Macrophages eat cancer cells using their own calreticulin as a guide: Roles of TLR and Btk

Abstract

Macrophage-mediated programmed cell removal (PrCR) is an important mechanism of eliminating diseased and damaged cells before programmed cell death. The induction of PrCR by eat-me signals on tumor cells is countered by don’t-eat-me signals such as CD47, which binds macrophage signal-regulatory protein α to inhibit phagocytosis. Blockade of CD47 on tumor cells leads to phagocytosis by macrophages. Here we demonstrate that the activation of Toll-like receptor (TLR) signaling pathways in macrophages synergizes with blocking CD47 on tumor cells to enhance PrCR. Bruton’s tyrosine kinase (Btk) mediates TLR signaling in macrophages. Calreticulin, previously shown to be an eat-me signal on cancer cells, is activated in macrophages for secretion and cell-surface exposure by TLR and Btk to target cancer cells for phagocytosis, even if the cancer cells themselves do not express calreticulin.

How One Thing Led to Another

“…All MDS that transition to AML upregulate CD47, so we conceivably have a roadmap for the development of MDS and its progression to AML. We and others have recently addressed the biochemical pathway for secreted calreticulin, in our studies in macrophages that can deposit calrecticulin on the surface of cancer cells lacking a calreticulin message (234). In these macrophages, stimulation of TLR3, 4, or 7 leads to phosphorylation and activation of BTK, which in turn leads to the phosphorylation of endoplasmic reticulum (ER)-resident calreticulin (234).…”

“…Anti-CD47 antibodies that block CD47-Sirpα interactions cause the tumor cells to be phagocytosed, and killed intracellularly (232). Antibody-blocked CD47 + Jurkat lymphoma cells are phagocytosed, whereas most normal blood and tissue cells that are CD47 + are not, presumably because most normal cells lack programmed cell removal via "eat me" signals (233,234).…”

“…As revealed by Rachel Weissman-Tsukamoto, Sidd Jaiswal, and Mark Chao in the lab, the "eat me" signal that is used is tumor cell surface calreticulin, which binds to the prophagocytic macrophage receptor LRP1 (CD91) (233,234). Gardai, Henson, Zitvogel, and Kroemer, among many others, had shown that calreticulin is moved to the cell surface of normal or tumor cells that are inflamed, infected, or treated with certain cytotoxic drugs (235)(236)(237)(238)(239).…”

“…We and others have recently addressed the biochemical pathway for secreted calreticulin, in our studies in macrophages that can deposit calrecticulin on the surface of cancer cells lacking a calreticulin message (234). In these macrophages, stimulation of TLR3, 4, or 7 leads to phosphorylation and activation of BTK, which in turn leads to the phosphorylation of endoplasmic reticulum (ER)-resident calreticulin (234). This leads to its cleavage from its KDEL ER retention signal, entry into the secretion pathway, binding to CD91 on the macrophages and on an as yet undetermined tumor cell calreticulin binding entity (234).…”

“…In these macrophages, stimulation of TLR3, 4, or 7 leads to phosphorylation and activation of BTK, which in turn leads to the phosphorylation of endoplasmic reticulum (ER)-resident calreticulin (234). This leads to its cleavage from its KDEL ER retention signal, entry into the secretion pathway, binding to CD91 on the macrophages and on an as yet undetermined tumor cell calreticulin binding entity (234). Thus, macrophages can be the most important source of molecules that guide them to phagocytose target cells with calreticulin binding sites for phagocytosis of tumor cells and efferocytosis of dying cells.…”