Absence of a prolonged macrophage and B cell response inhibits heart regeneration in the Mexican cavefish.

A balanced immune response after cardiac injury is crucial to successful heart regeneration, but knowledge of what distinguishes a regenerative from a scarring response is still limited. The Mexican cavefish Astyanax mexicanus provides a unique comparative model to study heart regeneration and scarring within a single species. Astyanax surface-dwelling fish are capable of heart regeneration, whereas, like the human heart, their cave-dwelling Pachón counterparts form a permanent scar. Using single-cell transcriptomics and immune perturbations, we identify two distinct and largely sequential immune mechanisms driving regeneration in surface fish. First, late-present macrophages are actively phagocytic, and pharmacological inhibition of phagocytosis disrupts scar clearance and regeneration. Second, B cells recruited from 7 days post-injury promote cardiomyocyte cell progression into mitosis, and their inhibition with Acalabrutinib significantly impairs cardiomyocyte proliferation and regeneration. This B cell-dependent mechanism is conserved in zebrafish. Our findings reveal two independent late immune mechanisms distinguishing regenerative from non-regenerative cardiac repair, offering insights for potential therapeutic strategies to enhance heart regeneration in mammals.