Sensitivity and specificity of anti-Jo-1 antibodies in autoimmune diseases with myositis

OBJECTIVE

To determine the sensitivity and specificity of anti-Jo-1 in systemic sclerosis (SSc) patients with and without myositis.

METHODS

Immunoblots on HeLa nuclei were used to screen sera from 554 consecutive connective tissue disease patients. Those who had 45-55-kd bands, all patients with polymyositis/dermatomyositis (PM/DM), and a random selection of SSc, Raynaud's disease, systemic lupus erythematosus, and rheumatoid arthritis patients were also studied by anti-Jo-1 enzyme-linked immunosorbent assay and by immunoblots on rabbit pooled aminoacyl-transfer RNA synthetase.

RESULTS

Anti-Jo-1 was present only in 8 of the 40 PM/DM patients.

CONCLUSION

Anti-Jo-1 is specific for PM/DM.

Epitope studies indicate that histidyl-tRNA synthetase is a stimulating antigen in idiopathic myositis

The most frequently found myositis-specific antibody, the anti-Jo-1 antibody (anti-HRS), binds to histidyl-tRNA synthetase (HRS). Although this antibody reacts with HRS, it is unclear whether HRS is the stimulating antigen or is merely a protein that cross-reacts with a yet undefined antigen. Because antibody directed against an unrelated antigen would not be expected to cross-react with HRS at multiple sites, we mapped the epitopes on HRS to resolve this issue. We found by Western blot analyses that immunoglobulins G (IgG) from 18 of 19 anti-HRS positive patient sera react with amino acids 2-44 and 286-509 of HRS. Patient IgG specific for these two epitopes were found not to inhibit HRS enzyme activity. Instead, the inhibitory property of anti-HRS was found to be associated with antibodies that do not react to HRS in immunoblots, indicating the presence of other epitopes. In addition, antibodies that react in immunoblots were found to represent only a small fraction of total anti-HRS antibody. Our finding that patient IgG recognized at least three distinct epitopes on HRS strongly suggests that the immunological response at some point in the disease is directed against HRS and not against a cross-reactive antigen.

Pulmonary fibrosis and myositis in a child with anti-Jo-1 antibody

This is the first report of a child with anti-Jo-1 antibody. At 10, she had a chronic cough, wheezing, dyspnea on exertion, a combined obstructive and restrictive ventilatory defect, and a decreased diffusion capacity compatible with alveolar fibrosis (lung biopsy). By age 12, she developed arthralgias, malar rash, frontal alopecia, Raynaud's phenomenon, and was seropositive for antinuclear antibodies, (SSA)Ro, Jo-1 and rheumatoid factor. At 16, symptomatic inflammatory myositis (elevated creatinine kinase, muscle biopsy) was documented with persistent anti-Jo-1. Her pulmonary symptoms have progressed despite appropriate therapy. Although her digits have become sausage shaped with swan neck deformities, joint pain is not frequent.

A motif in human histidyl-tRNA synthetase which is shared among several aminoacyl-tRNA synthetases is a coiled-coil that is essential for enzymatic activity and contains the major autoantigenic epitope

In myositis, disease-specific autoantibodies may be directed against an aminoacyl-tRNA synthetase, usually histidyl-tRNA synthetase. To explore the basis for this phenomenon, we have made recombinant histidyl-tRNA synthetase in the baculovirus system. It was enzymatically active and recognized by human autoantibodies. A truncated protein lacking the first 60 amino acids was inactive as an antigen and as an enzyme. This region is within the first two exons, is predicted to have a coiled-coil configuration, and is found in some other synthetases but not in Escherichia coli or yeast histidyl-tRNA synthetase. Circular dichroism showed that the peptides from this region (amino acids 1-60 and 1-47) have the predicted high alpha-helical content, but smaller fragments (1-30, 14-45, and 31-60) do not. The peptides with a high alpha-helical content could inhibit autoantibodies almost completely, whereas the smaller peptides were unable to do so. The amino acid sequence of this coiled-coil domain in human histidyl-tRNA synthetase resembles the sequence of the extended this coiled-coil arm near the NH2 terminus of bacterial seryl-tRNA synthetase as well as similar regions in some eukaryotic aminoacyl-tRNA synthetases, raising the possibility that this domain serves a similar tRNA-stabilizing role and has been preserved from a common ancestor.

