The complement cascade can be initiated by three distinct pathways: the classical, lectin, and alternative pathways. The classical and lectin pathways are triggered when recognition molecules bind to structures such as antibody complexes and carbohydrates on pathogen surfaces, leading to the activation of their associated proteases C1s/C1r and MASP-1/2. These proteases cleave C4 and C2, generating the C3 convertase (C4b2b), which then processes C3 into the anaphylatoxin C3a and the opsonin C3b. Accumulation of C3b induces the formation of the C5 convertase, which cleaves C5 into C5a and C5b. The subsequent interaction of C5b with C6, C7, C8, and C9 leads to the assembly of a lytic pore, known as the terminal complement complex (TCC). The alternative pathway is initiated when factor B (FB) interacts either with water-hydrolyzed C3, C3(H2O), or with deposited C3b to form the C3(H2O)Bb or C3bBb C3 convertases following factor D (FD) cleavage. These proteases, and in particular, the surface-bound and properdin/factor P–stabilized (FP-stabilized) C3bBbP C3 convertase act as an amplification loop for complement, generating most of the activated C3 fragments regardless of the initiating pathway.
SARS-CoV-2 infection triggers activation of the complement cascade through direct interaction with viral components or virus-specific antibodies, typically resolving once the infection is cleared. However, in patients with long COVID, this activation may persist, potentially contributing to ongoing symptoms. Several proposed mechanisms of long COVID can directly activate the complement system. For example, antiherpesvirus antibodies, likely the result of herpesvirus reactivation, or autoantibodies may drive activation via the classical pathway. Insertion of TCC in the endothelial cell wall causes activation and cell damage, causing the release of TSP1 and vWF. TSP1 promotes formation of monocyte-platelet aggregates, while vWF release — coupled with reduced levels of ADAMTS13, the metalloproteinase responsible for processing vWF multimers — leads to the accumulation of large or ultralarge vWF multimers on the endothelial surface. This, in turn, promotes platelet recruitment and thrombus formation. Additionally, vWF multimers on the endothelium, along with properdin and P selectin on activated platelets, can trap C3b, fueling complement activation via the amplification loop of the alternative pathway. Uncontrolled complement activation in the vasculature leads to red blood cell lysis, causing the release of heme and activation of the alternative pathway. Finally, tissue damage resulting from acute COVID-19, autoimmunity, or viral antigen reservoirs may all contribute to the persistent complement activation observed in patients with long COVID.
In this Review @virusesimmunity.bsky.social et al examine the evidence linking #complementsystem dysregulation to LC & explore its potential role in driving disease pathology.
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