Current insights into insect immune memory

Abstract

Traditionally, insects have been thought to be entirely dependent on their innate immune system, which has little capacity for the acquisition of experience from previous infections. However, much experimental evidence has challenged this view, showing that insects can develop long-term, pathogen-specific immune memory, which in some cases can be transmitted to offspring. Although significant progress has been made in this area, the underlying mechanism is still not fully understood, and a number of fundamental questions remain unanswered. In this review, we present an overview of documented cases of insect immune memory and summarize the experimental evidence in support of the prevailing hypotheses on the mechanism of antiviral and antibacterial immune memory in insects. We also highlight key questions that remain unanswered and discuss Drosophila melanogaster as a powerful model organism for investigating the mechanisms of innate immune memory formation. Finally, we evaluate the significance of this research and explore the potential for insect vaccination.

Keywords: Dscam; evolutionary biology; hemocytes; immune memory; immunology; inflammation; insect immunity; insect vaccination; trained immunity.

Figure 1.. Graphical representation of the progression of acquired immunity in insects (immune priming, immune training, and immune memory).

Immune priming is characterized by the persistent activation of the immune system following the first immune challenge, resulting in an enhanced immune response to the second challenge. In contrast, immune training and immune memory are biphasic, meaning immune response returns to baseline before the second challenge. While immune priming and immune training are pathogen-nonspecific responses, immune memory operates in a pathogen-specific manner, i.e., the same pathogen must be involved in both the first and second challenges (homologous infection; solid line). In the case of heterologous infection (where different pathogens are involved in the first and second challenges), immune memory is not induced (dashed line). The horizontal black dotted line indicates the immune response baseline. This figure was created using BioRender.com.

Figure 2.. Overview of the insect immune system.

The cuticle forms the first immune barrier, protecting insects from most pathogens in their environment. However, if this barrier is breached, the immune response is activated, consisting of a coordinated reaction between cellular and humoral immunity. The cellular immune response is mediated by several types of immune cells collectively known as hemocytes, which perform specialized immune functions such as phagocytosis, degranulation, extracellular trap formation, and encapsulation. The humoral immune response involves a diverse array of secreted factors that directly eliminate pathogens in circulation. These factors are produced not only by hemocytes but also by the fat body, an immunometabolic tissue. Circulating immune factors contribute to hemolymph clotting, melanization, and pathogen opsonization. Through the combination of these mechanisms, insects can effectively defend against a wide range of pathogens, including bacteria, fungi, viruses, parasitic nematodes, parasitoids, and protozoa. This figure was created using BioRender.com.

Figure 3.. Simplified phylogenetic tree of insect orders with documented cases of acquired immune responses.

Immune ‘adaptivity’ has been investigated in species highlighted in blue. NE, no evidence; PR, protection; TgIP, transgenerational immune priming. This figure was created using the following references - 1 González-tokman et al., 2010; 2 McNamara et al., 2014; 3 Faulhaber and Karp, 1992; 4 Cole et al., 2020.; 5 Rosengaus et al., 1999; 6 Vorburger et al., 2008; 7 Schwarz and Evans, 2013; 8 Hernández López et al., 2014; 9 Roth et al., 2009; 10 Salmela et al., 2015; 11 Riessberger-Gallé et al., 2015; 12 Sadd and Schmid-Hempel, 2007; 13 Hamilton et al., 2011; 14 Konrad et al., 2012; 15 Gálvez and Chapuisat, 2014; 16 Reber and Chapuisat, 2012; 17 Rosengaus et al., 2013; 18 Futo et al., 2017; 19 Roth et al., 2010; 20 Tate and Graham, 2015; 21 Eggert et al., 2014; 22 Futo et al., 2015; 23 Knorr et al., 2015; 24 Thomas and Rudolf, 2010; 25 Fisher and Hajek, 2015; 26 Dubuffet et al., 2015; 27 Zanchi et al., 2011; 28 Moreau et al., 2012; 29 Wu et al., 2015a; 30 Miyashita et al., 2015; 31 Wu et al., 2015b; 32 Wu et al., 2015c; 33 Freitak et al., 2014; 34 Trauer-Kizilelma and Hilker, 2015; 35 Trauer and Hilker, 2013; 36 Tidbury et al., 2011; 37 Freitak et al., 2009; 38 Rahman et al., 2004; 39 Mahbubur Rahman et al., 2007; 40 Ma et al., 2005; 41 Apidianakis et al., 2005; 42 Christofi and Apidianakis, 2013; 43 Cabrera et al., 2023; 44 Pham et al., 2007; 45 Mondotte et al., 2018; 46 Mondotte et al., 2020; 47 Linder and Promislow, 2009; 48 Longdon et al., 2013; 49 Moreno-García et al., 2015; 50 Vargas et al., 2020; 51 Rodriguez-Andres et al., 2024; 52 Rodrigues et al., 2010; 53 Ramirez et al., 2015, 54 Contreras-Garduño et al., 2015, 55 Bruner-Montero et al., 2023; 56 Patel and Oliver, 2024. This figure was created using BioRender.com.

