Platelet-derived CXCL12 regulates monocyte function, survival, differentiation into macrophages and foam cells through differential involvement of CXCR4–CXCR7

Abstract

Platelets store and release CXCL12 (SDF-1), which governs differentiation of hematopoietic progenitors into either endothelial or macrophage-foam cells. CXCL12 ligates CXCR4 and CXCR7 and regulates monocyte/macrophage functions. This study deciphers the relative contribution of CXCR4–CXCR7 in mediating the effects of platelet-derived CXCL12 on monocyte function, survival, and differentiation. CXCL12 and macrophage migration inhibitory factor (MIF) that ligate CXCR4–CXCR7 induced a dynamic bidirectional trafficking of the receptors, causing CXCR4 internalization and CXCR7 externalization during chemotaxis, thereby influencing relative receptor availability, unlike MCP-1. In vivo we found enhanced accumulation of platelets and platelet-macrophage co-aggregates in peritoneal fluid following induction of peritonitis in mice. The relative surface expression of CXCL12, CXCR4, and CXCR7 among infiltrated monocytes was also enhanced as compared with peripheral blood. Platelet-derived CXCL12 from collagen-adherent platelets and recombinant CXCL12 induced monocyte chemotaxis specifically through CXCR4 engagement. Adhesion of monocytes to immobilized CXCL12 and CXCL12-enriched activated platelet surface under static and dynamic arterial flow conditions were mediated primarily through CXCR7 and were counter-regulated by neutralizing platelet-derived CXCL12. Monocytes and culture-derived-M1–M2 macrophages phagocytosed platelets, with the phagocytic potential of culture-derived-M1 macrophages higher than M2 involving CXCR4–CXCR7 participation. CXCR7 was the primary receptor in promoting monocyte survival as exerted by platelet-derived CXCL12 against BH3-mimetic induced apoptosis (phosphatidylserine exposure, caspase-3 activation, loss of mitochondrial transmembrane potential). In co-culture experiments with platelets, monocytes predominantly differentiated into CD163+ macrophages, which was attenuated upon CXCL12 neutralization and CXCR4/CXCR7 blocking antibodies. Moreover, OxLDL uptake by platelets induced platelet apoptosis, like other platelet agonists TRAP and collagen-related peptide (CRP). CXCL12 facilitated phagocytosis of apoptotic platelets by monocytes and M1–M2 macrophages, also promoted their differentiation into foam cells via CXCR4 and CXCR7. Thus, platelet-derived CXCL12 could regulate monocyte-macrophage functions through differential engagement of CXCR4 and CXCR7, indicating an important role in inflammation at site of platelet accumulation.

Platelets as a Novel Source of Pro-Inflammatory Chemokine CXCL14

“…The membrane was mounted on glass slides; the number of migrated cells was counted for each well in randomly selected microscopic fields at ×20 magnification. Additionally, the number of migrated monocytes in the lower chamber was ascertained by running each sample for 2 minutes on a flow cytometer [4] at same flow rate (low).…”

“…Platelet-derived chemokines regulate migration and differentiation of monocytes into macrophages/foam cells [3, 4]. Furthermore, platelet-derived chemokines such as CXCL12 regulate migration and recruitment of circulating endothelial progenitor cells [5-8].…”

“…Upon activation platelets release macrophage-migration inhibition factor (MIF) or stromal cell-derived factor-1 (SDF-1/CXCL12) [5, 22], engage a wide range of chemokine receptors like CXCR2, CXCR4, CXCR7 to impose both autocrine and paracrine effects on interacting cells in the platelet micro-environment [23]. Platelet-derived CXCL12 is a chemokine, which acts as a regulator of monocyte adhesion, survival, and migration of monocytes through its receptors CXCR4 and CXCR7 [4]. CXCL12 and CXCL14 are similar in structure and considered primordial chemokines due to the sequence conservation among different species [24, 25].…”

Platelet Interaction with Innate Immune Cells

“…This process is mediated by CXCL4, CXCL12, platelet-derived growth factor (PDGF) and phagocytosis of lipid-laden platelets, which have taken up modified LDL [22,28,95,96].…”

“…5-HT = serotonin; CCL3 = chemokine (C-C motif) ligand 3; CD16 = cluster of differentiation 16; CXCL2 = chemokine (C-X-C motif) ligand 2; LXA 4 = Lipoxin A4; MCP-1 = monocyte chemoattractant protein 1; MIP-1β = macrophage inflammatory protein 1β; MMP9 = matrix metalloproteinase-9; MPO = myeloperoxidase; ROS = reactive oxygen species. tor 1 (SDF-1/CXCL12) further increase monocyte recruitment via chemotaxis [28,29]. Platelets also indirectly promote leukocyte migration via activation of endothelial cells.…”