Immune defence reaction in bumble‐bee workers after a previous challenge and parasitic coinfection

Abstract

Abstract A successful immune response requires the functioning of a range of physiological mechanisms and is therefore supposed to be costly in terms of limited resources that could be used otherwise, such as energy or nutrients. Because immunity is costly, immune defence should be reduced if appropriate multiple immunogenic challenges are present. The working hypothesis that multiple challenges will reduce the immune system’s capacity to respond is adopted. An existing infection by yet another parasite could also further reduce the strength of the immune response, since limited resources have to be divided among the different challenges. The bumble‐bee, Bombus terrestris L., and its parasites are used as a model system to study (1) whether the melanization response towards an antigen is reduced or increased after a preceding challenge (‘depletion/stimulation’); (2) if depletion occurs, how long it takes to mount a full defence response against the second challenge; and (3) whether the coinfection by another parasite reduces or increases the immune defence system’s capacity to respond. A standard technique was used where two novel antigens (nylon filaments of different sizes and time intervals) were implanted into the haemocoel of individual workers. Controls were individuals implanted with a single fixed sized implant. A gut parasite, the trypanosome Crithidia bombi, was used for coinfections. Size of the implant had no effect on the melanization response as well as time between primary and secondary implant. Primary and secondary implants differed from the controls. In particular, the primary implant was melanized more intensively than the controls and secondary, respectively. Furthermore, a coinfection by C. bombi had no effect on the encapsulation response. The results suggest that no general depletion of the encapsulation system occurs under the conditions of our experiment and that a similar immune response (melanization) independent on the magnitude of the challenge, occurs. However, there seems to be a depletion effect on the secondary implant. Furthermore, the dynamics of two different parasites seem to run independently as far as within‐host interactions are concerned.