Cardiac tissues generated from human induced pluripotent stem (iPS) cells can serve as platforms for patient-specific studies of physiology and disease1–6. The predictive power of these models remains limited by their immature state1,2,5,6. We show that this fundamental limitation could be overcome if cardiac tissues are formed from early iPS-derived cardiomyocytes (iPS-CM), soon after the initiation of spontaneous contractions, and subjected to physical conditioning of an increasing intensity. After only 4 weeks of culture, these tissues displayed adult-like gene expression profiles, remarkably organized ultrastructure, physiologic sarcomere length (2.2 μm) and density of mitochondria (30%), the presence of transverse tubules (t-tubules), oxidative metabolism, positive force-frequency relationship, and functional calcium handling for all iPS cell lines studied. Electromechanical properties developed more slowly and did not achieve the stage of maturity seen in adult human myocardium. Tissue maturity was necessary for achieving physiologic responses to isoproterenol and recapitulating pathological hypertrophy, in support of the utility of this tissue model for studies of cardiac development and disease.

The immunosuppressive capacity of human mesenchymal stromal cells (MSCs) renders them promising candidates for treating diverse immune disorders. However, after hundreds of clinical trials, there are still no MSC therapies approved in the United States. MSCs require specific cues to adopt their immunosuppressive phenotype, and yet most clinical trials use cells expanded in basic culture medium and growth conditions. We propose that priming MSCs prior to administration will improve their therapeutic efficacy. Interferon-gamma (IFN-γ) priming are cues common to situations of immune escape that have individually shown promise as MSC priming cues but have not been systematically compared. Using mixed lymphocyte reactions, we show that priming MSCs with either cue alone improves T-cell inhibition. However, combining the two cues results in additive effects and markedly enhances the immunosuppressive phenotype of MSCs. We demonstrate that IFN-γ induces expression of numerous immunosuppressive proteins (IDO, PD-L1, HLA-E, HLA-G), whereas hypoxia switches MSCs to glycolysis, causing rapid glucose consumption and production of T-cell inhibitory lactate levels. Dual IFN-γ/hypoxia primed MSCs display both attributes and have even higher induction of immunosuppressive proteins over IFN-γ priming alone (IDO and HLA-G), which may reflect another benefit of metabolic reconfiguration.

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The notion that both adaptive and maladaptive cardiac remodeling occurs in response to mechanical loading has informed recent progress in cardiac tissue engineering. Today, human cardiac tissues engineered in vitro offer complementary knowledge to that currently provided by animal models, with profound implications to personalized medicine. We review here recent advances in the understanding of the roles of mechanical signals in normal and pathological cardiac function, and their application in clinical translation of tissue engineering strategies to regenerative medicine and in vitro study of disease.