Clinical and histological features of 'mechanic's hands' in a patient with antibodies to Jo-1--a case report

The clinical and histological features of 'mechanic's hands' are described in a patient with polymyositis characterized serologically by antibodies to histidyl tRNA synthetase (Jo-1). Although described in the past in association with polymyositis, these distinctive cutaneous lesions have only recently been associated with the 'anti-synthetase syndrome'. It is becoming apparent that recognition of subsets within the spectrum of polymyositis/dermatomyositis characterized by certain clinical and serological features not only have prognostic significance, but also may provide insights into mechanisms of disease.

[Polymyositis and Jo-1 syndrome]

Polymyositis is a primarily clinical diagnosis. In 30-40% of patients, however, the specific anti-Jo-1 antibody (anti-histidyl-tRNA-synthetase) can be found. These cases are often associated with interstitial lung disease and other typical clinical manifestations. This symptomatology is called Jo-1-syndrome. Prognosis depends on early diagnosis, since Jo-1 associated interstitial lung disease resulting in pulmonary fibrosis is favorably influenced by immunosuppressive therapy, especially with corticosteroids. The present report concerns a patient with Jo-1-syndrome and beginning pulmonary fibrosis who was treated with methotrexate and prednisolone with good initial response. After 1 year the patient's condition had deteriorated, so that the therapy was changed to cyclophosphamide. Longterm results under this therapy remain to be evaluated.

[Pulmonary fibrosis as a presentation form of the Jo-1 syndrome]

Jo-1 syndrome is a disease recently described, included on the list of connective tissue diseases. Its clinical features are myositis and/or pulmonary fibrosis associated to the presence of precipitant antibodies against intracellular enzyme call histidine T-RNA synthetase. This antibody is related to pulmonary fibrosis associated to myositis and some scientist gave predictive value on the onset of pulmonary fibrosis in patients with myositis. However, isolated association of pulmonary fibrosis have been exceptionally described. A patient with severe interstitial pulmonary affliction and positive Jo-1 antibody without myositis is presented. The actual knowledge of the disease and its association is reviewed.

Cellular localization of the target structures recognized by the anti-Jo-1 antibody: immunofluorescence studies on cultured human myoblasts

Antibodies to Jo-1 (alpha Jo-1) are most characteristically detected in patients with the idiopathic inflammatory muscle disease polymyositis (PM). The Jo-1 antigen has previously been identified as histidyl-tRNA synthetase (HRS). In order to clarify the cellular localization of the antigenic targets recognized by the alpha Jo-1 antibody, immunofluorescence (IF) studies were performed with cultured human myoblasts. Incubation with alpha Jo-1 positive sera demonstrated granular cytoplasmic as well as nuclear staining, but only the cytoplasmic fluorescence was specifically inhibited by preabsorbing the sera with recombinant histidyl-tRNA synthetase (rHRS). A polyclonal rabbit anti-rHRS sera demonstrated granular cytoplasmic IF which was also specifically inhibited by preincubation with rHRS protein. Alpha Jo-1 negative healthy control or patient sera demonstrated nonspecific low intensity staining. 35S methionine biosynthetically labelled myoblast cell extracts immunoprecipitated with alpha Jo-1 positive sera and analyzed by SDS-PAGE revealed a specific band of the same molecular weight as the rHRS antigen. Our studies demonstrate that alpha Jo-1 specifically binds to antigen in the cytoplasm of cultured myoblasts. Alpha Jo-1 has been shown to inhibit HRS activity in vitro. Given the importance of aminoacyl tRNA synthetases such as HRS to intracellular protein assembly, intracytoplasmic binding and enzyme inhibition in vivo may potentially contribute to the pathogenesis of autoimmune muscle damage in PM.

Anti-Jo-1 autoantibodies and the immunopathogenesis of autoimmune myositis

Polymyositis and dermatomyositis are inflammatory myopathies characterized by proximal muscle weakness and myopathic electromyographic and histological findings. While the causes of myositis are not known, the close association of these disorders with a spectrum of autoantibodies suggests an etiologic and/or pathogenetic role for autoimmune processes. Of particular interest in this regard are antibodies directed against histidyl as well as other tRNA synthetases which are almost uniquely associated with myositis and may define a distinct subset of patients. Recently we isolated the histidyl tRNA synthetase gene which encodes the autoantigen representing the most frequent target of the myositis autoimmune response. The isolation and expression of this gene has allowed us to investigate both the autoreactive epitopes on histidyl-tRNA synthetase and the extent to which these correlate with functional epitopes on the molecule. As described here, the results of these studies as well as other recent data pertaining to the immunopathogenesis of myositis, provide a framework for delineating the mechanisms which render synthetases and other translation-related proteins autoantigenic in myositis, and allow one to examine the significance of such autoimmune responses in the etiology and pathogenesis of inflammatory myopathy.

