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Brouwer et al studied a very large group of European patients with idiopathic inflammatory myopathies for myositis associated autoantibodies with a variety of methods.1 This is the largest group of patients with myositis ever studied for autoantibodies. The results substantially increase the existing knowledge about frequencies of autoantibodies in patients with inflammatory myopathies.
Compared with the pre-existing data, clinical associations of certain myositis-specific autoantibodies seem to be widening and are becoming less specific. Whereas the overwhelming majority of patients with anti-SRP up until now were reported to have polymyositis (PM),2 3 and anti-Mi-2 was almost completely specific for dermatomyositis (DM),3-5 Brouwer et al mention several anti-SRP positive DM and anti-Mi-2 positive PM cases, and even some patients with these autoantibodies and inclusion body myositis. The question is whether this is a matter of test or autoantibody specificity or a problem of adequate clinical diagnosis or classification as PM or DM. Moreover, the specificity of myositis related autoantibodies is further questioned by several violations of the well established rule that coincidences of myositis-specific antibodies are almost non-existent6—for example, by patients being positive for anti-Jo-1 and anti-Mi-2 or anti-SRP at the same time.
As the authors correctly state, the detection of more positive results is mostly due to the assay design, especially of those tests with multiple recombinant fragments of the Mi-2 antigen. The consequence of this gain in sensitivity inevitably is a loss in specificity; whether this can be regarded as progress may be a matter for discussion. The authors do not give information about the diagnostic specificity of the new assays, evaluated by testing healthy controls and patients without myositis, therefore judgment about the relevance of the results is difficult.
The “new”, non-DM anti-Mi-2 sera differ from the “classical” ones: Brouwer et al mention that their fine specificity is different and that only 8/17 could be confirmed by western blotting. There is no information as to whether they are positive in the traditional Ouchterlony assay and if they show up as antinuclear antibodies (ANA) in HEp-2 cell immunofluorescence (as the “classical” anti-Mi-2 sera do). At least the single anti-Mi-2 positive patient with PM previously described by Rouxet al was ANA negative.7
We wonder if by broadening the anti-Mi-2 specificity its HLA association—previously described as very strong with HLA-DR7 and a tryptophan on residue 9 of DRB15—might also be altered. It would be especially useful to know the HLA type of the four anti-Mi-2/Jo-1 double positive patients described by Brouweret al because Jo-1 antibodies are highly DR3 associated,3 8 whereas DR3 is completely absent from all Mi-2 positive patients with DM whose HLA type has been published so far.3 9
In conclusion, we fear that by producing more and more positive results in very sensitive assays one does blur the picture of myositis-specific autoantibodies and diminishes their diagnostic value.
The conclusion of Drs Mierau, Dick, and Genth that by producing more and more positive results in very sensitive assays one blurs the picture of myositis-specific autoantibodies and diminishes their diagnostic value is not correct. The autoantibody specificities we measured remain very specific for myositis and as such still are a useful help for diagnosis. However, our data indeed show that anti-Mi-2 antibodies are not completely specific for dermatomyositis (DM) as has been suggested so far, but also occur in 9% of patients with polymyositis (PM).1-1 We explained this result by the fact that we used the complete Mi-2 protein (in four overlapping fragments) as antigen, whereas in all previous studies the NM fragment, containing only 26% of the protein, was coated in the enzyme linked immunosorbent assay (ELISA) assay1-1.
Mierau and colleagues argue that we did not provide information on the diagnostic specificity of the Mi-2 assay we used. The test, in fact, using the four overlapping fragments was evaluated with healthy subjects by testing 84 samples from blood donors. The absorbance values at 405 nm of the mean plus 3SD were 0.262 for the NT fragment, 0.260 for the M fragment, and 0.115 for the CT fragment. The cut off values for positivity were set distinctly above those values, that is 0.500 for both the NT and M fragments, and 0.450 for the CT fragment. When this design with rather high cut off values is used, these new tests are specific for the blood donor samples used. To strengthen the argument that anti-Mi-2 antibodies do occur in a significant percentage of patients with PM we successfully confirmed about 50% of the Mi-2 positives by immunoblotting. Because ELISA is much more sensitive than immunoblotting, or one of the other techniques mentioned by Mierau and colleagues, at this stage of our research we have to accept the fact that 100% confirmation cannot be reached. Our results thus show that anti-Mi-2 is not as specific for DM as was hitherto thought. The take home message for the clinician is that when anti-Mi-2 autoantibodies are found, the underlying disease might not necessarily be DM.
Mierau and colleagues also correctly state that reported coincidences of myositis-specific antibodies are rare. In our study, containing many patients, such coincidences were found more often, as expected, albeit still in a very low percentage of the patients. Again we think that we found these coincidences by virtue of the very sensitive assays we used for the detection of anti-Mi-2 and anti-SRP antibodies. We disagree, however, that such results inevitably mean a loss in specificity.
Apart from the biochemical refinements mentioned above, there could be a second reason for the discrepancy between previous studies and our results, and that is the problem of adequate diagnosis, as Mierau, Dick, and Genth suggest. In our study we distinguished PM from inclusion body myositis (IBM), which was not the case in previous studies on autoantibody associations. We used established criteria for the diagnosis PM, DM, and IBM.1-1-1-3 In addition, when anti-Mi-2 autoantibodies were found, patients were, to enable comparison with previous studies, also classified according to the criteria published by Bohan and Peter,1-4 which, however, did not change the diagnosis.
In conclusion, using more sophisticated serological analyses and precise clinical classifications based on established criteria in a large group of patients with myositis, we found that the presence of anti-Mi-2 antibodies is not as exclusive for DM as has been suggested in the past. Instead of blurring the picture of myositis-specific autoantibodies, we may have corrected it.