Introduction

Systemic lupus erythematosus (SLE) is a chronic, multifaceted rheumatic disease which is characterised by the generation of autoantibodies predominantly directed against nuclear proteins and nucleic acids [1, 2]. However, antibodies against cytoplasmatic antigens such as those binding to ribosomal P proteins (aRibPs) have been reported to be specific for SLE as well [2, 3]. In contrast to anti-Smith (anti-Sm) and anti-double-stranded DNA (anti-dsDNA) antibodies, anti-ribosomal P protein antibodies are not included in the current American College of Rheumatology (ACR) classification criteria for SLE [4, 5].

The human ribosomal phosphoproteins P0 (38 kDa), P1 (19 kDa) and P2 (17 kDa) are located within the 60S ribosomal subunit, forming a pentameric complex consisting of a P0 anchor and two P1/P2 heterodimers [3]. The subunits of that pentamer have a common immunodominant epitope at the carboxyl terminus [6], which can lead to cross-reactions of anti-ribosomal P antibodies with P0, P1 and P2 units. P proteins can also exist as ribosome-free P0, P1 and P2 forms in the cytoplasm [6, 7]. Notably, the P0-like protein is also detectable in the plasma membranes of hepatocytes, lymphocytes and other cells [8–11].

The prevalence of anti-ribosomal antibodies depends on the disease activity, the patient's ethnicity and the antigens used in detection systems [12–14]. There are reports about clinical associations of anti-ribosomal protein antibodies with short disease duration [15], rash [16, 17], lymphocytopenia [18] and lupus hepatitis [11, 19–23]. Ohira et al. [22] showed that patients with lupus hepatitis have significantly higher and more frequent levels of antibodies against recombinant ribosomal P0 protein (aRibPR0) than patients with autoimmune hepatitis. There are also contradictory reports of patients with juvenile onset SLE [24–27], neuropsychiatric SLE [3, 28, 29], lupus nephritis class V [3, 27, 30], high disease activity [15, 16, 26, 31] and low levels of complement component 3 (C3) or complement component 4 (C4) [16, 17, 22, 32].

A comparative investigation of the clinical laboratory associations of antibodies against recombinant ribosomal P0, P1 and P2 proteins (aRibPR0, aRibPR1 and aRibPR2) has never been conducted. Thus, the purpose of the present work was to determine the diagnostic value of antibodies against native ribosomal P heterocomplex (aRibPNH), aRibPR0, aRibPR1 and aRibPR2 for SLE and to analyse their associations with disease features and future damage.

Materials and methods

Study participants

Altogether 479 serum samples were obtained from the following groups: (1) patients with SLE (n = 163), who fulfilled the American College of Rheumatology (ACR) 1982 revised criteria for the classification of SLE [4], (2) patients with systemic sclerosis (SSc, n = 66) who met the ACR 1980 criteria for scleroderma [33], (3) patients with primary Sjögren's syndrome (pSS, n = 54) who fulfilled the preliminary European League Against Rheumatism criteria of Vitali et al. [34], (4) patients with rheumatoid arthritis (RA, n = 90) who met the ACR 1987 revised criteria for the classification of rheumatoid arthritis [35] and (5) healthy donors (HD, n = 100).

Disease activity of SLE patients was defined based on the Systemic Lupus Erythematosus Disease Activity Index 2000 (SLEDAI 2000) [36–38] in 101 patients: 6 of them had no activity (SLEDAI score 0), 35 were mildly active (0 < SLEDAI ≤ 5), 41 had moderate disease activity (5 < SLEDAI ≤ 10), 14 were highly active (10 < SLEDAI ≤ 20), and 5 had very high activity (SLEDAI > 20). Juvenile onset was diagnosed when the age at diagnosis was 18 years or younger according to the Pediatric Rheumatology International Trials Organization [39]. Twenty-four (14.7%) patients with juvenile onset SLE and 139 (85.3%) patients with adult onset SLE were studied. Disease damage was measured according to the criteria of the Systemic Lupus International Collaborative Clinics (SLICC) [40, 41] and the weighted damage score (WDS) [40]. All patients were recruited from the outpatient and inpatient facilities of the Department of Rheumatology and Clinical Immunology, Charité University Hospital, Berlin, Germany. The Ethics Committee of the Medical Faculty of Charité University Hospital approved the study, and written informed consent was obtained from all subjects. Sera from healthy donors were used in cooperation with University of Lübeck, Germany. Written informed consent was obtained from all healthy subjects.

