Selection for disease resistance in Oncorhynchus mykiss increases the myeloid to lymphoid balance and is affected by rapamycin as measured by flow cytometry and scRNAseq.

Abstract Heritable variation in disease resistance has been described in many species, but the effects of selection on innate immunity are unclear. Here, we compare the cellular response of two lines of rainbow trout that differ in survival due to five generations of selection for resistance to Flavobacterium psychrophilum(Fp), an important fish pathogen. We examined bacterial load, plasma biomarkers, whole blood cellular composition, and blood chemistry at days 1 and 5 after bacterial or PBS injection. We also examined the effect of 0.8 mg/kg rapamycin pretreatment, which results in immunosuppression and a ~100-fold reduction in the challenge LD 50. On day 1, bacterial load did not differ between genetic lines and treatment, but by day 5, the R-line exhibited no increase in Fpload, while rapamycin increased R- and S-line loads 33- and 506-fold, respectively. Biomarkers: cathelicidin 2 and C1q-LP3 significantly differed between genetic lines and by time, corresponding to Fpload. Analysis of whole blood by flow cytometry identified a response to infection and an ~8 percentage point higher frequency of myeloid-gated cells in R-line fish that was consistent in both naïve and challenged animals. S-line fish exhibited a higher percentage of IgM +B cells. Single-cell RNAseq was conducted on day 1 purified blood and identified 23 cell clusters including multiple B cell, neutrophil and macrophage subpopulations. In naïve animals, S-line fish harbored a cxcr4and mychexpressing B cell subpopulation absent in R-line fish. Rapamycin pretreatment caused profound loss of multiple B cell subpopulations in R- and S-line fish. In summary, selection of rainbow trout for resistance to Fpis correlated with a cellular shift in peripheral blood and is mTOR dependent. US Department of Agriculture, Agricultural Research Service; Project number 1930-32000-007