Phosphatidylserine receptors: Role of CD36 in binding of anionic phospholipid vesicles to monocytic cells

Exposure of phosphatidylserine (PtdSer) has been implicated in the recognit ion and phagocytosis of senescent and apoptotic cells, and CD36 has been pr oposed as one receptor protein that recognizes PtdSer and other anionic pho spholipids. We investigated the binding of phospholipid vesicles to the mon ocytic leukemia cell lines THP-1 and J774A.1 with a flow cytometric assay; vesicles contained 50 mol% PtdSer, phosphatidylinositol (PtdIns), or phosph atidylglycerol (PtdGro), with the balance being phosphatidylcholine. Specif ic, high affinity binding was observed for vesicles containing PtdSer, PtdI ns, or PtdGro, Specificity of the assay was confirmed by control experiment s with erythrocytes, which showed minimal vesicle binding, and with annexin V, which blocked the binding of PtdSer, PtdGro, and PtdIns vesicles to the THP-1 cells. However, O-phospho-L-serine (to 1 mM) had no effect on the bi nding of PtdSer vesicles, indicating that high affinity binding requires a surface containing multiple phosphoserine groups rather than a single molec ule. A monoclonal antibody to CD36 blocked up to 60% of the specific bindin g of PtdSer vesicles but had minimal to no effect on the binding of PtdGro or PtdIns vesicles. This antibody also selectively inhibited the phagocytos is of PtdSer-containing vesicles as measured by fluorescence microscopy, in dicating that CD36 is functionally significant for phagocytosis of this ves icle type. In addition, collagen and thrombospondin, two other putative lig ands of CD36, were unable to inhibit the binding of PtdSer vesicles. We con clude that CD36 is the primary protein responsible for the high affinity bi nding of PtdSer vesicles to these monocyte like cells. In addition, CD36 ap pears to be specific for PtdSer among anionic phospholipids, and non-phosph olipid ligands of CD36 do not share binding sites with PtdSer on CD36.