A Bug’s Life: How Insect Immune Systems Can Lead the Way

Questions about our ability to fight infections and develop immune systems that can cope with dangerous virus pandemics, now and in the future, have been at the heart of the COVID-19 crisis.

A vaccine appears to be the obvious solution, but it has been acknowledged that lifestyle and genetics can play a significant role in building immunity against virus and disease. A key to understanding how may lie in an unlikely source: insects.

“Insects are excellent models for the innate immune response in vertebrates, including humans,” explains Dr Sheena Cotter, who is a Senior Lecturer in the School of Life Sciences and an expert in the evolutionary ecology of insects. 

The innate immune response is the rapid and generalised response to infection that involves inflammation, fever, antimicrobial peptides, and white blood cells engulfing pathogens in the body. Humans and other vertebrates subsequently developed an antibody response, which is the secondary and highly specialised reaction to infection. Insects and other invertebrates do not have this second layer of immunity, but the initial response is common across the animal kingdom.

To find out more about this innate immune response, Dr Cotter examined two model systems: Spodoptera spp, or armyworms, and Nicrophorus vespilloides, the burying beetle, focusing on how each species responds to infection, and how these responses are modified by, for example, age or nutrition.

To test whether the availability of nutrients changes an animal’s response to infection, she set up a series of experiments, in collaboration with colleagues from Lancaster University, where food was chemically controlled, manipulating precisely the amounts of proteins, fats, micronutrients, and overall calories available. The insects were then infected with a natural pathogen.

The research found that more than twice as many caterpillars survived if the diet was high in protein, compared to those on the low protein food. When given a choice, the infected caterpillars would also actively choose the high protein food, thus improving their survival prospects.

“The choice of food was a crucial factor in the development of the immune response to the pathogens,” says Dr Cotter. “Pathogen growth can be slowed down either by the host organism producing what are called antimicrobial peptides, which limit pathogen reproduction, or through the food that the host consumes. By changing their diet, it is possible that the caterpillars are removing the fuel available to the pathogen, reducing its ability to grow. The highest protein diet appeared to give the best survival rates.”

Further tests on the burying beetle, which breeds on the carcasses of small mammals or birds and are regularly in contact with a highly pathogenic environment, confirmed the importance of diet on the immune system. However, as beetles already have a high protein diet, increasing protein content did not improve their already advanced immune system. Instead, a high fat, low protein diet increased their ability to tolerate infection.

The research found that diet not only influenced how well the beetles coped with infection but it also changed how rapidly they aged.

“We discovered that lifespan increased as the protein content of the diet declined, with median lifespan almost doubling as the protein content dropped from 74 per cent to 45 per cent,” explains Dr Cotter. “We know that low calorie diets have health benefits. However, many studies now show that, rather than the restriction of all nutrients, simply restricting protein can increase longevity."

The aim now is to expand this work by systematically manipulating the macronutrient composition of the diet at different ages, so we can predict which diet should be eaten at each life stage in order to maintain an adequate immune system, and a longer, healthier old age.

(Photo: Oliver Krueger)