Myositis autoantibodies detected by line blot immunoassay: clinical associations and correlation with antibody signal intensity

The aim of this study is to assess the relationship between myositis specific (MSA) and myositis associated (MAA) antibodies and diagnosis (including idiopathic inflammatory myopathies [IIM] and other systemic autoimmune diseases [SAID]), and to explore the impact of antibody signal intensity in diagnostic accuracy. We retrospectively reviewed all the serum samples obtained from patients tested for MSA/MAA by line immunoassay (LIA) between 01/01/2018 and 31/12/2020 in Ramón y Cajal University Hospital (Spain). Clinical true positive (CTP) MSAs and MAAs were defined as those patients with IIM or SAID with phenotypes expected of that MSA/MAA. Patients who did not have a phenotype compatible with that antibody were classified as clinical false positive (CFP). One hundred and thirty positive samples were analysed. Forty-six patients (33.38%) were classified as IIM, forty-two (32.3%) as SAID and forty-two (32.3%) as non-IIM/SAID. Among these 130 patients, 164 MSA/MAA were detected. Eighty-five (51.8%) positive MSA/MAA were classified as CTP, and seventy-nine (48.2%) as CFP. Strongly positive antibodies were more frequently CTP (35/47, 74.5%) than weak positives (54/68, 36.8%), (p ˂ 0.001). Antibodies classified as CTP had a higher signal intensity than CFP (36.77 AU vs 20.00 AU, CI95% 7.79-22.09, p ˂ 0.001). The probability of a CFP was associated to negative ANA, low ANA titer, and multiple positive MSA/MAA (p ˂ 0.001). In this study, we confirmed that CFP results using LIA are frequent, and are associated with low signal intensity MSA/MAA, negative ANA, lower titer ANA, and with multiple positive samples.

POS0847 MYOSITIS AUTOANTIBODIES DETECTED BY LINE BLOT IMMUNOASSAY: CLINICAL ASSOCIATIONS AND CORRELATION WITH ANTIBODY TITERS

Background

Idiopathic inflammatory myopathies (IIM) are a heterogeneous group of autoimmune diseases (AID) characterized by muscle inflammation and weakness, often accompanied with other organ involvement, such as skin rash or interstitial lung disease (ILD). Myositis specific (MSA) and myositis associated antibodies (MAA) can be detected in approximately 60% of patients with IIM. Besides, antibody titers have been suggested to be related with diagnostic accuracy, although it has not been widely studied. MSA are considered to be exclusive of patients with IIM, whilst MAA can occur in IIM and other systemic autoimmune diseases, nevertheless, most of the studies are focused exclusively on IIM patients.

Objectives

The aim of this study is to assess the relationship between MSA/MAA and diagnosis (including IIM and other AID), and to explore the impact of antibody titers in diagnostic accuracy.

Methods

We retrospectively reviewed all the serum samples obtained from patients tested for MSA/MAA between 01/01/2018 and 31/12/2020 in the Immunology department of Ramón y Cajal University Hospital (Spain). These antibodies were tested by line blot immunoassay (LIA) (EUROLINE Autoimmune Inflammatory Myopathies 16 Ag, Euroimmun, Lübeck, Germany). Positivity was stablished according to absorbance titer and adjusted by positive control of each test (arbitrary units, AU). True positive (TP) MSA and MAA were defined as those patients with IIM or AID with phenotypes expected of that MSA/MAA, according to the available information. The patients that did not have a phenotype compatible with that antibody were regarded as false positive (FP). Statistical analysis was carried out using IBM SPSS statistics version 22.

Results

We analyzed 130 positive samples which corresponded to 130 patients, 85 were women and mean age was 55.08 years. 44 patients (33.8%) were classified as IIM, 43 (33.1%) as AID, and 43 (33.1%) as non-IIM/AID. Among these 130 patients, 164 MSA/MAA were detected. 83 (50.6%) positive MSA/MAA were regarded as TP, and 81 (49.4%) as FP (positive predictive value [PPV] 50.6%). Antibodies regarded as TP had a higher antibody titer compared to FP (49,19 AU vs 26,96 AU, p<0.001). This difference was statistically significant for MSA and MAA when analysed separately (Figure 1). FP antibodies were associated with negative ANA and low titer ANA (p<0.001). Multiple positive antibodies (antibodies included in samples that were positive for > 1 MSA/MAA) were more frequently FP in comparison with isolated positive MSA/MAA (p<0.001).

Figure 1.

Autoantibody titers comparison between false positives and true positives. MSA=myositis specific antibody, MAA=myositis associated antibody, FP=false positive, TP=true positive

Conclusion

In this study we confirm that FP results using LIA are relatively frequent, and are associated with lower titer MSA/MAA, negative ANA, lower titer ANA, and with multiple positive MSA/MAA within one sample.

