LAUSR

Soon after our article published,1 we learned the sale of the myxovirus resistance A (MxA) polyclonal antibodies used in the study (Mx1/2/3 [H-285], sc-50509, Santa Cruz Biotechnology, Dallas, TX) had been discontinued. Furthermore, we received inquiries from several physicians concerning alternate MxA antibodies. We tested the company's monoclonal antibody alternate (Mx1/2/3 [C-1], sc-166412) on frozen muscle sections at various dilutions in 2% bovine serum albumin in PBS using the Ventana immunohistochemistry detection system (Ventana Medical Systems, Tucson, AZ) with or without the enhancement mode. Muscle samples tested included MxA-positive dermatomyositis (n = 3, including 1 juvenile participant), MxA-negative dermatomyositis (n = 3), anti-Jo-1 myopathy (n = 3, MxA-negative), and immune-mediated necrotizing myopathy (n = 3, comprising 1 with anti-signal recognition particle antibodies [MxA-negative], 1 with anti-3-hydroxy-3-methylglutaryl-CoA reductase antibodies [MxA-negative], and 1 without those antibodies [MxA-positive]). We observed essentially the same staining pattern at comparable signal intensity as the original polyclonal antibodies at 1:10 dilution with the enhancement mode although the signal was barely detected at the manufacturer's recommended dilution (starting dilution: 1:50), indicating that the monoclonal antibody alternate can be similarly used to detect sarcoplasmic MxA expression on frozen muscle sections for the diagnosis of dermatomyositis (although higher concentration is necessary).