Measurement of antibodies

Microtiter plates (Nunc, Roskilde, Denmark) were coated with 1 μg/ml full-length recombinant ribosomal protein P0, P1 or P2 expressed in insect cells (DIARECT, Freiburg, Germany). Sera diluted 1:201 in phosphate-buffered saline (PBS) and 0.1% (wt/vol) casein were added and allowed to react for 30 minutes, followed by three washing cycles with PBS 0.05% (vol/vol) and Tween 20. For detection of bound antibodies, the plates were incubated with antihuman immunoglobulin (IgG) peroxidase conjugate (EUROIMMUN, Lübeck, Germany) for 30 minutes, washed three times and allowed to react with tetramethylbenzidine (EUROIMMUN) for 15 minutes. After addition of acidic stopping solution (EUROIMMUN), the optical density (OD) was read at 450 nm using an automated spectrophotometer (Spectra Mini; Tecan, Crailsheim, Germany). All steps were performed at room temperature. A highly positive index patient serum was used to generate a standard curve consisting of three calibrators (2, 20 and 200 relative units (RU)/ml). Relative units per milliliter were calculated for all samples using this three-point standard curve. The analytical reproducibility of all aRibP assays was evaluated by repeated testing of two serum samples (10 determinations each) in the same run, giving intraassay coefficients of variation (CV) of 2.4% (aRibPR0), 2.1% (aRibPR1) and 2.7% (aRibPR2), respectively. Relationships between sensitivity and specificity at different cutoff values were examined for all assays by receiver-operating characteristics (ROC) curve analyses, allowing also for the determination of test characteristics at predefined specificities.

The anti-RibPNH enzyme-linked immunosorbent assay (ELISA) (IgG, CV 2.6%), anti-Sm ELISA, anti-dsDNA radioimmunoassay (RIA) (Farr assay) and anti-dsDNA ELISA are commercially available assays from EUROIMMIUN and were performed following the manufacturer's instructions.

Statistical analysis

Statistical analyses were performed using GraphPad Prism 5 software (GraphPad Software, La Jolla, CA, USA). The diagnostic significance of antiribosomal proteins N, P0, P1 and P2 antibodies was assessed and areas under the curve (AUCs) were created using ROC analysis. To determine associations, the Mann-Whitney U test (for comparing medians between groups; MWT), Fisher's exact test (FET) and Spearman's rank test (SRT) were used. Two-tailed t-tests were used throughout with an α set at 0.05.

Results

Reactivity and diagnostic significance of antiribosomal proteins N, P0, P1 and P2 antibodies

Antibodies against ribosomal PNH, PR0, PR1 and PR2 proteins (Figure 1), Sm and dsDNA (ELISA and Farr assays) were measured in sera from 163 SLE patients, 210 disease controls and 100 healthy donors to define and compare the sensitivity and specificity in ROC curve analysis (Table 1). For aRibPNH, a sensitivity of 5.5% and a specificity of 100% were calculated using the manufacturer's cutoff (20 RU/ml). At a predefined specificity of 98% among 210 patients with other rheumatic diseases (SSc, pSS and RA), only five (2.4%), four (1.9%), four (1.9%) and four (1.9%) had elevated aRibPNH, aRibPR0, aRibPR1 and aRibPR2 titers, respectively. At the same specificity among 100 healthy donors, only zero (0%), one (1.0%), two (2.0%) and two (2.0%) patients had high titers of aRibPNH, aRibPR0, aRibPR1 and aRibPR2. Among antiribosomal P protein antibodies, aRibPR0 had the highest performance with regard to criteria such as AUC and maximum sum of sensitivity and specificity, followed by aRibPNH (Table 1). All test criteria of aRibPR0 were inferior to those of the anti-dsDNA ELISA or the Farr assay, but were almost equal to those of the anti-Sm ELISA.

