COS 74-7
Immune strategies as life history traits: A theoretical framework for understanding how animal life histories determine optimal defenses against pathogens
Life history should have profound impacts on the architecture of immune systems: the ‘faster’ an animal lives its life (i.e., the more rapidly it develops / the more prolifically it breeds), the fewer resources it should invest in immunity. Moreover, there is growing evidence that life histories affect HOW animals combat infections, with fast-living animals emphasizing rapid, non-specific (“innate”) defenses and slow-living animals relying more on highly specific (“adaptive”) defenses and immunological memory. These empirical findings motivate development of theory focused on understanding optimality in allocation to immunity versus other life history components, and among different components of the immune system.
We have developed a theoretical framework for exploring optimal immune strategies in hosts with different life histories, when exposed to different types of parasites. We express the fitness of hosts exposed to parasitic challenges as a function of immune strategy, and find the strategy that maximizes fitness for a given host life history and parasite type. Beyond this, we will model multi-parasite host systems, to understand immune strategies as host life history-dependent solutions to coping with diverse and variable parasitic challenges. Ultimately, this work paves the way towards understanding how species’ traits determine their roles in infectious disease dynamics.
Results/Conclusions
Using our models, we determined optimal immune strategies for ungulate hosts with well-described contrasting life histories. We also obtained an assessment of each species’ potential for pathogen amplification, expressed as expected lifetime pathogen production. We used four representative parasites differing in their dynamic behavior and virulence, to illustrate how optimal immunity depends on the disease risk profile presented by the parasite.
Our models produced patterns of optimal immunity that are consistent with empirical observations: Overall immune investment, and investment in adaptive immunity, followed a gradient from fast to slow-living hosts; whereas innate immunity was predicted to be stronger in faster-living species. Innate immunity was most important for pathogens with epidemic dynamics, while endemic pathogens provoked strong adaptive responses. This makes intuitive sense because adaptive immunity is cheap and effective upon re –exposure, which happens continually for endemic pathogens. Interestingly, for our fastest-living ungulate, the best response to a highly virulent pathogen with unpredictable dynamics was to invest its resources solely in growth and reproduction, rather than in immunity. These modeling results are encouraging in that they suggest that we are capturing relevant functional variation in our representation of the immune response of different hosts to a range of pathogens.
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