Macrophage infiltration into subcutaneous adipose tissue in overconditioned cows after excessive fat accumulation

Abstract Text: Adipose tissue (AT) secretes adipokines regulating both metabolism and immune function. In monogastrics, diet-induced obesity is associated with changing adipokine profiles and increased macrophage (Ms) infiltration. However, in early lactating dairy cows we found virtually no Ms infiltration in different AT depots; non-lactating overconditioned heifers had increased accumulation of Ms in AT related with larger adipocytes, albeit in low total numbers. We hypothesized that the portion of Ms in bovine AT will remain small, even if fat is excessively accumulated in short time. Therefore we aimed to investigate the Ms infiltration in subcutaneous (sc)AT after rapid, diet-induced fat accumulation in cows. Eight non-pregnant, non-lactating pluriparous German Holstein cows, were adapted to diets with increasing concentrate feed proportions (from 0 to 60% of total dry matter intake) during the first 6 weeks of the experiment which was maintained for 9 weeks at 60% concentrate feeding. The body condition score (5-point-scale) increased from 2.31 ± 0.12 to 4.53 ± 0.14 and the body weight increased from 540 ± 20 kg to 792 ± 29 kg. Three biopsies were taken every 8 weeks of the entire experimental period from scAT of the tailhead region. Immunohistochemistry was performed on cryosections (12 µm) using the Ms-specific marker CD68. Bovine lymph nodes were used for positive and negative controls. The number of Ms and adipocytes per mm² were counted (100-fold magnification; 10 fields per sample). The portion of Ms was calculated from the mean number of positive stained cells/mean number of total adipocytes x 100. In total, 12 out of 23 samples yielded CD68-positive stainings. Considering the different time points, 5 out of 7, 5 out of 8 and 2 out of 8 samples were Ms-positive at the beginning, the middle and the end of the experiment, respectively. However, the average portion of Ms was only marginal: 3.7 ± 3.0% (n=7) at the beginning, 0.9 ± 0.3% (n=8) at the middle and 0.5 ± 0.3% (n=8) at the end of the experiment. Thus even a rapid and pronounced increase of fat mass was not accompanied by Ms infiltration into subcutaneous AT. In consideration of the virtual absence of Ms in AT in earlier studies about cows during the first weeks of lactation, and the low portion of Ms in overconditioned heifers and in the present study, we assume that Ms infiltration is of no importance for bovine AT.

Keywords: macrophages, adipose tissue, cow