Experimental design
The experiment was performed on 14 Polish Holstein-Friesian calves originating from a private dairy herd located in the north-eastern region of Poland. We are aware that the studied population was rather small; however, the evaluated animals are large, and the number of animals per group fully meets the criteria for conducting research of the type. The calves (aged 30±2 days) were fed colostrum within 1 hour after birth. Colostrum was administered in the amount of 2 kg/animal/day for 5 days. After that period, the animals were fed milk replacers until the age of 8 weeks. Immediately after birth, the calves were moved to wooden sheds outside the cow barn. At the age of one month, calves were included in a 60-day study. Prior to the experiment, the animals were weighed and subsequently blindly allocated into two groups by the analogue method. The control group (I) comprised calves fed a standard farm-made diet. Between the age of 5 days to 8 weeks, they were fed the Mlekowit (Polmass, Bydgoszcz, Poland) milk replacer in the amount of 4 L/day/animal in two portions. In the experimental group (II), the same quantity of the milk replacer was supplemented with β-hydroxy-β-methylbutyrate (HMB, Metabolic Technologies Inc. Ames, IA, USA) at 40 mg/kg BW. Beginning from the first week, the animals had unlimited access to solid feed (maize silage, meadow hay and Jösera Kälberkost prestarer) which was supplied in increasing quantities. Fresh water was supplied ad libitum. All calves enrolled in to the study survived the experimental period and were still breed in the herd.
Sample collection
Blood was sampled from the jugular vein prior to HMB supplementation of feed and on days 15, 30 and 60 of the experiment to determine and compare: chemotactic activity (MIGRATEST® kit) (Glycotope Biotechnology GmbH, Heidelberg, Germany), phagocytic activity (PHAGOTEST® kit) (Orpegen Pharma, Heidelberg, Germany) and respiratory burst (BURSTTEST® kit) (Orpegen Pharma, Heidelberg, Germany) of peripheral blood granulocytes and monocytes by flow cytometry.
Determination of the chemotactic activity of blood granulocytes and monocytes in calves with the MIGRATEST® kit
The chemotactic activity of blood granulocytes and monocytes was determined with the MIGRATEST® kit (Orpegen Pharma, Heidelberg, Germany) based on the attached instructions. The assay involved leucocyte-rich plasma (LRP) that was isolated from heparinised whole blood by spontaneous sedimentation. The LRP was extracted from each sample, and it was transferred to two cell culture inserts with pore size of 3.0 μM. One insert was placed in a well containing N-formyl-methionyl-leucyl-phenylalanine (fMLP) (Glycotope Biotechnology GmbH, Heidelberg, Germany) as the chemoattractant. The other insert was used as negative control, and it was placed in a buffer solution that did not contain the chemotactic peptide. Chemotaxis was quantified over a period of 30 min at a temperature of 37°C. An antibody reagent (FITC-labelled anti-CD62L (Glycotope Biotechnology GmbH, Heidelberg, Germany) containing counting beads was applied for 10 minutes to stain the cells. The samples were placed on ice, and vital DNA dye (Glycotope Biotechnology GmbH, Heidelberg, Germany) was added for 5 min before flow cytometry. The number of migrated neutrophils and L-selectin shedding from the surface of activated cells were determined by flow cytometry. The results were expressed by the chemotactic index which was calculated by dividing the number of cells migrating towards fMLP by the number of migrating cells in the absence of fMLP [47].
Determination of the phagocytic activity of blood granulocytes and monocytes in calves with the PHAGOTEST® kit
The phagocytic activity of blood granulocytes and monocytes was evaluated with the use of the PHAGOTEST® kit (Orpegen Pharma, Heidelberg, Germany) in line with the attached instructions. The manufacturer’s specifications were also observed in the process of preparing the reagents. The negative control was a 5 mL test tube (blue, Beckman Coulter, Fullerton, CA, USA) filled with 100 μL of whole heparinised blood chilled to 0°C. The experimental test tube (5 mL, blue, Beckman Coulter, Fullerton, CA, USA) contained 20 μL of chilled E. coli bacteria (Orpegen Pharma, Heidelberg, Germany). Both tubes were shaken at low speed for around 3 seconds. The experimental test tube was incubated at 37°C for 10 minutes. The negative control was incubated at 0°C on ice. The incubated samples were combined with 100 μL of the quenching solution (Orpegen Pharma, Heidelberg, Germany), and the test tubes were shaken. The washing solution (Orpegen Pharma, Heidelberg, Germany) chilled to 0ºC was added in the amount of 3 ml. Both samples were centrifuged for 5 min at 4ºC (250 x g), and the supernatant was removed. The samples were rinsed twice, after which, 2 ml of the lysing solution (Orpegen Pharma, Heidelberg, Germany) with room temperature was added to each sample. The samples were shaken, incubated at room temperature for 20 min, and centrifuged for 5 min at 4ºC (250 x g). The supernatant was removed. Each sample was combined with 3 mL of the washing solution (Orpegen Pharma, Heidelberg, Germany) with a temperature of 0ºC. The samples were centrifuged for 5 min at 4ºC (250 x g), and the supernatant was removed. A DNA staining solution (Orpegen Pharma, Heidelberg, Germany) chilled to 0ºC was added in the amount of 200 µL. The samples were shaken and incubated on ice for 10 min. The phagocytic activity of the evaluated cells was measured in a cytometer (Beckmann Coulter, Epics XL, USA) within less than 60 min after the addition of the last reagent. The Phagotest kit (Orpegen Pharma, Heidelberg, Germany) contains fluorescein (FITC)-stained E. coli bacteria which are phagocytised by macrophages. Cell nuclei are also stained. The number of phagocytising cells, granulocytes and monocytes are determined separately. Phagocytic activity is evaluated based on the mean fluorescence intensity (MFI) of individual cells that ingest bacteria.
