Background: IL6 is a cytokine known to mark adverse prognosis after myocardial infarction (MI) and approaches to block IL6 early post MI were reported to improve the outcome. IL6 produced as response to cardiac injury induces a protective program in cardiomyocytes that can lead to tissue preservation. Here, we studied which cardiac cell type produces IL6 and explored the mechanisms that regulate IL6 formation in a murine model of ischemia and reperfusion.

Methods & Results: Analysis of IL6 expression in FACS-sorted cardiac cell types at day 1, 3 and 7 post MI by qPCR revealed IL6 to be strongly and preferentially expressed by cardiac fibroblasts (CFs). These findings were confirmed by RNAscope, in addition showing that particularly Periostin-expressing cells located in the border zone of the left ventricle were positive for Il6. To obtain a deeper insight into Il6 expression and signaling in different non-cardiomyocyte cell populations of the heart, we carried out single cell transcriptomics (5d post MI). The most highly IL6 expressing CFs show a proliferative phenotype and myo-fibroblast-like characteristics. While IL6 is mainly formed by CFs, the IL6 receptor was predominantly expressed by cardiac immune cells. In contrast, the IL6 family member IL11 was also predominantly produced by CFs that also highly expressed the receptor. As to the regulation of IL6, previous work from us and others showed IL6 induction by adenosine A2b receptor (A2bR) activation. Using authentic adenosine (20µM), IL6 was strongly induced in CFs which was absent in CFs not expressing A2bR a condition that was also observed for IL11. In a final experimental series, we found that CFs had very little CD73 activity, so that degradation of extracellular ATP does not proceed beyond AMP to form adenosine. Accumulating AMP is likely to be a substrate for CD73 which is most highly expressed on T cells. This view is supported by experiments in which the release of IL6 into the effluent coronary perfusate of isolated hearts was significantly decreased in a mouse model lacking CD73 on T cells.

Conclusion: In conclusion, post-ischemic IL6 is mainly formed by activated CFs and is controlled by T cell (CD73)-derived adenosine, which signals via A2bR. Metabolic cooperation between AMP producing CFs and CD73 on T cells is very likely important for the effective production of adenosine by the injured heart.