Type 2 innate immunity drives distinct neonatal immune profile conducive for heart regeneration.

Neonatal immunity is functionally immature and skewed towards a T2-driven, anti-inflammatory profile. This neonatal immunotolerance is partly driven by the type 2 cytokines: interleukin-4 (IL-4) and interleukin-13 (IL-13). Studies on neonatal cardiac regeneration reveal the beneficial role of an anti-inflammatory response in restoring cardiac function after injury. However, the role of an imbalanced immune repertoire observed in neonates on tissue regeneration is poorly understood; specifically, whether IL-4 and IL-13 actively modulate neonatal immunity during cardiac injury. Neonatal mice lacking IL-4 and IL-13 (DKOs) examined at 2 days after birth exhibited reduced anti-inflammatory immune populations with basal cardiac immune populations like adult mice. Examination of neonates lacking IL-4 and IL-13 at 2 days post cardiac ischemic injury, induced on the second day after birth, showed impaired cardiac function compared to their control counterparts. Treatment with either IL-4 or IL-13 cytokine during injury restored both cardiac function and immune population profiles in knockout mice. Examination of IL-4/IL-13 downstream pathways revealed the role of STAT6 in mediating the regenerative response in neonatal hearts. As IL-4/IL-13 drives polarization of alternatively activated macrophages, we also examined the role of IL-4/IL-13 signaling within the myeloid compartment during neonatal cardiac regeneration. Injury of IL-4Rα myeloid specific knockout neonates 2 days after birth revealed that loss of IL-4/IL-13 signaling in macrophages alone was sufficient to impair cardiac regeneration. Our results confirm that the T2 cytokines: IL-4 and IL-13, which skews neonatal immunity to a T2 profile, are necessary for maintaining and mediating an anti-inflammatory response in the neonatal heart, in part through the activation of alternatively activated macrophages, thereby permitting a niche conducive for regeneration.