Mapping the functional form of the trade-off between infection resistance and reproductive fitness under dysregulated immune signaling.

Immune responses benefit organismal fitness by clearing parasites but also exact costs associated with immunopathology and energetic investment. Hosts manage these costs by tightly regulating the induction of immune signaling to curtail excessive responses and restore homeostasis. Despite the theoretical importance of turning off the immune response to mitigate these costs, experimentally connecting variation in the negative regulation of immune responses to organismal fitness remains a frontier in evolutionary immunology. In this study, we used a dose-response approach to manipulate the RNAi-mediated knockdown efficiency of cactus (IκBα), a central regulator of Toll pathway signal transduction in flour beetles (Tribolium castaneum). By titrating cactus activity across four distinct levels, we derived the shape of the relationship between immune response investment and traits associated with host fitness, including infection susceptibility, lifespan, fecundity, body mass, and gut homeostasis. Cactus knock-down increased the overall magnitude of inducible immune responses and delayed their resolution in a dsRNA dose-dependent manner, promoting survival and resistance following bacterial infection. However, these benefits were counterbalanced by dsRNA dose-dependent costs to lifespan, fecundity, body mass, and gut integrity. Our results allowed us to move beyond the qualitative identification of a trade-off between immune investment and fitness to actually derive its functional form. This approach paves the way to quantitatively compare the evolution and impact of distinct regulatory elements on life-history trade-offs and fitness, filling a crucial gap in our conceptual and theoretical models of immune signaling network evolution and the maintenance of natural variation in immune systems. Author summary: The immune system provides defense against infection but demands substantial energy and can inadvertently harm the host. To limit these costly side effects, hosts use regulatory proteins that dampen immune responses after they respond to infection. Despite the importance of these regulating proteins, we do not understand how their variation influences infection defense and the broader effects on an organism's health, reproduction, and fitness. We used a novel approach to vary the expression of the regulatory protein cactus, thereby adjusting the magnitude of Toll pathway activation in the red flour beetle. We found that while increasing the intensity of immune pathway activation enhances immune output and survival to bacterial infection, it comes at a disproportionally severe cost to female egg production, gut health, body mass, and lifespan. This reveals that even small variations in negative regulation and immune pathway activation can have disproportionate health and reproductive consequences, helping us better understand how immune systems evolved and the diversity of rules governing their regulation in nature. [ABSTRACT FROM AUTHOR]