MEMBRANE ATTACK COMPLEX DEPOSITS IN CUTANEOUS LESIONS OF DERMATOMYOSITIS

Background and Design: The presence of membrane attack complex of comp lement (MAC) deposits in the intramuscular vasculature of biopsy speci mens taken from patients with dermatomyositis (DM) has been implicated in the pathogenesis of this myopathy. The purpose of this study was t o investigate the presence of MAC deposition in the skin lesions of pa tients with DM. Using immunohistochemical methods, we examined 22 biop sy specimens from lesional skin, six biopsy specimens from uninvolved skin, and 12 muscle biopsy specimens from patients with DM for the pre sence of MAC and vitronectin and CD59, two regulatory proteins of comp lement. Results: The deposition of MAC was demonstrated in a large per centage of biopsy specimens obtained from the lesional skin of patient s with DM. Deposits were found along the dermoepidermal junction in 19 (86%) of 22 biopsy specimens. Deposits on the vessel walls of the der mis were found in 17 (77%) of 22 biopsy specimens; but only in six of these biopsy specimens (27%) were deposits present in more than 10% of blood vessels. In contrast, deposits along the dermoepidermal junctio n and the vessel walls of the dermis were absent in specimens from uni nvolved skin. In 12 muscle biopsy specimens obtained simultaneously fr om these patients, MAC deposits were found on the vessel walls in nine (75%), but only in six (50%) were deposits found in more than 10% of the intramuscular vessels. The pattern of vitronectin immunoreactivity in skin and muscle biopsy specimens obtained from patients with DM wa s similar to MAC deposits. The expression of CD59 was normal in all sk in and muscle biopsy specimens. Conclusions: The deposition of MAC was found in a high percentage of biopsy specimens from the lesional skin of patients with DM; it was absent in uninvolved skin. These findings suggest that the complement system may be involved in the pathogenesi s of the skin lesions of DM.