthesized. Circulating antibodies against the peptides were detected by enzyme linked immunosorbent assay (ELISA).

Results

We found that all sera with anti-P14 antibodies reacted with the 13-mer sequence in the C-terminus of P14 (P14c) exclusively. The level of antibodies against P14 was highly correlated with the level of antibodies against P14c (r=0.970, P<0.001). P14c was the core immunogenic region and the amino acid sequence (ISLWKGFSFIMFT) was highly hydrophobic. Each amino acid residue in P14c was sequentially replaced by alanine. Three residues of glycine142, phenylalanine143, and phenylalanine145 were identified crucial for antibody binding based on the remarkable decline (P<0.001) of antibody reaction after each residue replacement.

Conclusions

We defined GFxF (α3142, 143,145) as the critical motif of P14. It may provide some clues for understanding the etiology of anti-GBM disease.

Glomerulopathy induced by immunization with a peptide derived from the goodpasture antigen α3IV-NC1

Mouse experimental autoimmune glomerulonephritis, a model of human antiglomerular basement membrane disease, depends on both Ab and T cell responses to the Goodpasture Ag noncollagenous domain 1 of the α3-chain of type IV collagen (α3IV-NC1). The aim of our study was to further characterize the T cell-mediated immune response. Repeated immunization with mouse α3IV-NC1 caused fatal glomerulonephritis in DBA/1 mice. Although two immunizations were sufficient to generate high α3IV-NC1-specific IgG titers, Ab and complement deposition along the glomerular basement membranes, and a nephrotic syndrome, two additional immunizations were needed to induce a necrotizing/crescentic glomerulonephritis. Ten days after the first immunization, α3IV-NC1-specific CD4(+) cells producing TNF-α, IFN-γ, or IL-17A were detected in the spleen. With the emergence of necrotizing/crescentic glomerulonephritis, ∼0.15% of renal CD4(+) cells were specific for α3IV-NC1. Using peptides spanning the whole α3IV-NC1 domain, three immunodominant T cell epitopes were identified. Immunization with these peptides did not lead to clinical signs of experimental autoimmune glomerulonephritis or necrotizing/crescentic glomerulonephritis. However, mice immunized with one of the peptides (STVKAGDLEKIISRC) developed circulating Abs against mouse α3IV-NC1 first detected at 8 wk, and 50% of the mice showed mild proteinuria at 18-24 wk due to membranous glomerulopathy. Taken together, our results suggest that autoreactive T cells are able to induce the formation of pathologic autoantibodies. The quality and quantity of α3IV-NC1-specific Ab and T cell responses are critical for the phenotype of the glomerulonephritis.

Combination peptide immunotherapy suppresses antibody and helper T-cell responses to the RhD protein in HLA-transgenic mice

The offspring from pregnancies of women who have developed anti-D blood group antibodies are at risk of hemolytic disease of the newborn. We have previously mapped four peptides containing immunodominant T-helper cell epitopes from the RhD protein and the purpose of the work was to develop these into a product for suppression of established anti-D responses. A panel of each of the four immunodominant RhD peptides was synthesized with modifications to improve manufacturability and solubility, and screened for retention of recognition by human T-helper cells. A selected version of each sequence was combined in a mixture (RhDPmix), which was tested for suppressive ability in a humanized murine model of established immune responses to RhD protein. After HLA-DR15 transgenic mice had been immunized with RhD protein, a single dose of RhDPmix, given either intranasally (P=0.008, Mann-Whitney rank sum test) or subcutaneously (P=0.043), rapidly and significantly suppressed the ongoing antibody response. This was accompanied by reduced T-helper cell responsiveness, although this change was less marked for subcutaneous RhDPmix delivery, and by the recruitment of cells with a regulatory T-cell phenotype. The results support human trials of RhDPmix peptide immunotherapy in women with established antibody responses to the RhD blood group.

The immune system and kidney disease: basic concepts and clinical implications

The kidneys are frequently targeted by pathogenic immune responses against renal autoantigens or by local manifestations of systemic autoimmunity. Recent studies in rodent models and humans have uncovered several underlying mechanisms that can be used to explain the previously enigmatic immunopathology of many kidney diseases. These mechanisms include kidney-specific damage-associated molecular patterns that cause sterile inflammation, the crosstalk between renal dendritic cells and T cells, the development of kidney-targeting autoantibodies and molecular mimicry with microbial pathogens. Conversely, kidney failure affects general immunity, causing intestinal barrier dysfunction, systemic inflammation and immunodeficiency that contribute to the morbidity and mortality of patients with kidney disease. In this Review, we summarize the recent findings regarding the interactions between the kidneys and the immune system.