Figure 4.. Schematic representation of the hypothetical mechanism underlying the formation of antiviral immune memory in insects.

Virus-infected cells produce signals that attract macrophage-like plasmatocytes (1), which eliminate them through a process known as efferocytosis (2). The engulfed cellular debris, along with viral particles, is degraded in the phagolysosome, where viral RNA is acquired (3). This viral RNA is then reverse transcribed (4) and integrated into the genome of host plasmatocytes through the activity of co-opted retrotransposons (5). The secondary viral RNA produced (6) can either undergo amplification in the cytosol via the ping-pong biogenesis mechanism (7) and serve as a template for sequence-specific cellular immunity mediated by the RNA interference (RNAi) pathway (8) or be packaged into exosomes for distribution to other cells within the organism (9). Through this mechanism, plasmatocytes provide systemic antiviral immunity to tissues and cells before they encounter the virus (10), while also retaining imprints of past viral infections in the form of integrated viral sequences. This figure was created using BioRender.com.

Figure 5.. Structure and variability of the Drosophila Dscam1 gene.

(A) Schematic representation of the Dscam1 genomic region, highlighting the hypervariable exons 4, 6, and 9. (B) Diversity in the immunoglobulin domains of Dscam1 arises through mutually exclusive alternative splicing of these hypervariable exons, exemplified here by exon 4. This splicing process is tightly regulated by serine/arginine-rich (SR) proteins and heterogeneous nuclear ribonucleoproteins (hnRNPs). (C) A single Dscam1 gene can generate thousands of isoforms, which may exist as full-length, membrane-bound proteins or as truncated, soluble variants circulating in the hemolymph. (D) The DSCAM protein structure consists of 10 immunoglobulin domains, 6 fibronectin type III (FNIII) repeats, a transmembrane domain, and a cytoplasmic tail involved in signaling and regulatory functions. This figure was created using BioRender.com.

Figure 6.. Schematic representation of the hypothetical mechanism underlying the formation of Dscam-based antibacterial immune memory in insects.

The recognition of pathogenic bacteria by plasmatocytes triggers the activation of key classical immune pathways, leading to the production of humoral immune factors, increased cell motility, and metabolic polarization (1). Additionally, changes in epigenetic regulators, including serine-arginine protein (SR-protein) and heterogeneous nuclear ribonucleoproteins (hnRNPs), induce high variability in the alternative splicing of the Dscam gene, resulting in the production of a diverse repertoire of DSCAM molecules (2). DSCAM proteins are produced both as circulating opsonization factors and as membrane-bound receptors (3). When a specific DSCAM variant exhibits high affinity for pathogen surface antigens, signaling through the DSCAM receptor is initiated via serine/threonine kinases (MAPK, JNK, ERK). (5). This leads to the stabilization of alternative splicing, ensuring the production of only DSCAM variants that specifically recognize the pathogen (6). Subsequently, the DSCAM receptor undergoes proteolytic cleavage by ADAM metalloproteases and γ-secretase, releasing its cytosolic domain (7). This cleaved cytosolic domain translocates to the nucleus, where it directly participates in the regulation of gene transcription involved in cell persistence, maintenance, and proliferation (8). These processes contribute to the long-term persistence of immune-activated cells and the maintenance of immune memory (9). While a significant portion of this hypothetical model is supported by robust scientific data, further experimental validation is required to confirm its mechanisms. This figure was created using BioRender.com.