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Myocardial infarction causes unrecoverable loss of cardiomyocytes. Engineered heart tissue (EHT) is an in vitro model of three-dimensional, force generating cardiomyocyte network with morphological and functional similarity to native heart tissue. In this study we transplanted EHTs from human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes (CM) on cryo-injured guinea pig hearts and investigated whether hiPSC-CM-EHTs support left ventricular function.Human iPSC were generated by retroviral reprogramming of dermal fibroblasts. Cardiac differentiation of hiPSC was performed by an embryoid body-based three-stage differentiation protocol. EHTs were created from hiPS-CM (5*10^6 cardiomyocytes and 2*10^6 GFP+-HUVECs per EHT) and cultivated for 3 weeks under auxotonic stretch between flexible silicone posts. Development of contractile force was monitored prior to transplantation. Left ventricular myocardial cryo-injury was induced in adult guinea pigs (n=21). 7 days after injury EHTs (2 per animal, n=12) or cell-free constructs (n=9) were implanted. Animals received ciclosporin and methylprednisolon for immunosuppression. Functional parameters were examined by echocardiography and histology at baseline, before and 28 days after transplantation.The cardiac differentiation protocol resulted in a cell population with ~50% cardiomyocytes, which was further enriched by lactate-based selection to >90% purity and directly used for EHT generation. HiPSC-CM-EHTs developed contractile force and displayed morphological properties of native heart tissue. Cryo-injury resulted in large transmural scars (~30% of ventricular wall) which were verified histologically. Immunohistochemical staining for dystrophin and MLC2v showed the formation of large islets of cross-striated muscle tissue in the scar. The human origin was demonstrated by fluorescent-in-situ-hybridization. The new myocardium was vascularized with endothelium partly being of human origin. Animals receiving cell-free constructs showed left ventricular dilatation 28 days after transplantation. The EHT-group showed less dilatation (8.12 mm ± 0.21 basal, 8.18 mm ± 0.23 7d post cryo-injury, 8.87 mm ± 0.41 28d EHT vs. 9.74 mm ± 0.72 28d control) and significantly better fractional area shortening (42.20% ± 1.92 basal, 26.07% ± 2.13 7d post cryo-injury, 41.98% ± 4.48 28d EHT vs. 23.00% ± 3.24 28d control).Transplantation of hiPSC-derived EHTs in a guinea pig cryo-injury model provides early evidence that human EHTs survive after transplantation and support cardiac function. [ABSTRACT FROM AUTHOR]