Macrophages eat cancer cells using their own calreticulin as a guide: Roles of TLR and Btk
Significance Macrophage mediated programmed cell removal (PrCR) plays an essential role in tumor surveillance and elimination. Blockade of the don't eat me signal CD47 on tumor cells allows already expressed eat me signals to induce PrCR to eliminate tumor cells. To date the molecular mechanism by which macrophages recognize and phagocytose tumor cells remains unclear. This paper demonstrates that the activation of Toll like receptor (TLR) pathways in macrophages induces the phosphorylation of Bruton's tyrosine kinase (Btk) which catalyzes cell surface exposure of calreticulin. Calreticulin on or secreted by macrophages plays a critical role in mediating adjacent tumor cell recognition and phagocytosis. These findings reveal a strategy to enhance the efficacy of PrCR through a combination of TLR/Btk activation and CD47 blockade, and advance our understanding of the underlying mechanism of macrophage mediated PrCR of tumor cells. 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 a 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.
Proceedings of the National Academy of Sciences