The majority of the studies evaluated the direct effect of antiphospholipid antibodies isolated from APS patients in platelets obtained from healthy volunteers. The literature is scarce about platelet activity obtained from patients with primary antiphospholipid syndrome with thrombosis (t-PAPS).

To evaluate platelet function in platelet-rich plasma (PRP) obtained from patients with t-PAPS or healthy volunteers.

Forty patients with t-PAPS (62.5% females, mean age: 44 years) and 62 volunteers with no history of thrombosis (64.5% females, mean age: 38 years) were included. Firstly, PRP was obtained and stimulated with adenosine diphosphate (ADP, 3 or 10 μM), collagen (1 μg/mL) or arachidonic acid (AA, 300 μM). Next, PRP was pre-incubated with platelets inhibitors, as nitric oxide donor, sodium nitroprusside (SNP, 3 or 10 μM) or the stable analogue of prostacyclin, iloprost (ILO, 3 or 10 nM), and then stimulated with ADP 3 μM. In another set of experiment, ticagrelor, a P2Y12 receptor antagonist (2 μM) was also incubated in PRP. Second, washed platelet (WP) from t-PAPS patients was obtained for P2Y12 protein and cyclic nucleotides quantification. In another assay, WP from healthy volunteers was incubated with IgG antibodies (500 μg/mL, 30 min) isolated from the serum of healthy volunteers or t-PAPS patients and then stimulated with ADP 30 μM in the absence and in the presence of ticagrelor (2 μM).

ADP-induced platelet aggregation was significantly higher in t-PAPS group than in controls (ADP 3 μM: 69.3% ±23.8% vs 57.8% ± 24.3%, p = 0.02). No difference in AA- (52% ± 33.2% vs 56% ± 29.1%, P = 0.95) or collagen- (72% ± 19.3% vs 72.4% ± 17.3%, p = 0.51) -induced aggregation was seen between groups. The amplitude of platelet inhibition induced by SNP 3 μM (46.4% ±25.8% vs 34.7% ± 20.9%, p = 0.02) and 10 μM (31.4% ± 23.1% vs 18.8% ±14.9%, p = 0.008) or ILO 3 nM (23.3% ± 23.3% vs 12.1% ± 14%, p = 0.02) was less prominent in platelets from t-PAPS than in control volunteers. The intraplatelet levels of cAMP (6.3 ± 5.1 pmol/108 platelets vs 14.6 ± 8.1 pmol/108 platelets, p = 0.01) and cGMP (2.3 ± 1.7 pmol/108 platelets vs 4.6 ± 1.9 pmol/108 platelets, p = 0.01) was significant reduced in platelets from t-PAPS than in healthy volunteers. In platelets from t-PAPS patients the protein expression for the P2Y12 receptor was significantly greater than in platelets from healthy volunteers (0.81 ± 0.23 vs 0.53 ± 0.24, p = 0.01). The IgG antibodies obtained from t-PAPS patients produced greater amplitude of aggregation in ADP-stimulated platelet than in platelets stimulated with IgG antibodies obtained from healthy volunteers (66.2% ± 7.9% vs 52.2% ± 10.6%, P = 0.006). Ticagrelor practically abolished ADP-induced aggregation in both PRP from control and t-PAPS patients (12.9% ±6.7% vs 10.2% ± 6.4%, p = 0.09) and in WP treated with IgG antibodies (0.3% ±0.6% vs 0.0% ±0.0%, p = 0.4).

Platelet activity from t-PAPS patients was enhanced upon ADP stimulation, but not by collagen or AA, mainly due to higher protein expression for P2Y12 receptor subtype and lower intraplatelet levels of cAMP and cGMP.

Our findings suggest that ADP receptor antagonists can be considered as first-line therapy to treating patients with t-PAPS or in those who have contraindications to receiving aspirin.