Abstract
The complement system represents a crucial part of innate immunity. It contains a diverse range of soluble activators, membrane-bound receptors, and regulators. Its principal function is to eliminate pathogens via activation of three distinct pathways: classical, alternative, and lectin. In the case of viruses, the complement activation results in effector functions such as virion opsonisation by complement components, phagocytosis induction, virolysis by the membrane attack complex, and promotion of immune responses through anaphylatoxins and chemotactic factors. Recent studies have shown that the addition of individual complement components can neutralise viruses without requiring the activation of the complement cascade. While the complement-mediated effector functions can neutralise a diverse range of viruses, numerous viruses have evolved mechanisms to subvert complement recognition/activation by encoding several proteins that inhibit the complement system, contributing to viral survival and pathogenesis. This review focuses on these complement-dependent and -independent interactions of complement components (especially C1q, C4b-binding protein, properdin, factor H, Mannose-binding lectin, and Ficolins) with several viruses and their consequences.
Highlights
The innate immune system is characterised by its ability to distinguish between “self” and “non-Self”
This review focuses on the complement activation-dependent and independent functions of complement components as soluble pattern recognition receptors for several viruses (Table 1)
C1q, C4b-binding protein (C4BP), Properdin, factor H, and virus complement control protein (VCP) have individually been shown to inhibit the entry of viruses, such as the H1N1 subtype of the Influenza A Virus (IAV), into the cell and downregulate inflammatory cytokines and chemokines (TNF-α, IL-6, IL-12, NF-κB, RANTES)
Summary
The innate immune system is characterised by its ability to distinguish between “self” and “non-Self”. The complement system plays a crucial part in the innate immune surveillance against viruses through several mechanisms that prevent host viral infection. It can be activated through three pathways: the classical, the alternative, and the lectin, depending upon the recognition subcomponents and the ligand that trigger its activation. The binding of C1q to either antibodies or pathogen surface triggers the autoactivation of serine protease, C1r, which subsequently cleaves and activates another serine protease, C1s [1] This generates a C1-complex consisting of one molecule of C1q and two molecules each of.
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