Neutrophils, platelets, and inflammatory pathways at the nexus of sickle cell disease pathophysiology

Abstract

Sickle cell disease (SCD) is a severe genetic blood disorder characterized by hemolytic anemia, episodic vaso-occlusion, and progressive organ damage. Current management of the disease remains symptomatic or preventative. Specific treatment targeting major complications such as vaso-occlusion is still lacking. Recent studies have identified various cellular and molecular factors that contribute to the pathophysiology of SCD. Here, we review the role of these elements and discuss the opportunities for therapeutic intervention.

Figure 1

Multicellular and multistep model of sickle cell vaso-occlusion. Sickle cell vaso-occlusion arises from a cascade of interactions among RBCs, neutrophils, and endothelial cells. Activation of endothelial cells leads to the recruitment of neutrophils, which is initiated by rolling of neutrophils on endothelial selectins, followed by adhesion mediated by integrins. Adherent neutrophils receive a secondary wave of signals transduced through E-selectin, leading to the activation of α_Mβ₂ (Mac-1) integrin on the leading edge. Activated Mac-1 on adherent neutrophils mediates the capture of circulating sickle RBCs, producing a temporary or prolonged obstruction of venular blood flow. Circulating neutrophils exhibit considerable heterogeneity in their proinflammatory properties. Signals derived from the microbiota drive the neutrophil aging in the circulation, generating an overly active aged subset that exhibits enhanced Mac-1 activation and NETs formation. Aged neutrophils play an important role in promoting sickle cell vaso-occlusion. Currently, whether NET formation plays a role in the vaso-occlusive process remains unclear. ESL-1, E-selectin ligand-1; MADCAM-1, mucosal vascular addressin cell adhesion molecule-1; PSGL-1, P-selectin glycoprotein ligand-1; VLA-4, very late antigen-4.

Figure 2

Inflammation in SCD. SCD has been recognized as a chronic inflammatory disease. Multiple cell types and molecules are involved in its inflammatory pathways. Hemolysis of RBCs leads to the release of heme into the circulation, which can activate endothelial cells through the TLR4 pathway and induce PlGF release from RBCs. PlGF activates monocytes, leading to the production of proinflammatory cytokines TNF-α and IL-1β. The adhesion of platelets or sickle RBCs can also activate endothelial cells, producing increased expression of adhesion molecules including ICAM-1, VCAM-1, P-selectin, and E-selectin, and thus promoting the recruitment of neutrophils. iNKT cells exhibit an activated phenotype and contribute to pulmonary dysfunction in SCD by producing IFN-γ and inducing CXCR3 chemokines. Platelets can form aggregates with circulating leukocytes, including neutrophils and monocytes. Activated neutrophils roll and adhere to the endothelium, and initiate VOC by capturing sickle RBCs. Heme can also induce NET formation, which promotes acute pulmonary injury in SCD. HMGB1, high mobility group box 1; iNKT cell, invariant natural killer T cell.