Human autoantibodies to poly(adenosine diphosphate-ribose) polymerase recognize cross-reactive epitopes associated with the catalytic site of the enzyme

Nucleic acid-associated autoantigens: pathogenic involvement and therapeutic potential

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

Human tankyrases are aberrantly expressed in colon tumors and contain multiple epitopes that induce humoral and cellular immune responses in cancer patients

PURPOSE

Tankyrases 1 and 2 are telomere-associated poly(ADP-ribose) polymerases (PARP) that can positively regulate telomere elongation and interact with multiple cellular proteins. Recent reports implicated tankyrases as tumor antigens and potential targets of anticancer treatment. We examined expression of tankyrases in colon tumors and immune response to these enzymes in patients with different types of cancer.

METHODS

mRNA and protein expression was evaluated by quantitative real-time RT-PCR and Western blotting, respectively. Humoral immune response to recombinant tankyrases was investigated by modified enzyme-linked immunoassays. Cellular immune response was analysed by ELISPOT and (51)Cr release assays.

RESULTS

We found that both mRNA and protein levels of tankyrase 2 (TNKL) are upregulated in colon tumors. In contrast, protein level of tankyrase 1 (TNKS) is downregulated, while mRNA level shows variable changes. More than a quarter of colon cancer patients develop humoral immune response to at least one of the two tankyrases. In this study we mapped common and unique B-cell epitopes located in different domains of the two proteins. Additionally, we present evidence for T-cell responses both to epitopes that are unique for TNKL and to those shared between TNKL and TNKS.

CONCLUSION

Our study favors a biomarker usage of antibody response to tankyrases. Spontaneous CD8(+) T-cell responses to these enzymes are rare and further investigation is needed to evaluate tankyrases as potential targets for cancer immunotherapy.

Identification of Anti-Plasmin Antibodies in the Antiphospholipid Syndrome That Inhibit Degradation of Fibrin

The combined presence of anti-phospholipid Ab (aPL) and thrombosis is recognized as the antiphospholipid syndrome (APS). The aPL represent a heterogeneous group of Ab that recognize various phospholipids (PL), PL-binding plasma proteins, and/or PL-protein complexes. Recently, we found the presence of antithrombin Ab in some APS patients and that some of these anti-thrombin Ab could inhibit thrombin inactivation by antithrombin. Considering that thrombin is homologous to plasmin, which dissolves fibrin, we hypothesize that some APS patients may have Ab that react with plasmin, and that some anti-plasmin Ab may interfere with the plasmin-mediated lysis of fibrin clots. To test this hypothesis, we searched for anti-plasmin Ab in APS patients and then studied those found for their effects on the fibrinolytic pathway. The results revealed that seven of 25 (28%) APS patients have IgG anti-plasmin Ab (using the mean OD plus 3 SD of 20 normal controls as the cutoff) and that six of six patient-derived IgG anti-thrombin mAb bind to plasmin with relative K(d) values ranging from 5.6 x 10(-8) to 1 x 10(-6) M. These K(d) values probably represent affinities in the higher ranges known for human IgG autoantibodies against protein autoantigens. Of these mAb, one could reduce the plasmin-mediated lysis of fibrin clots. These findings suggest that plasmin may be an important driving Ag for some aPL B cells in APS patients, and that the induced anti-plasmin Ab may act either directly, by binding to plasmin and inhibiting its fibrinolytic activity, or indirectly, by cross-reacting with other homologous proteins in the coagulation cascade to promote thrombosis.

Identification of autoantibody against poly (ADP-ribose) polymerase (PARP) fragment as a serological marker in systemic lupus erythematosus

OBJECTIVES

By utilizing serological analysis of a recombinant cDNA expression library (SEREX), we previously found that autoantibodies to poly (ADP-ribose) polymerase (PARP) are specifically present in the sera of patients with SLE. In this study, recombinant proteins of various domains of PARP were used to determine the PARP domain that is associated with SLE.

