Different ways to die: harnessing variation in Drosophila reveals loss of tolerance and resistance over age and sex-specific association between infection susceptibility and lifespan

It is widely accepted that susceptibility to infection increases with age. The reason often invoked is the dysregulation of the immune system, which is both cause and consequence of ageing. However, we do not all age in the same way, and increased susceptibility may not be solely due to immune dysregulation affecting resistance to infection. There are many possible ways to make a host less tolerant to infection by dysregulating key physiological or metabolic processes. We hypothesised that the increase in susceptibility to infection over age can be linked to both immune ageing and decreased tolerance, and importantly, that it will depend on genotype and sex of the host. We assessed susceptibility to Gram-negative bacterial challenge in both sexes in 22 Drosophila isolines at young and old ages, and leveraged variation between genotypes to investigate how frequently an increase in susceptibility to infection was more associated with a decline in resistance or disease tolerance mechanisms. To achieve this, we assessed pathogen load to report on host immune decline. Strikingly, in most cases, greater infection susceptibility at old age was driven by reduced tolerance, although we also frequently identify cases that suffered bona fide immunosenescence, e.g. impaired resistance. We screened across bacterial pathogens during systemic and oral infections and found sex-specific signatures in survival in young and old individuals, but increased susceptibility with age occurred in both sexes. Pairing infection survival with lifespan data, we find that transcending genotype variation, susceptibility at old age predicts lifespan in males only, regardless of the existence or direction of sex bias in longevity. This work highlights that increased infection susceptibility is an early-arising ageing phenotype that occurs in both sexes, but only predicts lifespan in males, paralleling burgeoning evidence in mammals for male-biased effects of age on infection and its connection with mortality. Our data support a model where infection susceptibility increases with age following the same multiplicative pattern as organismal mortality, with existing failures making new failures more consequential. We propose that the term "immunosenescence" be used specifically to describe proven dysregulation of immune tissue resistance mechanisms. We argue that to fully understand the drivers of age-related susceptibility to infection, it is essential to consider genotype, sex, and their interaction, as well as the dysregulation of non-immune functions that influence the ability to control pathogens.