Innate Immune Memory in Invertebrate Metazoans: A Critical Appraisal

Abstract

The ability of developing immunological memory, a characteristic feature of adaptive immunity, is clearly present also in innate immune responses. In fact, it is well known that plants and invertebrate metazoans, which only have an innate immune system, can mount a faster and more effective response upon re-exposure to a stimulus. Evidence of immune memory in invertebrates comes from studies in infection immunity, natural transplantation immunity, individual, and transgenerational immune priming. These studies strongly suggest that environment and lifestyle take part in the development of immunological memory. However, in several instances the formal correlation between the phenomenon of immune memory and molecular and functional immune parameters is still missing. In this review, we have critically examined the cellular and humoral aspects of the invertebrate immune memory responses. In particular, we have focused our analysis on studies that have addressed immune memory in the most restrictive meaning of the term, i.e., the response to a challenge of a quiescent immune system that has been primed in the past. These studies highlight the central role of an increase in the number of immune cells and of their epigenetic re-programming in the establishment of sensu stricto immune memory in invertebrates.

Keywords: immune priming; immunological memory; innate immunity; innate memory; invertebrates.

Figure 1

Innate memory in invertebrates and vertebrates. (A) Infections or stressors can prime the innate immune system so that, after a phase of extinction of the response, it will respond more potently to a subsequent challenge with the same or a different stimulus. In invertebrates, this is defined as a recall response, while in vertebrates it is called potentiation or trained immunity (red line). Only in vertebrates, the secondary response can be less intense than the first one, a phenomenon known as tolerance (green line). (B) In invertebrates, a second challenge in primed animals can lead to an immune shift, i.e., the shift from a type of response (dotted line) to a different, more efficient one (solid line). (C) In invertebrates, priming could result in a medium- or long-term immune activation state, which can further increase upon challenge. This is identified as sustained or unique response. Adapted from Coustau et al. (48) and Pradeu and Du Pasquier (15).

Figure 2

The main invertebrate haemocytes involved in immune response. Pro-haemocyte. Immature cell identified as pro-haemocyte or lymphocyte-like cell. These cells, present in ascidians, crustaceans, insects and probably in the haemopoietic tissues of other invertebrates, are able to differentiate in mature haemocytes. The undifferentiated pro-haemocyte is small, with a big nucleus containing a large amount of heterochromatin and a prominent nucleolus. Amoeboid phagocytes are motile vacuolated cells present in annelids, insects, echinoderms and ascidians. Depending on the species, amoeboid phagocytes are involved in phagocytosis, migration, wound repair, non-self-recognition, transplant reaction, cytotoxicity, encapsulation, endocytosis, and enzymatic digestion of engulfed material. Granular cells are mature cells found in ascidians, crustaceans, insects and bivalves. They are able to synthesize a number of cytotoxic and defense factors and store them in granules. Degranulation occurs upon challenge with stressors. Hyaline cells are vacuolated or non-vacuolated cells, abundant mostly in ascidians, crustaceans, and bivalve molluscs. They are mainly involved in phagocytosis. In ascidians, hyaline cells rapidly clump together in vitro. Oenocytoid cell. These cells are widely present in insect species. They are large cells with a low nuclear-cytoplasmic ratio, which show phenoloxidase activity in the cytoplasm. This suggests that oenocytoid cells could play a role in the melanization process. Spherule or morula cell. These haemocytes, present in some cnidarians, annelids, insects, echinoderms, and ascidians, are berry-shaped cells, sometimes pigmented, with highly refractive cytoplasmic inclusions. They are actively involved in encapsulation and synthesize, transport and release various defensive factors during infections, including antimicrobial proteins, cytotoxic factors, and opsonins. Lamellocyte. These flat cells with adhesive properties are present in insects, in particular in Diptera. Lamellocytes appear in the lymph glands and haemolymph during larval development and differentiate in response to parasite infection. They are active in neutralizing and encapsulating materials recognized as “non-self,” too large to be phagocytosed.

Figure 3

Immune defensive responses in invertebrates. Humoral and cellular effectors cooperate to achieve parasite/pathogen clearance. The immune system recognizes foreign agents (parasites, viruses, bacteria) and responds with the migration and production of immune cells (cellular response) and proteins (humoral response). More specifically, following recognition of pathogen-associated molecular patterns (PAMPs) or other molecules by PRRs on immune cells, circulating haemocytes within the haemolymph or immune cells in injured tissues neutralize the intruder by either phagocytosis or encapsulation/melanization. In parallel, the same or different immune cells release factors that are directly toxic for pathogens (antimicrobial peptides, agglutinins, etc.), or that improve or facilitate cell-mediated pathogen killing (PO, opsonins, complement components, etc.).

Figure 4

The major mechanisms of innate memory in invertebrates. The two major phenomena underlying the capacity of invertebrates to mount a more effective defensive response after priming, speaking sensu stricto, are the recall response (upper part) and the immune shift (lower part). As general mechanism (center part), priming is expected to induce epigenetic reprogramming that, upon challenge, determines improved clearance of parasites and enhanced survival. At the level of the whole organism (left part), memory can encompass mechanisms leading to an increase in the number of immune cells at the site or reaction (haematopoiesis, mitosis, haemocytosis), and also the capacity of transferring resistance across generations. At the cellular level (right part) it is also possible to observe increased effector functions in individual cells (e.g., an increased phagocytic rate vs. phagocytic index). Mechanisms that are observed both at the global and cellular levels (lower part) encompass the increased production of soluble immune mediators and the shift of immune response from an initial protective reaction (e.g., phagocytosis) to a more effective mechanism (encapsulation).