Longitudinal characterization of bovine monocyte-derived dendritic cells from mid-gestation into subsequent lactation reveals nadir in phenotypic maturation and macrophage-like cytokine profile in late gestation

• Dynamic changes to monocyte composition and moDC maturation over bovine pregnancy.
• Reduction in intM and ncM proportions, and increase in cM proportions in pregnancy.
• moDC have weak CD80 and MHC II upregulation, high CD14 expression in late gestation.
• moDC produced highest levels of IL-10 and IL-1β in late gestation.

Changes in monocyte and dendritic cell populations during bovine pregnancy and lactation remain poorly described despite the key roles these cells play in immune tolerance and activation. Using a prospective longitudinal study, we characterized CD14+ monocyte-derived dendritic cell (moDC) differentiation and maturation and captured monocyte composition dynamics from mid-gestation through calving and into the subsequent lactation in dairy cows (n = 7). First, we measured absolute counts of classical (CD14 + CD16−, cM), intermediate (CD14 + CD16+, intM), and nonclassical (CD14-CD16+, ncM) monocytes in the blood and determined proportions of individual subsets within the total monocyte population. We found the proportion of cM decreased and intM increased significantly by early lactation, whereas there was a nadir in the proportion of ncM in late gestation, two weeks prepartum. Monocyte composition appears to be regulated in pregnancy, possibly to limit the proportion of highly inflammatory monocytes i.e. intM. Ultimately, we found that moDC differentiated from CD14+ monocytes isolated in the early dry period of late gestation had impaired E. coli-induced maturation, with nadirs in upregulation of CD80 and MHC II, and downregulation of CD14. The moDC from late gestation also had altered cytokine profiles with greatest production of pro-inflammatory IL-1β and anti-inflammatory IL-10. These data suggest monocytes in late gestation, in contrast to other stages of pregnancy and lactation, differentiate and maturate into moDC less capable of eliciting strong T cell activation, and have macrophage-like cytokine profiles. These results provide insight into maternal immune modulation and elucidate potential immune changes necessary to facilitate bovine pregnancy.