Cerebral amyloid angiopathy (CAA) is a vascular pathology characterized by the thickening of small and medium arteries in the brain due to the deposition of amyloid β (AB) within the media and adventitia of vessel walls. Over time, the smooth muscle (SM) tissue is replaced with acellular AB plaques, accompanied by development of structural fragility and vulnerability to rupture [1]. Alternatively, severe cases of microvascular AB deposition are also associated with cerebral microinfarcts [2]. CAA pathology is associated with several diseases and is estimated to coincide with 75–90% of all cases of Alzheimer’s disease [1, 3–5]. Cerebrovascular dysfunction is now recognized as a risk factor for the onset of dementia. CAA-induced microhemorrhages may contribute to the progression of dementia by increasing the amount of iron present in the brain, resulting in oxidative damage and neurodegeneration [1, 6–8]. By one statistical estimate, the average contribution of CAA to the total cognitive decline of afflicted individuals is 15.7% [5]. No effective treatment for CAA currently exists [9, 10].
Our prior work suggested a role for the membrane attack complex (MAC) in the progression of CAA [3]. The MAC, also referred to as C5b-9, is an assembly of plasma proteins which together form a trans-membrane pore structure directly connecting the internal and external cellular environments (Fig. 1a). The resultant unrestricted osmotic exchange initiates cytolysis [11–13]. MAC formation is the convergent endpoint of the complement cascade, which is a series of protein interactions that is initiated through three distinct pathways with their specific activation mechanisms: classical, alternative, and lectin (Fig. 1b). The classical pathway is activated by antigen-antibody complexes, the alternative pathway can be activated spontaneously, and the lectin pathway is activated via recognition of conserved pathogenic carbohydrate motifs [14]. The complement cascade represents an aspect of the innate immune response, primarily targeting susceptible pathogens such as gram-negative bacteria [15]. Host cell membranes are likewise vulnerable to the cytolytic activity of MAC. To protect themselves, most human cell types and tissues express surface membrane-bound complement-inhibiting molecules. One of the most important is the glycophosphatidylinositol-anchored surface protein CD59, also referred to as protectin [16–21]. When an incomplete MAC complex (C5b-8) inserts itself into a host cell membrane, a functional CD59 protein binds the complex and blocks the spontaneous incorporation of multiple C9 molecules. This prevents the completion of the MAC and its characteristic transmembrane pore structure, thereby preserving membrane integrity [22–24]. While incidental MAC insertions may still occur, causing mild membrane damage, this remains sub-lytic due to cellular repair mechanisms [25]. Other surface membrane-bound complement regulatory proteins can intervene at earlier points in the complement cascade to prevent MAC formation. CD46 is a cofactor which enables the inactivation of the C3b and C4b complement proteins, and CD55 disrupts C3 and C5 convertases, which are required for C3b deposition [26].
Figure 2 The complement system protein cascade is initiated through antigen-antibody complexes (classical pathway), recognition of conserved pathogenic carbohydrate motifs (lectin pathway), or spontaneous hydrolysis of the C3 complement protein (alternative pathway). All three pathways converge by promoting C3 hydrolysis into C3a and C3b protein fragments. C3b promotes additional C3 hydrolysis, forming a self-amplification loop. C3b also promotes the cleavage of the C5 protein to form the protein fragment C5b, which complexes with other complement proteins to form the membrane attack complex (MAC)
Vulnerability to complement-mediated cytolysis can vary across cell types according to their levels of CD59 surface expression [11, 23]. Low CD59 expression can sensitize cells to complement damage while high expression confers resistance [18, 21, 27]. Normal expression levels of CD59 vary with tissue location but can also fluctuate under abnormal conditions, such as Alzheimer’s disease, organ transplantation, or cancer [16, 18, 20, 27–29]. Regardless of expression level, CD59 protection is species-restricted and will not inhibit complement proteins endogenous to other species [23]. A number of reports documented decreased CD59 protective potential. Genetic mutations can compromise CD59’s anti-MAC functionality [27, 30]. Insufficient CD59 expression is believed to be associated with a number of conditions including: paroxysmal nocturnal hemoglobinuria, Alzheimer’s disease, age-related macular degeneration, post-transplant organ rejection, and genetic demyelinating neuropathy in some patients [16, 27, 31–33]. Lower CD59 expression in the intracranial artery is associated with complement activation, inflammation, and possible weakening of the arterial wall [34].
Earlier work suggested that CAA-afflicted cerebral blood vessels have increased MAC deposition without a compensatory upregulation of surface CD59 [3]. Over time, the cumulative cytotoxic and cytolytic damage could play a role in the gradual characteristic destruction of human cerebral vascular smooth muscle cells (HCSM). However, this potential sensitivity of primary HCSM cells to complement attack has not yet been reported. In this study, the primary HCSM cells were isolated from small blood vessels of the brain, obtained during routine temporal lobe biopsies. Their surface-expressed CD59 proteins were then inhibited in a controlled dose-dependent manner, to evaluate the role of CD59 for cellular resistance against complement-dependent cytotoxicity.