"Find Me" and "Eat Me" signals: tools to drive phagocytic processes for modulating antitumor immunity

Abstract

Phagocytosis, a vital defense mechanism, involves the recognition and elimination of foreign substances by cells. Phagocytes, such as neutrophils and macrophages, rapidly respond to invaders; macrophages are especially important in later stages of the immune response. They detect "find me" signals to locate apoptotic cells and migrate toward them. Apoptotic cells then send "eat me" signals that are recognized by phagocytes via specific receptors. "Find me" and "eat me" signals can be strategically harnessed to modulate antitumor immunity in support of cancer therapy. These signals, such as calreticulin and phosphatidylserine, mediate potent pro-phagocytic effects, thereby promoting the engulfment of dying cells or their remnants by macrophages, neutrophils, and dendritic cells and inducing tumor cell death. This review summarizes the phagocytic "find me" and "eat me" signals, including their concepts, signaling mechanisms, involved ligands, and functions. Furthermore, we delineate the relationships between "find me" and "eat me" signaling molecules and tumors, especially the roles of these molecules in tumor initiation, progression, diagnosis, and patient prognosis. The interplay of these signals with tumor biology is elucidated, and specific approaches to modulate "find me" and "eat me" signals and enhance antitumor immunity are explored. Additionally, novel therapeutic strategies that combine "find me" and "eat me" signals to better bridge innate and adaptive immunity in the treatment of cancer patients are discussed.

Keywords: CARL; CX3CL1; Fc; LPC; Phagocytosis; PtSer; SLAMF7; cancer immunotherapy; “Eat me” signal; “Find me” signal.

FIGURE 1

“Find me” and “eat me” signals and their receptors. The primary “find me” signals, including ATP, LPC, S1P, CX3CL1, RP S19, LPC, TryRS, and EMAPII, interact with receptors such as adenosine, G2A/GPR4, S1PR, CX3CR1, CD88, CXCR3, and CXCR1. Similarly, the main “eat me” signals, consisting of CALR, SLAMF7, Fc, and PtdSer, engage with receptors such as LRP1, SLAMF7, FcR, αVβ3, and BAI1/PSR1/TIM1/3/4. ADORA receptors, adenosine A receptors; ATP, adenosine triphosphate; BAI1, brain‐specific angiogenesis inhibitor 1; CALR, calreticulin; CX3CL1, C‐X3‐C motif chemokine ligand 1; CX3CR1, CX3C chemokine receptor1; CXCR1, CXC‐chemokinereceptor1; CXCR3, CX3C chemokine receptor 1; EMAPII, endothelial monocyte activating polypeptide II; G2A, G‐protein coupled receptor 2A; GPR4, G‐protein coupled receptor 4; LPC, lysophosphatidylcholine; LRP1, low‐density lipoprotein receptor‐related protein 1; MFG‐E8, milk fat globule‐epidermal growth factor 8; PSR1, phosphatidylserine receptor 1; PtdSer, phosphatidylserine; RP S19, ribosomal protein S19; S1P, Sphingosine‐1‐phosphate; S1PR, sphingosine‐1‐phosphate receptor; SLAMF7, signaling lymphocytic activation molecule family member 7; TIM1/3/4, T cell immunoglobulin and mucin domain 1/3/4; TryRS, tyrosyl tRNA synthetase; αVβ3, integrin alpha V beta 3.

FIGURE 2

“Find me” signals. The different “find me” signals released from apoptotic cells, their known or putative mechanisms of release, and the possible receptors on phagocytes that can regulate chemotaxis. Apoptotic cells release “find me” signals, such as nucleotides, LPC, S1P, CX3CL1, RP S19, TryRS, and EMAPII, to the extracellular space; these signals can interact with P2Y2, GPR4, S1PR1‐5, CD88, CXCR1/CXCR3, and CXCR3 on macrophages, respectively. Pannexin channels, activated by caspase‐3/7 during apoptosis, release “find me” signals like nucleotides. The released ATP induces phagocyte migration via P2Y2 receptors. ABCA1‐transported PLA2 transforms into sPLA2, hydrolyzing phospholipids to generate LPC. LPC binds GPR4, inducing macrophage migration. TG2 acts as a chemoattractant for macrophages by cross‐linking RP S19 monomers. CD88 senses RP S19, mediating monocyte migration. CX3CL1's release mechanism is unclear, but its chemotactic effect on phagocyte relies on CX3CR1. Post‐proteolysis, EMAPI and TyrRS exhibit chemotactic properties. EMAPI may result from caspase‐7 cleavage, while elastase from neutrophils generates TyrRS. TyrRS stimulates phagocyte migration through CXCR1 and CXCR3. EMAPII promotes endothelial progenitor cell migration via CXCR3, which is unrelated to apoptotic cell clearance. Intracellular S1P, synthesized by SphKs, is released through Mfsd2b. The extracellular SphK1 levels remain stable, indicating that mainly intracellular S1P is produced. During apoptosis, SphK2 can be secreted; this mechanism explains why extracellular S1P production primarily depends on SphK2. Released S1P can bind to S1PR1‐5. ABCA1, ATP‐binding cassette transporter A1; CD88, cluster of differentiation 88; CX3CL1, C‐X3‐C motif chemokine ligand 1; CX3CR1, CX3C chemokine receptor 1; CXCR1, C‐X‐C motif chemokine receptor 1; CXCR3, C‐X‐C motif chemokine receptor 3; EMAPII, endothelial monocyte‐activating polypeptide II; GPR4, G‐protein coupled receptor 4; iPLA2, independent phospholipase A2; LPC, lysophosphatidylcholine; P2Y2, P2Y purinoceptor 2; PANX1, pannexin‐1; PLA2, phospholipase A2; RP S19, ribosomal protein S19; S1P, sphingosine‐1‐phosphate; S1PR1‐5, sphingosine‐1‐phosphate receptors 1‐5; SphK1, sphingosine kinase 1; sPLA2, secretory phospholipase A2; TG2, transglutaminase 2; TyrRS, tyrosyl‐tRNA synthetase.

