Authors

Y Corripio-Miyar¹; A Hayward¹; H Lemon⁴; X Bal³; J Pilkington²; JM Pemberton³; D Nussey²; TN McNeilly¹;  ¹ Moredun Research Institute, UK;  ² University of Edinburgh, Institute of Evolutionary Biology, UK;  ³ University of Edinburgh, UK;  ⁴ University of Edinburgh, Institute of Ecology and Evolution, UK

Discussion

T-helper (Th) cells co-ordinate the vertebrate immune response to ensure that infections with a diverse array of parasites are met with appropriate and effective responses. Helminths are typically associated with a T-helper type 2 (Th2) cell-mediated immune response, while intracellular parasites such as protozoans are associated with a Th1 response. Laboratory models have typically reported that Th1 and Th2 responses are antagonistic, such that a strong Th2 response is associated with a downregulated Th1 response. These results, however, have been challenged by studies of natural infections that suggest much less clear-cut results in resource-limited, genetically diverse, hosts infected with multiple parasite species. Since 1985, a wild population of Soay sheep living in the St Kilda archipelago, NW Scotland, has been the subject of an individual-based study, with animals monitored from birth until death and repeatedly captured and sampled. The sheep are infected with gastrointestinal nematodes (GIN) and coccidia, which are highly prevalent and known to influence host body weight and survival. GIN and coccidian infections have been monitored for >30 years, and for the last 4 years we have measured immune phenotypes associated with different aspects of cell-mediated and humoral immunity. We found that counts of Th cells of different phenotypes were generally positively correlated with each other, as were levels of different cytokines and different antibodies, but there were no strong positive associations among cell counts, cytokines or antibodies. Cell counts generally had low repeatability (among individual variation) across the four years, while antibodies were highly repeatable; the Th1 cytokine IFN-γ and the Th2 cytokine IL-4 were moderately repeatable. IFN-γ and IL-4 were positively correlated at both the between- and within-individual levels, suggesting individuals with high levels of one cytokine tended to have high levels of the other, although this result was not apparently mediated by body condition or parasite exposure. IL-4 was negatively associated with GIN faecal egg count, while IFN-γ was negatively associated with coccidian faecal oocyst count, suggesting that these cytokines reflect resistance to these respective parasites. None of our immune markers were strongly associated with lamb survival once body weight was accounted for. Our results provide some glimpses of how different aspects of immune function interact to produce effective responses to complex infection, but more data will enable these interactions to be dissected.