Predominance of activated, clonally expanded T helper type 17 cells within the CD4+ T cell population in psoriatic lesions

Recent evidence points to the T helper type 17 (Th17) subset as key in the pathogenesis of psoriasis, but cells of this type in lesions remain to be fully characterized. Here we isolated, enumerated, functionally tested and clonotyped the CD4(+) Th cell population ex vivo from lesional biopsies and paired peripheral blood samples from psoriasis patients. Th17 cells were over-represented dramatically in lesions from all patients, representing 49-93% of CD4(+) Th cells compared with 3-18% in blood. Most lesional Th17 cells produced interleukin (IL)-17A ex vivo without further stimulation and expressed the CD45RO(+) phenotype characteristic of activated or memory cells. There was no increase in 'natural' [CD25(hi) forkhead box protein 3 (FoxP3(+))] regulatory T cells in lesions versus peripheral blood, but there was enrichment of 'induced' IL-10(+) regulatory T cell numbers in biopsies from some patients. The lesional Th17 cells exhibited a bias in T cell receptor Vβ chain usage, suggestive of specific expansion by antigen. The therapeutic challenge is to overcome the dominance of overwhelming numbers of such antigen-specific Th17 cells in psoriatic lesions.

The HLA-DRB1*15:01-restricted Goodpasture's T cell epitope induces GN

Human anti-glomerular basement membrane (GBM) disease strongly associates with HLA-DRB1*15:01. The target autoantigen in this disease is the noncollagenous domain of the α3 chain of type IV collagen, α3(IV)NC1, but critical early T cell epitopes presented by this human MHC class II molecule are unknown. Here, by immunizing HLA-DRB1*15:01 transgenic mice with whole recombinant α3(IV)NC1 and with overlapping α3(IV)NC1 peptides, we defined a HLA-DRB1*15:01-restricted α3(IV)NC1 T cell epitope (α3136-146) with four critical residues. This peptide was not immunogenic in HLA-DRB1*01:01 transgenic or C57BL/6 mice. The T cell epitope is naturally processed from α3(IV)NC1. CD4(+) T cell clones, generated from HLA-DRB1*15:01 transgenic mice and specific for α3136-146, transferred disease into naive HLA-DRB1*15:01 transgenic mice, evidenced by the development of necrotizing crescentic GN, albuminuria, renal impairment, and accumulation of CD4(+) T cells and macrophages in glomeruli. Because Fcγ receptors are implicated in disease susceptibility, we crossed HLA transgenic mice onto an FcγRIIb-deficient background. Immunization with either α3136-146 or α3(IV)NC1 induced GN in HLA-DRB1*15:01 transgenic FcγRIIb-deficient mice, but HLA-DRB1*01:01 transgenic FcγRIIb-deficient mice were unaffected. Taken together, these results demonstrate that the HLA-DRB1*15:01-restricted T cell epitope α3136-146 can induce T cell responses and injury in anti-GBM GN.

Antibodies against linear epitopes on the Goodpasture autoantigen and kidney injury

BACKGROUND AND OBJECTIVES

Linear epitopes on the Goodpasture autoantigen involved in human anti-glomerular basement membrane (GBM) disease are not fully defined. This study investigated the linear epitopes recognized by circulating antibodies in anti-GBM patients, aiming to identify the potential nephrogenic linear epitopes and their clinical significance.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Sixty-eight patients with anti-GBM disease were enrolled. Twenty-four overlapping linear peptides were synthesized across the whole sequence of the human Goodpasture autoantigen. ELISA detected circulating antibodies against linear epitopes. Their associations with clinical features were further analyzed.

