Microscopical assessment of explanted allograft heart valves: a limited contribution of histopathology to the pathogenic mechanism of the graft failure in long-term explants

Background: Cryopreserved allograft heart valves (CAHV) currently represent conduits of choice showing longer event-free survival compared to other types of prostheses. However, almost all patients develop early or late allograft failure. The exact mechanism of the failure and eventual role of cellular or antibody-mediated rejection remain uncertain. In this study, we targeted the microscopical structure of CAHV in correlation to the duration of implantation, assessed the degree of degenerative changes and searched for signs of cellular rejection. Methods: 24 patients with CAHV and one patient with xenograft heart valve who required conduit explantation in the period of 11/2017-5/2020 were prospectively enrolled in the study. Age of the patients at the time of implantation, duration of implantation, type of conduit, main diagnosis and number of previous implantations were recorded. Conduits were subjected to light microscopy and the microscopic structure was assessed using basic staining methods and immunohistochemistry. Results: All conduits were completely non-vital, with areas of necrosis, hyalinization, calcification, metaplastic bone formation and pannus overgrowth. Foci of rejection-type inflammatory infiltrate were noted in three cases. Three other patients showed perioperative and microscopical signs of subacute infective endocarditis. Both severity of the microscopical changes and presence of rejection-type infiltrate were independent of duration of implantation or any other clinical variable. Conclusions: Histopathological assessment of explanted conduits may contribute to understanding of exact pathogenic mechanisms of allograft failure. However, the microscopic appearance of long-term explants is often non-specific and signs of cellular rejection are sparse. Examination of short-term and intermediate-term explants may be more beneficial.