TREM2 Regulates the Removal of Apoptotic Cells and Inflammatory Processes during the Progression of NAFLD

Abstract

Nonalcoholic fatty liver disease (NAFLD) is the most common liver pathology worldwide. In mice and humans, NAFLD progression is characterized by the appearance of TREM2-expressing macrophages in the liver. However, their mechanistic contributions to disease progression have not been completely elucidated. Here, we show that TREM2⁺ macrophages prevent the generation of a pro-inflammatory response elicited by LPS-laden lipoproteins <i>in vitro</i>. Further, <i>Trem2</i> expression regulates bone-marrow-derived macrophages (BMDMs) and Kupffer cell capacity to phagocyte apoptotic cells <i>in vitro</i>, which is dependent on CD14 activation. In line with this, loss of <i>Trem2</i> resulted in an increased pro-inflammatory response, which ultimately aggravated liver fibrosis in murine models of NAFLD. Similarly, in a human NAFLD cohort, plasma levels of TREM2 were increased and hepatic <i>TREM2</i> expression was correlated with higher levels of liver triglycerides and the acquisition of a fibrotic gene signature. Altogether, our results suggest that TREM2⁺ macrophages have a protective function during the progression of NAFLD, as they are involved in the processing of pro-inflammatory lipoproteins and phagocytosis of apoptotic cells and, thereby, are critical contributors for the re-establishment of liver homeostasis.