Figure 1
figure 1

Graphs showing levels of antiribosomal P protein antibodies in SLE, other rheumatic diseases and healthy donors. Autoantibodies directed against (a) native ribosomal P heterocomplex (aRibPNH), (b) recombinant ribosomal P0 protein (aRibPR0), (c) recombinant ribosomal P1 (aRibPR1) and (d) recombinant ribosomal P2 protein (aRibPR2) were measured using enzyme-linked immunosorbent assay. Dotted lines represent the distinct cut-offs based on ROC curve analysis at specificities of 95% (dotted line), 98% (broken line) and 99% (dotted and broken line). Values >30 RU/ml were set to 30 RU/ml for clearer arrangement of the figures. SLE, systemic lupus erythematosus; SSc, systemic sclerosis; pSS, primary Sjögren's syndrome; RA, rheumatoid arthritis; HD, healthy donors. RU, relative units.

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Table 1 Test values of antiribosomal PNH, PR0, PR1 and PR2 antibodies calculated in receiver-operating characteristics analysisa
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Patients negative for aRibPNH but positive for aRibPRP0-2

Although the native heterocomplex of ribosomal P contains all immunological domains of the subunits P0, P1 and P2, there were considerable differences in the cutoffs and in sensitivities for the detection of aRibPNH, aRibPR0, aRibPR1 and aRibPR2 (Table 1), with outstanding results for aRibPR0.

Thus, we further investigated whether there were patients negative for aRibPNH but positive for aRibPR0, aRibPR1 or aRibPR2 (Figure 2). Sera fulfilling these criteria would point out that there are some epitopes of ribosomal P proteins that are not accessible to autoantibodies because of the spatial conformation of the native heterocomplex.

Figure 2
figure 2

Frequencies of aRibPR0, aRibPR1 and aRibPR2 in aRibPNH-negative lupus patients. (a) Results according to specificity of 99% are shown in a Venn diagram. (b) Corresponding to Figure 2a, those sera were selected that were exclusively positive for aRibPR0, aRibPR1 or aRibPR2 among aRibPNH-negative SLE patients. To further show exact and comparable signal intensities, fold change indices in relation to the given cutoffs of each recombinant aRibP protein (see also Table 1) were calculated. ARibPNH, antibodies against native ribosomal P heterocomplex; aRibPR0, antibodies against recombinant ribosomal P0 protein; aRibPR1, antibodies against recombinant ribosomal P1 protein; aRibPR2, antibodies against recombinant ribosomal P2 protein; aRibPs, anti-ribosomal P protein antibodies.

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At 99% specificity, among 141 aRibPNH- patients there were 19 (13.5%) positive for aRibPR0, six (4.3%) positive for aRibPR1 and 11 (7.8%) positive for aRibPR2. Some of those sera were further exclusively positive for one of the recombinant aRibPs and showed an increased titer up to twofold of the corresponding cutoff (Figure 2b).

Diagnostic value of anti-ribosomal P protein antibodies in SLE

To investigate the auxiliary diagnostic value of antiribosomal P protein antibodies in SLE, we searched for patients who were negative for antibodies against dsDNA and Sm, but positive for aRibPNH, aRibPR0, aRibPR1 or aRibPR2 at a specificity of 100% (Figure 3). This analysis was performed twice, taking either the results of the anti-dsDNA ELISA (Figure 3a) or those of the Farr assay (Figure 3b).

Figure 3
figure 3

Additional diagnostic benefit of antiribosomal P protein antibodies in lupus patients. Both flow charts aim to demonstrate the additional diagnostic value of antiribosomal P protein antibodies (anti-RibP) in the absence of anti-double-stranded DNA (anti-dsDNA) and anti-Smith (anti-Sm) antibodies for lupus diagnostics. The cutoffs of all test systems were set to ensure an absolute specificity of 100% (see Table 1 for cutoffs). Anti-dsDNA and anti-Sm antibodies were selected because they are highly specific for systemic lupus erythematosus (SLE) (instead of, for example, anti-phospholipid antibodies) and are part of American College of Rheumatology classification criteria for SLE [4]. Flowcharts differ only in the test system used for the detection of anti-dsDNA antibodies. (A) Anti-dsDNA enzyme-linked immunosorbent assay (ELISA) and (B) Farr assay. RibPNH+, native ribosomal P heterocomplex-positive; RibPR0+, recombinant ribosomal P0 protein-positive; RibPR1+, recombinant ribosomal P1 protein-positive; RibPR2+, recombinant ribosomal P2 protein-positive.