RESULTS

Antibodies against linear peptides were detected in sera from 55 patients (80.9%). Three major epitopes with high frequencies were identified: P14 (41%), P16 (36.8%), and P18 (57%). P14, a formerly defined T cell epitope, was a mutual B cell epitope. Antibodies against P14 were frequently detected in patients with positive antineutrophil cytoplasmic antibodies (39.3% versus 12.5%; P=0.01). Patients with anti-P16 antibodies presented with higher serum creatinine on diagnosis (665.5±227.2 versus 443.7±296.8 μmol/L; P=0.001) and worse renal outcome during follow-up (hazard ratio, 2.10; 95% confidence interval, 1.10-3.90; P=0.02). The level of anti-P18 antibodies positively correlated with the percentage of crescents in glomeruli (r=0.54; P=0.008). Recognition of P22 was an independent predictor for patient death (hazard ratio, 3.02; 95% confidence interval, 1.20-7.57; P=0.02).

CONCLUSIONS

Antibodies against linear epitopes on the Goodpasture autoantigen could be detected in human anti-GBM disease and were associated with kidney injury. P14 was a mutual T and B cell epitope, implying its nephrogenic role in disease initiation.

Characterization of the renal CD4+ T-cell response in experimental autoimmune glomerulonephritis

Autoimmunity against the Goodpasture antigen α3IV-NC1 results in antiglomerular basement membrane glomerulonephritis. Although antibodies are central to the pathogenesis, there is good evidence for the participation of T cells in this disease. To define the contribution of T cells, we used the model of experimental autoimmune glomerulonephritis. Immunization of DBA/1 mice with α3IV-NC1 resulted in proteinuria, a biphasic course of the disease, and an eventual loss of kidney function. In the initial phase, the mice developed an α3IV-NC1-specific IgG response, had IgG deposition along the glomerular basement membrane, and steadily increased proteinuria, but only marginal signs of inflammation with limited leukocyte infiltration. After 9-13 weeks, mice proceeded to develop crescentic glomerulonephritis, extensive tubulointerstitial damage, and massive macrophage infiltration. T-cell infiltration was less pronounced, mostly confined to the interstitium, and T cells displayed an activated phenotype with a significant fraction of Th1 or Th17 CD4(+) T cells. Close examination revealed the presence of autoreactive T cells producing IFNγ upon restimulation with α3IV-NC1. Thus, our results suggest that accumulation of effector T cells, including autoreactive T cells, represents a critical step in the progression from mild glomerulonephritis, with limited glomerular damage, to severe crescentic glomerulonephritis accompanied by tubulointerstitial inflammation and loss of kidney function.

Production of the effector cytokine interleukin-17, rather than interferon-γ, is more strongly associated with autoimmune hemolytic anemia

BACKGROUND

Interleukin-17A is the signature cytokine of the Th17 subset and drives inflammatory pathology, but its relevance to autoantibody-mediated diseases is unclear. Th1 cells secreting interferon-γ have been implicated in autoimmune hemolytic anemia, so the aim was to determine which cytokine is more closely associated with disease severity.

DESIGN AND METHODS

Interferon-γ and interleukin-17A were measured in the sera of patients with autoimmune hemolytic anemia and healthy donors, and in peripheral blood mononuclear cell cultures stimulated with autologous red blood cells, or a panel of peptides spanning red blood cell autoantigen.

RESULTS

Serum interleukin-17A, but not interferon-γ, was significantly raised in patients with autoimmune hemolytic anemia (P<0.001), and correlated with the degree of anemia. Interleukin-17A was also more prominent in the responses of peripheral blood mononuclear cells from patients with autoimmune hemolytic anemia to red blood cells, and, again unlike interferon-γ, significantly associated with more severe anemia (P<0.005). There were no interleukin-17A responses to red blood cells by peripheral blood mononuclear cells from healthy donors. Specific autoantigenic peptides were identified that elicit patients' interleukin-17A responses.

CONCLUSIONS

Interleukin-17A makes a previously unrecognized contribution to the autoimmune response in autoimmune hemolytic anemia, challenging the model that the disease is driven primarily by Th1 cells. This raises the possibility that Th17, rather than Th1, cells should be the target for therapy.

Crescentic glomerulonephritis: new aspects of pathogenesis

This review provides a summary of recent advances in the understanding of crescentic glomerulonephritis, focusing on antineutrophil cytoplasm antibody (ANCA)-associated vasculitis and anti-glomerular basement membrane (anti-GBM) antibody disease. In ANCA-associated vasculitis (AAV), four main conceptual advances are discussed as follows: (1) evidence for the pathogenicity of ANCA, (2) molecular mimicry and the role of infection in AAV, (3) evidence for aberrant T-cell responses and T-cell regulation in AAV, and (4) advances in understanding of genetic predisposition to AAV. In relation to anti-GBM disease we discuss the following: (1) the nature of the Goodpasture autoantigens, (2) T-cell responses and regulation in anti-GBM disease, and (3) human leukocyte antigen and non-human leukocyte antigen genetic associations.