The role of an autoantigen, histidyl-tRNA synthetase, in the induction and maintenance of autoimmunity

Patients with systemic autoimmune diseases make specific autoantibodies that are directed against self structures. According to one view, these autoantibodies arise as a result of an immune response to foreign antigens such as infectious agents that share, by molecular mimicry, common structures with host proteins. An alternative view is that the target autoantigen itself initiates, selects, and sustains autoantibody synthesis. We show here that anti-Jo-1 autoantibodies directed against histidyl-tRNA synthetase in the human autoimmune muscle disease polymyositis undergo, in addition to spectrotype broadening and class switching, the sine qua non of an immune response to the target antigen--affinity maturation to that antigen. We demonstrate further that these autoantibodies, unlike anti-synthetase antibodies induced in mice immunized with heterologous antigen, bind only nonlinear epitopes on the native human synthetase that remain exposed when the enzyme is complexed to tRNA(His). These data suggest that the native target autoantigen itself has played a direct role in selecting and sustaining the autoantibody response and sharply restrict the time and the way in which a molecular mimic might act to provoke autoantibodies.

Lymphocyte activation markers in idiopathic myositis: changes with disease activity and differences among clinical and autoantibody subgroups

We studied the immunologic correlates of disease activity and differences among subgroups of patients with idiopathic inflammatory myopathy by analysing phenotypic and activation marker expression on peripheral blood mononuclear cells (PBMC). Compared with controls, myositis patients with clinically active disease (n = 51) had significantly lower proportions of CD8+ cells and higher proportions of PBMC that expressed DR, CD3- DR, CD14- DR, interleukin-2 receptors, and the late T cell activation markers CD26 and TLiSA1. TLiSA1 expression, a marker for cytotoxic differentiation, correlated significantly with both clinical activity indices and serum levels of muscle-associated enzymes. In serial studies of seven patients, the proportion of PBMC expressing MHC class II antigen and late T cell activation markers decreased as myositis disease activity decreased, independent of type of therapy. Among the clinical subgroups, polymyositis (n = 21) and inclusion body myositis (n = 11) were virtually indistinguishable; dermatomyositis patients (n = 19) showed decreased proportions of CD3+DR+ and TLiSA1+ cells, and increased proportions of CD20+ and CD20+DR+ cells compared with the other two groups. Patients with autoantibodies to histidyl-tRNA synthetase (Jo-1 antigen, n = 11) had significantly lower proportions of CD3+ and CD4+ cells, lower CD4/CD8 ratios, and higher proportions of CD+ cells expressing CD20, compared with patients without anti-Jo-1 antibodies. These findings support the concept that activated lymphocytes, especially cells undergoing anamnestic responses and cytotoxic differentiation, are important in the pathogenesis of idiopathic myositis. Moreover, taken together with other studies, these data suggest that groups of patients segregated by clinical or autoantibody status have different mechanisms of systemic immune activation and immunopathology.

Origin and regulation of a disease-specific autoantibody response. Antigenic epitopes, spectrotype stability, and isotype restriction of anti-Jo-1 autoantibodies

Anti-Jo-1 antibodies (AJoA), which bind to and inhibit the activity of histidyl-transfer RNA synthetase (HRS), are found in a genetically and clinically distinct subset of myositis patients. This specificity suggests that understanding the antigenic epitopes and immunoregulation governing the production of AJoA may result in clues to disease pathogenesis. Limited digestion of human HRS by V8 protease resulted in four major antigenic polypeptides of 35, 34, 21, and 20 kD; digestion with subtilisin gave four fragments of the same sizes and two additional major antigenic polypeptides of 28 and 17 kD. Sera from 12 AJoA positive patients reacted indistinguishably with these proteolytic fragments by Western blotting, and AJoA elution studies suggested a common epitope(s) on all six. Isoelectric focusing showed a different polyclonal pattern of AJoA in each patient, although serial analyses in individual patients revealed stable AJoA spectrotypes over years of observation. Enzyme-linked immunosorbent assays showed that the AJoA response was mainly restricted to the IgG1 heavy chain isotype. The levels of IgG1 AJoA varied in proportion to disease activity over time but were independent of total IgG1 levels, and three patients became AJoA negative as their myositis remitted after treatment. These findings suggest that AJoA are induced by an antigen-driven mechanism, bind to a common epitope or epitopes on HRS, and are modulated by an immune response closely linked to that which is responsible for myositis in these patients.