FIGURE 3

“Eat me” signals and downstream responses upon binding to phagocyte receptors. Stress‐induced and dying tumor cells expose CALR on the cell surface, which binds to LRP1 on phagocytes, possibly recruiting GULP1 to regulate phagocytosis. SLAMF7 on macrophages binds to MAC‐1, which, in turn, recruits FCRγ and DAP12, activating Syk and Btk kinases to promote phagocytosis. Macrophages express Fcγ receptors (FcγR‐IIb, FcγR‐I, FcγR‐IIIa, and FcγR‐IIa). Crosslinking these receptors with IgG complexes triggers ITAM phosphorylation, activating the Syk, Src, and Pkc pathways, and leading to actin remodeling, which is crucial for the phagocytosis of IgG immune complexes. BAI1, brain‐specific angiogenesis inhibitor 1; Btk, Bruton's tyrosine kinase; C1q/C3b/C4, complement components 1q/3b/4; CD300, cluster of differentiation 300; Cdc42, cell division control protein 42; CR1/3/4, complement receptors 1/3/4; DAP12, DNAX‐activating protein of 12 kDa; FcRγ, Fc receptor gamma chain; Gas6, growth arrest‐specific protein 6; GULP1, Engulfment adaptor PTB domain‐containing 1; GULP1, engulfment adaptor PTB domain‐containing 1; LRP1, low‐density lipoprotein receptor‐related protein 1; MAC‐1, macrophage‐1 antigen; MFG‐E8, milk fat globule epidermal growth factor 8; Pkc, protein kinase C; ProS, Protein S; PSR1, phosphatidylserine receptor 1; PtdSer, Phosphatidylserine; Rac1, Ras‐related C3 botulinum toxin substrate 1; RAGE, receptor for advanced glycation end products; RhoA, Ras homolog family member A; Sfk, Src family kinases; SLAMF7, signaling lymphocytic activation molecule family member 7; Src, Src kinase; Syk, spleen tyrosine kinase; TAM, Tryo3/Axl/Mer; TIM1/3/4, T cell immunoglobulin and mucin domain 1/3/4; Vav1, vav guanine nucleotide exchange factor 1; αVβ3/αVβ5, integrin alpha V beta 3/ alpha V beta 5.

FIGURE 4

Related drug types and specific examples that regulate “find me” and “eat me” signals to exert anti‐tumor immunity. These categories include chemotherapy, oncolytic peptides, small‐molecule drugs, liposomes, shRNAs/siRNAs, antibodies, enzymes, nanomedicines, radiotherapy, oncolytic viruses, and others. Chemotherapy drugs such as paclitaxel, etoposide, cisplatin, and gemcitabine inhibit cancer cell growth and division. Oncolytic peptides such as LTX‐315 and RT53 selectively target and kill cancer cells. Small‐molecule drugs such as osimertinib, nilotinib, gefitinib, and bortezomib target molecular pathways in cancer cells. Liposomal formulations, including CAR@aCD47/aPD‐L1‐SSL, serve as effective drug delivery systems. Nucleic acid‐based therapies like Xkr8 shRNA and TI589875 silence genes involved in cancer progression. Monoclonal antibodies such as elotuzumab, bavituximab, obinutuzumab, and cetuximab activate immune responses against tumors. Enzyme drugs such as RLH and ADA modulate immune responses and inhibit tumor growth. Nanoparticle‐based therapies like BiTNs and LNP‐Rep‐(IL‐12) deliver drugs specifically to cancer cells. Radiation therapy techniques like SBRT, 3D‐CRT, IMRT, and IGRT precisely target and destroy cancer cells. Abbreviations: 3D‐CRT, three‐dimensional conformal radiation therapy; ADA, adenosine deaminase; IGRT, image‐guided radiation therapy; IMRT, intensity‐modulated radiation therapy; RLH, RIG‐I‐like helicases; SBRT, stereotactic body radiation therapy.