Muscling in on immunity: the role of muscle in the insect immune response, demonstrating the value of a whole organism perspective.

Insects have robust immune systems to protect themselves against pathogens. However, the immune system relies on the support of a number of organs for effective immune defense. Muscle plays an unexpectedly key role in both cell-mediated and humoral immunity in insects. During an immune response, muscle responds to increasing cytokine levels by producing antimicrobial peptides. Muscle also shifts resources toward the immune system by releasing myokinins and other factors. These compounds reduce insulin-like peptide release from the brain. In addition, muscle undergoes insulin resistance, further reducing its uptake of glucose. Muscle also donates its own resources, breaking down glycogen to support the hemocytes' need for glucose during cell-mediated immunity. However, this support of immune function results in a decline in muscular capacity, leading to reduced anti-predator behavior and increased predation. This physiological trade-off between muscle and immunity may help explain why sick animals typically reduce exposure to predators by increasing shelter use. In addition, muscle's ability to regulate the flow of resources in the body suggests that it may also play a role in mediating trade-offs between immune function and other life history traits, such as reproduction. Muscle should be considered as a research target by ecoimmunologists. Some immune-mediated effects, such as insulin resistance and chronic inflammation, may have adaptive functions when viewed from the perspective of fitness maximization under adverse conditions.