Modulation of the immune response after myocardial infarction to improve cardiomyocyte engraftment

The decline in cardiac pump function due to the formation of a fibrotic scar is one of the most severe consequences of myocardial infarction (MI) and can ultimately result in terminal pump failure of the organ. Currently available therapies primarily target the remaining viable myocardium and aim to reduce myocardial workload but do not directly treat the infarct scar. Re-population of the scar area with transplanted cardiomyocytes is therefore an attractive therapeutic strategy, However, previous studies have demonstrated that the majority of injected cardiomyocytes are lost very early after delivery into the freshly infarcted myocardium. Loading of cells with magnetic nanoparticles (MNP) prior to intramyocardial injection in the presence of an external magnetic field has emerged as a promising approach to enhance cellular retention within the tissue, yet a substantial loss of transplanted cells over time has still been observed. The NLRP3 inflammasome is a multi-protein complex that assembles in response to endogenous signals. Its terminal effector, caspase 1, mediates the maturation of interleukin 1β (IL-1β), a potent pro-inflammatory cytokine, that plays a key role in initiating the post-MI immune response. Pharmacological inhibition using the NLRP3 specific inhibitor MCC950 has been shown in animal models to improve cardiac pump function after MI and to preserve graft function following heart transplantation. On the basis of these findings, the present study investigated whether NLRP3 inflammasome inhibition can enhance short- and long-term engraftment of transplanted embryonic cardiomyocytes (eCM). In a mouse model of permanent coronary ligation (LADx), 2 × 105 MNP-loaded eCM were injected intramyocardially under the influence of an external magnetic field. Additionally, animals received MCC950 intraoperatively and every other day until day 14 post-MI. In the absence of NLRP3 inhibition, only 1.1% of the injected cells remained within the scar 14 days after transplantation, whereas MCC950 treatment significantly increased this proportion to 5.5%. At this time point, mice treated with the combination of eCM and MCC950 showed an improvement in left ventricular ejection fraction (EF), which was not observed in animals receiving eCM alone. Interestingly, MCC950 monotherapy also led to an improvement in EF. Infarct scar volumes did not differ between groups; however, no transmural lesions were detected in MCC950-treated mice 14 days post-MI. Histological analyses confirmed a reduced number of dying cells in the infarct area on day 1 post-MI and a decrease in leukocyte and fibroblast proliferation on day 3 post-MI in MCC950 treated animals. The efficacy of MCC950 treatment was further supported by reduced expression of NLRP3 and IL-1β on day 3 post MI. Eight weeks after MCC950 treatment, the number of engrafted eCM remained 4.3-fold higher compared to animals without NLRP3 inhibition, although a loss of approximately 75% of the cells relative to the 2 week time point was still observed. The beneficial effect of MCC950 on EF persisted long term, both in animals receiving cell transplantation and in those treated with MCC950 alone. In summary, MCC950 attenuates the post-infarction immune response and improves both short- and long-term engraftment of intramyocardially transplanted eCM. Moreover, cardiomyocyte transplantation and MCC950 treatment, either in combination or as standalone interventions, enhance cardiac pump function following MI.