Abstract Body: After myocardial infarction (MI), adult mammalian hearts fail to regenerate, and the cardiac microenvironment is irreversibly disrupted. In contrast, MI in neonatal mammals induces proliferation of existing CMs, resulting in near-complete cardiac renewal with minimal fibrosis. We found that C3, the central component of the innate immune complement pathway, is increased in epicardial cells (EpiCs) and cardiac fibroblasts (CFs) after neonatal MI. We used single-cell RNA-sequencing combined with spatial transcriptomics to examine the cellular relationships within the microenvironment of regenerating neonatal murine hearts. We conditionally knocked out C3 in EpiCs and CFs of murine hearts to determine the role of C3 in neonatal cardiac renewal. Loss of C3 in EpiCs and CFs resulted in a metabolic shift towards oxidative phosphorylation, aberrant activation of interferon-mediated signaling, increased fibrosis, and impaired cardiac function after neonatal MI. These results indicate that C3 and the complement pathway are required for conditional a pro-renewal immunosuppressive microenvironment during cardiac renewal.