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We previously reported that activated human platelets release small and large macromolecular activators of phagocytosis from platelets (S-MAPP and L-MAPP, respectively), which enhance Fcγ receptor-mediated phagocytosis by neutrophils. We subsequently investigated the production and release of S-MAPP and L-MAPP by platelets as well as the structure of MAPPs. Experiments using expired platelet concentrates revealed that precursors of MAPPs (dimer or tetramer holo-transferrin) were converted to MAPPs by HATKTAK cleaved from Apo CIII on HDL in or on platelets. Immunohistoche-mistry showed that a rabbit IgG antibody against HATKTAK visualized not only platelets in blood coagula and thrombi, but also the foamy macrophages and smooth muscle cells of atheroma, some endothelial cells of blood vessels, glomerular filtrate, tubular epithelial cells of kidneys, and some fibers and cell bodies of neurons. HATKTAK appears to be widely distributed in the human body and plays some roles in biological reactions other than lipid metabolism. We herein review the process of investigation and describe perspectives.
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