The Macrophage αMβ2 Integrin αM-Lectin Domain Mediates the Phagocytosis of Chilled Platelets.

αMβ2 integrin receptors on myeloid cells mediate the adhesion or uptake of diverse ligands, and particular sequences on this integrin’s alpha subunit are responsible for recognition specificity. Inhibition of neutrophil adhesion to platelet monolayers by αM-derived peptides has implicated an αM sequence (I-domain) that also binds iC3b, fibrinogen, ICAM-1 and other ligands. However, αMβ2 also recognizes βN-acetylglucosamine (βGlcNAc) residues on the platelet von Willebrand factor receptor (GPIbα) that are clustered after cooling, suggesting that the αM lectin domain is important for this recognition. The phagocytosis of chilled platelets can be reconstituted when CHO cells are transfected with αMβ2. Removal of the N-T portion of GPIbα, containing the exposed βGlcNAc residues, from the chilled platelet surface using the snake venom mocarhagin inhibits this phagocytosis, as does the addition of 1 mM β-methyl-N-acetyl-D-glucosamine. Replacement of the I domain, or the lectin domain of the αM-chain with the corresponding domain from the αx-chain (p150), reveals that the activity of the αMβ2 integrin toward chilled platelets resides within the lectin-domain, and does not require the I domain. Other evidences for this conclusion include: (1) Sf9 cells expressing αM lectin domain but not β2 bind 3-4 x more chilled platelets than control cells; βGlcNAc inhibits this binding. (2) Soluble recombinant αM lectin domain inhibits the phagocytosis of chilled platelets by αMβ2-expressing THP-1 cells while I-domain substrates (fibrinogen or LPM19c), recombinant I-domain protein, or a synthetic peptide that inhibits neutrophil adhesion to platelet monolayers, are without inhibitory effect. We conclude that chilled platelets are removed from blood by an interaction between βGlcNAc residues on clustered GPIbα and the lectin domain of αM-chain of the αMβ2 integrin. Since the phagocytosis and clearance of chilled platelets by hepatic macrophages has precluded refrigeration of platelets procured for transfusion, a more precise molecular definition of the recognition mechanism could inform strategies for its inhibition, thereby accommodating cold platelet storage.