Myeloid MAS-driven macrophage efferocytosis promotes resolution in ischemia-stressed mouse and human livers

Liver ischemia-reperfusion injury (LIRI) is an inevitable detrimental event after liver transplantation. The MAS receptor plays a protective role in various diseases. However, the specific roles of MAS in myeloid cell innate immunity and the maintenance of hepatic tissue homeostasis remain unclear. Here, we showed that mice with systemic, Kupffer cell-specific, or myeloid cell-specific Mas1 deficiency were vulnerable to LIRI. Single-cell RNA sequencing, spatial transcriptomics, and intravital imaging revealed that myeloid deficiency of Mas1 resulted in impaired macrophage efferocytosis by down-regulating MER tyrosine kinase (MERTK), leading to the accumulation of aged neutrophils and exacerbation of inflammation and pathology. Mechanistic studies indicated that the MAS receptor regulated the Krüppel-like factor 4 (KLF4)/MERTK axis in macrophages via the protein kinase A (PKA)/cAMP response element-binding protein (CREB) signaling pathway. KLF4 directly bound to the promoter region of MERTK and transcriptionally promoted its expression in macrophages, leading to attenuation of the liver inflammatory response. Macrophage-specific knockout of KLF4 and MERTK in the mice also resulted in impaired macrophage efferocytosis with the accumulation of aged neutrophils. Macrophage-specific overexpression of KLF4 in vivo effectively reversed the phenotype exacerbated by myeloid Mas1 deficiency. In addition, we demonstrated that MAS⁺MERTK⁺ macrophages actively migrated toward aged neutrophils in ischemia-stressed human livers, thereby promptly clearing aged neutrophils. In summary, this study documented the regulatory function of the MAS/KLF4/MERTK axis in macrophage efferocytosis via PKA/CREB signaling. This axis may thus serve as a therapeutic target and checkpoint regulator of homeostasis in response to LIRI.