Characterizing the inflammatory reaction in explanted Medtronic Freestyle stentless porcine aortic bioprosthesis over a 6-year period.

BACKGROUND: The Medtronic Freestyle valve is a stentless porcine valve with reportedly excellent clinical and hemodynamic results, but little has been reported about its long-term pathology. METHODS: Seventeen Freestyle valves were explanted (from 2003 to 2009) and reviewed to assess reasons for bioprosthesis failure. All valves were examined in detail, using histochemistry and immunohistochemistry to identify morphological changes, as well as cellular and humoral responses. RESULTS: One Freestyle valve, explanted for mitral valve endocarditis on the fifth postoperative day, was excluded from analysis. The average implant duration was 71.1±35.2 months. Six valves were explanted for infective endocarditis, six for aortic insufficiency, and four for aortic stenosis. Calcification was seen in 11 explants, pannus in 15, thrombus in 12, cusp tears in 9, and 10 explants showed needle tract-like injuries. A chronic inflammatory reaction involving the xenograft arterial wall was seen in 15 of 16 valves. The cells were composed of macrophages and lymphocytes, including T cells (CD8 positive) and B cells. Significant damage to the porcine aortic wall was seen in 15 cases, and cusp myocardial shelf damage in 7 cases. All cases stained positively for IgG and C4dpar. CONCLUSIONS: The porcine aortic tissue showed T cell-mediated rejection and significant aortic medial damage, consistent with dilatation of the porcine aortic root. The demonstration of IgG suggests the likelihood of humoral rejection, in addition to cellular rejection. One of the underlying possibilities is that the porcine aortic tissues are inadequately fixed, hence the retained antigenicity.

Determining the fate of seeded cells in venous tissue-engineered vascular grafts using serial MRI.

A major limitation of tissue engineering research is the lack of noninvasive monitoring techniques for observations of dynamic changes in single tissue-engineered constructs. We use cellular magnetic resonance imaging (MRI) to track the fate of cells seeded onto functional tissue-engineered vascular grafts (TEVGs) through serial imaging. After in vitro optimization, murine macrophages were labeled with ultrasmall superparamagnetic iron oxide (USPIO) nanoparticles and seeded onto scaffolds that were surgically implanted as inferior vena cava interposition grafts in SCID/bg mice. Serial MRI showed the transverse relaxation times (T(2)) were significantly lower immediately following implantation of USPIO-labeled scaffolds (T(2) = 44 ± 6.8 vs. 71 ± 10.2 ms) but increased rapidly at 2 h to values identical to control implants seeded with unlabeled macrophages (T(2) = 63 ± 12 vs. 63 ± 14 ms). This strongly indicates the rapid loss of seeded cells from the scaffolds, a finding verified using Prussian blue staining for iron containing macrophages on explanted TEVGs. Our results support a novel paradigm where seeded cells are rapidly lost from implanted scaffolds instead of developing into cells of the neovessel, as traditionally thought. Our findings confirm and validate this paradigm shift while demonstrating the first successful application of noninvasive MRI for serial study of cellular-level processes in tissue engineering.