Host-parasite interactions of an avian blood parasite elucidated by single-cell transcriptomics

To date, the intricate cellular and molecular interplay between hosts and parasites is well described for only a limited number of model systems. Recent methodological advances now allow deep insights into non-model host-parasite interactions, including intense reciprocal manipulations. We applied single cell RNA-sequencing to blood samples of wild, long-lived avian hosts, infected with the malaria-like parasite Leucocytozoon . We identified host cell types and the molecular toolkits involved in host immunity and parasite virulence. Comparing blood cell-type specific transcriptomic profiles between hosts with low and high parasitaemia indicates tools of parasitic immune evasion and potential modulation of the host immune response. In line with the reportedly low pathogenicity of the parasite, the response of immune cells was overall low. This lack of antagonism likely represents a result of long-lasting coevolution. Our study illuminates in rare detail a rather benign non-model host-parasite interaction and bridges the large phylogenetic gap along the evolutionary gradient from parasitism to mutualism. The cell-type specific markers identified will be useful in advancing our understanding of the evolutionary forces shaping host immunity.