IL-23, not IL-12, directs autoimmunity to the Goodpasture antigen

The autoantigen in Goodpasture disease is the noncollagenous domain of alpha3 type IV collagen [alpha3(IV)NC1]. We previously demonstrated that IL-12p40(-/-) mice are protected from experimental autoimmune anti-glomerular basement membrane (anti-GBM) glomerulonephritis, seemingly defining a role for IL-12 in this disease; however, the recent identification of IL-23, a heterodimer composed of IL-12p40 and IL-23p19 subunits, raises the possibility that IL-23, rather than IL-12, may modulate this disease instead. We immunized wild-type, IL-12p35(-/-) (IL-12 deficient, IL-23 intact), IL-12p40(-/-) (deficient in both IL-12 and IL-23), and IL-23p19(-/-) (IL-12 intact, IL-23 deficient) mice with recombinant mouse alpha3(IV)NC1. Wild-type mice developed autoreactivity to alpha3(IV)NC1: Humoral responses, cellular responses, renal histologic abnormalities, leukocyte accumulation, autoantibody deposition, and IL-17A mRNA expression (a cytokine produced by the IL-23-maintained Th17 subset). IL-23 but not IL-12 was detected in the immune system. Regardless of the presence of IL-12, mice deficient in IL-23 were protected, but mice with IL-23 were not. Both IL-23-deficient strains exhibited lower autoantibody titers, reduced cellular reactivity, diminished cytokine production (IFN-gamma [Th1], IL-17A [Th17], TNF, and monocyte chemoattractant protein 1), and less renal disease and glomerular IgG deposition. The deficient responses in the absence of IL-23 were not due to increased regulatory T cells; IL-12p40(-/-) and IL-23p19(-/-) mice did not show increased proportions of CD4(+)CD25(+)FoxP3(+) cells or IL-10 levels early in the immune response. In conclusion, autoreactivity to the Goodpasture antigen is directed primarily by IL-23, absence of which results in hyporeactivity including but extending beyond a deficient Th17 response.

Identification of a nephritogenic immunodominant B and T cell epitope in experimental autoimmune glomerulonephritis

Experimental autoimmune glomerulonephritis (EAG) can be induced in Wistar Kyoto (WKY) rats by immunization with the non-collagenous domain (NC1) of the alpha 3 chain of type IV collagen, alpha3(IV)NC1. In patients with Goodpasture's disease, the major B cell epitope is located at the N-terminus of alpha3(IV)NC1. In order to investigate whether B and T cell responses in EAG are directed towards immunodominant peptides within the same region of rat alpha3(IV)NC1, we immunized WKY rats with recombinant rat alpha3(IV)NC1 (positive control) and five 15-mer overlapping synthetic peptides from the N-terminus of rat alpha3(IV)NC1: pCol(17-31), pCol(24-38), pCol(31-45), pCol(38-52) and pCol(45-59). Positive control animals immunized with alpha3(IV)NC1 produced an antibody response directed towards alpha3(IV)NC1 and pCol(24-38). Splenic T cells from these animals proliferated in response to alpha3(IV)NC1 and pCol(24-38). No significant antibody or T cell responses were observed to the other peptides examined. Animals immunized with pCol(24-38) developed linear deposits of immunoglobulin G on the glomerular basement membrane, albuminuria and focal necrotizing glomerulonephritis with crescent formation by week 6 after immunization. Circulating antibodies from these animals recognized pCol(24-38) and alpha3(IV)NC1, and their T cells proliferated in response to pCol(24-38) and alpha3(IV)NC1. Animals immunized with the other peptides developed no significant immune response to alpha3(IV)NC1 and no disease. In conclusion, these results demonstrate that a 15-mer peptide from the N-terminus of alpha3(IV)NC1 [pCol(24-38)] is recognized by B and T cells from rats immunized with recombinant alpha3(IV)NC1, and that the same peptide is capable of inducing crescentic glomerulonephritis. Identification of this immunodominant peptide will be of value in designing new therapeutic strategies for inducing mucosal tolerance in EAG, which may be applicable to patients with glomerulonephritis.

