Membrane attack complex deposits in cutaneous lesions of dermatomyositis

The presence of membrane attack complex of complement (MAC) deposits in the intramuscular vasculature of biopsy specimens taken from patients with dermatomyositis (DM) has been implicated in the pathogenesis of this myopathy. The purpose of this study was to investigate the presence of MAC deposition in the skin lesions of patients with DM. Using immunohistochemical methods, we examined 22 biopsy specimens from lesional skin, six biopsy specimens from uninvolved skin, and 12 muscle biopsy specimens from patients with DM for the presence of MAC and vitronectin and CD59, two regulatory proteins of complement.

The deposition of MAC was demonstrated in a large percentage of biopsy specimens obtained from the lesional skin of patients with DM. Deposits were found along the dermoepidermal junction in 19 (86%) of 22 biopsy specimens. Deposits on the vessel walls of the dermis 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 junction and the vessel walls of the dermis were absent in specimens from uninvolved skin. In 12 muscle biopsy specimens obtained simultaneously from 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 was similar to MAC deposits. The expression of CD59 was normal in all skin and muscle biopsy specimens.

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 pathogenesis of the skin lesions of DM.