Immune Systems in Insects and Vertebrates

Insect Immune Priming and Transgenerational Immunity

Insects possess a unique form of immune memory known as immune priming, which allows them to mount a more robust defense against pathogens they have previously encountered. This is not due to adaptive immunity as seen in vertebrates, since insects lack specialized immune cells like B and T cells. Instead, their immune memory relies on innate immune mechanisms. Priming occurs when insects survive an infection or are exposed to non-lethal doses of pathogens. Their immune cells, called hemocytes, become better at recognizing and neutralizing the same pathogens in future encounters. Intriguingly, this enhanced immune response can be passed down to their progeny, a phenomenon known as transgenerational immune priming. This provides the offspring with a survival advantage against pathogens that their parents have encountered.

The Dscam Gene and Immune Specificity in Insects

The Down syndrome cell adhesion molecule (Dscam) gene is pivotal in the insect immune system, contributing to its ability to specifically recognize pathogens. The Dscam gene can produce a vast diversity of protein isoforms through alternative splicing of its exons, which encode immunoglobulin domains. These different isoforms allow hemocytes to distinguish between a wide array of pathogens. When an insect encounters a pathogen, it generates Dscam isoforms that bind specifically to that pathogen. If the insect survives and the pathogen reappears, the presence of the matching Dscam isoforms enables a more effective immune response. This specificity is a key feature of the insect immune system, allowing it to adapt to the diverse microbial environment.

RNA Interference as an Antiviral Defense in Insects

RNA interference (RNAi) is a crucial antiviral defense mechanism in insects, targeting viral RNA to prevent replication. The RNAi pathway involves several steps, starting with the recognition and cleavage of viral double-stranded RNA into small interfering RNAs (siRNAs). These siRNAs are incorporated into the Argonaute 2 (Ago2)-containing RNA-induced silencing complex (RISC), which then degrades viral RNA molecules that are complementary to the siRNA. Other RNAi pathways include the microRNA (miRNA) and PIWI-interacting RNA (piRNA) pathways, which also contribute to the regulation of gene expression and defense against viruses and transposable elements. These RNAi mechanisms are highly specific and form an integral part of the insect's immune defense.

Adaptive Immunological Memory in Vertebrates

Vertebrates have a complex adaptive immune system characterized by the presence of memory B cells and T cells. These cells are capable of remembering specific antigens and mounting a rapid and potent response upon subsequent exposures. Immunological memory in vertebrates can be acquired actively through natural infection or vaccination, leading to long-lasting protection. Alternatively, passive immunity can be conferred temporarily through the transfer of antibodies, such as maternal antibodies passed to offspring. This adaptive immunity is a hallmark of the vertebrate immune system, providing a sophisticated and targeted response to a wide array of pathogens.

Evolution and Function of the Acquired Immune System

The acquired, or adaptive, immune system is a relatively recent evolutionary development, first appearing in jawed vertebrates around 500 million years ago. It is characterized by the presence of highly specialized cells and molecules, such as lymphocytes, immunoglobulins, T cell receptors, and the major histocompatibility complex (MHC). These components are essential for the specific recognition of antigens. The diversity of the immune response is generated through a process known as V(D)J recombination, which allows for the production of a vast array of unique antigen receptors from a limited genetic repertoire. This enables the immune system to recognize and respond to an almost limitless variety of pathogens. The adaptive immune system operates in conjunction with the innate immune system, with each branch enhancing the effectiveness of the other.