Positive association of HLA-DRB1*15 with Dupuytren's disease in Caucasians

Dupuytren's disease (DD) is a permanent nodular condition affecting the palms and digits of the hands, leading to progressive shortening and contractures of the digits often resulting in diminished function and severe deformity. DD is thought to be one of the most common hereditary connective tissue disorders in Caucasians. To elucidate further the aetiology of DD, we compared the HLA-DRB1 phenotype frequencies of DD patients (n=67) against the HLA-DRB1 phenotype frequencies observed in a control population (n=537). HLA-DRB1*15 phenotype frequency was higher in DD positive Caucasoids (37.3%) when compared with control data (20.9%) (corrected P=0.029): we conclude that in Caucasoids of European origin, HLA-DRB1*15 is associated with risk of developing DD.

Regulatory immune cells in kidney disease

Lymphocytes and macrophages act as effector immune cells in the initiation and progression of renal injury. Recent data have shown that subpopulations of these immune cells (regulatory T lymphocytes and alternately-activated or regulatory macrophages) are potent modulators of tissue injury and repair in renal disease. Recent animal studies examining the therapeutic effect of these cells raise the exciting possibility that strategies targeting these cell types may be effective in treating and preventing kidney disease in humans. This review will describe their biological role in experimental kidney disease and therapeutic potential in clinical nephrology.

Advances in the pathogenesis of Goodpasture's disease: from epitopes to autoantibodies to effector T cells

Goodpasture's disease, an "organ-specific" autoimmune disease is manifest by rapidly progressive glomerulonephritis and pulmonary hemorrhage. Studies into the pathogenesis of this disease have shed light on the autoantigen (the non-collagenous domain of the alpha3 chain of type IV collagen, alpha3(IV)NC1) and its epitopes, as well as the involvement of autoantibodies and cellular effectors in disease. The discovery of alpha3(IV)NC1 lead to studies that defined the structure and biology of type IV collagen and are defining B and T cell epitopes. Goodpasture autoantibody epitopes are "cryptic" in that they are structurally sequestered by adjacent non-collagenous domains of alpha4 and alpha5 type IV collagen. T cell epitope studies in rats demonstrated that a 13-mer could induce experimental autoimmune glomerulonephritis. T cells from patients with Goodpasture's recognize two epitopes, in regions which are highly susceptible in antigen processing by endosomal proteases. Goodpasture's disease is strongly associated with HLA DRB1 genes, whereby DRB1*1501 confers susceptibility and the DRB1*0701 and DRB1*0101 are dominantly protective. Experimental data implicate both autoantibodies and cell mediated immunity as disease effectors. Observations in humans suggest that regulatory T cells are associated with the development of self-immunoregulation in the convalescent phase of disease.

Healthy individuals have Goodpasture autoantigen-reactive T cells

Autoreactive T cells in patients with Goodpasture's disease are specific for epitopes in the Goodpasture antigen (the NC1 domain of the alpha3 chain of type IV collagen) that are rapidly destroyed during antigen processing to a degree that diminishes their presentation to T cells. We hypothesized that patients' autoreactive T cells exist because antigen processing prevents presentation of the self-epitopes they recognize, circumventing specific tolerance mechanisms. We predicted that autoreactive T cells specific for these peptides should also exist in healthy individuals, albeit at low frequency and in an unprimed state. We obtained blood from healthy unrelated donors and, using a panel of 45 alpha3(IV)NC1 peptides, identified alpha3(IV)NC1-specific T cells in all donors. Thirty-six of 45 peptides elicited a proliferative T cell response from at least one subject, and 6 of the peptides evoked a response in >50% of the individuals. This consistency was not caused by selectivity of HLA class II molecules because the donors expressed a diversity of HLA antigens, but was largely a result of the substrate-specificity of the endosomal proteases Cathepsin D and E. There was a significant correlation between high susceptibility to Cathepsin D digestion and the capacity to stimulate primary T cell responses (P = 0.00006). In summary, healthy individuals have low frequencies of unstimulated alpha3(IV)NC1-reactive T cells with similar specificities to the autoreactive T cells found in patients with Goodpasture disease. In both cases, existence of the alpha3(IV)NC1-reactive T cells can be accounted for by destructive processing.

