Innate immune cells, major protagonists of sickle cell disease pathophysiology

Abstract

Sickle cell disease (SCD), considered the most common monogenic disease worldwide, is a severe hemoglobin disorder. Although the genetic and molecular bases have long been characterized, the pathophysiology remains incompletely elucidated and therapeutic options are limited. It has been increasingly suggested that innate immune cells, including monocytes, neutrophils, invariant natural killer T cells, platelets and mast cells, have a role in promoting inflammation, adhesion and pain in SCD. Here we provide a thorough review of the involvement of these novel, major protagonists in SCD pathophysiology, highlighting recent evidence for innovative therapeutic perspectives.

Figure 1.

Monocytes in sickle cell disease. Interaction of sickle red blood cells (RBC) with endothelial cells enhances cellular oxidant stress, resulting in increased transendothelial migration of blood monocytes. Suspected mechanisms of monocyte activation in sickle cell disease (SCD) involve hypoxemia, platelet-monocyte aggregates mediated by P-selectin/P-selectin glycoprotein ligand 1 interaction, RBC/reticulocyte-monocyte interactions, placental growth factor released from RBC and co-stimulation of toll-like receptor 4 by heme and lipopolysaccharide. SCD activated monocytes display increased expression of CD1, CD11b and tissue factor on their surfaces, as well as increased production of interleukin-1β and tumor necrosis factor-α. In turn, activated monocytes activate endothelial cells through the nuclear factor-κB pathway, resulting in enhanced expression of intercellular adhesion molecule 1, vascular cell adhesion molecule 1 and E-selectin. Patrolling monocytes uptake cellular debris derived from heme-exposed endothelial cells, thus leading to high expression of heme oxygenase-1. Patrolling monocytes also scavenge endothelium-adherent sickle RBC. HO-1: heme oxygenase-1; ICAM-1: intercellular adhesion molecule 1: IL1-β: interleukin 1 beta; LPS: lipopolysaccharide; Lu/BCAM: Lutheran/basal cell adhesion molecule; NF-κB: nuclear factor kappa B; PlGF: placental growth factor; PSGL-1: P-selectin glycoprotein ligand 1; TF: tissue factor; TLR4: toll-like receptor 4; TNF-α: tumor necrosis factor alpha; VCAM-1: vascular cell adhesion molecule 1.

Figure 2.

Neutrophils in sickle cell disease. Neutrophils rolling on E-selectin under shear stress promote catch-bond formation between E-selectin and L-selectin via sLe^x expressed on L-selectin. This interaction triggers activation of high-affinity β₂-integrins, which leads to shear-resistant bonds with intercellular adhesion molecule 1. Neutrophil adhesion also occurs through interactions between P-selectin glycoprotein ligand 1 (PSGL-1) and endothelial P-selectin, which is upregulated in response to toll-like receptor 4 activation by heme released from red blood cells (RBC). E-selectin induces a secondary wave of activating signals, which leads to the clustering of activated macrophage-1 antigen (Mac-1) on the leading edge of adherent neutrophils, allowing for the capture of sickle RBC. Endothelin-1 promotes Mac-1 expression through the endothelin B receptors on neutrophils. Activated platelets express P-selectin, which binds PSGL-1, thereby enhancing the formation of platelet-neutrophil aggregates. The platelet-neutrophil association is also mediated by interactions between glycoprotein Ibα and Mac-1, which is positively regulated by AKT2 in neutrophils during vascular inflammation. Neutrophil aging is promoted by the microbiota via TLR/Myd88 signaling and is positively correlated with Mac-1 expression and neutrophil extracellular trap (NET) formation. Heme also promotes NET formation, possibly via generation of reactive oxygen species in neutrophils, and induces expression of heme oxygenase-1 during neutrophilic differentiation, thereby impairing the bactericidal oxidative burst. ESL-1: E-selectin ligand-1; ET: endothelin; GP: glycoprotein; HO-1: heme oxygenase-1; ICAM-1: intercellular adhesion molecule 1; ROS: reactive oxygen species; TLR: toll-like receptor.

Figure 3.

