Immune Response: Evolution


Innate immune processes arose from specific host–pathogen coevolutive interactions and the ability to recognise self and nonself structures is extremely conserved from invertebrates to vertebrates. In this context, phagocytes represent the most ancient defensive line against foreign materials, already present in earliest protostomes. Moreover, although more complex functions evolved in vertebrates, both cellular and humoral innate immune mechanisms are extremely conserved, leading to identify some forms of innate immune memory also in invertebrates.

Key Concepts

  • Phagocytosis represents the first mechanism of defence in metazoans.
  • Immune system has evolved to discriminate self and nonself structures.
  • Immune mechanisms present a high level of conservation throughout phyla.
  • Vertebrate innate immune memory derives from ancient forms and mechanisms already present in invertebrates.
  • Alternative animal models are crucial for investigating the molecular basis of the immune processes.

Keywords: invertebrate; innate immunity; adaptive immunity; comparative immunology; immune memory

Figure 1. Representation to describe the HvRNASET2 antibacterial role in the medicinal leech. After 30 min, both PAMPs or DAMPs recognition by TLR2 and TLR4 positive cells induces granulocyte activation. These immune cells secrete HvRNASET2, whose primary role is to perform an antibacterial activity against harmful invaders. Moreover, HvRNASET2 allows to recruit numerous macrophages in the stimulated area especially after 6–24 h, and these cells subsequently secrete both AIF‐1 and HvRNASET2, which maintain the inflammatory state by the recruitment of other macrophages. These phagocytic cells not only represent the main defensive line against pathogens but also play a pivotal role in cellular and bacterial debris clearance.
Figure 2. The humoral cytolytic factor presents in the oligochetes coelom (CCF) is involved in peptidoglycan, b‐1,3‐glucan and LPS recognition. These molecules are highly present on bacterial and yeast membranes and once detected induce the prophenoloxidase cascade activation, which in turn leads to melanin production.
Figure 3. (a) Hirudinea immunocytes. TEM images representing a transversal section of LPS stimulated leech and the relative immune cells detected in the stimulated area. Underneath the epithelium (e), numerous vessels (v) are clearly visible after LPS treatment and different types of migrating cells are easily recognizable (arrowheads). NK cells, macrophages and two types of granulocytes are shown in TEM details. A scheme of Hirudinea immunocytes and of their inherent roles is represented below. (b) Schemes of both body wall transversal sections and gut anterior portion of oligochetes. Hyaline (LC) and granular (SC) coelomocytes, shown in TEM details, derive from somatopleure (in yellow) and splanchnopleure (in green) respectively. These cells express specific markers and possess phagocytic and cytotoxic activities. Around the gut, chloragocytes (or eleocytes) play an antibacterial role by releasing several molecules that directly act against pathogens.


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Further Reading

Grimaldi A, Tettamanti G and de Eguileor M (2018) Annelida: Hirudinea (Leeches): heterogeneity in leech immune responses. In: Cooper E (ed.) Advances in Comparative Immunology, pp 173–191. Springer: Cham.

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Baranzini, Nicolò, Engelmann, Péter, and Grimaldi, Annalisa(Dec 2020) Immune Response: Evolution. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0029225]