A requirement for membrane-associated phospholipase A2 in platelet cytotoxicity activated by receptors for immunoglobulin G and complement.

Abstract

SunliitlsryPlatelets are potent antibody-and complement-dependent cytotoxic effector cells. We showed previously that a single platelet can lyse a target cell sensitized with immunoglobulin G (IgG) and complement components up to C3 (C,,o3b denotes the target cell-bound fragment of complement up to C3; the precise nature of the bound C3 fragment has not been established), and that the complete cytotoxic system capable of specific recognition and lysis resides in platelet membranes. To define the components of platelet membranes required for cytotoxicity, a set of inhibitors of phospholipase A2 (PLA2) that act by different chemical mechanisms was tested. The lytic reaction is blocked at appropriate concentrations of bromophenacylbromide, mepacrine, and manoalide. When platelets are treated with bromophenacylbromide, inhibition of cytolytic activity and that of PLA2 enzymatic activity occur in parallel. Platelets release arachidonate when incubated with target cells bearing IgG and C~3b, confirming that FcyR and complement receptor trigger both PLA2 action and efficient lysis. Inhibition by thimerosal of a reverse reaction, i.e., reacylation catalyzed by acyltransferase, causes increased target cell lysis, presumably by increasing the products of PLA2 action. Platelet cytotoxicity is increased when platelets are pretreated with some products of PLA2: exogenous lysophospholipids and not free arachidonic acid increase cytotoxicity. Electron microscopy suggests that platelets and target ceils may fuse, possibly as a result of the formation of lysophospholipids which are well-known membrane fusogens. Fixation with paraformaldehyde does not affect platelet cytotoxicity, suggesting that the complete cytotoxic system resides as a preformed complex in platelet membranes. The results indicate that platelet membrane-associated PLAz, together with receptors for Fc and complement, are required for platelet cytotoxicity. lthough small in size and lacking nuclei, platelets are potent cytotoxic effector cells present in numbers and total surface area that are vast in comparison with other bloodborne immune effectors (1, 2). Both mouse and human platelets bear receptors for the constant domains of IgG molecules (Fc3'R) and CR (3-6). Platelets are antibody-dependent tumoricidal effectors of the immune system in vivo (7-9). They also have been shown to lyse IgE-coated schistosomulae (10) and Brugia malayi microfilariae (11). In the absence of specific antibody, platelets can lyse Toxoptasma gondii (12) and tumor cells (13) in vitro. In these antibody-independent processes, the activation of phospholipase A2 (PLA2) 1 and the involvement of arachidonic acid metabolites have been implicated.

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Phospholipase A2 from plasma of patients with septic shock is associated with high-density lipoproteins and C3 anaphylatoxin: some implications for its functional role

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