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.
ABSTRACT
Micronutrients, such as vitamins and trace minerals, are critical for supporting growth, performance, health and maintaining redox balance. Zinc (Zn), an essential micronutrient, aids the functioning of innate and adaptive immune cells. This scoping review aims to assemble and evaluate the evidence available for the role of Zn within calf immunity. Relevant literature was identified within Web of Science, PubMed, and CABI using search terms specific to the major innate and adaptive immune cell populations. There was no evidence that Zn supplementation altered neutrophil, natural killer cell, or T-cell functions. However, there was limited evidence to support Zn supplementation with reduced monocyte numbers, but there was no evidence to associate the monocytopenia with improvements in monocyte function. There is moderate evidence to suggest that Zn supplementation was beneficial for maintaining epithelial barriers of integumental and mucosal surfaces. The evidence supports supplementation above the current industry recommendations for improving immunoglobulin (Ig) production, with the strongest results being observed for IgG and IgM. Moreover, Zn supplementation was associated with reduced proinflammatory cytokine production, which may reduce inflammation-associated hypophagia and warrants further investigation. Furthermore, Zn reduced the duration of clinical signs in animals facing respiratory disease and diarrhea. However, consensus is needed about the optimal dose, route, and Zn formulation most appropriate for supporting immunity. In conclusion, while the literature supports that Zn could enhance calf immunity, there is insufficient evidence to adequately determine the extent to which Zn impacts innate immune cell and T-cell functions. Determination of the immune cell functions susceptible to modification by Zn supplementation is an important knowledge gap for enhancing the understanding of Zn and calf immunity.
