Yolk-sac derived macrophages contribute to the adaptive response of the neonatal mouse heart to pressure overload

Introduction: Cardiovascular diseases are the leading cause of death worldwide. This is also because of insufficient regenerative ability in the adult heart. In contrast, the neonatal mammalian heart has the ability to regenerate. We have shown in earlier work that the neonatal mouse heart at postnatal day 0 (P0) is able to adapt to pressure overload (POL) and that his adaptive response is characterized by preserved cardiac function, enhanced cardiomyocyte proliferation, and angiogenesis in the absence of interstitial fibrosis and cardiac hypertrophy. However, this regenerative ability is lost at P7 and POL, leading to cardiomyocyte hypertrophy, interstitial fibrosis, and impaired cardiac function. Transcriptomics has revealed a unique response of innate immunity in the adaptive response of P0 compared to P7 to POL. Since macrophages are the main cells involved in innate immunity and those originating from the embryonic phase, especially from the yolk sac (YS) have been proposed to have pro-regenerative and pro-angiogenic properties, we hypothesized that YS-derived macrophages contribute to the adaptive response of P0 hearts to POL.

Method: To investigate the differences in the response of YS-derived macrophages in POL at P0 and P7 we performed genetic fate mapping using Tnfrsf11aCre x Rosa26YFP. POL was induced using a neonatal model of transverse aortic constriction (nTAC) at P0 or P7. Hearts were harvested 7 and 14 days after injury, non-injured hearts were also collected at P0, P7, and P14 as controls. YS-derived macrophages were identified and quantified in the right and the left ventricle using immunofluorescence staining against F4/80, Iba1, and GFP. In addition, immunostaining against LYVE1 and Ki67 were performed to assess the macrophage subpopulation and the proliferation rate, respectively.

Results: Quantitation of YS-derived macrophages in non-injured postnatal hearts revealed decreased numbers in the hearts (P0-P14) with age. In contrast, the number of these macrophages increased at P7 and P14 upon POL (P0). This increase was attributed to their enhanced proliferation rate after POL at P0. The response of these YS-derived macrophages was different, as number and proliferation rate did not change 7 and 14 days after POL at P7. Interestingly, LYVE1+ was not exclusive to YS-derived macrophages and found to label a small subpopulation of YS-derived macrophages in the heart, and this subpopulation remained unchanged upon POL in both P0 and P7.

Conclusion: Our data suggest that the number of YS-derived macrophages in the heart decreases after birth with age. However, these macrophages increase in number upon POL at P0, but not P7. These differences in the response of YS-derived macrophages could potentially contribute to the adaptive response of P0 to POL and lack thereof at P7. In ongoing work, we FACS-sort these cells and will investigate the transcriptomics of YS-derived macrophages upon POL in P0 and P7 to explore potential signals and pathways impacting the adaptive response to POL.