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Fig. 4 | Microbiome

Fig. 4

From: Host-microbe interactions in octocoral holobionts - recent advances and perspectives

Fig. 4

Current knowledge on the immune system of octocorals. (I) Microbe-associated molecular patterns (MAMP) are recognized by pattern recognition receptors (PRR), which subsequently activate signaling cascades that induce (II) expression of genes involved in the immune system. (III) Immune effector molecules are produced and secreted, including antimicrobial peptides (AMP). (IV) Chitinases degrade chitin, an important component of the cell wall of fungi. (V) The host also uses protease inhibitors to neutralize protease virulence factors secreted by pathogenic microbes. (VI) One of the main immune system components is the prophenoloxidase (proPO)-activating pathway. It is activated following the binding of MAMPs to their respective binding proteins (BP), leading to the activation of a protease cascade that ultimately cleaves proPO into PO. Subsequently, PO oxidizes phenolic compounds (e.g., dihydroxidephenylalanine) that undergo further non-enzymatic reactions to form a microbe-immobilizing barrier of melanin. Cytotoxic molecules are also formed during this process. Octocorals are also known to possess lectins, which can be used in (VII) the lectin-complement system that leads to the deposition of complement C3 on the target microbe, and/or to (VIII) aggregate microbes into large aggregates. Both systems facilitate (IX) the rapid phagocytosis of microbes following binding to lectin, C3-receptors or various scavenger PRRs. Once internalized, the phagosome matures and becomes microbicidal with (X) bacterial cell wall degrading lysozyme as well as AMPs and oxidative burst of reactive oxygen species (ROS). (XI) The ROS may be also damaging to the host cell and antioxidant enzymes, such as superoxide dismutase (SOD) and peroxidase (POX), are used to neutralize it

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