UVB radiation exposes fibrinogen binding sites on platelets by activating protein kinase C via reactive oxygen species.

Previous studies have shown that ultraviolet B (UVB) radiation causes platelet aggregation by exposing fibrinogen binding sites via activation of an intracellular mechanism. In the present study we have further investigated the routes of platelet activation following UVB exposure. Evidence is provided that UVB radiation does not activate the platelets via the classical Phospholipase A2 and Phospholipase C routes. Despite this observation, UVB-induced fibrinogen binding was found to be correlated with a 40% increase in phosphorylated 47 kD protein. Both findings could be completely inhibited in the presence of staurosporine, a potent inhibitor of protein kinase C (PK-C). In efforts to explain the mechanism of PK-C activation by UV radiation we found that both UV-induced PK-C activation and platelet aggregation were significantly reduced in the presence of specific scavengers for reactive oxygen species including superoxide dismutase and catalase. We conclude that exposure of platelets to UVB radiation can activate PK-C via oxygen radicals, resulting in exposure of fibrinogen binding sites and subsequent platelet aggregation.

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.

Evaluation of a new UVB source for irradiation of platelet concentrates.

The application of ultraviolet B (UVB) radiation has been proposed as a new technology to decrease immunogenicity of leucocytes in platelet transfusions. UV radiation also induces platelet aggregation, which occurs most effectively at wavelengths between 240 and 280 nm and falls off sharply above 300 nm. In order to minimize the effects of UV energy on the platelets we evaluated in this study the expected benefit of a new narrow-band UVB source, emitting a narrow peak around 312 nm. Exposure of platelet or lymphocyte suspensions to this source induced in the platelets both aggregation and functional defects at a dose of 12 J/cm2 and in the lymphocytes inhibition of in vitro function at a dose of 2 J/cm2. A conventional UVB source, emitting a broad spectrum between 280 and 340 nm, was more deleterious for the cells and induced similar defects in the platelets at a dose of 6 J/cm2 and inhibition of lymphocyte function at a dose of 1 J/cm2. These data indicate no benefit for the new UVB source, since the ratio of the doses to induce platelet defects and inhibition of lymphocyte function is identical for each of the two sources. Absorption of UV energy by plasma and the plastic material of platelet containers is another criterium for selection of UV sources. In view of the better transmission characteristics of long wavelength UV energy we propose that there is a preference for the new narrow-band UVB source.

Filtration. A method to prepare white cell-poor platelet concentrates with optimal preservation of platelet viability.

HLA alloimmunization is a major problem for thrombocytopenic patients receiving long-term platelet support. It is caused by white cells (WBCs) that are present as contaminants in platelet concentrates (PCs). Recent data have shown that filtration is an effective means to reduce WBC contamination, but it has little effect on the recovery of platelets. The present report evaluates two filters, a cellulose acetate (CA) filter requiring the inactivation of platelets with prostacyclin and a cotton wool (CW) filter requiring no platelet inactivation. The results show that, using fresh pooled PCs from six random donors, both filters reduce WBC contamination below 10(7) per PC, the likely threshold below which alloimmunization does not develop. With platelets stored for 2 to 3 days the efficacy of the CW filter decreases. Neither filter inflicts important damage to the platelets, as there is no considerable platelet activation or cell disruption. Moreover, PCs prepared by both filters show normal survival and effectively reduce the bleeding times. Thus, filtration of PCs results in platelets with optimal responsiveness both in vitro and in vivo.