Abstract
The transition from late pregnancy (LP) to early lactation (EL) in dairy cows is characterized by a major reorganization of the metabolic activities of liver and adipose tissue in support of milk synthesis. This reorganization has been attributed in large part to variation in the plasma concentration and actions of growth hormone, insulin, and other metabolic hormones. A role for the immune system has also been suggested by a near-universal rise in circulating levels of liver-derived acute-phase proteins (APP) in early lactating cows. However, less attention has been devoted to the possibility that resident macrophages of liver and adipose tissue adopt a proinflammatory state (referred herein as inflammatory tone) in parallel with the rise in plasma APP. We addressed this question by measuring the expression of genes expressed predominantly in the resident macrophage population of liver and adipose tissue and indicative of a proinflammatory (tumor necrosis factor α, IL-6, IL-12, resistin, and cluster of differentiation 80 [CD80]) or anti-inflammatory state (IL-10 and chitinase-3-like protein 1 [CHI3L1]). In a first group of cows, none of these inflammatory gene markers were regulated in liver between LP on d -29 (relative to parturition) and on d 8 of EL despite 1.7 to 5.6-fold upregulation in the expression of the APP (haptoglobin, serum amyloid α, and orosomucoid 1). In a second group of healthy cows, expression of the inflammatory gene markers did not differ between livers with low (<5.3%) or high (>11.5%) triglyceride content on d 7 of EL. In adipose tissue, a modest increase in inflammatory tone was suggested between LP and EL by increased CD80 expression and decreased CHI3L1 expression in EL. To assess the possibility that inflammatory tone would be more prominent if assayed in a cell compartment enriched with macrophages, adipose tissue was obtained in LP on d -28 and in EL on d +10 from cows experiencing a healthy transition period and fractionated into its adipocyte and stromal vascular cell (SVC) compartments. Expression of inflammatory gene markers was higher in SVC than adipocytes but remained unregulated in SVC between LP and EL. Overall, these results suggest little change in the inflammatory tone of resident macrophages in liver and adipose tissue of healthy transition dairy cows and do not support a role for the local immune system in the reorganization of metabolism in these tissues at the onset of lactation.