Upregulation of the complement anaphylatoxin receptor C5aR1 on platelets in patients with atherosclerotic disease and in ischemic tissue. (A) Dot plot depicting C5aR1 expression on murine platelets (right); the left side shows the dot plot obtained with an immunoglobulin G (IgG)-type matched isotype. The plot is representative of the analysis of 4 independent platelet samples. (B) Analyzing human platelets in whole blood by flow cytometry confirmed C5aR1 expression. Displayed is the staining with C5aR1 antibody (right), showing C5aR1⁺ platelets (CD41⁺CD45⁻C5aR1⁺) vs isotype control (left). (C) In patients with coronary artery disease, expression of C5aR1 and activated GPIIb/IIIa (activation-specific antibody PAC-1) on platelets was measured by flow cytometry. The correlation of C5aR1 with activated GPIIb/IIIa is depicted. Spearman rank coefficient, rs = 0.59, P < .001, n = 349 patients. (D) Baseline characteristics of the analyzed patients with coronary artery disease (CAD). (E) In patients with PAD, C5aR1 expression was decreased in asymptomatic disease compared with symptomatic patients and healthy controls. A total of 20 patients were included in the study, and 7 controls. Data are shown as the mean ± standard error of the mean (SEM) and are displayed as the fraction of C5aR1-expressing platelets (%), ∗P < .05 (F) Furthermore, we observed that the fraction of platelets expressing C5aR1 correlates with pain-free walking distance in patients with PAD. r = −0.5062, P < .05. (G) Baseline characteristics of the analyzed patients with PAD.