Mitochondrial function of dairy calf lymphocytes from birth to immunologic maturity

Abstract

The inability of dairy calves to fully respond to immune stimuli until they reach maturity at 6 mo of age severely limits the use of parenteral vaccines to protect calves against disease. Immune responses are metabolically demanding, and immune cells rely on mitochondrial metabolites for their functionality. Due to the essential role of mitochondria in driving T-cell responses necessary for vaccine efficacy, we hypothesized that the mitochondrial function of dairy calf lymphocytes changes with age, from birth to immunologic maturity. In this cross-sectional study, groups of dairy calves (n = 4/group) were blood sampled at birth before colostrum intake and at 1, 2, 3, 4, 6, 8, 16, and 24 wk of age. Mid-lactation adult cows (n = 4) were also sampled to reference fully mature immune cell populations. B, CD4+, CD8+, and γδ T lymphocytes were enriched using magnetic-activated cell sorting, and their mitochondrial function was assessed with an extracellular flux analyzer. Non-mitochondrial oxygen consumption, basal respiration, maximal respiration, spare respiratory capacity, proton leak, and the oxygen consumption rate (OCR) to extracellular acidification rate (ECAR) ratio were reported. Results were compared among groups using a Kruskal-Wallis test. The OCR to ECAR ratio is an indicator of the relative proportions of oxidative phosphorylation and aerobic glycolysis which is associated with effector functions in lymphocytes. The ratio was lower in 0 wk than adults in CD4+ T-cells. For CD8+ T-cells, the OCR to ECAR ratio for the 2 wk group was lower than the 3 wk group. A lower OCR to ECAR ratio indicates more reliance on glycolytic metabolism than oxidative phosphorylation. Maximal respiration is an indication of mitochondrial efficiency and is often associated with mitochondrial mass. For γδ T-cells, the 3 wk group had higher maximal respiration than the 16 wk group, whereas for B cells maximal respiration was higher in the 1 wk compared with the 16 wk group. Basal respiration indicates all cell functions that require oxygen and was lower in the 0 wk group than the 1 wk and 3 wk groups for CD4+ T-cells. γδ T-cells exhibited lower basal respiration in the 2 wk group than the 24 wk one. Although we found minimal differences in the mitochondrial outcomes reported from non-stimulated lymphocytes from birth through 6 mo of age and mid-lactation adults who served as mature immune cell populations, these results align with previous reports from weaning aged calf and adult CD4+ T-cells. In conclusion, there was insufficient evidence to suggest that the mitochondria in the lymphocytes of dairy calves from birth through immunologic maturity had functional changes associated with age. In conclusion, the capacity of unstimulated calf mitochondria to perform oxidative phosphorylation is not associated with age.