Asymptomatic bacterial invasion as the reason for inflammation in bioprosthetic heart valves on later stages of their functioning

Background. The average lifespan of a bioprosthetic heart valves (BHVs) is limited to 10-15 years due to structural valve degeneration (SVD). One of the causes of SVD is the infiltration of immune cells. Inflammation like this usually develops several years after valve replacement, the cause of it remains unknown. Presumably, the main stimulus of the immune response is an asymptomatic colonization by bacterial agents.Aim: To study bacterial invasion of BHVs explanted due to SVD, and to identify patterns in microorganism and cellular infiltrate localization.Material and Methods. 23 BHVs excised during replacement due to SVD were included in the study. The identification of bacteria in the samples was carried out by Gram staining and PCR analysis. Immunohistochemical staining with antibodies to markers CD45 (pan-leukocyte marker), CD68 (macrophage marker), CD3 (T-lymphocyte marker) and neutrophil elastase (ELA2) were used for phenotyping of immune cells. The composition of BHVs tissues was studied using Russell-Movat pentachrome staining.Results. All BHVs showed traditional signs of SVD (perforations and large calcifications at the base and dome of the leaflets, ruptures in the commissural zone), but had no signs of prosthetic valve endocarditis (no vegetation, blood culture and swab results were negative). Using the histological method, we detected bacterial cells in 19 of the 23 studied valves (82.6%), whereas PCR data indicated the presence of bacteria in 11 (47,8%) valves (Streptococcus ssp. and Staphylococcus ssp. without species identification). Moreover, we identified the co-localization of colonies of microorganisms and inflammatory infiltrates composed of macrophages (CD45+, CD68+) and neutrophils (CD45+, ELA2+).Conclusion. Bacteria commonly cause asymptomatic colonization of BHVs which is strongly associated with inflammatory cell infiltration.