Epitope mapping of the cloned human autoantigen, histidyl-tRNA synthetase. Analysis of the myositis-associated anti-Jo-1 autoimmune response

Autoantibodies that bind aminoacyl-tRNA synthetases are strongly associated with the human inflammatory myopathies polymyositis and dermatomyositis, but their molecular origins and relationship to pathogenesis are not known. To address these issues, we wished to identify the autoantigenic epitopes which react with these autoantibodies and to this end, we previously isolated a full length cDNA clone encoding the target Ag recognized most frequently by myositis sera, histidyl-tRNA synthetase (HRS). In the present study, we have analyzed the HRS autoepitopes by two amino acid insertion linker mutagenesis of HRS proteins expressed in Cos 1 cells. A series of mutant HRS cDNA were constructed and the expressed proteins were tested for enzyme activity and for immune reactivity with a panel of sera with anti-Jo-1 antibodies. Immunoblotting and immunoprecipitation analyses revealed that anti-Jo-1 antibodies recognize multiple conformation-dependent and independent epitopes on HRS and that the autoepitopes vary among different myositis patients.

Sequence homology between encephalomyocarditis virus protein VPI and histidyl-tRNA synthetase supports a hypothesis of molecular mimicry in polymyositis

The amino acid sequence of histidyl-tRNA synthetase, the Jo-I antigen of polymyositis, has been compared with that of the polyprotein of encephalomyocarditis virus (EMCV), an agent shown recently to produce polymyositis symptoms in mice. It is shown that there is significant sequence homology between a region of the synthetase formerly identified by us as a possible epitope and a region on the coat protein VPI of EMCV. The viral protein also shows significant sequence homology with several muscle proteins. Identification of the likely spatial location of the VPI sequence on the shell of the virus by comparison with two other picornaviruses whose three dimensional structures have been determined, reveals it to be homologous with known immunogenic sites on these viruses. Independent structural localisation of the possible epitope by comparison with two aminoacyl-tRNA synthetases, whose three dimensional structures are known, also reveals that the sequence in question is likely to be favourably placed on histidyl-tRNA synthetase to crossreact with appropriate antibodies. These findings provide more definitive evidence in support of our previous suggestion that molecular mimicry could explain the origin of serum antibodies to aminoacyl-tRNA synthetases in polymyositis and suggests a likely viral etiology for the disease.

Purification of mammalian histidyl-tRNA synthetase and its interaction with myositis-specific anti-Jo-1 antibodies

Histidyl-tRNA synthetase is purified to near homogeneity from rat liver. The subunit molecular weight of histidyl-tRNA synthetase is 50,000, as determined by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. The Stokes radius and the sedimentation coefficient of histidyl-tRNA synthetase are 38 A and 6.0 S, respectively. The native molecular weight of histidyl-tRNA synthetase is calculated to be 96,000 on the basis of its hydrodynamic properties. The purified histidyl-tRNA synthetase reacts with the myositis-specific anti-Jo-1 antibodies. Anti-Jo-1 immunoglobulin G reacts with the native form of histidyl-tRNA synthetase and does not react or only weakly reacts with the denatured form. The anti-Jo-1 antibodies exhibit stronger inhibition toward histidyl-tRNA synthetase that has been preincubated with tRNA than that without preincubation. Anti-Jo-1 antibodies behave as a noncompetitive inhibitor with respect to tRNA in the aminoacylation reaction catalyzed by histidyl-tRNA synthetase. The structural features of the antigen of the anti-Jo-1 antibodies in light of these results are discussed.

