Proximate and ultimate causes of immune/behavioural interactions

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Stress (both acute and chronic) decreases the resistance of humans and animals to disease. Immune cells have evolved receptors for stress hormones, suggesting that stress-induced changes in immune function provide some benefit. However, these benefits are often unclear, in part because the vertebrate immune system is difficult to study. Our lab examines this question in a simpler insect system. A simpler system is helpful, because answering this question requires experiments spanning multiple levels of analysis (e.g. molecular to behavioural), and an examination of both the physiological mechanisms and their evolutionary significance. As in mammals, insects have a stress hormone (i.e. octopamine). It also depresses disease resistance. In this study, we test whether the stress response selectively depresses immune functions that conflict with it. For example, both the stress and immune responses generate oxidative stress. If both functions were fully activated, the resulting tissue damage may cause even greater declines in survival than stress-induced reductions in disease resistance. I predict that stress hormones reconfigure the immune response so that it relies less on tissue damaging mechanisms. This hypothesis could help explain why stress hormones can both enhance and depress disease resistance. This study has the potential to offer fresh insights into the perennial issue of why stress hormones alter immune function and why chronic stress reduces disease resistance. There are also practical reasons for knowing why stress hormones reduce immunity. It has been suggested that these effects should be blocked by drugs. Without knowing their function, preventing the expression of these ancient conserved responses may bring unintended negative consequences. This study also examines how climate change will alter the impact of chronic stress on disease resistance in insects. I predict that increased temperature will exacerbate the decline in disease resistance caused by chronic stress. Reduced disease resistance in insects will have profound effects on the terrestrial ecosystem and human health (e.g. if it increases the chance that insects will become infected with diseases like West Nile Virus).