Increased Platelet-Derived CD40L May Modulate Endothelial Cell ICAM-1 Expression and Interact With Leukocytes In Sickle Cell Anemia

Vaso-occlusive events are a major cause of morbidity in sickle cell anemia (SCA) and attributable to the abnormal adhesion of red cells and leukocytes to the endothelium. Platelets may contribute to the chronic inflammation and endothelial activation that initiates the vaso-occlusive process. We hypothesized that platelet-associated CD40 ligand (CD40L) may contribute to platelet-mediated inflammatory responses in SCA.

This study evaluated the platelet (PLT) release of CD40L, the expression of its receptor (CD40) on platelets, neutrophils, lymphocytes and monocytes of control individuals (CON) and SCA patients, and also the ability of platelet-derived CD40L to activate endothelial cells.

IL-8, soluble ICAM-1, VCAM-1 and CD40L were determined in PLT-free plasma or the supernatant of stimulated (ADP or Collagen) and unstimulated PLTs (2•10⁸/mL in Kreb’s buffer), from CON individuals and steady-state SCA patients, by ELISA. Flow cytometry was used to analyze CD40 expression on platelets, neutrophils, lymphocytes and monocytes from the peripheral blood of the study’s subjects. Human umbilical vein endothelial cells (HUVECs) were cultured (1x10⁶cells/well; 37°C, 5% CO₂) together with PLTs (3x10⁸PLTs/well) from CON individuals or steady-state SCA patients for 24h, 37°C, 5%CO₂, in the presence, or not, of blocking antibodies against CD40L. After incubation, PLTs were removed and HUVECs analyzed by flow cytometry for CD54 (ICAM-1) surface expression.

SCA individuals presented elevated levels of plasma CD40L (724.4± 55.7 pg/ml; n=90) compared to CON (241.5±34.6 pg/ml; n=41; P<0.0001) and these levels correlated with PLT counts (r_s=0.255; P=0.015). No correlation was found between plasma CD40L and plasma IL-8, ICAM-1 or VCAM-1. PLT release of CD40L (90 min, 37°C, 5%CO₂) was evaluated; PLTs of SCA patients released higher quantities of CD40L (8347±1464 pg/10⁸ PLTs; n=10) than PLTs of CON individuals (3652±568 pg/10⁸ PLTs; n=5; P=0.019). CD40L release from SCA PLTs was augmented by incubation with collagen (P<0.001), but not ADP. Expression of the CD40 receptor on the platelet surface was elevated in the SCA group (52.4±2.7% positive cells; n=23), compared to the CON group (36.8±3.7% positive cells; n=9; P=0.005). The surface expression of CD40 was also elevated on neutrophils (SCA, 10.4±1.5% positive cells, n=14; CON, 5.5±1.1% positive cells, n=13; P=0.03), lymphocytes (SCA, 8.3±0.8% positive cells, n=16; CON, 3.6±0.4% positive cells, n=14; P<0.001) and monocytes (SCA 69.6±5.9% positive cells, n=16; CON, 49.9±5.8% positive cells, n=14; P=0.03) of SCA patients, compared to controls. ICAM-1 expression on the surface of HUVECs (Basal expression 32.8±1.8%, n=11) was significantly increased following incubation with SCA PLTs (54.0±4.8%, n=11, p<0.0001) and slightly augmented after incubation with CON PLTs (40.8±3.1%, n=11, p<0.05; Repeated measures ANOVA). Interestingly, when HUVECs and SCA PLTs were incubated with a blocking antibody against CD40L, the increase in ICAM-1 expression was significantly reversed on HUVECs (HUVECs, 28.1±0.2%, n=6; HUVECs+SCA PLTs, 42.0±3.3%, n=6; HUVECs+SCA PLTs+anti-CD40L 28.9±1.5%, n=6; P<0.01).

Plasma levels and platelet release of CD40L were found to be significantly elevated in SCA, in association with increased expressions of the CD40 receptor on SCA PLTs, neutrophils, lymphocytes and monocytes, possibly indicating a CD40L-mediated crosstalk between platelets and leukocytes in SCA. Platelets from SCA patients can induce adhesion molecule expression on the surface of endothelial cells in vitro, and this up-regulation may be modulated by platelet-derived CD40L. Results suggest that the CD40/CD40L pathway may be altered in SCA and that platelets may participate in this up-regulation. Given the potent inflammatory effect of this cytokine, a role for platelets and this cytokine in endothelial activation, inflammation and consequent vaso-occlusion, is likely.

No relevant conflicts of interest to declare.