Anti-CD9 monoclonal antibodies (MoAbs) are reported to activate human platelets through stimulation of the FcγII receptor. We show here that nonstimulatory F(ab′)2 fragments of the anti-CD9 MoAb 50H.19 induce dense-granule release and dose-dependent platelet aggregation when attached to polystyrene latex beads. Cross-linking F(ab′)2 fragments of MoAb 50H. 19 by F(ab′)2 fragments of goat anti-mouse IgG does not result in platelet aggregation unless the second antibody is bound to latex beads, indicating that immobilization, and not cross-linking of the stimulus, is critical to the initiation of the CD9 signal. In contrast, F(ab′)2 fragments of the second antibody readily induce the aggrega-tion of platelets treated with the anti-FcγII receptor MoAb IV.3. Immobilization of MoAb per se is insufficient to induce an activation signal because intact and F(ab′)2 fragments of nonstimulatory MoAb directed to glycoprotein lb and HLA class I do not become stimulatory when attached to beads. CD9-induced activation requires cytoskeletal rearrangement because it is inhibited by cytochalasin B. Aggregation is blocked by inhibitors of the thromboxane pathway, indicating that CD9 activates phospholipase C indirectly through prior activation of phospholipase A2.
|Original language||English (US)|
|Number of pages||7|
|State||Published - Oct 1 1991|
ASJC Scopus subject areas
- Cell Biology