Partial Heart Transplants Require Immunosuppression in Growing Piglets

Partial heart transplants (PHTs) are a new type of transplant that provides growing heart valve substitutes for children.1 PHTs differ from heart transplants because only the part of the heart containing the necessary valve is transplanted while the native ventricles are preserved. Preserving the native ventricles eliminates the risk of graft ventricular dysfunction from immune rejection.2 Moreover, heart valves benefit from immune privilege relative to the myocardium.3 Therefore, we hypothesized that PHTs may allow for withdrawal of immune suppression. A total of 15 piglets (sus domesticus) were used. Donor (n = 6) and recipient piglets were ABO matched. The pulmonary valve was transplanted as previously described.4 PHT recipients (n = 3) were immunosuppressed with twice daily oral mycophenolate (10 mg/kg), twice daily oral prednisolone tapered to 0.5 mg/kg, and daily intramuscular tacrolimus titrated to a trough level of 5 ng/mL. In these piglets, immunosuppression was withdrawn after 1 mo (Figure 1A). Negative control recipients (n = 3) underwent pulmonary valve transplant without immune suppression. Positive control piglets (n = 3) did not undergo surgery. The pulmonary valves were examined by serial echocardiography (EPIQ 7G, Phillips Ultrasound, Bothell, WA) according to standard clinical protocols.5 Briefly, valve growth was quantified using annular diameter, valve stenosis was quantified using valve gradient, and valve regurgitation was quantified using the width of the diastolic Doppler jet relative to annular diameter. Echocardiography measurements before and after withdrawal of immunosuppression were compared with the control groups using the linear mixed-effects model for interrupted time series (R version 4.4.3, R Foundation, Vienna, Austria). Data are reported as mean ± SD. Once piglets had doubled their weight after surgery, the valves were analyzed histologically.FIGURE 1.: Partial heart transplants require immunosuppression in growing piglets. A, The illustration shows the experimental design. B, The scatter plots show valve growth and valve gradient in PHT recipients while immunosuppressed (blue circles) and after withdrawing immunosuppression (red squares), in negative control pulmonary valve transplant recipients without immunosuppression (gray triangles) and in unoperated positive control piglets (green diamonds). C, Micrographs of positive control piglets show normal heart valves. Micrographs of PHT recipients after withdrawing immunosuppression and negative control pulmonary valve transplant recipients without immunosuppression show endothelial cell hyperplasia, which was most prominent overlying the fibrosa layer of the hinge region on H&E staining, T-cell infiltration, also most prominent in the hinge region, on anti-CD3 immunohistochemical staining, and increased collagen deposition but otherwise no intrinsic structural disarray on Movat Pentachrome and Van Gieson staining. H&E, hematoxylin and eosin; PHT, partial heart transplant.PHT was performed for recipients of age 40 ± 6.5 d and weight 11.8 ± 2.0 kg. PHT recipients doubled their weight 43 ± 3.5 d after surgery. The average tacrolimus trough level was 4.4 ± 2.9 ng/mL. While PHT recipients were immunosuppressed, the slopes for valve gradient and valve growth were not significantly different from unoperated positive controls (P = 0.76 and P = 0.56). After withdrawing immunosuppression, the slopes for valve gradient and valve growth were significantly different from unoperated positive controls (P = 0.04 and P = 0.03) but not negative controls without immunosuppression (P = 0.57 and P = 0.28; Figure 1B). No echocardiogram showed more than mild valve regurgitation. Histology of unoperated positive control piglets showed normal valves. PHT recipients after withdrawal of immunosuppression and negative control recipients without immunosuppression had endothelial cell hyperplasia and T-cell infiltration, which was most prominent at the hinge region. The hinge region also showed increased collagen deposition, but there was no other intrinsic structural disarray (Figure 1C). These data indicate that immunosuppressed PHTs grow normally. Without immune suppression, PHT valve growth diminishes, and valve gradients increase as the recipient piglets continue to grow (Figure 1B). This is associated with immune rejection, especially at the valve hinge region (Figure 1C). These results are clinically relevant because infections or lymphoproliferative disorders may mandate withdrawal of immune suppression in children with PHTs. Moreover, increasing valve gradients could serve as a clinical biomarker to guide escalation of immunosuppression. This is valuable because PHTs do not easily lend themselves to biopsy. ACKNOWLEDGMENTS The authors thank Dr Andrew W. Brown from the Department of Biostatistics at the University of Arkansas for Medical Sciences for his consultation on the statistical analysis.