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Among 163 SLE patients, there were 11 (6.7%) individuals who could be diagnosed only by detection of aRibPs, while 63 (38.7%) patients were regularly diagnosed by the presence of anti-dsDNA or anti-Sm antibodies. Considering the excellent Farr assay, these relations adjusted to 89 (54.6%) individuals with regular diagnosis and five (3.1%) individuals with additional diagnosis only by the presence of aRibP.

Comparison of disease features in aRibP+ vs. aRibP-SLE patients

To determine the special characteristics of lupus patients with elevated aRibPs, we compared medical records, including ACR criteria, SLEDAI 2000 items and laboratory parameters, including autoantibodies, immunosuppressants and antimalarials, with those of aRibP- lupus patients. All clinical laboratory results and detailed demographic information about the study cohort are shown in Table 2.

Table 2 Comparison of the frequency: demographical and clinical data in aRibP-positive and negative SLE patientsa
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ARibPNH+ patients fulfilled significantly more ACR criteria and more often had photosensitivity. Moreover, the frequency of patients with decreased C3 levels was higher among aRibPNH+ patients. Lymphocytopenia was associated with the presence of aRibPR0, and a higher γ-glutamyl transpeptidase (GGT) level was found in aRibPR1+ patients. Anti-Sm, anti-dsDNA and anti-U1-ribonucleoprotein (anti-U1-RNP) antibodies were much more frequent in all aRibP+ patients.

Comparison of disease damage in aRibP+ vs. aRibP-SLE patients

To study the prognostic role of ribosomal P protein antibodies, SLICC scores and WDS were assessed in aRibP+ patients and in an age-, sex- and nephritis-matched group of aRibP- patients at the time of blood sampling and 3 years later. Changes in damage scores (ΔSLICC, ΔWDS) were calculated, and both groups were separately compared. Damage scores from 41 of all 58 aRibP+ patients were completely assessable at the time of blood sampling and 3 years later. There were 22 aRibPNH+, 27 aRibPR0+, 18 aRibPR1+ and 23 aRibPR2+ patients. SLICC and WDS correlated significantly with disease duration and the ages of patients, but not with ACR scores or with anti-dsDNA, anti-Sm or any antiribosomal P protein antibodies. Total disease damage and damage to every organ system separately was not significantly higher in aRibP+ patients than in their aRibP- counterparts within these 3 years. Thus, we found no prognostic role for aRibP.

Discussion

In this study, the diagnostic potential, clinical laboratory associations and correlations with disease damage of antibodies directed against the native ribosomal heterocomplex and its recombinantly produced constituents P0, P1 and P2 were investigated. ARibPR0 revealed the best diagnostic performance among all aRibP types and offered the most diagnostic benefit among sera negative for anti-dsDNA and anti-Sm antibodies. Furthermore, aRibPR0+ lupus patients had significantly lower lymphocyte counts than their aRibPR0- counterparts. Finally, no prognostic relevance was found for any of the aRibPs during a 3-year period.

Our results concerning the prevalence and high specificity of aRibPs for SLE agree with data described before [3, 42]. We further found sensitivities of PR0 > PNH > PR2 > PR1 at specificities of 98% to 99% and PNH > PR0 = PR2 > PR1 at a specificity of 100% in a cohort of 163 lupus patients. This is in contrast to another study where sensitivities of PR2 = PR1 = PR0 were determined at a specificity of 100% in a cohort of 50 SLE patients [13]. Different detection systems and patient cohorts might have contributed to these divergent observations. Since all three subunits of aRibPs share the carboxyl-terminal epitope, it is of interest to note that an ELISA (referred as anti-C22 ELISA) detecting antibodies against this shared epitope reached the same sensitivity of 22% at a specificity of nearly 99% as aRib PR0 in our Berlin patient cohort [43].

We have additionally demonstrated that negativity of aRibPNH does not automatically imply negativity of antibodies against its subunits, especially those against ribosomal P0. This could be due to immunologically relevant epitopes that are freely accessible using RibPR0 alone, but not within the spatial conformation of the native heterocomplex. A biological reason for the higher frequency of aRibPR0 might be the disposability of ribosomal P0-like protein in the cell membranes of many cells, which could contribute to an increased immunogenicity [8–11].

