C5b-9 deposits on endomysial capillaries in non-dermatomyositis cases

•Endomysial capillary C5b-9 deposits are rather unspecific for dermatomyositis (DM).•Classical DM cases show capillary C5b-9 accumulation in a perifascicular pattern.•C5b-9-positive non-DM cases display rather diffuse endomysial capillary C5b-9 deposits.•60% of non-DM cases with C5b-9-positive capillaries suffered from auto-immune diseases.•Endomysial capillary C5b-9 deposits are a marker for (auto-) immune diseases.

Deposits of the terminal-membrane-attack-complex (MAC) C5b-9 on perfascicular endomysial capillaries are generally regarded as diagnostic hallmark of dermatomyositis (DM). Although the pathophysiology is not clear, C5b-9 deposits on capillaries seem to be associated with microinfarctions and vascular damage. Here, we report on a series of 19 patients presenting with C5b-9 accumulation on endomysial capillaries in the absence of features for DM. To decipher differences in the capillary C5b-9 accumulation pattern between DM and non-DM cases, we assessed the extent of endomysial capillary C5b-9 deposits related to capillary density and extent of myofiber necrosis by immunohistochemistry in 12 DM and 8 control patients. We found similar numbers of C5b-9-positive myofibers in both DM and non-DM C5b-9+ cases. The distribution pattern differed as DM cases showed significantly more perifascicular capillary C5b-9 deposits as compared to non-DM cases, which presented stronger endomysial capillary C5b-9 deposits in a diffuse pattern. While total capillary density was not differing, DM patients displayed significantly more C5b-9+ necrotic fibers as compared to non-DM C5b-9+. In summary, endomysial capillary C5b-9 deposits are present in a variety of non-DM cases, however with differing distribution pattern. In conclusion, capillary C5b-9+ deposits should be assessed critically, taking into consideration the distribution pattern.