Different Forms of Helper Tolerance to Carcinoembryonic Antigen: Ignorance and Regulation

PURPOSE

Understanding the mechanisms of immune tolerance to tumor-associated antigens (TAA) is an important step in the design of cancer immunotherapy. The aim was to determine how T helper (Th) cell tolerance is mediated for a prototypic TAA, carcinoembryonic antigen (CEA).

EXPERIMENTAL DESIGN

Peripheral blood mononuclear cells from 50 healthy volunteers were stimulated with CEA, and the type and fine specificity of any Th cell responses were identified. The inhibitory effects of T regulatory (Tr) populations were determined by depleting "natural" CD25(+) Tr cells or neutralizing cytokine produced by the "induced" Tr form.

RESULTS

Proliferative Th cell responses were consistently induced by CEA in 22 of 50 individuals. Responding cells were drawn from the CD45RA(+) "naive" or quiescent population. Depleting the CD25(+) fraction did not enhance CEA responsiveness. However, CEA elicited secretion of the Tr cytokine interleukin-10 (IL-10) in 23 of 50 donors, including 20 of 22 where no proliferation was induced. Neutralizing IL-10 revealed previously unseen proliferation to CEA by CD45RO(+) "memory" Th cells. Epitope maps revealed differences in the fine specificities of Th cells capable of proliferating or secreting IL-10.

CONCLUSIONS

There are at least two major forms of CEA tolerance in different individuals. One is "ignorance," a failure of specific Th cells to respond to antigen presented in vivo. The other, seen when ignorance is lost, is mediated by IL-10-secreting Tr cells that recognize CEA. TAA tolerance, for example to colorectal carcinoma cells expressing CEA, may be overcome by peptide vaccines that exploit the differences in epitopes recognized by effector and Tr responses.

Autoepitopes and alloepitopes of type IV collagen: role in the molecular pathogenesis of anti-GBM antibody glomerulonephritis

Anti-glomerular basement membrane (anti-GBM) antibodies elicited by autoimmune or alloimmune mechanisms are associated with aggressive forms of rapid progressive glomerulonephritis. Pathogenic anti-GBM autoantibodies and alloantibodies target the noncollagenous (NC1) domains of the alpha3alpha4alpha5(IV) collagen, a major GBM component. In autoimmune anti-GBM glomerulonephritis, a breakdown of immune self-tolerance leads to the activation of autoreactive B and T cells recognizing epitopes within the alpha3NC1 subunit. In the GBM, the conformational epitopes targeted by anti-GBM autoantibodies are structurally sequestered within the alpha3alpha4alpha5NC1 hexamer complex formed upon assembly of collagen IV chains into trimeric molecules and networks. Autoantibodies selectively bind to and dissociate a subset of alpha3alpha4alpha5NC1 hexamers composed of monomer subunits, whereas hexamers containing NC1 dimer subunits are resistant to dissociation by autoantibodies. The crypticity of alpha3NC1 autoepitopes suggests that self-tolerance to alpha3(IV) collagen is broken by structural alterations of the native alpha3alpha4alpha5NC1 hexamer that unmask normally sequestered epitopes, triggering an autoimmune reaction. Post-transplant anti-GBM nephritis in the renal allograft of transplanted Alport patients is mediated by an alloimmune reaction to the NC1 domains of alpha3alpha4alpha5(IV) collagen, present in the allograft GBM but absent from Alport basement membranes. Alloantibodies from patients with autosomal-recessive Alport syndrome predominantly bind to the alpha3NC1 domain, whereas alloantibodies from X-linked Alport patients target preferentially, though not exclusively, epitopes within the alpha5NC1 subunit. The accessibility of the alloantigenic sites within the alpha3alpha4alpha5NC1 hexamers, contrasting with the crypticity of autoantigenic sites, suggest that different molecular forms of alpha3alpha4alpha5(IV) collagen initiate the immunopathogenic responses in the two forms of anti-GBM disease. Advances in elucidating the structure of the GBM antigen and the identification of the pathogenic B and T cell epitopes, along with new insights into the pathogenic mechanisms at cellular and molecular level will facilitate the development of targeted strategies for prevention, detection, and treatment of human anti-GBM antibody glomerulonephritis.