Among the vast quantity of antibodies that are detectable in SLE, antibodies against dsDNA and Sm are highly specific and therefore most useful for the verification of the diagnosis. However, aRibPs are also discussed as a diagnostic criterion. Therefore, we asked whether aRibPs provide additional diagnostic benefit in direct comparison to anti-dsDNA and anti-Sm antibodies. Exactly 10% of sera negative in the anti-Sm and anti-dsDNA ELISAs were positive for aRibPR0 at a specificity of 100%. Even the comparison including the Farr assay revealed that 5.4% of all anti-Sm ELISA and anti-dsDNA RIA (Farr assay) negative sera were positive for aRibPR0 at 100% specificity. Thus, laboratories using less sensitive assays seem to benefit more from testing for aRibP in suspected cases of SLE. However, to be sure, all patients with suspected diagnosis of SLE should be tested for aRibP. Finally, we conclude that the determination of antibodies against ribosomal P proteins, especially those against P0, would improve the classification and diagnosis of SLE.

By comparing disease features of lupus patients with elevated aRibPs to their seronegative counterparts, we could not confirm an association of aRibP positivity with lupus nephritis, short disease duration, high disease activity or juvenile onset. These results might be influenced by the Caucasian ethnicity of the study cohort and differences of the test systems. Cases of neuropsychiatric lupus [28, 29] and subtypes of lupus nephritis were not recorded in our study.

The most striking association of aRibPs with disease features was that aRibPR0+ lupus patients had significantly lower lymphocytes than aRibPR0- lupus patients. Interestingly, a P0-like protein is also detectable in the plasma membranes of different cells, including lymphocytes [11]. Further, the aRibPs are able to bind and penetrate T-cell lines [44, 45], and especially aRibPR0 can induce apoptosis in Jurkat T-cells [46]. In that context, our data confirm the thesis of Sun et al. [46] that aRibPR0 contributes in a clinically relevant manner to lymphocytopenia in SLE. Thus, clinicians should keep aRibPR0 in mind as one differential diagnosis for lymphocytopenia in SLE, along with viral status, drug side effects, hematologic malignancies and other factors.

Another remarkable, significant clinical laboratory association was that aRibPR1+ patients had an elevated GGT value. The participation of aRibPs in liver pathology of SLE was previously reported in cell cultures [9, 11, 46] and in case reports [19–21]. However, aRibPR0 were most frequently in focus because of their membrane-bound isoform [8–11]. As such, in a study of 61 Japanese patients [22], no significant association was found between aRibPR0 and liver enzymes alanine aminotransferase or aspartate aminotransferase, but the GGT level was not assessed. The correlation shown here between GGT and aRibPR1 indicates a possible association of aRibP with lupus hepatitis. However, we do not have a clear definition of lupus hepatitis, and it is hard to rule out other causes, such as nutrition, drugs and other autoimmune hepatitis forms. Longitudinal analysis of aRibPs with liver function tests, including GGT in parallel, might reveal this association best.

Up to now, accepted prognostic factors in SLE have only been lupus nephritis and neuropsychiatric SLE. No prognostic laboratory parameter is known. In this study, we investigated whether aRibP+ lupus patients would develop more or specific disease damage measured by SLICC or WDS after 3 years than their aRibP- counterparts. However, no significant correlations with any of the antiribosomal P protein antibodies could be found over a 3-year period. Conclusively, we first show that aRibPs are not a prognostic parameter for damage in SLE. Further study with more patients and over longer observation time frames could strengthen this result.

Conclusions

In summary, antiribosomal P protein antibodies are very specific for SLE, can also be found in patients with negative anti-dsDNA and anti-Sm antibodies and therefore have to be discussed in the upcoming classification and diagnostic criteria for SLE. Among all four investigated aRibPs, aRibPR0 was the most abundant and should be used for the diagnosis of SLE. High aRibPR0 titers can be associated with lymphocytopenia, and high aRibPR1 titers can be associated with an elevated GGT level. A prognostic role of antiribosomal P protein antibodies is unlikely.