References

[1]Betteridge Z, Tansley S, Shaddick G, et al (2019) Frequency, mutual exclusivity and clinical associations of myositis autoantibodies in a combined European cohort of idiopathic inflammatory myopathy patients. J Autoimmun 101:48–55. https://doi.org/10.1016/j.jaut.2019.04.001

[2]Cavazzana I, Fredi M, Ceribelli A, et al (2016) Testing for myositis specific autoantibodies: Comparison between line blot and immunoprecipitation assays in 57 myositis sera. J Immunol Methods 433:1–5. https://doi.org/10.1016/j.jim.2016.02.017

[3]Tansley SL, Li D, Betteridge ZE, McHugh NJ (2020) The reliability of immunoassays to detect autoantibodies in patients with myositis is dependent on autoantibody specificity. Rheumatol (United Kingdom) 59:2109–2114. https://doi.org/10.1093/rheumatology/keaa021

[4]Mahler M, Betteridge Z, Bentow C, et al (2019) Comparison of Three Immunoassays for the Detection of Myositis Specific Antibodies. Front Immunol 10:848. https://doi.org/10.3389/fimmu.2019.00848

Disclosure of Interests

None declared

FRI0250 “ARE MYOSITIS ANTIBODIES SPECIFIC FOR IDIOPATHIC INFLAMMATORY MYOPATHY DIAGNOSIS?” CLINICAL CORRELATION OF A COHORT OF PATIENTS POSITIVE FOR MYOSITIS ANTIBODIES

Background:

Idiopathic inflammatory myopathies (IIM) are a group of immune-mediated diseases characterized my muscle weakness, skin rash and systemic involvement. Myositis-specific antibodies (MSA) and myositis-associated antibodies (MAA) play a major role in IIM diagnosis, classification and prognosis. Nevertheless, MSA/MAA testing is not standardized and there very few studies addressing their relationship with other diseases.

Objectives:

To describe a cohort of patients tested positive for MSA/MAA, and to explore it´s relationship with IIM and other autoimmune diseases.

Methods:

We retrospectively review all the serum samples obtained from patients tested for MSA/MAA during 2019 in the Immunology department of Ramón y Cajal University Hospital (Madrid, Spain). These antibodies were tested by specific immunoblot (EUROLINE: Autoimmune Inflammatory Myopathies 16 Ag) with highly purified MSA/MAA. Positivity was stablished according to absorbance titer and adjusted by positive control of each test (arbitrary units, AU). Patients were diagnosed with IIM according to their clinician diagnosis. Diagnosis and classification were confirmed by an independent rheumatologist (JL) according to current understanding of IIM classification.

Results:

Three-hundred-seventy-five samples were tested for MSA during the study period. Two-hundred-seventy-nine were negative for all antibodies tested. Ninety-six samples were positive for one or more MSA/MAA, corresponding to 74 patients (11 patients had 2 different samples). Forty-nine (66.2%) of the patients who tested positive were female and 25 (33.8%) were male. Mean age was 58.65 years. Only 22 patients (29.7%) had a confirmed diagnosis of IIM, 24 (32.4%) had a diagnosis of other autoimmune disease, and 11 (14.9%) were diagnosed with interstitial lung disease (ILD) (Figure 1). Six ILD patients had anti-PM-Scl or anti-Ku antibodies, which are associated with scleroderma or overlap-CTD myositis, nevertheless, they remained classified as ILD as no other features were described in this group.

Seventeen patients were positive for more than 1 MAA or MSA, including 14 patients positive for anti Ro-52. Antibody titer was higher in the IIM group compared to non-myositis group (59.59 vs 44.16, p=0.015). Anti Mi-2 was positive in 4 ILD without any other myositis features, and high titer anti-SRP (n=4, mean 59.75 AU) was found in primary biliary cirrhosis (PBC) patients. Additionally, 5 patients positive for antiJo-1 using ELIA (Thermo Fisher) were diagnosed with antisynthetase syndrome. IIM diagnosis and its relationship with antibody titer is represented in table 1.

Table 1.

Autoantibody titer according to diagnosis.

Antibody (Number of patients)Number of samplesIIMIIM antibody titer (AU)Non-IIMNon-IIM antibody titer (AU)Other AI diseasesAnti Ro-52Anti Jo-1 (n=1)11 (100%)92.70-01 (100%)Anti PL-7 (n=8)113 (37.5%)29.595 (62.5%)25.595 (62.5%)2 (25%)Anti PL-12 (n=3)21 (33.3%)53.952 (66.6%)69.971 (33.3%)2 (50%)Anti EJ (n=1)11 (100%)99.460-01 (100%)Anti OJ (n=2)40-2 (100%)23.041 (50%)0Anti Ku (n=14)193 (21.4%)107.2511 (78.6%)31.855 (35.7%)0Anti Tif1gamma (n=5)63 (60%)40.122 (40%)23.841 (20%)0Anti NXP2 (n=2)30-2 (100%)13.991 (50%)1 (50%)Anti Mi2 (n=12)173 (25%)48.659 (75%)26.674 (33.3%)0Anti SAE (n=1)10-1 (100%)1800Anti MDA5 (n=2)32 (100%)30.5401 (50%)1 (50%)Anti SRP (n=9)124 (44.4%)42.695 (55.6%)68.165 (55.5%)2 (22.2%)Anti PM-Scl75 and PM-Scl100 (n=2)21 (50%)68.61 (50%)36.6700Anti PM-Scl75 (n=8)10016.268 (100%)22.723 (37.5%)1 (12.5%)Anti PM-Scl100 (n=4)50-4 (100%)23.622 (50%)0

Conclusion:

Only 28.7% of the patients that were MAA/MSA positive had a diagnosis of IIM. Other autoimmune diseases and ILD were commonly found in this group of MSA/MAA positive patients.

References:

[1]Damoiseaux J, Vulsteke JB, Tseng CW, Platteel ACM, Piette Y, Shovman O, et al. Autoantibodies in idiopathic inflammatory myopathies: Clinical associations and laboratory evaluation by mono- and multispecific immunoassays. Vol. 18, Autoimmunity Reviews. Elsevier B.V.; 2019. p. 293–305.

Disclosure of Interests:

None declared