Cytokines in Glomerulonephritis

Cytokines play central roles in both innate and adaptive immune responses that lead to renal inflammation. They are involved systemically in cross-talk between antigen-presenting cells, leukocytes, and regulatory cells to initiate and modulate nephritogenic immunity. Within the kidney, cytokines play a central role in signaling between infiltrating leukocytes and intrinsic renal cells and orchestrate the effector responses that lead to renal damage. Glomerulonephritis (GN) is an important cause of renal inflammation leading to renal failure that results from adaptive responses targeted at the kidney. Animal models of GN have shown that cytokines play critical roles in initiation and modulation of renal inflammatory responses through their ability to modulate the T helper 1/T helper 2 balance of nephritogenic immune responses. Evidence from clinical studies is now confirming the importance of this paradigm in directing the inflammatory mechanisms, histologic patterns, and clinical consequences of human GN. Cytokines also have critical intrarenal effector roles in the development, perpetuation, and resolution of GN. The proinflammatory role of intrarenal cytokine production by leukocytes in GN is well recognized, but, more recently, the role of intrinsic renal cell cytokine production in amplifying renal inflammation has been shown in animal models of GN. Studies showing benefits of specific anticytokine therapies directed at tumor necrosis factor in human GN are now appearing.

Presentation of the Goodpasture Autoantigen Requires Proteolytic Unlocking Steps That Destroy Prominent T Cell Epitopes

The most abundant autoreactive T cells in patients with Goodpasture's disease are specific for peptides in the autoantigen that have high affinity for the disease-associated HLA class II molecule, DR15. How can such T cells escape self-tolerance mechanisms? This study showed that these peptides are highly susceptible to destruction in the earliest stages of antigen processing, and some must be cleaved for antigen digestion to be possible ("unlocking"). Goodpasture autoantigen [collagen alpha3(IV)NC1; approximately 31 kD] that was incubated with B cell lysosomes was cleaved within a few minutes to form approximately 9- and approximately 22-kD fragments, then increasing quantities of smaller peptides. The processing was completely abrogated by pepstatin A, a specific inhibitor of cathepsin D/E, even though lysosomal extracts contain a rich array of proteases. Purified cathepsin D generated the same major alpha3(IV)NC1 fragments as entire lysosomes, suggesting that cathepsin D cleavages are required to initiate alpha3(IV)NC1 processing. The initial unlocking cleavages destroyed two major self-epitopes, and subsequent preferred cleavages destroyed all of the other T cell epitopes that are recognized by most patients' autoreactive T cells. The responses of T cell clones that are specific for a major disease-associated peptide to antigen-pulsed intact antigen-presenting cells were substantially enhanced by pepstatin A treatment. Therefore, cathepsin D activity significantly diminishes presentation of alpha3(IV)NC1 peptides that are recognized by patients' T cells by destroying the peptides in early processing. These observations can explain why the mature T cell repertoire includes reactivity toward these self-peptides and suggests that a key factor in disease initiation is likely to be a shift in antigen processing.

Mapping helper T-cell epitopes on platelet membrane glycoprotein IIIa in chronic autoimmune thrombocytopenic purpura

Chronic autoimmune thrombocytopenic purpura (AITP) is associated with autoantibodies specific for platelet membrane components, often including glycoprotein GPIIIa. T helper (Th) cells reactive with GPIIIa, which are capable of driving the autoantibody response, are activated in AITP, and the aim here was to map the epitopes that they recognize. Peripheral blood mononuclear cells (PBMCs) were obtained from 31 patients with AITP and 30 control donors and stimulated with a panel of 86 overlapping synthetic 15-mer peptides spanning the complete sequence of GPIIIa. One or more peptides elicited recall proliferation by PBMCs from 28 of the patients, and, typically, multiple sequences were stimulatory. In contrast, responses in healthy control donors were rare (chi-square test = 115.967; P ≤ .001). It was confirmed that the proliferating PBMCs from patients were cells of the CD3+CD4+ helper phenotype that were MHC class II restricted. Despite variation between different cases of AITP, particular sequences were commonly recognized with PBMCs from 24 patients (77%) responding to 1 or more of the 4 most dominant peptides. Mapping such dominant autoreactive helper epitopes is the first step in the development of new approaches to the treatment of AITP, based on the use of peptides to tolerize Th cells specific for platelet glycoproteins.