4 Development of a humanized, functional, tissue engineered, in vitro model of healthy myocardium

<h3>Introduction</h3> Cardiac research over the past decade has significantly improved our understanding of the interaction between cardiomyocytes and immune cells, illuminating the importance of cardiac resident macrophages (CRMs) in maintaining homeostasis within the myocardium. In particular, macrophages in the heart perform a number of critical functions such as facilitating electrical conduction, capture and elimination of cardiac exophers and routine immunosurveillance.¹ Here, we aim to fabricate a tissue engineered model of healthy myocardium, integrating an innate immune response for patient-specific drug screening, regenerative medicine and drug discovery applications. <h3>Methods</h3> iPSCs were differentiated to obtain macrophages (iMacs) and cardiomyocytes (iCMs). To understand how macrophages respond to a cardiac environment, iMacs were exposed to distinct cardiac cues, such as conditioned media from iCMs, co-culture with iCMs and electrical stimulation. Analyses were performed using flow cytometry, qRT-PCR and immunofluorescence imaging. Finally, an engineered heart tissue (EHT) model was developed using type I collagen and Matrigel to fabricate 3D models of the myocardium, incorporating an innate immune response. <h3>Results</h3> The iMacs that were generated were found to have a resident macrophage phenotype, with ability to perform phagocytosis as well as polarize to become classically and alternatively activated macrophages. iMacs displayed unique gene expression profiles when in culture with iCM conditioned media as well as in co-culture with iCMs. Interestingly, when in co-culture with iCMs, iMacs expressed CRM-specific genes, as well as cardiac developmental genes. Additionally, iCMs in co-culture with iMacs displayed an elongated morphology, with an upregulation in maturation markers. Furthermore, iMacs were also found to electrically couple with iCMs and facilitate synchronous beating, demonstrating great potential to improve iCM function in vitro. <h3>Reference</h3> Suku M, Forrester L, Biggs M, Monaghan MG. Resident macrophages and their potential in cardiac tissue engineering. <i>Tissue Eng Part B Rev.</i> 2022.