Putting the brakes on phagocytosis: "don't-eat-me" signaling in physiology and disease

Abstract

Timely removal of dying or pathogenic cells by phagocytes is essential to maintaining host homeostasis. Phagocytes execute the clearance process with high fidelity while sparing healthy neighboring cells, and this process is at least partially regulated by the balance of "eat-me" and "don't-eat-me" signals expressed on the surface of host cells. Upon contact, eat-me signals activate "pro-phagocytic" receptors expressed on the phagocyte membrane and signal to promote phagocytosis. Conversely, don't-eat-me signals engage "anti-phagocytic" receptors to suppress phagocytosis. We review the current knowledge of don't-eat-me signaling in normal physiology and disease contexts where aberrant don't-eat-me signaling contributes to pathology.

Keywords: ITIM; anti-phagocytic receptor; efferocytosis; phagocytosis; ‘don't-eat-me’.

Figure 1. Inhibition of phagocytosis by SIRP⍺

Engagement of the CD47‐SIRP⍺ axis occurs in cis and in trans, which induces tyrosine phosphorylation of the ITIMs in the cytoplasmic tail of SIRP⍺. Phosphorylation at Y429 and Y453 of human SIRPα mediate binding of tyrosine phosphatase SHP‐1 (Myers et al, 2020). Dephosphorylation of non‐muscle myosin IIA is one proposed substrate of SHP‐1, resulting in disassembly of the actomyosin cytoskeleton. (top) Viable cells express don't‐eat‐me signals such as CD47 and lack expression of eat‐me signals, thus limiting phagocytic clearance. (bottom‐left) Antibody‐dependent cellular phagocytosis (ADCP) is also negatively regulated by the CD47‐SIRPα axis (Zent & Elliott, 2017). (bottom‐right) Efferocytosis by alveolar macrophages in the lung can be negatively regulated by SIRPα activation in response to binding CD47 and surfactant proteins, SP‐A and SP‐D. The mechanism of this suppression is thought to involve activation of the GTPase RhoA (Janssen et al, 2008).

Figure 2. The structure of human and mouse anti‐phagocytic receptors

Several anti‐phagocytic receptors have been identified in humans and mice. These are single‐pass type I transmembrane receptors belonging to the immunoglobulin (Ig) superfamily and contain one or more immunoreceptor tyrosine‐based inhibitory motifs (ITIM) in their cytoplasmic tails. Many of these receptors also have additional binding sites for signaling molecules that can endow these receptors with activation functions.

Figure 3. Therapeutically blocking the CD47‐SIRP⍺ axis to enhance cancer cell phagocytosis

Many cancer cells have increased expression of CD47. Preclinical and clinical studies have shown promise in enhancing anti‐tumor immunity when anti‐CD47 blockade is used in combination with other immunotherapies such as the anti‐CD20 monoclonal antibody Rituximab. This strategy provides an eat‐me signal via activation of antibody‐dependent cellular phagocytosis (ADCP) while concurrently blocking the CD47‐SIRPα don't‐eat‐me signaling axis.

Figure 4. Impaired efferocytosis in cardiovascular disease

Several mechanisms contribute to impaired efferocytosis in cardiovascular diseases, such as atherosclerosis (Doran et al, 2019). Tumor necrosis factor (TNF) signaling through the TNF receptor activates the nuclear factor‐κB pathway leading to enhanced CD47 expression on apoptotic cells in atherosclerotic lesions. Enhanced expression of the “myocardial infarct‐associated transcript” (MIAT) long non‐coding RNA inhibits processing of the microRNA, miR‐149‐5p, and in turn, results in increased CD47 expression. CD47 can bind SIRPα in cis on the phagocyte surface to inhibit phagocytosis. Loss of pro‐phagocytic receptors on the phagocyte surface including LRP1 and MerTK contributes to impaired efferocytosis. Metalloproteinase ADAM17 cleaves MerTK, resulting in soluble Mer that may act to sequester growth arrest‐specific protein 6 (Gas6) needed for recognition of phosphatidylserine (PS) on the surface of apoptotic cells. Parts of this figure were adapted from Doran et al (2019).