Abstract TP351: Axl/Mer Receptor Tyrosine Kinase Mediates Erythrophagocytosis-Induced Macrophage Reparative Phenotype and Brain Recovery in Experimental Intracerebral Hemorrhage

Introduction: Local inflammation contributes to both brain injury and recovery after intracerebral hemorrhage (ICH). Our previous studies have shown brain-infiltrating macrophages (BIMs) aggravate early brain injury after ICH; however, BIMs increase scavenger receptor CD36 levels over time, and hematoma clearance is delayed in the absence of BIMs. The mechanism that mediates BIMs phenotypic change in the ICH brain is elusive. In this study, we delineate the dynamic transcriptome profile of BIMs after ICH and test potential mediator that might modulate BIMs polarity in ICH. Methods: Autologous blood injection ICH model and thrombin-treated bone marrow-derived macrophages (BMDM) were used to mimic ICH in vivo and in vitro . BIMs were isolated by FACS, and the 780 transcriptome of BIMs were determined using NanoString. Flow cytometry and RT-qPCR were performed to detect the frequency of phosphatidylserine-positive (eryptotic) RBCs and to assess BIMs phenotype in the perihematomal tissue. Erythrophagocytosis of eryptotic RBCs was identified by immunofluorescence and microscopy. Neurologic deficit was evaluated by cylinder test. Axl/Mer receptor tyrosine kinase double knockout (AM DKO) mice, AM DKO bone-marrow chimeras, and AM DKO BMDM were used to evaluate the function of Axl/Mer on macrophage phenotype and on brain recovery after ICH. Results: BIMs highly expressed proinflammatory transcripts such as cd86 , tlr2 , nlrp3 , and tnf at days 1 and 3 post-ICH; these were decreased at days 7 and 10. Transcripts relevant to efferocytosis ( axl ) and lysosome formation ( cd63 ) increased from days 3 to 10 post-ICH. At days 1 and 3, phosphatidylserine levels was increased on RBCs in the ICH brain. Engulfment of eryptotic RBCs reduced proinflammatory phenotype of BMDM. Thrombin-stimulated AM DKO BMDM had reduced erythrophagocytosis ability and increased tnf and il-6 gene expression. AM DKO mice and AM DKO chimeras had low CD36 and high MHC II levels on BIMs and had worse functional outcome after ICH. Conclusions: BIMs initially express proinflammatory phenotype and then switch to a reparative phenotype after ICH. Axl/Mer is involved in regulation of macrophage polarity through modulating erythrophagocytosis ability and contributes to ICH brain recovery.