Antony Rosen

Autoantigens as substrates for apoptotic proteases: implications for the pathogenesis of systemic autoimmune disease

Systemic autoimmune diseases are a genetically complex, heterogeneous group of diseases in which the immune system targets a diverse, but highly specific group of intracellular autoantigens. The clustering and marked concentration of these molecules in the surface blebs of apoptotic cells, and their modification by apoptosis-specific proteolytic cleavage and/or phosphorylation at these sites, has focused attention on a unique apoptotic setting as the potential initiating stimulus for systemic autoimmunity. This apoptotic event is likely to (i) occur in a microenvironment containing high concentrations of the targeted antigens, (ii) be pro-immune in nature (e.g. viral infection), and (iii) allow suprathreshold concentrations of antigen with non-tolerized structure (either novel fragments, post-translational modifications, or complexes) to enter the class II processing pathway and initiate a primary immune response. Defective clearance or reduced anti-inflammatory consequences of apoptotic material may be important susceptibility factors in this group of diseases. Once the primary immune response to apoptotic antigens has been initiated, other apoptotic events (occurring in the course of homeostasis or damage) may stimulate the secondary immune response with less stringency, resulting in flares.

Autoantibodies against 3‐hydroxy‐3‐methylglutaryl‐coenzyme A reductase in patients with statin‐associated autoimmune myopathy

OBJECTIVE In addition to inducing a self-limited myopathy, statin use is associated with an immune-mediated necrotizing myopathy (IMNM), with autoantibodies that recognize ∼200-kd and ∼100-kd autoantigens. The purpose of this study was to identify these molecules to help clarify the disease mechanism and facilitate diagnosis. METHODS The effect of statin treatment on autoantigen expression was addressed by immunoprecipitation using sera from patients. The identity of the ∼100-kd autoantigen was confirmed by immunoprecipitation of in vitro-translated 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR) protein. HMGCR expression in muscle was analyzed by immunofluorescence. A cohort of myopathy patients was screened for anti-HMGCR autoantibodies by enzyme-linked immunosorbent assay and genotyped for the rs4149056 C allele, a predictor of self-limited statin myopathy. RESULTS Statin exposure induced expression of the ∼200-kd/∼100-kd autoantigens in cultured cells. HMGCR was identified as the ∼100-kd autoantigen. Competition experiments demonstrated no distinct autoantibodies recognizing the ∼200-kd protein. In muscle biopsy tissues from anti-HMGCR-positive patients, HMGCR expression was up-regulated in cells expressing neural cell adhesion molecule, a marker of muscle regeneration. Anti-HMGCR autoantibodies were found in 45 of 750 patients presenting to the Johns Hopkins Myositis Center (6%). Among patients ages 50 years and older, 92.3% had taken statins. The prevalence of the rs4149056 C allele was not increased in patients with anti-HMGCR. CONCLUSION Statins up-regulate the expression of HMGCR, the major target of autoantibodies in statin-associated IMNM. Regenerating muscle cells express high levels of HMGCR, which may sustain the immune response even after statins are discontinued. These studies demonstrate a mechanistic link between an environmental trigger and the development of sustained autoimmunity. Detection of anti-HMGCR autoantibodies may facilitate diagnosis and direct therapy.

Granzyme B Directly and Efficiently Cleaves Several Downstream Caspase Substrates: Implications for CTL-Induced Apoptosis

Caspase-mediated proteolysis of downstream substrates is a critical element of the execution pathway common to all forms of apoptosis studied to date. While this caspase-dependent pathway is activated during cytotoxic lymphocyte granule-induced cell death, recent studies have also provided evidence for caspase-independent pathways. However, the mechanisms mediating these additional pathways have not been defined. The current study demonstrates that DNA-PKcs and NuMA are directly and efficiently cleaved by granzyme B in vitro and in vivo, generating unique substrate fragments not observed during other forms of apoptosis. This direct, caspase-independent ability of granzyme B to cleave downstream death substrates constitutes an apoptotic effector mechanism that is insensitive to inhibitors of the signaling or execution components of the endogenous apoptotic cascade.

Enhanced autoantigen expression in regenerating muscle cells in idiopathic inflammatory myopathy.

