Inflammation resolution is a fundamental process orchestrated by the host in response to tissue injury and pathogenic insults, aimed at attenuating inflammation and facilitating tissue repair to restore homeostasis across various organisms. This intricate biological phenomenon involves a cascade of cellular and molecular events[1]. Concurrently, programmed cell death, particularly apoptosis, plays indispensable roles in various physiological processes and our healthy living, including embryogenesis and nervous system development, but died cells are hardly observable[2], [3]. To maintain the homeostasis, cell death must be controlled; the efficient clearance of apoptotic cells(ACs), termed efferocytosis, is indispensable to prevent the accumulation of apoptotic debris and the initiation of diverse pathological conditions[4]. Failure of defects in the disposal of ACs, lead to the accumulation of billions of cells in numerous tissues, inducing autoimmune diseases, infections, cancer and other diseases[5], [6]. Uncleared ACs can undergo secondary necrosis exacerbating inflammation and tissue damage, activating Fc receptor (FcR) and Toll-like receptors (TLRs) signaling pathway[3], [7].

Efferocytosis is a multistep and finely regulated process involving the recognition, engulfment, and digestion of ACs by phagocytes, primarily macrophages[6], [8]. During these processes, the first step is critical, i.e. macrophages “sense” the ACs. ACs release manifold ‘find-me’ signals including the redistribution of phosphatidylserine (PS) to the outer leaflet of the plasma membrane and danger-associated molecular patterns (DAMPs)[8], [9], [10]. Recent research has identified several receptors on macrophages involved in the recognition of PS, including Bai1, MerTk, Tim1, and Tim4. These receptors play crucial roles in mediating the efficient recognition and clearance of ACs by macrophages, thereby preventing the development of inflammation and tissue injury[11]. Our studies determined the treatment with LPS upregulates the expression of Tim4 in the macrophages, boosting the binding with ACs.

The interplay between innate and adaptive immunity is essential for effective immune responses, with TLRs playing a central role in both arms of the immune system[12], [13], [14]. TLRs recognize one or more pathogen-associated molecular patterns (PAMPs) and DAMPs initiating inflammatory responses[15], [16], and are highly expressed in macrophages and dendritic cells[17], [18]. LPS, a key component of the outer membrane in Gram-negative bacteria (GNB), interacted with Toll-like receptor 4 (TLR4) to form an LPS signaling complex. This complex triggers myeloid differentiation primary response 88 (MYD88)-dependent and −independent pathways and subsequent activation of the nuclear factor kappa B (NF-κB) or interferon regulatory factor (IRF) transcription factors. Consequently, the activation of these pathways leads to the synthesis of proinflammatory cytokines, including interleukin 1β (IL-1β), IL-6, and tumor necrosis factor-alpha (TNF-α), thereby instigating an inflammatory cascade[19], [20], [21]. It has been reported that TNF-α signaling reduces the expression of MFGE8, MerTK, and LRP1 by activating TLRs in atherosclerosis. It also increases the expression of CD47, which is a 'don't eat me' signal that inhibits phagocytosis and clearance of ACs by phagocytes. This exacerbates inflammation and creates a vicious circle[6]. Precious study also demonstrated that activation of macrophages with Poly(I:C) (TLR3 ligand) enhances efferocytosis of macrophages[22]. Thus, we aimed to investigate whether LPS influences efferocytosis, thereby impacting the inflammatory response.

In this study, we investigated the influence of TLR ligands on efferocytosis. Remarkably, pretreatment with LPS enhanced macrophage phagocytosis ability. Further exploration revealed that LPS accelerated macrophage efferocytosis during resolution responses, both in vitro and in vivo. This study unveiled that LPS modulates efferocytosis by regulating the expression of cell membrane receptor Tim4, thereby mitigating tissue damage. These findings offer valuable insights into potential therapeutic strategies for resolving inflammatory conditions.