*Purpose: Ischemia reperfusion injury (IRI) remains an unresolved clinical problem. A sterile inflammatory response drives the disease pathogenesis. Although mechanisms of liver innate immune activation by IR have been studied extensively, the contextual impact of tissue injury/cell death on the local inflammatory response has not been fully appreciated.

*Methods: In a murine liver partial warm ischemia (90m) model, we dissected functional roles of efferocytosis receptor Axl and MerTK in the activation of liver IRI using myeloid-specific (Lyz-Cre) gene knock-out (KO) mice. The differential regulation of liver resident (Kupffer cells KCs) vs. infiltrating macrophages by these receptor kinases was studied by selective depletion and reconstitution approaches. Liver IRI was evaluated at 6h post reperfusion.

*Results: Both Myeloid Axl or MerTK single deficiency increased hepatocellular damages and enhanced pro-inflammatory activation in IR-livers. The Axl and MerTK double deficiency resulted in the most severe liver IRI than any single gene deficiency. KC depletion by clodronate liposomes (CL, 200ul at 48h prior to the onset of liver ischemia) aggravated liver IRI in WT mice. In the Axl/MerTK double KO mice, however, the same CL treatment alleviated liver IRI, diminishing the differences between the KO and WT mice in their liver inflammatory responses. The reconstitution of CL-treated WT mice with KCs or of diphtheria toxin-treated CD11b-DTR mice with bone marrow-derived macrophages (WT or KO types, 2 millions/i.v., at 0h post reperfusion) revealed that Axl and MerTK regulated selectively KCs but not BMMs in the activation of liver IRI. Mechanistically, both local IL-13 gene expression (in IR-livers by qRT-PCR) and IL-13 cytokine levels in serum (by ELISA) were significantly lower in the double KO vs. control mice post IR. In vitro, Axl/MerTK activation by either apoptotic cells or MerTK agonist Abs increased IL-13 production by TLR4-stimulated KCs. Axl/MerTK deficient KCs were unable to produce IL-13 in response to these stimulations. Administration of exogenous IL-13 (1ug at 0h post reperfusion, i.v.) mitigated liver IRI in these KO mice.

*Conclusions: These results indicate that both Axl and MerTK are critical for KC homeostatic functions in liver IRI by upregulating IL-13 production.

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