Irradiation of platelets with UV-B light exposes fibrinogen binding sites via an intracellular mechanism.

Previous studies have shown that ultraviolet irradiation (UVI) causes platelet aggregation. In the present study we exposed platelet suspensions to a relatively high dose of UV-B (8 J/cm2) under conditions comparable to those of UVI of platelet concentrates in order to obtain more insight into the UV-induced aggregation response and to evaluate the significance of this phenomenon for the clinical use of UV-irradiated platelet concentrates. This study provides evidence that UV-B induced aggregation is mediated by a Ca2(+)-dependent process of fibrinogen binding to an intact glycoprotein IIb-IIIa complex on platelet membranes. Although UV-induced platelet aggregation is independent of thromboxane A2 formation and ADP secretion, it requires metabolic energy, cytosolic Ca2+ and a low cyclic-AMP level. Thus, UV-B irradiation causes platelet aggregation by exposing fibrinogen binding sites via an intracellular mechanism. Since the amount of bound fibrinogen following UVI is relatively low (about 2,300 molecules platelet) and the binding remains reversible, its effect on platelet behaviour after transfusion may be minor.