Unique autoantibody specificities are strongly associated with distinct clinical phenotypes, making autoantibodies useful for diagnosis and prognosis. To investigate the mechanisms underlying this striking association, we examined autoantigen expression in normal muscle and in muscle from patients with autoimmune myositis. Although myositis autoantigens are expressed at very low levels in control muscle, they are found at high levels in myositis muscle. Furthermore, increased autoantigen expression correlates with differentiation state, such that myositis autoantigen expression is increased in cells that have features of regenerating muscle cells. Consistent with this, we found that cultured myoblasts express high levels of autoantigens, which are strikingly down-regulated as cells differentiate into myotubes in vitro. These data strongly implicate regenerating muscle cells rather than mature myotubes as the source of ongoing antigen supply in autoimmune myositis. Myositis autoantigen expression is also markedly increased in several cancers known to be associated with autoimmune myositis, but not in their related normal tissues, demonstrating that tumor cells and undifferentiated myoblasts are antigenically similar. We propose that in cancer-associated myositis, an autoimmune response directed against cancer cross-reacts with regenerating muscle cells, enabling a feed-forward loop of tissue damage and antigen selection. Regulating pathways of antigen expression may provide unrecognized therapeutic opportunities in autoimmune diseases.

Hsp60 accelerates the maturation of pro‐caspase‐3 by upstream activator proteases during apoptosis

The activation of caspases represents a critical step in the pathways leading to the biochemical and morphological changes that underlie apoptosis. Multiple pathways leading to caspase activation appear to exist and vary depending on the death‐inducing stimulus. We demonstrate that the activation of caspase‐3, in Jurkat cells stimulated to undergo apoptosis by a Fas‐independent pathway, is catalyzed by caspase‐6. Caspase‐6 was found to co‐purify with caspase‐3 as part of a multiprotein activation complex from extracts of camptothecin‐treated Jurkat cells. A biochemical analysis of the protein constituents of the activation complex showed that Hsp60 was also present. Furthermore, an interaction between Hsp60 and caspase‐3 could be demonstrated by co‐immunoprecipitation experiments using HeLa as well as Jurkat cell extracts. Using a reconstituted in vitro system, Hsp60 was able to substantially accelerate the maturation of procaspase‐3 by different upstream activator caspases and this effect was dependent on ATP hydrolysis. We propose that the ATP‐dependent ‘foldase’ activity of Hsp60 improves the vulnerability of pro‐caspase‐3 to proteolytic maturation by upstream caspases and that this represents an important regulatory event in apoptotic cell death.

Histidyl–tRNA Synthetase and Asparaginyl–tRNA Synthetase, Autoantigens in Myositis, Activate Chemokine Receptors on T Lymphocytes and Immature Dendritic Cells

Autoantibodies to histidyl–tRNA synthetase (HisRS) or to alanyl–, asparaginyl–, glycyl–, isoleucyl–, or threonyl–tRNA synthetase occur in ∼25% of patients with polymyositis or dermatomyositis. We tested the ability of several aminoacyl–tRNA synthetases to induce leukocyte migration. HisRS induced CD4+ and CD8+ lymphocytes, interleukin (IL)-2–activated monocytes, and immature dendritic cells (iDCs) to migrate, but not neutrophils, mature DCs, or unstimulated monocytes. An NH2-terminal domain, 1–48 HisRS, was chemotactic for lymphocytes and activated monocytes, whereas a deletion mutant, HisRS-M, was inactive. HisRS selectively activated CC chemokine receptor (CCR)5-transfected HEK-293 cells, inducing migration by interacting with extracellular domain three. Furthermore, monoclonal anti-CCR5 blocked HisRS-induced chemotaxis and conversely, HisRS blocked anti-CCR5 binding. Asparaginyl–tRNA synthetase induced migration of lymphocytes, activated monocytes, iDCs, and CCR3-transfected HEK-293 cells. Seryl–tRNA synthetase induced migration of CCR3-transfected cells but not iDCs. Nonautoantigenic aspartyl–tRNA and lysyl–tRNA synthetases were not chemotactic. Thus, autoantigenic aminoacyl–tRNA synthetases, perhaps liberated from damaged muscle cells, may perpetuate the development of myositis by recruiting mononuclear cells that induce innate and adaptive immune responses. Therefore, the selection of a self-molecule as a target for an autoantibody response may be a consequence of the proinflammatory properties of the molecule itself.

The mucocutaneous and systemic phenotype of dermatomyositis patients with antibodies to MDA5 (CADM-140): A retrospective study

