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IntroductionExpression of human complement pathway regulatory proteins (hCPRP's) such as CD46 or CD55 has been associated with improved survival of pig organ xenografts in multiple different models. Here we evaluate the hypothesis that an increased human CD46 gene dose, through homozygosity or additional expression of a second hCPRP, is associated with increased protein expression and with improved protection from injury when GTKO lung xenografts are perfused with human blood.MethodsTwenty three GTKO lungs heterozygous for human CD46 (GTKO.heteroCD46), 10 lungs homozygous for hCD46 (GTKO.homoCD46), and six GTKO.homoCD46 lungs also heterozygous for hCD55 (GTKO.homoCD46.hCD55) were perfused with human blood for up to 4 h in an ex vivo circuit.ResultsRelative to GTKO.heteroCD46 (152 min, range 5–240; 6/23 surviving at 4 h), survival was significantly improved for GTKO.homoCD46 (>240 min, range 45–240, p = .034; 7/10 surviving at 4 h) or GTKO.homoCD46.hCD55 lungs (>240 min, p = .001; 6/6 surviving at 4 h). Homozygosity was associated with increased capillary expression of hCD46 (p < .0001). Increased hCD46 expression was associated with significantly prolonged lung survival (p = .048),) but surprisingly not with reduction in measured complement factor C3a. Hematocrit, monocyte count, and pulmonary vascular resistance were not significantly altered in association with increased hCD46 gene dose or protein expression.ConclusionGenetic engineering approaches designed to augment hCPRP activity — increasing the expression of hCD46 through homozygosity or co‐expressing hCD55 with hCD46 — were associated with prolonged GTKO lung xenograft survival. Increased expression of hCD46 was associated with reduced coagulation cascade activation, but did not further reduce complement activation relative to lungs with relatively low CD46 expression. We conclude that coagulation pathway dysregulation contributes to injury in GTKO pig lung xenografts perfused with human blood, and that the survival advantage for lungs with increased hCPRP expression is likely attributable to improved endothelial thromboregulation.

Purpose: Blockade of the CD40/CD154 T cell costimulation pathway is a promising approach to replace conventional clinical immunosuppressive therapy. TNX-1500 (TNX)* is a novel humanized anti-CD154 mAb that contains the hu5c8 Fab region and an Fc region engineered to modulate FcγR2 binding to reduce the risk of thromboembolic events seen with hu5c8 IgG1 in previous clinical trials. Its efficacy was evaluated in cynomolgus monkey heterotopic heart allograft recipients. Methods: Cynomolgus monkey heterotopic cardiac allograft recipients were treated with TNX 30mg/kg iv 2x weekly for 2 weeks, then either a 'standard-dose' regimen of 20mg/kg weekly from days 21-180 (sTNX, n=5) or a reduced-dose TNX maintenance regimen (10 mg/kg weekly x6, then 20mg/kg monthly; loTNX), with (loTNX+MMF, n=4) or without mycophenolate mofetil (MMF) at 200mg/d po (loTNX, n=4). In 3 sTNX animals therapy was discontinued on day 175. Results were compared to historical data with hu5c8 monotherapy dosed as for loTNX (n=5). Biopsies or explanted hearts were evaluated at three protocol-defined time-points (d45, d90, d180), at earlier explant. Results: sTNX (MST >232d, range 116-305) significantly prolonged cardiac allograft survival relative to hu5c8 alone (MST 133d; p=0.016), loTNX (MST 99d; p=0.014), or loTNX+MMF (MST 88d; p=0.011). All grafts treated with loTNX () or with loTNX+MMF () exhibited acute rejection histologically within 90 days, and 7 of 8 grafts failed before day 180. One sTNX graft rejected at 116d in association with bacterial infection; one graft was electively explanted at 181d; and three grafts survived for 57, 90, and 130 days after discontinuing TNX-1500. Neither sustained thrombocytopenia nor micro-or macro-vascular thrombosis were observed. At 90 days, loTNX+MMF had a significantly higher median ISHLT score (2.0) than sTNX grafts (0.2; p=0.016); loTNX+MMF also had a significantly higher median CAV score (1.8) at d90 than either loTNX (0.2; p=0.022) or sTNX (0.3; p=0.026). At subsequent graft explant or end-of-study biopsy (180d), loTNX+MMF had a higher median CAV score (2.4) than loTNX (0.4; p=0.002) or sTNX (0.8; p=0.002). sTNX suppressed the rise in Teff/Tregs ratio at the final time-point (180d or <180d if graft failure) more effectively than loTNX+MMF (3.5 ± 0.9 vs 9.5 ± 6.1, p=0.031). sTNX (5/5) but not loTNX (1/4) or loTNX+MMF (2/4) consistently prevented anti-donor antibody elaboration. Conclusions: Blockade of CD154 with sTNX monotherapy was not associated with thrombotic events, well tolerated, and consistently prevented pathogenic alloimmunity in this stringent preclinical model. Lower TNX maintenance dosing, with or without MMF, was less effective. *TNX-1500 is a pre-IND molecule and not approved for any clinical indication.

Background:The first clinical genetically-engineered pig heart transplant has stimulated consideration of which patients might be medically and ethically appropriate to enroll in the initial clinical trials. Beyond exceptional 'compassionate use' applications, as in the University of Maryland case, which patients might be selected for formal 'qualifying' trials of 'destination' or 'bridging' heart xenotransplantation? Approach and Results: Patients eligible for a heart allograft and in whom mechanical circulatory support is contraindicated or associated with a high risk of mortality or morbid complications, such as presence of a mechanical valve prosthesis, restrictive or hypertrophic cardiomyopathy, and refractory ventricular arrhythmias, are at high risk of dying before transplant, and might be eligible for either a 'bridging' or 'destination' trial. Patients with high titers of panel-reactive anti-HLA antibodies, including prior allotransplant recipients with graft vasculopathy, are at high risk of sudden death, experience long waiting times and inferior post-allograft survival; those without antibodies that cross-react with pig cells might expect better survival after a successful heart xenograft than after desensitization to enable a heart allograft. Infants and children with complex CHD have limited access to allotransplantation due to the scarcity of size-matched donor organs. In these patients the results of mechanical support are poor, and both survival and quality of life after multiple staged surgical reconstructive procedures remain limited, particularly for those relying upon univentricular 'Fontan' physiology. A genetically-engineered pig heart could be used as a life-supporting 'bridge' for a clinically deteriorating infant or child until a heart from a size-appropriate deceased human donor can be obtained. Pediatric or adult patients presenting in extremis and unable to participate, with their caregivers, in a complete, robust informed consent process would be inappropriate to enroll on ethical grounds. Patients whose prognosis is poor based on risk factors not directly related to their heart pathology, such as frailty or malignancy, that would disqualify them as heart allograft candidates should also be avoided, as this approach is likely to yield poor outcomes and undermine public and peer support for xenotransplantation. Conclusion:We propose that patients meeting these inclusion and exclusion criteria are appropriate to consider for enrollment in initial heart xenograft clinical trials.

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