METHODS

We produced four recombinant PARP proteins, which contained various PARP domains, and then carried out enzyme linked immunosorbent assay (ELISA) using these recombinant proteins to identify domains useful for SLE diagnosis. The recombinant proteins used in this analysis were; ADPNF (amino acids 1-234), ET-L2 (amino acids 339-680), ET-L3 (amino acids 681-1014), and ADPCF (amino acids 300-1014).

RESULT

ELISA with ADPNF or ET-L2 showed low sensitivity in the sera of patients with SLE (14.3% and 17.0% respectively), whereas ELISA with ET-L3 or ADPCF showed high sensitivity in the sera of patients with SLE (34.0% and 49.1%, respectively). Autoantibodies to ADPCF were not found in the sera of patients with rheumatoid arthritis (0/30), systemic sclerosis (0/30) or healthy donors (0/54) and were rarely found in polymyositis/dermatomyositis (1/30) and Sjogren syndrome (1/14). Autoantibodies to ADPCF were closely associated with the presence of an oral ulcer in SLE (P=0.03, by the chi-square test).

CONCLUSION

The high sensitivity and specificity shown by autoantibodies to ADPCF protein could be used as a valuable serologic maker for the diagnosis of SLE.

Characterization of the necrotic cleavage of poly(ADP-ribose) polymerase (PARP-1): implication of lysosomal proteases

The poly(ADP-ribose) polymerase (PARP-1), a 113 kDa nuclear enzyme, is cleaved in fragments of 89 and 24 kDa during apoptosis. This cleavage has become a useful hallmark of apoptosis and has been shown to be done by DEVD-ase caspases, a family of proteases activated during apoptosis. Interestingly, PARP-1 is also processed during necrosis but a major fragment of 50 kDa is observed. This event is not inhibited by zVAD-fmk, a broad spectrum caspase inhibitor, suggesting that these proteases are not implicated in the necrotic cleavage of PARP-1. Since lysosomes release their content into the cytosol during necrosis, the proteases liberated could produce the cleavage of PARP-1. We therefore isolated lysosomal rich-fractions from Jurkat T cells. Our results reveal that the in vitro lysosomal proteolytic cleavage of affinity purified bovine PARP-1 is composed of fragments corresponding, in apparent molecular weight and function, to those found in Jurkat T cells treated with necrotic inducers like 0.1% H2O2, 10% EtOH or 100 microM HgCl2. Moreover, we used purified lysosomal proteases (cathepsins B, D and G) in an in vitro cleavage assay and found that cathepsins B and G cleaved PARP-1 in fragments also found with the lysosomal rich-fractions. These findings suggest that the necrotic cleavage of PARP-1 is caused in part or in totality by lysosomal proteases released during necrosis.

The role of bacterial DNA in autoantibody induction

Bacterial DNA has potent immunological properties that can stimulate the immune system in SLE in both specific and non-specific ways. As such, this molecule may play an important role in disease pathogenesis, because it can exert immunomodulatory activity and function as a molecular mimic. Future studies will hopefully both determine the role of foreign nucleic acids in the induction of autoantibodies and lead to strategies for their elimination.

Inhibition of caspase-3-mediated poly(ADP-ribose) polymerase (PARP) apoptotic cleavage by human PARP autoantibodies and effect on cells undergoing apoptosis

Autoantibodies directed to nuclear antigens are serological hallmarks of autoimmune rheumatic diseases such as systemic lupus erythematosus. Although much more is known about the molecular identity and functions of targeted self-antigens, with few exceptions, evidence that autoantibodies to these targets have a particular function and contribute directly to the pathological process is lacking. Here we show that human autoantibodies reacting with the zinc fingers of poly(ADP-ribose) polymerase involved in the recognition of damaged DNA totally prevent the cleavage of poly(ADP-ribose) polymerase by caspase-3, a process that normally occurs during early apoptosis. Furthermore, these antibodies, which are frequent in certain autoimmune rheumatic and bowel diseases, affect the characteristic features of apoptosis and increase cell survival ex vivo. This new observation is important, because failure to remove autoimmune or abnormal cells can give rise to prolonged autoimmune stimulation and tumor formation.