Mast cells in sickle cell disease. Histamine released from mast cells (MC) stimulates endothelial H₂ and H₄ receptors, thereby inducing the release of von Willebrand factor and expression of P-selectin. Tryptase released from MC activates protease-activated receptor 2 on peripheral nerve endings, thus contributing to nociceptor sensitization and stimulating the release of substance P (SP). SP released from MC and from sensory nerve endings increases plasma extravasation via neurokinin 1 receptor (NK1R) and promotes neurogenic inflammation. SP also acts on MC via NK1R and MAS-related G-protein-coupled receptor X2 (MRGPRX2), thus inducing more SP release in an amplification loop of MC activation. MRGPRX2 stimulation by SP induces the release of several cytokines and chemokines, which promotes immune cell recruitment. MC degranulation in response to morphine is also mediated by MRGPRX2. Hemolysis in sickle cell disease (SCD) may contribute to MC activation because it is responsible for nitric oxide depletion, which is known to activate MC. MC activation appears to contribute to endothelial dysfunction in SCD, via endoplasmic reticulum stress-mediated P-selectin expression and increased endothelial permeability. NO: nitric oxide; PAR-2: protease-activated receptor 2; RBC: red blood cell; VWF: von Willebrand factor.

Figure 4.

Main roles of innate immune cells in sickle cell disease. Innate immune cells promote inflammation, adhesion and pain in sickle cell disease (SCD). Monocytes may be activated by several mechanisms, including interactions with red blood cells (RBC), reticulocytes and platelets, as well as toll-like receptor 4 (TLR4) stimulation by heme and lipopolysaccharide. Activated monocytes produce pro-inflammatory cytokines, such as interleukin (IL)-1β and tumor necrosis factor-α, which activate endothelial cells, resulting in enhanced expression of adhesion molecules. The interaction of sickle RBC with endothelial cells induces cellular oxidant stress, which leads to transendothelial migration of blood monocytes. Patrolling monocytes scavenge endothelium-adherent sickle RBC and take up cellular debris derived from heme-exposed endothelial cells, thereby leading to high expression of heme oxygenase-1. Macrophages may be activated by heme, released from RBC in SCD, resulting in increased production of pro-inflammatory cytokines, especially IL-1β, through activation of the NLRP3 inflammasome. Elevated plasma levels of sphin-gosine-1-phosphate (S1P) induce IL-6 expression via S1P receptor 1 (S1PR1), and IL-6 in turn promotes S1PR1 expression by macrophages, leading to a vicious cycle of chronic inflammation. Neutrophil adhesion involves endothelial P-selectin, which is upregulated in response to TLR4 activation by heme in SCD. Heme may also promote neutrophil extracellular trap formation via the generation of reactive oxygen species (ROS) in neutrophils. Endothelial E-selectin induces the clustering of macrophage-1 antigen (Mac-1) on the leading edge of adherent neutrophils, allowing for the capture of sickle RBC. P-selectin is also expressed by platelets, which promotes the formation of platelet-neutrophil aggregates. The platelet NLRP3 inflammasome is activated in SCD, via HMGB1/TLR4 and Bruton tyrosine kinase, leading to enhanced production of pro-inflammatory cytokines, including IL-1β. Increased platelet activation may also result from depletion of nitric oxide (NO), secondary to the release of free hemoglobin from RBC. Activated platelets release soluble CD40L and thrombospondin, which binds CD36 on both endothelial cells and RBC, thereby promoting RBC adhesion to microvascular endothelium. Mast cell activation in SCD may be mediated by ischemia-reperfusion, hemolysis with NO depletion, and chronic inflammation. Substance P released from mast cells promotes neurogenic inflammation and pain, but it also activates mast cells themselves via neurokinin 1 receptor and MAS-related G-protein-coupled receptor X2, thereby inducing a vicious cycle of substance P release as well as the release of pro-inflammatory cytokines and chemokines, which promotes immune cell recruitment. Histamine released from mast cells also induces plasma extravasation and endothelial P-selectin expression. Invariant natural killer T-cell activation in SCD is associated with increased interferon-γ production and A_2A receptor expression. Adhesion of circulating eosinophils to fibronectin in SCD is mediated by several integrins, including Mac-1, and activated eosinophils are responsible for enhanced ROS production. A_2AR: adenosine A_2A receptor; BTK: Bruton tyrosine kinase; Hb: hemoglobin; HO-1: heme oxygenase-1; IFN: interferon; iNKT cell: invariant natural killer T cell; LPS: lipopolysaccharide; MRGPRX2: MAS-related G-protein-coupled receptor X2; NET: neutrophil extracellular traps; NK1R: neurokinin 1 receptor; PlGF: placental growth factor; TLR: toll-like receptor 4; TNF-α: tumor necrosis factor alpha.