Determination of the oxidative metabolism of blood granulocytes and monocytes in calves with the BURSTTEST® Kit
The oxidative metabolism of blood granulocytes and monocytes was determined with the BURSTTEST (Orpegen Pharma, Heidelberg, Germany) based on the attached instructions. The manufacturer’s specifications were also observed in the process of preparing the reagents. Whole heparinised blood was split into four portions and placed in four 100 μL test tubes (blue, Beckman Coulter, Fullerton, CA, USA) chilled to 0°C. The first sample (experimental) was combined with 20 μL of chilled E. coli bacteria (Orpegen Pharma, Heidelberg, Germany); the second sample (negative control) was combined with 20 μL of the washing solution (Orpegen Pharma, Heidelberg, Germany); the third sample (low control) was combined with 20 μL of fMLP (N-formyl-methionyl-leucyl-phenylalanine) (Orpegen Pharma, Heidelberg, Germany); the fourth sample (high control) was combined with 20 μL of PMA (4-phorbol-12-β-myristate-13-acetate) (Orpegen Pharma, Heidelberg, Germany). The contents of each test tube were stirred and incubated at 37°C for 10 min, excluding the sample containing fMLP which was incubated for 7 min. Twenty μL of substrate solution (Orpegen Pharma, Heidelberg, Germany) was added to each incubated sample, and the tubes were thoroughly shaken. The samples were incubated at 37°C for 10 min, and 2 mL of the lysing solution (Orpegen Pharma, Heidelberg, Germany) at room temperature was added to each test tube. The samples were shaken and incubated at room temperature for 20 min. The test tubes were centrifuged at 4°C for 5 min (250× g), and the supernatant was removed. The samples were rinsed once with 3 ml of the washing solution (Orpegen Pharma, Heidelberg, Germany), they were centrifuged at 4°C for 5 min (250× g), and the supernatant was removed. Each sample was combined with 200 μL of the staining solution chilled to 0°C, it was shaken and incubated for 10 min on ice. The intracellular killing activity of phagocytes was measured in a cytometer (BD Biosciences, San Jose, California, USA) within less than 30 min after the addition of the last reagent. The cells were stimulated with three activators: E. coli bacteria (Orpegen Pharma, Heidelberg, Germany), PMA (Orpegen Pharma, Heidelberg, Germany) as the strong activator, and fMLP (Orpegen Pharma, Heidelberg, Germany) as the weak activator. Oxidative stress was induced with H2O2, and mitochondria-generated reactive oxygen species converted dihydrorodamine (123-DHR) to cation rhodamine 123 (R123), the fluorescence emitter.
FACS Acquisition and Analysis
Flow cytometry was performed with the FACSCanto II cytometer (BD Biosciences, San Jose, California, USA). Data were acquired with FACSDiva version 6.1.3 software (BD Biosciences, San Jose, California, USA) and analysed in FlowJo 10 software (Tree Star, Ashland, Oregon, USA). The cytometry setup and tracking beads (CST; BD Biosciences, San Jose, California, USA) were used to initialise the photomultiplier tube (PMT). Unstained control cells and a single stain control for every fluorochrome were prepared and used to establish flow cytometric compensation. Granulocytes and monocytes were gated based on forward and side scatter (FSC/SSC) parameters (Figure 6A).
Granulocytes and counting beads were identified in PerCP to SSC scatter and depicted in FSC to SSC scatter. Data acquisition ended after the acquisition of exactly 2000 events in the region of counting beads. The number of events in the region of granulocytes was counted, and the number of granulocytes in the control sample was compared with the number of granulocytes in the positive control sample after stimulation with fMLP. The decrease in L-selectin expression can be measured simultaneously. Downregulation of this cell adhesion molecule correlates directly with the activation of neutrophils under exposure to chemotactic factors. Changes in cell shape precede cell migration and can be measured by analysing changes in forward scatter signals during flow cytometry (Figure 6B).
Statistical Analysis
Numerical results were presented as the arithmetic mean ± SD. The obtained results were processed statistically by two-way ANOVA for orthogonal design. In post-hoc analysis, Dunnett’s test was used to compare day 0 with days 15, 30, and 60 in group II (significance of differences between days: (A) p < 0.05; (B) p < 0.01; (C) p < 0.001; (D) p < 0.0001), and Tukey’s test for equal groups to compare group II with group I at each time point (significance of differences between groups: * p < 0.05; ** p < 0.01; *** p < 0.001; **** p < 0.0001) with the use of GraphPad Prism 7 software. The significance level has been set to 5% HMB.