Mapping of topoisomerase II alpha epitopes recognized by autoantibodies in idiopathic pulmonary fibrosis

Autoantibodies against DNA topoisomerase II alpha have been identified in the sera of patients with idiopathic pulmonary fibrosis (IPF). To map topoisomerase II autoepitopes, we tested by ELISA and immunoblotting the IPF anti-topoisomerase II-positive sera against a series of recombinant proteins which covered the full length of topoisomerase II alpha. Specific patterns of reactivity were observed, indicating the existence of multiple epitopes on topoisomerase II, either highly complex or conformational/discontiguous or conformational/contiguous ones. The latter resided in amino acid residues 854-1147 and 1370-1447. A detailed analysis of these regions was undertaken, but we were not able to pinpoint a sequential peptide-sized epitope, or any significant homology with foreign pathogens. Further, we observed a significant correlation between the progression from a contiguous to a quaternary/tertiary structure-dependent autoepitope and the disease duration but not with the disease severity. Therefore, this result supports the hypothesis that anti-topoisomerase II autoreactivity evolves following an antigen-driven process.

Zinc is an essential cofactor for recognition of the DNA binding domain of poly(ADP-ribose) polymerase by antibodies in autoimmune rheumatic and bowel diseases

OBJECTIVE

To characterize autoantibody response to poly(ADP-ribose) polymerase (PARP) and to assess the significance of autoantibodies to the 2 zinc fingers of this enzyme in patients with autoimmune rheumatic and bowel diseases.

METHODS

The specificity of antienzyme autoantibodies was established by dot immunoassay with recombinant human PARP and by enzyme-linked immunosorbent assay using the recombinant N-terminal fragment containing the DNA binding domain of PARP, the recombinant C-terminal catalytic domain (40-kd fragment), a peptide containing the nuclear localization signal (NLS) of PARP, 2 synthetic peptides (and mutated peptides) corresponding to zinc-finger motifs F1 and F2 that are present in the DNA binding domain, zinc fingers from other self antigens (e.g., peptides from Ro60, Ro52, and U1C proteins), and poly(ADP-ribose). Sera from patients with autoimmune rheumatic and bowel diseases were tested, as were affinity-purified antibodies. Histocompatibility typing of systemic lupus erythematosus (SLE) patients was performed by serology.

RESULTS

Antibodies from the patient sera reacted only weakly with the recombinant N- and C-terminal domains and with the NLS peptide. In contrast, the 2 synthetic peptides corresponding to zinc-finger motifs F1 and F2 represented immunodominant targets for IgG antibodies from patients with SLE, mixed connective tissue disease (MCTD), Crohn's disease, and ulcerative colitis. The sera from patients with SLE and MCTD showed much weaker reactivity with mutant peptides F1 and F2, which contain mutations at the cysteine residues involved in zinc coordination. F1/F2 antibodies did not cross-react with zinc fingers from other self proteins. No correlation was found between the presence of F1/F2 autoantibodies in SLE sera and the presence of other autoantibodies typical of this disease (e.g., anti-double-stranded DNA and poly[ADP-ribose] antibodies). The presence of F2 antibodies in the serum of SLE patients was negatively associated with HLA-DR6.

CONCLUSION

An autoimmune response to PARP is potentially important because this enzyme is involved in DNA repair and is rapidly cleaved during the "execution phase" of apoptosis. The high prevalence in certain autoimmune rheumatic and bowel diseases of antibodies to F1 and F2, which are directly involved in this process, is further evidence implicating involvement of the DNA repair system in chronic inflammatory diseases.

Autoantibodies to RNA polymerase II are common in systemic lupus erythematosus and overlap syndrome. Specific recognition of the phosphorylated (IIO) form by a subset of human sera