Measurement of antibody to Jo-1 by ELISA and comparison to enzyme inhibitory activity

Antibody to the Jo-1 antigen (histidyl-tRNA synthetase) is found almost exclusively in myositis patients, usually those with adult PM, but has been found in only 30% of that group by immunodiffusion or other techniques thus far reported. We have reexamined the prevalence of antibody to Jo-1 in sera from 130 patients and 82 controls by using the sensitive ELISA technique. The ELISA used affinity-purified, enzymatically active bovine Jo-1 antigen. A wide range of antibody level by ELISA was found among 24 immunodiffusion positive sera. Six myositis and two control sera had apparent specific antibody detectable only by ELISA. Overall, however, the antibody continued to show high myositis specificity with predominance in adult PM (35.8% in that group). Because the antibody inhibits enzymatic activity of the synthetase antigen, we also studied the quantitative inhibitory activity of these sera to compare with the antibody activity as determined by ELISA. Twenty-four immunodiffusion-positive sera, 29 immunodiffusion-negative sera, and 15 normal sera were tested at 1/50 dilution in the reaction mixture. There was background inhibition by all normal sera tested that averaged 30.5%. All but one immunodiffusion negative myositis sera (a high binder by ELISA) inhibited less than 50% of the average with normal serum. Twenty-three of 24 immunodiffusion positive sera inhibited greater than 80% of this normal average; the other inhibited 66%. The serum dilution giving 50% inhibition was highly correlated (R = 0.83) with the ELISA activity. Thus, inhibition of histidyl-tRNA synthetase activity is a relatively accurate measure of Jo-1 antibody. This method should be applicable to measuring antibody to other aminoacyl-tRNA synthetases.

Purification of bovine liver histidyl-tRNA synthetase, the Jo-1 antigen of polymyositis: size of the whole enzyme and its characteristic proteolytic fragments

We have recently reported the marked increase in frequency which can be achieved in the detection of the anti-Jo-1 antibody of polymyositis in serum samples by replacing commercial mixtures of cytoplasmic and nuclear antigens with the purified antigen, histidyl-tRNA synthetase. The present paper describes a method for purifying this antigen and an investigation of its size. Molecular masses previously reported for the enzyme have varied from 85-154 kDa and subunit molecular masses varying from 40-77 kDa have been observed. Several of these fragments are of sizes similar to those of a number of other autoantigens commonly observed in connective tissue diseases. Since the clinical identification of these autoantigens often relies exclusively on size determination by Western blotting, we have characterized the commonly occurring fragments of histidyl-tRNA synthetase lest they confuse such identification. It is concluded that histidyl-tRNA synthetase, like many other aminoacyl-tRNA synthetases, is subject to severe proteolysis during extraction procedures. Several characteristic fragments (Mr = 80, 75, 61, 55, 50 and 45 kDa) result, a finding that provides a satisfactory explanation of the various values previously reported. The intact bovine enzyme is a dimer of molecular mass close to 160 kDa.

An enzyme-linked immunosorbent assay for the detection and quantitation of anti-Jo-1 antibody in human serum

We have developed an enzyme-linked immunosorbent assay (ELISA) specific for autoantibodies directed against the autoantigen Jo-1 (histidyl-tRNA synthetase) using antigen prepared biochemically from HeLa cells. No other patient sera, including those containing antibodies directed at threonyl-tRNA synthetase or alanyl-tRNA synthetase, reacted in the assay. Screening of sera from 169 patients with a variety of autoimmune and neuromuscular diseases confirmed that anti-Jo-1 antibodies are confined to a subgroup of patients with pure polymyositis, pure dermatomyositis, or myositis associated with another rheumatic disease.

Improved detection of anti-Jo-1 antibody, a marker for myositis, using purified histidyl-tRNA synthetase

The increased detection of anti-Jo-1 antibody afforded by the use of the purified antigen, histidyl-tRNA synthetase, in counterimmunoelectrophoresis is demonstrated. Using purified antigen, anti-Jo-1 antibody was detected in the sera of 16/33 (48.5%) patients with confirmed myositis and in 20/45 (44.5%) patients with confirmed or possible myositis. This rate is approximately double that obtained with commercial thymus extracts both in this study and seven others reported in the literature. The presence of antibody shows marked correlation with the activity of myositis at the time of serum sampling and with the presence of interstitial lung disease. Detection rates are similar in patients with polymyositis and dermatomyositis both with and without additional connective tissue diseases.

Polymyositis and molecular mimicry, a mechanism of autoimmunity

The amino acid sequences of Escherichia coli histidyl-tRNA synthetase and alanyl-tRNA synthetase, two proteins recently identified as autoantigens in polymyositis, were compared by a computer alignment procedure with those of the 3600 proteins tabulated in the National Biomedical Research Foundation protein sequence database. Both proteins contain sequences long enough to function as epitopes that match sequences on viral and muscle proteins. The homology thus revealed not only lends strong support to mechanisms of autoimmunity that invoke the theory of molecular mimicry of viral proteins, but also suggests a rationale for the skeletal muscle target of polymyositis.

