**4. Etiology and mechanisms of stroke in APS**

#### **4.1 Pathophysiology of stroke in APS**

Vascular thrombosis in APS can affect a wide variety of organ systems, but cerebrovascular thrombosis leading to stroke and transient ischemic attack is the most prevalent and perhaps the most consequential arterial event [61]. In a retrospective study of 135 APS patients, the highest morbidity was linked to neurologic involvement especially due to arterial thrombosis [62]. APS is also an important cause of stroke in the young, but as described can also affect older individuals [60]. The mechanisms of stroke in APS are diverse and include thrombosis in arteries, veins, and the microvasculature, as well as cardioembolism from non-bacterial thrombotic endocarditis.

The pathophysiology of vascular thrombosis in APS is not completely understood, but several studies suggest multiple converging pathways involving not only antibodies but also endothelial cells, platelets, monocytes, coagulation cascade proteins, and complements [63] producing a systemic thrombo-inflammatory state. The presence of aPL is not the sole cause for the significant clinical manifestations of APS as there can be asymptomatic "carriers" [17, 60]. Therefore, as previously mentioned, a "two-hit" hypothesis has been theorized, where the first-hit involves the presence of circulating aPL and associated endothelial dysfunction, and the second-hit presents an inflammatory insult such as trauma, surgery, or infection, leading to upregulation of β2GPI receptors on endothelial cells, as schematically demonstrated in **Figure 2**.

Even though aPL can be detected either by clotting tests, such as LA, or by an ELISA, such as aCL and anti-β2GPI, they are predominantly directed against β2GPI [17] and prothrombin [64]. Other important antigens recognized by aPL are annexin V, phosphatidylethanolamine, and phosphatidylserine [65]. Mechanistically these autoantibodies target phospholipid-binding plasma proteins bound to the surface of vascular endothelial cells and thrombocytes [60]. Plasma proteins predominantly bind to phosphatidylserine [17]. Normally located in the inner surface of cell membranes, phosphatidylserine becomes externalized when endothelial cells, platelets, and monocytes are activated. The avidity with which β2GPI binds to *Antiphospholipid Syndrome and Stroke DOI: http://dx.doi.org/10.5772/intechopen.101777*

#### **Figure 2.**

*The pathophysiology of vascular thrombosis in APS is not completely understood, but a 2-hit hypothesis is widely proposed. The first hit involves the presence of circulating aPL and endothelial injury, while the second hit requires an inflammatory insult such as trauma, surgery, or infection, leading to upregulation of beta-2 glycoprotein 1 (β2-GP1) receptors on endothelial cells. The aPLs-β2-GP1 receptor interaction unleashes multiple converging downstream pathways culminating in a thrombo-inflammatory state. VEGF: vascular endothelial growth factor; neutrophil extracellular traps (NETosis); GP: glycoprotein; TF: tissue factor (adapted [64, 66, 67]).*

phosphatidylserine is further enhanced by the 'β2GPI'- 'β2GPI antibody dimerization' [66]. The downstream effect of β2GPI antibodies on endothelial cells and monocytes includes increased expression of tissue factor and thromboxane A2 which trigger the extrinsic coagulation pathway [64, 67]. Furthermore, the antibody binding inhibits the tissue factor pathway inhibitor and protein C activity [64, 67]. Taken together, the net effect is the synergistic production of a prothrombotic state. Endothelial cells, upon stimulation with aPL, also downregulate their nitric oxide production and increase the surface expression of adhesion molecules such as E-selectin leading to pro-inflammatory and procoagulation endothelial phenotype [17, 57, 67, 68]. This antibody-induced endothelial injury can lead to intimal hyperplasia, micro-vasculopathy, and accelerated atherosclerosis [69]. Activated platelets increase their surface expression of GPIIb-IIIa, synthesis of thromboxane A2 and platelet factor-4a, all acting to facilitate thrombosis [67]. Activation of neutrophils with accompanying release of Neutrophil Extracellular Traps (NETosis) and IL-8 may also play a role [67]. Annexin V, a natural anticoagulant, binds to phosphatidylserine (a procoagulant) forming an anticoagulant shield in the physiologic state in APS, this shield is disrupted tipping the system in favor of coagulation [70]. Upregulation in the mTOR (mechanistic target of rapamycin) pathway on endothelial cells may partly explain the microvascular thrombosis seen in APS.

In addition to vascular thrombosis, up to one-third of patients with APS develop non-bacterial thrombotic endocarditis (NBTE) in which there is a deposition of sterile platelet thrombi on heart valves, particularly the mitral and aortic valves, which can be a source of cardioembolic strokes [66].

#### **4.2 Genetic considerations**

Population and family studies, as well as animal studies, have suggested genetic disposition may be relevant to the development of APS. Like many autoimmune

disorders, predisposition to APS has been mapped to genes in the major histocompatibility complex (MHC), among others. Also, epigenetic phenomena such as altered microRNA biogenesis in neutrophils, leading to accelerated atherosclerosis, have been implicated in APS [63].