Autoantibodies to RNA polymerases (RNAP) I, II, and III are reported to be highly specific for the diagnosis of scleroderma (systemic sclerosis, SSc). In the present study, the specificity of autoantibodies to RNAP I and III for SSc was confirmed by immunoprecipitation of 35S-labeled proteins. However, we report here the previously unrecognized production of anti-RNAP II autoantibodies by 9-14% of patients with SLE and mixed connective tissue disease/overlap syndrome. 12 out of 32 anti-RNAP II positive sera (group 1) immunoprecipitated a diffuse 220-240-kD band identified as the largest subunit of RNAP II whereas the remaining 20 (group 2) immunoprecipitated preferentially the 240-kD phosphorylated (IIo) form of the large subunit. After pulse labeling, group 1 sera immunoprecipitated only the 220-kD (IIa) RNAP II subunit, whereas the diffuse IIa/IIo band plus the 145-kD second largest RNAP II subunit (IIc) were immunoprecipitated after several hours of cold chase, suggesting that these sera recognized primarily the largest subunit of RNAP II. Group 2 sera recognized the IIc subunit after pulse labeling, and immunoprecipitated the IIc and IIo, but not the IIa, subunits after cold chase. Although it has been suggested that autoantibodies to RNAP II are usually accompanied by anti-RNAP I/III in SSc, all but one of the anti-RNAP II positive sera from SLE or mixed connective tissue disease/overlap syndrome patients, as well as most of the SSc sera, were negative for anti-RNAP I/III. Moreover, in contrast to previous reports suggesting that anti-RNAP antibodies rarely coexist with other SSc subset marker antibodies, anti-RNAP II antibodies were often accompanied by anti-Ku, anti-nRNP, or anti-topoisomerase I autoantibodies in the present study. We conclude that autoantibodies to RNAP II are not a specific marker for SSc, whereas autoantibodies to RNAP I/III are associated primarily with SSc. In addition, we have identified two distinctive patterns of RNAP II antigen recognition by autoantibodies, one of them characterized by specific recognition of the transcriptionally active (phosphorylated) form of RNAP II. The clinical significance of these different patterns remains to be determined.

Molecular and biochemical features of poly (ADP-ribose) metabolism

In the past five years, poly(ADP-ribosyl)ation has developed greatly with the help of molecular biology and the improvement of biochemical techniques. In this article, we describe the physico-chemical properties of the enzymes responsible for the synthesis and degradation of poly(ADP-ribose), respectively poly(ADP-ribose) polymerase and poly(ADP-ribose) glycohydrolase. We then discuss the possible roles of this polymer in DNA repair and replication as well as in cellular differentiation and transformation. Finally, we put forward various hypotheses in order to better define the function of this polymer found only in eucaryotes.

Laboratory evaluation of patients with Sjögren's syndrome

Sjögren's syndrome (SS) is a chronic autoimmune disorder characterized by lymphocytic infiltration of the lacrymal and salivary glands, leading to severe dryness of eyes (keratoconjunctivitis sicca) and mouth (xerostomia). SS may exist as a primary disorder (1 degree-SS) or in association with other autoimmune diseases including rheumatoid arthritis (RA), systemic lupus erythematosus or progressive systemic sclerosis (scleroderma). Diagnosis of 1 degree-SS is confirmed by minor salivary gland biopsy and the presence of circulating autoantibodies. Minor salivary gland biopsies exhibit focal lymphocytic infiltrates that are present in the majority of lobules. Incorrect methods of biopsy and failure to determine the average focus score are common causes for false-positive and false-negative biopsies. SS patients frequently have a positive antinuclear antibody test due to presence of SS-A (Ro) and SS-B (La) autoantibodies. Molecular analysis has revealed multiple "SS-A" proteins (60 kd, 54 kd, 52 kd) that react with sera from SS patients, as well as a 48 kd SS-B protein. Rheumatoid factor (anti-IgG Fc antibody) in 1 degree-SS patients exhibits restriction in its light chain-associated idiotype, in contrast to RA patients where no restriction of idiotype was detected. Other autoantibodies found in a subpopulation of SS patients include anti-ADP ribose polymerase, anti-cardiolipin, anti-mitochondrial, anti-mitotic spindle apparatus, anti-parietal cell, and anti-thyroid associated antibodies. Due to the high frequency of dryness syndromes in patients due to other causes (ranging from drug side effects to normal aging processes), the use of strict criteria for diagnosis of SS will lead to improved cost-efficient medical care avoiding needless anxiety in the patient.