BACKGROUND Dermatomyositis (DM) is a multisystem autoimmune disease, in which serologic evidence of immune responses to disease-specific antigenic targets is found in approximately 50% to 70% of patients. Recently, melanoma differentiation-associated gene 5 (MDA5) has been identified as a DM-specific autoantigen that appears to be targeted in patients with DM and mild or absent muscle inflammation and with an increased risk of interstitial lung disease. OBJECTIVE We wished to understand the role of MDA5 in DM skin inflammation by testing it to determine if a specific cutaneous phenotype is associated with MDA5 reactivity. METHODS We retrospectively screened plasma from 77 patients with DM in the outpatient clinics at the Stanford University Department of Dermatology in California. RESULTS We found that 10 (13%) patients had circulating anti-MDA5 antibodies, and had a characteristic cutaneous phenotype consisting of skin ulceration, tender palmar papules, or both. Typical areas of skin ulceration included the lateral nailfolds, Gottron papules, and elbows. Biopsy specimens of the palmar papules showed a vasculopathy characterized by vascular fibrin deposition with variable perivascular inflammation. Patients with anti-MDA5 antibodies also had an increased risk of oral pain and/or ulceration, hand swelling, arthritis/arthralgia, and diffuse hair loss. Consistent with previous reports, these patients had little or no myositis and had increased risk of interstitial lung disease. LIMITATIONS This study was conducted at a tertiary referral center. Multiple associations with MDA5 antibodies were tested retrospectively on a relatively small cohort of 10 anti-MDA5-positive patients. CONCLUSION We suggest that MDA5 reactivity in DM characterizes a patient population with severe vasculopathy.

Association of the autoimmune disease scleroderma with an immunologic response to cancer.

Cancer Immunosurveillance Gone Bad? A subset of patients who develop scleroderma, a debilitating autoimmune disease, have an elevated risk of developing cancer. These patients harbor autoantibodies to RPC1, an RNA polymerase subunit encoded by the POLR3A gene. Joseph et al. (p. 152, published online December 5; see the Perspective by Teng and Smyth) explored whether the RPC1 autoantibodies target a “foreign” antigen derived from a mutated POLR3A gene. Sequence analysis revealed that POLR3A mutations were present in tumors from six of eight patients with RPC1 autoantibodies but in no tumors from eight control patients who lacked RPC1 autoantibodies. Cell culture data suggested that the POLR3A mutations triggered cellular and humoral immune responses in the patients. These results provide support for the “immunosurveillance” hypothesis, which posits the continual eradication of nascent tumor cells via immune responses. The immune system’s response to a mutant protein produced by an incipient cancer may help to explain an autoimmune disease. [Also see Perspective by Teng and Smyth] Autoimmune diseases are thought to be initiated by exposures to foreign antigens that cross-react with endogenous molecules. Scleroderma is an autoimmune connective tissue disease in which patients make antibodies to a limited group of autoantigens, including RPC1, encoded by the POLR3A gene. As patients with scleroderma and antibodies against RPC1 are at increased risk for cancer, we hypothesized that the “foreign” antigens in this autoimmune disease are encoded by somatically mutated genes in the patients’ incipient cancers. Studying cancers from scleroderma patients, we found genetic alterations of the POLR3A locus in six of eight patients with antibodies to RPC1 but not in eight patients without antibodies to RPC1. Analyses of peripheral blood lymphocytes and serum suggested that POLR3A mutations triggered cellular immunity and cross-reactive humoral immune responses. These results offer insight into the pathogenesis of scleroderma and provide support for the idea that acquired immunity helps to control naturally occurring cancers.

Annexin V Binds to Viable B Cells and Colocalizes with a Marker of Lipid Rafts upon B Cell Receptor Activation

Recombinant annexin V (rAnV) has been used to identify apoptotic cells based on its ability to bind phosphatidylserine (PS), a lipid normally restricted to the cytoplasmic face of the plasma membrane, but externalized early during apoptosis. However, this association of rAnV binding and apoptosis is not an obligatory one. We demonstrate that rAnV binds to a large fraction of murine B cells bearing selectable Ag receptors despite the fact that these cells are not apoptotic. Phosphatidylserine, which is uniformly distributed on resting B cells, is mobilized to co-cap with IgM on anti-IgM-treated B cells and to colocalize with GM1, a marker of lipid rafts. Cross-linking PS before anti-IgM treatment sequesters this lipid and alters signaling through IgM. Thus, PS exposed on the majority of B cells in vivo does not reflect early apoptosis, but, instead, plays a role in receptor-mediated signaling events.

Ultraviolet light-induced keratinocyte apoptosis: A potential mechanism for the induction of skin lesions and autoantibody production in LE

Systemic lupus erythematosus (SLE) is a prototype systemic autoimmune disease which is characterized clinically by pleiotropy and periodicity. The immune features which accompany the characteristic flares of the disease have strongly suggested that the autoimmune response is driven by self antigen, and is T cell-dependent. These features have prompted the search for potential initiating process(es) which induce the release of self-antigens in a form which causes T cell tolerance to those self molecules to be broken. We review here several recent observations which implicate apoptotic cells as an important potential source of clustered and concentrated autoantigens in SLE, and present our current model whereby the novel autoantigen fragments generated in apoptotic surface blebs initiate and drive the autoimmune response in this disease.