Cyclosporin-associated microfibrils in cardiac transplant patients

Endomyocardial biopsies from 21 cyclosporin-treated cardiac transplant patients were studied at the ultrastructural level. Eight patients exhibited focal interstitial accumulation of filaments, which most commonly measured 8-10 nm in diameter (range 7-16 nm). The filaments were straight, hollow, and nonbranching. Filament accumulation was observed as early as 4 days posttransplant. No filament accumulation was observed in 17 donor endomyocardial biopsies obtained at the time of transplantation. The ultrastructure of the filaments was similar to that of amyloid fibrils. Stains for amyloid at the light microscopic level were uniformly negative, however. Until the exact nature of the filaments is determined, we propose that they be referred to as cyclosporin associated microfibrils.