An acquired antithrombin autoantibody directed toward the catalytic center of the enzyme

Antibody inhibitors against human thrombin are rare and have remained poorly characterized. We report the case of a 40-yr-old patient who developed a potent thrombin inhibitor revealed by mild bleeding symptoms and marked prolongation of most laboratory clotting times. After two years of evolution, he died from cerebral hemorrhage. The inhibitor, a polyclonal IgG, was associated with hematological and immunological criteria of autoimmune disorder. Antithrombin IgG was isolated from the patient's plasma by protein A- and thrombin-affinity chromatography. Fab fragments inhibited amidolytic activity of alpha thrombin, and thrombin-thrombomodulin catalyzed protein C activation with a Ki of approximately 10(-8) M in a noncompetitive manner. Alpha to gamma conversion of thrombin resulted in a moderate loss of affinity for the inhibitor. Upon complex formation of thrombin with staphylocoagulase or alpha 2-macroglobulin (alpha 2M), inhibition was decreased by two orders of magnitude and acquired an apparent competitive character. In Western blot experiments, the antibody reacted with active alpha-thrombin, did not react with chloromethylketone-inhibited thrombin and reacted with a lower affinity with iPr2P-thrombin. The inhibitor did not block thrombin binding to benzamidine-, heparin-, or fibrin-Sepharose, but displaced proflavin from its complex with thrombin. Taken together, these results indicate that the patient's autoantibody recognized a conformational structure which includes, at least in part, the apolar binding site adjacent to the catalytic site of thrombin.

Insights into native epitopes of proliferating cell nuclear antigen using recombinant DNA protein products

A cDNA clone encoding full-length human proliferating cell nuclear antigen (PCNA) was used to generate a panel of in vitro translated labeled protein products with COOH-terminal deletions and to construct a set of fusion proteins with COOH- and NH2-terminal deletions. A rabbit antiserum raised against an NH2-terminal peptide, a well-characterized murine monoclonal antibody (mAb), and 14 human lupus sera with autoantibody to PCNA were analyzed for their reactivity with the constructs using both immunoprecipitation and immunoblotting techniques. The rabbit antiserum reacted in immunoprecipitation and immunoblotting with constructs containing the appropriate NH2-terminal sequence and mAb reacted with a sequence from the midregion of PCNA. These experimentally induced antibodies also reacted with 15-mer synthetic peptides in enzyme-linked immunosorbent assay (ELISA). In contrast, none of the lupus sera reacted with synthetic peptides in ELISA. 9 of the 14 lupus sera also failed to react in Western immunoblotting with any recombinant fusion protein, although they all immunoprecipitated in vitro translated full-length protein. Four of the nine had variable patterns of immunoprecipitation with shorter constructs. The remaining five lupus sera were able to immunoprecipitate translation products as well as Western blot recombinant fusion proteins. From analysis of the patterns of reactivity of human lupus sera, it was deduced that the apparent heterogeneity of human autoantibodies to PCNA could be explained by immune response to highly conformational epitopes. These observations demonstrate that there might be special features in "native" epitopes of intranuclear antigens that are recognized by autoantibodies, and that these special features of native epitopes might not be present in prepared antigen used for experimental immunization. These features may be related to protein folding or to association of the antigen with other intranuclear proteins or nucleic acids, as might occur with antigens that are components of subcellular particles.

Expression in E. coli of the catalytic domain of rat poly(ADP-ribose)polymerase

A 2 kilobase pair cDNA coding for the entire C-terminal catalytic domain of rat poly(ADP-ribose)polymerase has been expressed in E. coli. The overproduced 55 kDa polypeptide is active in synthesizing poly(ADP-ribose) and the 4 kDa N-terminal region of this domain is recognized by the monoclonal antibody C I,2 directed against the calf enzyme. Also, the minor alpha-chymotrypsin cleavage site found in the human catalytic domain is not present in the rat enzyme as revealed by the absence of the 40 kDa specific degradation product in the E. coli cells expressing the rat domain. The expression of this partial rat cDNA should thus permit the rapid purification and subsequent crystallization of the catalytic domain of the enzyme.