Immunology of Invertebrates: Cellular

Abstract

Circulating (free) cells are the main mediators of immunity in invertebrates, carrying out the phagocytic, encapsulating and microbicidal responses that enable these animals to protect themselves against infection without a specific adaptive immune system. The cells are specialised for these functions, with division of labour between types. In general, immune cell diversity tends to increase with body complexity and life histories but common to the majority of species, however, are phagocytic and granular cell types that synthesise and store bioactive proteins. The release of these compounds occurs by exocytosis following interaction of pattern recognition receptors and nonself motifs carried on the surface of pathogens or parasites. Cell death of one form or another appears to be inherent in immune reactivity of many invertebrates with new cells produced, at least in coelomates, by mesodermal haemopoietic tissues. Key concepts Immunity in invertebrates is confined to nonspecific inflammatory responses, mediated to a large extent by the circulating blood cells (haemocytes or coelomocytes) or their products. All coelomate invertebrates contain populations of freely circulating cells dedicated to host defence, some well developed for specialist purposes. There are no true vertebrate‐type lymphocytes and no long‐term, highly specific immune memory in invertebrates. The main invertebrate cellular immune responses include phagocytosis, encapsulation, cytotoxicity and the synthesis and release of microbicidal agents. These cellular reactions often involve the death of immune cells or their removal from the circulation, requiring new cells to be synthesised in the haemopoietic tissue and released into the blood. Genomic and proteomic studies have enabled many of the proteins responsible for cellular defence in invertebrates to be identified, especially in arthropods. Although many of the responses and pathways are ancient and highly conserved, there is great diversity in the effector molecules across the invertebrates as a whole.

Echinochrome A Release by Red Spherule Cells Is an Iron-Withholding Strategy of Sea Urchin Innate Immunity

“…4). Activation of invertebrate defences generally leads to an increase in free-floating immune cell numbers and the liberation of bioactive compounds (e.g., antimicrobial peptides and lysozyme) [33]. Likewise, exposure of sea urchin phagocytes to LPS in vitro induces cellular aggregation (reminiscent of encapsulation) and de novo synthesis of SpTransformer proteins (formerly Sp185/333) [9].…”

Do all creatures possess an acquired immune system of some sort?

“…Arthropods have several types of hemocytes that are involved in immunity; for example, their plasmatocytes are analogous to the phagocytes of other taxa . In addition to plants and nematodes, acquired RNAi immunity has also been identified in many insects , including Drosophila melanogaster .…”

“…Molluscs, similarly to arthropods, possess different types of hemocytes involved in immunity, some of which are phagocytic . Acquired diversity generated somatically by alternative splicing has been identified in fibrinogen‐related proteins (FREPs) in the snail Biomphalaria glabrata .…”

A combined characterization of coelomic fluid cell types in the spiny starfishMarthasterias glacialis– inputs from flow cytometry and imaging

“…Coelomocytes are mostly known as effectors of the immune response and, thus, they are key contributors to the monitoring and restoration of missing tissues (4). How these processes occur is still a matter of debate, due to the still limited knowledge of the fluid's composition and the mechanistic/molecular basis of coelomocytes functions.…”