Abstract Body (Do not enter title and authors here): Background: The mRNA vaccines against COVID-19 are highly effective but have been associated with a rare non-infective form of myocarditis, particularly in young males after receiving the second dose. Understanding the mediators of this adverse effect is crucial to enhance the safety of future mRNA vaccines.

Hypothesis: Myocardial injury following COVID-19 mRNA vaccination is mediated by overproduced cytokines, and estrogens have a protective effect on this adverse effect.

Approach: Candidate cytokine mediators were identified through analysis of proteomics data from plasma samples of vaccinated individuals. Human iPSC-derived macrophages and cardiomyocytes were used to model cytokine-induced effects. An in vivo mouse model of cytokine-induced myocardial injury was employed to assess the impact of the cytokine cocktail and estrogens.

Results: CXCL10 and IFN-γ were consistently upregulated in vaccinated individuals on day 1 and further elevated in patients with myocarditis following mRNA vaccination. Consistently, iPSC-derived macrophages exposed to COVID-19 mRNA vaccines produced these cytokines. Next, iPSC-derived cardiomyocytes exposed to these cytokines showed impaired contractility, arrhythmogenicity, and pro-inflammatory gene expression. The phytoestrogen genistein mitigated these effects in vitro, reducing cytokine-induced proteasomal degradation of cardiac proteins and preserving contractile function. In vivo, genistein significantly decreased cardiac injury markers and immune cell infiltration in a mouse model of cytokine-induced myocardial injury.

Conclusion: CXCL10 and IFN-γ are key mediators of myocardial injury post-mRNA vaccination. Genistein shows potential as a therapeutic agent to mitigate associated cardiovascular risks.