Antibodies to La, Jo-1, nRNP and Sm detected by multi-track immunoblotting using a novel filter holder: a comparative study with counterimmunoelectrophoresis and immunodiffusion using sera from patients with systemic lupus erythematosus and Sjögren's syndrome

The use of Western blotting or immunoblotting to detect autoantibodies in the serum of patients with autoimmune connective tissue diseases was investigated. An apparatus suitable for simultaneously screening 16 sera on immunoblots was used to show that a complex pattern of antibody binding polypeptides was present in whole HeLa cells. A simpler and readily interpreted pattern of binding was achieved using affinity-purified rabbit thymus antigens. Seventy-seven patients with systemic lupus erythematosus, 44 with primary Sjögren's syndrome and 50 normals were screened for anti-Sm, anti-La, anti-nRNP and anti-Jo-1 by immunoblotting and the results compared with those obtained by counterimmunoelectrophoresis and immunodiffusion. It was shown that both IgG and IgM antibodies must be analysed on immunoblots to detect the maximum number of positive sera, and that the immunoblot detects many anti-La sera which do not form precipitins.

Histidyl-tRNA synthetase, the myositis Jo-1 antigen, is cytoplasmic and unassociated with the cytoskeletal framework

The myositis-specific anti-Jo-1 autoantibody, which is directed against histidyl-tRNA-synthetase, is found in 30% of polymyositis patients. The Jo-1 antigen has been reported to be a nuclear antigen by some authors. On the contrary we show that less than 2% of the total histidyl-tRNA and lysyl-tRNA synthetase activities are associated with purified rat liver nuclei or the hepatocyte intermediate filament-nuclear fraction. In the presence of polyethylene glycol, in which the high Mr multi-enzyme complex containing lysyl-tRNA synthetase is insoluble, 65% of the lysyl-tRNA synthetase and only 15% of histidyl-tRNA synthetase activities remained associated with the cytoskeletal framework. The Jo-1 antigen exhibited a diffuse granular cytoplasmic distribution in cultured rat hepatocytes as determined by indirect immunofluorescent microscopy. Hence, the Jo-1 antigen is cytoplasmic and unassociated with the cytoskeletal framework or high Mr synthetase complex in situ.

Cellular protein and RNA antigens in autoimmune disease

Antibodies directed against soluble cellular antigens are a distinctive feature of systemic autoimmune disease. We have examined 22 autoantibodies in sera from 1111 patients and present the disease associations together with a biochemical analysis of the antigens. The data emphasize the clinical specificity of the antibodies and the restricted number of cellular components that commonly elicit an immune response. In several instances, serological relationships between antibodies mirror biochemical relationships between the corresponding antigens. The antigens are mainly proteins and are often present in complexes with additional protein or nucleic acid molecules. In myositis the antibodies react chiefly with cytoplasmic antigens such as aminoacyl-tRNA synthetases, in contrast to the mainly antinuclear response in SLE. It is argued that both environmental stimuli and genetic factors govern autoantibody specificity, and that molecular characterization of the cellular antigens may yield clues to the aetiology of the disease and of the concomitant, specific autoimmune response.

Myositis autoantibody inhibits histidyl-tRNA synthetase: a model for autoimmunity

In autoimmune disorders such as rheumatoid arthritis and systemic lupus erythematosus (SLE), autoantibodies are generated against a variety of macromolecules. Myositis is a human autoimmune disease characterized by weakness and wasting of muscle. In American studies, antibodies directed against soluble cellular constituents were detected by immunodiffusion in about 60% of cases; the commonest of these, found in 25% of patients, was antibody to the Jo-1 antigen. An antibody system referred to as PL-1 was recognized at a similar frequency in a series of patients studied at Hammersmith Hospital, London. We show here that this system is identical with the Jo-1 system and demonstrate that the antigen is a polypeptide of molecular weight (Mr) 50,000. The protein is immunoprecipitated with tRNA His and appears to be histidyl-tRNA synthetase. The identity of the Jo-1 antigen, the first of the RNA-associated antigens familiar in autoimmune disease to be characterized as a specific enzyme, suggests a model for virus involvement in autoantibody generation.