Studying Xenograft Rejection of Bioprosthetic Heart Valves

Millions of patients with valvular heart disease have benefitted from heart valve replacement since the procedure was first introduced in the 1960s; however, there are still many patients who get early structural valve deterioration (SVD) of their bioprosthetic heart valves (BHV). BHV are porcine, bovine, or equine tissues that have been glutaraldehyde fixed to preserve the tissue and presumably make the tissue immunologically inert. These glutaraldehyde-fixed BHV with anti-calcification treatments last long periods of time in older adults but develop early SVD in younger patients. The consensus at present is that the early SVD in younger patients is due to more "wear and tear" of the valves and higher calcium turnover in younger patients. However, as younger patients likely have a more robust immune system than older adults, there is a new hypothesis that BHV xenografts may undergo xenograft rejection, and this may contribute to the early SVD seen in younger patients.At present, the technology to noninvasively study in vivo whether an implanted BHV in a human patient is undergoing rejection is not available. Thus, a small animal discordant xenotransplant model in young rodents (to match the young patient getting a pig/bovine/equine BHV) was developed to study whether the hypothesis that glutaraldehyde-fixed BHV undergo xenograft rejection had any merit. In this chapter, we describe our model and its merits and the results of our investigations. Our work provides clear evidence of xenograft rejection in glutaraldehyde-fixed tissue, and our small animal model offers an opportunity to study this process in detail.

Human ABO blood group is important in survival and function of porcine working hearts

Pig organs express alphaGal antigen and thus are hyperacutely rejected if perfused by human blood. Human B/A antigens are similar to pig alphaGal antigen, suggesting that the corresponding antibodies may cross-react. Our purpose was to determine if there is a human ABO blood-group difference in porcine-human xenotransplantation. Plasma from six A, five B, seven AB, and six O individuals pooled by blood group were tested in an ex-vivo porcine working heart model. Blood-group A plasma-perfused hearts survived 20 +/- 14 min (n = 5), B 241 +/- 9 min (n = 3), AB 151 +/- 37 min (n = 5), and O 9 +/- 1 min (n = 8). A and O were different (p < 0.001) from B and AB. Function was significantly better in group B. Edema accumulation and creatine kinase change was highest in A and O. All groups had comparable levels of anti-alphaGal antibody, as well as comparable perfusion and operative conditions. Multivariate linear regression analysis showed the anti-B antibody levels to be predictive of survival (p < 0.001). At higher plasma concentrations, hearts perfused with B plasma survived longer (p = 0.01) than AB (218 +/- 45 min, n = 4 vs. 6 +/- 0 min, n = 3). These results suggest a human ABO blood-group difference in porcine-to-human xenotransplantation, which may be mediated by the anti-A and anti-B antibodies.

Timing of hyperoxic exposure during alveolarization influences damage mediated by leukotrienes

Hyperoxic exposure of rat pups during alveolarization (postnatal days 4-14) severely retards alveolar development. Some aspects of this inhibition are mediated by leukotrienes (LTs) and may be time sensitive. We determined 1) the effects of exposure to hyperoxia (O(2)) during discrete periods before and during alveolarization on developing alveoli and 2) whether a relationship exists between O(2) and LTs in these periods. Pups were exposed to >95% O(2) from days 1 to 4, 4 to 9, 9 to 14, or 4 to 14 in the absence and presence of the LT synthesis inhibitor MK-0591. Both the level of in vitro lung tissue LT output on days 4, 9, and 14 and the degree of alveolarization on day 14 were determined. Pups exposed to O(2) from days 4 to 9 had a more profound inhibition of alveolarization on day 14 compared with those exposed to O(2) from days 1 to 4 or 9 to 14. Peptido-LT levels were significantly higher in pups exposed to O(2) on days 9 and 14 compared with pups in air and returned to normal once normoxia was restored. LT inhibition from days 4 to 14, 4 to 9, or 9 to 14 in pups exposed to O(2) from days 4 to 14 prevented the O(2)-induced inhibition of alveolarization. These data suggest that developing alveoli are sensitive to LTs shortly before and after day 9, significantly retarding certain parameters of alveolarization on day 14. We conclude that some of the effects of O(2) are not uniform throughout different stages of alveolarization and that this is likely related to the timing of LT exposure.