Platelet Activation Promotes Formation of Platelet-Leukocyte Conjugates (PLC) in Non-Valvular Atrial Fibrillation (AF).

Background: Atrial fibrillation (AF) is associated with thromboemboli, which is due to blood stasis in the left atrium and hypercoagulability. Several studies have shown that both platelet and leukocyte activation are increased in AF. Formation of platelet-leukocyte conjugates (PLC) is an important mechanism by which leukocytes contribute to thrombosis. Increased levels of P selectin expression in platelets and platelet-leukocyte conjugates (PLC) have been correlated with the risk of cerebral infarction in AF (

Methods.

Patient population: Fourteen patients with chronic AF (8 men and 6 women; mean age = 62±9 years) were studied. AF was documented by a 12-lead electrocardiogram and all patients underwent an echocardiogram to assess cardiac function and rule out valvular AF. The mean left ventricular ejection fraction was 49±16%. Comorbidities included hypertension (85%), diabetes mellitus (21%), coronary artery disease (35%) and congestive heart failure (28%). The mean heart rate was 80±13 bpm. The mean BP was 127±9 / 75±6 mmHg. All patients were anticoagulated with warfarin and 78% were taking aspirin. Since we have previously shown that digitalis use is associated with platelet activation in this population, patients taking digitalis were excluded.

Laboratory methods: Venous blood from patients with AF and from healthy volunteers was examined by flow cytometry using fluorescent mAbs. We measured platelet expression of P selectin and leukocyte expression of CD11b. Coexpression of CD41 and CD45 was used to measure PLC.

Results: Patients with AF had markedly increased levels of P selectin (22.1 vs 5.0 fluorescence units; p=0.008), PLC (61.2±13.6% vs 43.1±21.6%; p=0.001) and CD11b expression in leukocytes (16.4 vs. 7.9 fluorescence units; p=0.0003). PLC strongly correlated with P selectin expression (R= 0.80; R²=0.64; p=0.0006) but not with leukocyte activation as measured by CD11b expression (R=0.41; R²= 0.17; p=0.14).

Conclusion: Our findings indicate that (1) Patients with AF demonstrate increased levels of platelet and leukocyte activation; (2) Increased PLC formation occurs in AF; (3) Levels of PLC closely correlate with platelet activation but not with leukocyte activation, suggesting than PLC formation is driven by platelet activation rather than by leukocyte activation in this population. Aspirin has been shown to be only marginally effective to prevent thromboembolism in AF. Clopidogrel, but not aspirin, has been shown to decrease the formation of PLC (