**4.2 Eat-me signals**

By binding to their receptors on the surface of phagocytes, eat-me molecules produce intracellular signal cascades, rearranging the cytoskeleton and contributing to swelling. Eat-me signals are usually divided into two groups [33–35]. The first group consists of new emerging molecules, such as phosphatidylserine (PtdSer) or annexin I, on the surface of apoptotic cells. The second category consists of existing molecules changed on the surface of the apoptotic cells [e.g., conversion of the intercellular adhesion molecule (ICAM)-3 to CD31] or various modifications in the plasma membrane components (e.g., loss of plasma membrane asymmetry and increased external negative charges) (**Figure 2**). Among the new evolving molecules on the surface of apoptotic cells, PtdSer is the most popular and well-known eat-me signal. PtdSer is actually located naturally on the inner surface of the plasma


#### **Figure 2.**

*Apoptotic cell clearance-a multi-stage process. ATP-binding cassette transporter (ABCA1), complement component 1q (C1q), chemokine C-C motif ligand 5 (CCL5), C-C chemokine receptor type 2 (CCR2), chemokine C-X-C motif ligand (CXCL), erythropoietin (EPO), epiregulin (EREG), growth arrest-specific 6 (Gas6), heme oxygenase-1 (HO-1), interferon (IFN), interleukin (IL), Mer tyrosine kinase (MerTK), milk fat globule-EGF factor 8 protein (MFGE8), nitric oxide synthase 2 (NOS2), nuclear receptor subfamily 4 group A (Nr4a1), thrombospondin-1 (TSB-1), transforming growth factor* β *(TGF-*β*), tumor necrosis factor (TNF), vascular endothelial growth factor (VEGF).*

membrane, but is transmitted to apoptotic cells on the outer surface of the plasma membrane. Flippases are responsible for preserving an asymmetric distribution of phospholipids within the cell membrane in non-apoptotic cells. However, when the apoptosis message is given and caspases in apoptotic cells are activated, flippases become cleaved and inactive. Caspases also simultaneously activate scramblases in apoptotic cells and, as a result, PtdSer rapidly migrates to the outer surface of the plasma membrane [36–40].

PtdSer can bind directly to various surface receptors of phagocytes, including T-cell immunoglobulin mucin (TIM)-1, TIM-4, BAI-1, stabilin-2, and advanced glycation end-product receptors (RAGE). Among them, by binding to PtdSer, TIM-4 is crucial in the tethering of apoptotic cells. Of note, among all phagocytes, some of these receptors do not have the Same distribution [41–46]. Therefore, in the clearance of specific apoptotic cells, each phagocyte with a particular set of these receptors plays a certain function [32, 37–40, 47].
