Elevated Plasma Levels and Platelet-Associated Expression of the Pro-Thrombotic and Pro-Inflammatory Protein, LIGHT (TNFSF14), In Sickle Cell Anemia (SCA)

LIGHT (TNFSF14; CD258), a recently-identified member of the TNF superfamily, is found associated with and produced from platelets, amongst other immune cells. Increased circulating levels of this protein have been observed in patients with myocardial infarction and acute atherothrombotic stroke and LIGHT has been proposed as a potential therapeutic target in atherosclerosis, due to its reported pro-thrombotic and pro-inflammatory effects upon human endothelial cells. This study evaluated whether production of LIGHT is altered in patients with sickle cell anemia (SCA), and the participation that the platelets (PLTs) may have in this production, since SCA is characterized by a significant chronic inflammation and endothelial activation that may initiate the vaso-occlusive process.

Soluble LIGHT (sLIGHT) was determined in PLT-free plasma (obtained by sequential centrifugations and ultrafiltration) from healthy control individuals (CON), SCA patients in steady state (SCA) and SCA patients on hydroxyurea therapy (SCAHU; 20–30 mg/kg/day HU) by ELISA. Expressions of PLT-membrane LIGHT and PLT activation markers were evaluated by flow cytometry using anti-CD258-PE, anti-CD62P-FITC or anti-PAC1-FITC. Subjects had not taken ASA during the previous 14 days.

sLIGHT was significantly elevated in the plasma of SCA and SCAHU individuals, compared to CONs (SCA; 35.4 ± 9.4 pg/ml; SCAHU, 37.3 ± 7.5 pg/ml; CON, 9.7 ± 1.5 pg/ml, n=27, 27, 19, resp.; P<0.01 for SCA/SCAHU, compared to CON; Kruskal-Wallis/Dunn's). Plasma sLIGHT in SCA/SCAHU individuals presented no correlation with hematological variables, such as PLT counts (r_s=-0.079, P=0.65) and fetal hemoglobin (r_s=0.107, P=0.51). In contrast, plasma sLIGHT levels in SCA patients presented an impressive correlation with plasma levels of CD40L, another important PLT-derived inflammatory protein (r_s=0.817, P<0.0001 for SCA group and r_s=0.651, P<0.0001 for SCA+SCAHU) and correlated with IL-8, an endothelium-derived inflammatory mediator (r_s=0.900, P<0.05 for SCA and r_s=0.455, P=0.06 for SCA+ SCAHU). Expression of LIGHT protein (CD258) was significantly higher on the membrane of PLTs from SCA and SCAHU individuals, compared to CON PLTs (SCA, 27.1 ± 4.5 %; SCAHU, 33.7 ± 5.8 %; CON, 5.5 ± 1.6 % positive PLTs, n=15, 20, 15, resp.; P<0.001 for SCA/SCAHU comp. CON). Notably, when PLTs were activated by incubation with ADP (20 μM, 30 min), PLTs from SCA/SCAHU individuals still expressed significantly more surface LIGHT than CON PLT (SCA, 41.8 ± 4.5 %; SCAHU, 41.3 ± 5.4 %; CON, 11.1 ± 2.3 % positive PLTs; n=15, 20, 14, resp. P<0.001 for SCA/SCAHU comp. CON); successful activation of PLTs from all groups was confirmed by increased PAC-1 (anti-activated α_IIbß₃ integrin) binding and increased P-selectin (CD62P, a PLT activation marker) expression (data not shown); furthermore LIGHT expression on ADP-activated cells correlated significantly with both PAC-1 binding (r_s=0.483, P=0.03) and P-selectin expression (r_s=0.502, P=0.02, n=20) on SCA PLTs. sLIGHT release from SCA PLTs during 90 min (37°C, 5% CO₂ in Krebs) was evaluated and demonstrated that PLTs of SCA individuals may be an important source of circulating sLIGHT, releasing 22.2 ± 4.7 pg LIGHT/10⁸ PLTs (n=10); release of sLIGHT from SCA PLT was significantly augmented by incubation with collagen (P<0.01), but not ADP (P>0.05); basal and collagen-stimulated sLIGHT release correlated significantly with CD40L release (r_p=0.654; r_p=0.764, resp. P<0.05).

The pro-inflammatory and atherogenic protein, LIGHT, is found significantly elevated in the plasma of SCA patients. The correlation of plasma LIGHT with IL-8 indicates that LIGHT may participate in, or reflect, endothelial activation, whilst the correlation of circulating LIGHT and PLT release of LIGHT with CD40L indicates that the production of these two proteins may be tightly coupled. LIGHT protein is highly expressed on the PLT surface in SCA and this expression appears to be associated with PLT activation, rather than PLT number. Interestingly, HU therapy was not associated with any significant alteration in circulating LIGHT, nor PLT surface LIGHT. Future investigations will determine the extent of the contribution of LIGHT to endothelial activation, inflammation and vaso-occlusion in SCA and whether this protein holds promise as a potential therapeutic target for the disease.

No relevant conflicts of interest to declare.