Cellular immune senescence in mosquitoes accelerates when the temperature is warmer

Mosquitoes are poikilotherms and ectotherms, so their body temperature is predicated by the temperature of their environment. As the temperature rises, development and metabolism quicken. At the same time, immune strength weakens with aging, a process called senescence. Aging can be characterized as a function of time (chronological age) or as a function of how well the body operates (physiological age), and we predict that warmer temperature decouples chronological and physiological aging, accelerating immune senescence. We evaluated how warmer temperature and aging interact to alter cellular immunity in the mosquito, Anopheles gambiae, by rearing them at three temperatures and quantifying the number of immune cells, called hemocytes, and their phagocytic activity at four ages. We discovered that the number of circulating hemocytes decreases with warmer temperature and aging, and that the aging-dependent decrease occurs faster when the temperature is warmer. However, the number of sessile hemocytes attached to the dorsal abdominal wall increases with infection and warmer temperature but decreases with aging. When a mosquito is infected, the aging-dependent decrease in the number of sessile hemocytes occurs faster when the temperature is warmer. Although the number of hemocytes decreases with aging, the phagocytic activity of individual hemocytes increases, with the aging-dependent increase occurring earlier when the temperature is warmer. Overall, warmer temperature accelerates senescence of the cellular immune response in mosquitoes, which has implications for how poikilotherms and ectotherms fight infections as they age in our warming world.