MxA Protein Expression and Histological Characteristics on Muscle Biopsy in Myositis
Jaya Trivedi1, Changhong Xing1, Nicole Bitencourt1, Chunyu Cai1
1UT Southwestern Medical Center
Objective:
To evaluate both myofiber and capillary MxA expression in a wide range of potentially type 1 interferon driven myositis.
Background:
The myxovirus resistance protein A (MxA) is one of the most highly induced protein by type I interferons. The current international neuromuscular workshop recognizes MxA reactivity on myofibers as a definitive marker for dermatomyositis, with 98-100% specificity on muscle biopsies. However, many type I interferon driven inflammatory myositis have not been tested in previous studies.
Design/Methods:
We evaluated both myofiber and capillary MxA expression in a wide range of potentially type 1 interferon driven myositis, including lupus myositis, HIV myositis, Sjogren’s disease associated myositis, systemic sclerosis associated myositis and antisynthetase syndrome associated myositis. Dermatomyositis and immune mediated necrotizing myopathy served as positive and negative control, respectively. We also cross compared the presence of endothelial tubuloreticular inclusions (TRI), another type 1 interferon activation marker, with MxA expressions.
Results:
Myofiber MxA expression was present in 78.9% (15/19) of active lupus myositis but was uncommon in active myositis associated with HIV (1/18), Sjogren’s disease (1/12), systemic sclerosis (2/14), and antisynthetase syndrome (2/11). Capillary MxA expression was much more prevalent and less specific, present in lupus (18/19), HIV (8/18), Sjogren’s disease (9/12), systemic sclerosis (11/14), and antisynthetase syndrome (9/11). There was a positive correlation between myofiber MxA expression and presence of endothelial tubuloreticular inclusions. Histologically, lupus myositis was characterized by a “pan-fascicular” necrotizing myopathy pattern.
Conclusions:
MxA myofiber expression is not a specific marker for dermatomyositis. Nevertheless, MxA is still a highly useful tool in muscle biopsy whose myofiber expression is strongly suggestive of either dermatomyositis or lupus myositis. The “pan-fascicular” necrotizing myopathy pattern in lupus myositis may serve as a useful histological feature to differentiate from the “perifascicular” injury pattern in dermatomyositis.