Diet modulates the relationship between immune gene expression and physiological immune responses

Nutrition is vital to health and the availability of resources has long been acknowledged as a key factor in the ability to fight off parasites, as investing in the immune system is costly. Resources have typically been considered as something of a “black box”, with the quantity of available food being used as a proxy for resource limitation. However, food is a complex mixture of macro- and micronutrients, the precise balance of which determines an animal’s fitness. To unpick the effects of the balance and amount of nutrients on an animal’s ability to mount an immune response, we used the Geometric Framework for Nutrition (GFN). Spodoptera littoralis caterpillars were assigned to one of 25 diets that varied in the ratio of macronutrients (protein and carbohydrate) and their energy density to cover a large region of nutrient space. Caterpillars were then handled or injected with either live or dead Xenorhabdus nematophila bacterial cells. The expression of 9 genes (6 immune, 3 non-immune) was measured 24 h post immune challenge. For two of the immune genes (PPO and Lysozyme) we also measured the relevant functional immune response in the blood. Gene expression and functional immune responses were then mapped against nutritional intake. The expression of all immune genes was upregulated by injection with dead bacteria, but only those in the IMD pathway (Moricin and Relish) were substantially upregulated by both dead and live bacterial challenge. Functional immune responses increased with the protein content of the diet but the expression of immune genes was much less predictable. Nonetheless, immune gene expression did predict the relevant functional immune response, but the relationship between the two varied over nutrient space, such that you could infer functional activity from gene expression only when protein availability was not limiting. Our results indicate that diet does play an important role in the ability of an animal to mount an adequate immune response, with the availability of protein being the most important predictor of the functional immune response. However, gene expression alone responds quite differently to functional immunity and we would caution against using gene expression as a proxy for immune investment, as it is unlikely to be reliable indicator of the immune response, except under specific dietary condition.