Myocarditis After mRNA Vaccination: A Metabolic-Innate Immune Cascade Centered on Lipid Nanoparticles.

mRNA vaccine-associated myocarditis is a rare but clinically important adverse event whose pathogenesis remains incompletely understood. Initial hypotheses focused primarily on the spike protein antigen, with growing preclinical evidence implicating the lipid nanoparticle (LNP) delivery system as an additional and potentially important contributor to myocardial inflammation. Here, we propose a multi-hit model that integrates LNP-driven mechanisms as a central pathogenic axis, initiated by the systemic distribution and accumulation of LNPs in the heart. While the mRNA payload is cleared within days, the synthetic ionizable lipids -ALC-0315 (BNT162b2) and SM-102 (mRNA-1273) persist significantly longer than the mRNA payload itself. These two lipids differ in biodegradability and pharmacokinetic distinctions, together with differences in lipid dose and formulation, they may contribute to the divergent myocarditis rates observed between the two vaccine products. In this suggesting review, the first hit" involves the disruption of myocardial energy metabolism by these lipids, which can integrate into cellular membranes and impair mitochondrial fatty acid oxidation. This is compounded by a second hit of direct innate immune activation, preclinical studies demonstrate that LNPs engage pattern-recognition receptors (PRRs) like toll-like receptors (TLRs) and the NLRP3 inflammasome, leading to the release of pro-inflammatory cytokines such as IL-1β and IL-18. Inflammation is then amplified via Damage-associated molecular patterns (DAMPs) released from stressed cardiomyocytes. The clinical outcome-ranging from self-limited mild myocarditis to fulminant disease with diverse histopathological patterns-is likely shaped by host susceptibility factors, including sex hormones, genetic predisposition, and prior immune priming, that modulate the intensity of this pathogenic cascade.