Considerable literature exists on the best diet formulation strategies for dairy cows during the initial dry period, and subsequent new lactation
[20][47][48]. These strategies focus on satisfying nutrient requirements avoiding metabolic challenges driven by sudden changes in diet composition. Furthermore, there is a growing interest regarding the inclusion of nutraceuticals on the diet of transition dairy cows due to their specificity in modulating metabolic pathways (i.e., energy metabolism and the immune functions), allowing a fine tuning on the metabolic processes that are more likely to exceed the control of homeorhetic mechanisms
[49][50]. These compounds include probiotics, prebiotics, dietary lipids, functional peptides, phytoextracts, microminerals, vitamins, and methyl donors. However, the requirement of dairy cows for most of these active principles is very low and their direct dietary inclusion is hard to fit with the feeding routine adopted in most commercial dairy farms. Furthermore, each active principle mentioned above affects few metabolic pathways through a highly specific mode of action, while molecules interacting with multiple body compartments could likely provide a greater benefit against the multifactorial challenges affecting the metabolism of transition dairy cows. Thus, several commercial products containing a mixture of active substances have been developed to address these limitations, allowing direct dietary inclusion at the farm level. For example, Omnigen-AF (
OAF) is known as an immune modulator that could be included in the diet of dairy cows from dry-off to the first month of lactation, providing positive effects on leukocyte function and a consequent reduction of the incidence of infectious diseases in TP
[51][52]. Although a direct effect of OAF on the gene expression of neutrophils has been reported
[53], a recent study found no effects on neutrophils diapedesis measured in-vivo or on the amelioration of liver metabolism and inflammation of dairy cows receiving the feed additive
[54][55]. Nevertheless, the same study described dairy cows receiving OAF to have an increased abundance of lymphocytes at the blood level and improved leukocyte function after ex-vivo stimulation with LPS (lower lactate production and lower glucose consumption). These outcomes, paired with the improved rumination time and energy metabolism found in dairy cows receiving the additive, suggest that OAF has an indirect effect on immune function, likely mediated by positive effects on energy metabolism and feeding behavior.
Besides nutritional aspects related to diet formulation, recent interest in the alterations occurring in the immune functions of dairy cows during the peripartum period have promoted the development of specific treatments aimed at reducing their negative effects. Pegbovigrastim (Imrestor; Elanco Animal Health Greenfield, IL, USA) is a commercial product consisting of pegylated recombinant bovine granulocytes-colony stimulating factor (PEG-rbG-CSF), aimed at stimulating the circulating numbers, and possibly function of neutrophils. G-CSF is a cytokine that induces a state of neutrophilia, characterized by a ‘left-shift’ towards progenitor cells with a release of mature neutrophils and band cells from storage pools in bone marrow
[56]. PEG-rbG-CSF only needs to be injected two times (a 15 mg injection approximately a week before parturition and a second injection within 24 h after parturition), addressing the limitations of rbG-CSF treatment (i.e., daily injections, leading to an impractical on-farm application). Some studies using this product have demonstrated substantial increases in circulating neutrophil, monocyte, and eosinophil counts for several weeks
[57][31][58][59][60][61]. These were paired with modest increases in per-cell function, most consistently of extracellular release of myeloperoxidase
[59][62]. From a molecular perspective, the abundance of mRNA in most genes involved in the cell adhesion (
ITGB2,
ITGAL,
TLN1,
SELL,
SELPLG, and
CD44), recognition and immune stimulation (
CD14,
CD16,
MYD88,
TLR2, and
TLR4), antimicrobial activity (
MMP9,
LTF, and
LCN2), and inflammation cascade (
CASP1,
TNFRSF1A,
IL1B,
IL1R,
IL18,
IRAK1,
NLRP3, and
S100A8), together with the expression of
SOD2 and
ALOX5, increased in whole blood leukocytes from dairy cows treated with PEG-rbG-CSF during the TP
[57][58]. Conversely, treated cows showed lower expression of
RPL13A,
ALOX15,
IL8, and
TNF. In another study, isolated neutrophils from the blood of cows receiving PEG-rbG-CSF had an increased expression of genes related to the chain of functional steps (
ICAM1,
TLR2 and
PTGS2), while those isolated from the uterus at 4 and 7 days postpartum showed 11 differentially expressed genes, which collectively suggested enhanced antimicrobial capacity
[63]. Despite occurring only at the transcriptional level, these differences in gene expression suggest a possible improvement of migration, adhesion, and antimicrobial ability, and enhanced inflammatory response of leukocytes (especially neutrophils) with PEG-rbG-CSF. In turn, this could trigger immune cell activation and enhanced function allowing a complete activation of the immune machinery against the challenges occurring in post-partum. Recently, Lopreiato et al.
[14] showed, for the first time, the effect of PEG-rbG-CSF in maintaining stable (with no drops) PIC levels during the first month after parturition. This reflects greater regulation of neutrophil recruitment, tracking, and maturation during the inflammatory response, providing evidence of the immunomodulatory action of PEG-rbG-CSF around parturition, when dairy cows are highly immune hypo-reactive. Several randomized controlled trials have demonstrated an approximately one-third relative reduction in the incidence of clinical mastitis in early lactation following treatment with PEG-rbG-CSF
[62][64][65]. Conversely, two studies reported no differences in the incidence risk for retained placenta
[60][62], and one study reported PEG-rbG-CSF to increase the incidence risk of metritis at 21 days postpartum
[62]. Taken together, these results suggest that PEG-rbG-CSF do not consistently lead to a reduction in the incidence of diseases thought to be related to neutrophil function(i.e., retained placenta, metritis, and endometritis)
[66], suggesting that a massive increase in the number of neutrophils available in circulation does not correspond to fewer diseases.
Finally, treatments that are classically adopted in transition cows to modulate rumen fermentation, recently revealed to have a potential role in mitigating the immune dysfunction occurring immediately before and after parturition. Monensin is known for its effect on rumen bacteria populations, resulting in increased propionic acid and reduced lactic acid and methane production
[25][67][68]. Thus, modulation of rumen fermentation through monensin is an effective strategy to address the risk of both ketosis and acidosis in dairy cows at the onset of lactation
[69]. These positive effect of monensin on rumen fermentation is reflected by trends of several plasma analytes (i.e reduced BHB and NEFA concentrations and higher plasma glucose) in cows treated with monensin
[67][68][70]. A recent study
[71][72] found reduced infiltration of T and B leukocytes in the rumen fluid from transition dairy cows that received a controlled-release capsule of monensin 21 days before the expected calving date (expected release rate, 335mg/d for 95 days). Such an outcome suggests monensin promotes the stabilization of rumen milieu, probably through mitigating pH alterations related to the adaptation of dairy cows to the lactation diet. This provides a new perspective on the utilization of ionophores to modulate the immune system of early lactating cows.