Immunomodulation in Heifers Fed Endophyte-Infected Tall Fescue Seed

Effects of heat stress and endophyte-infected fescue seed consumption upon changes in cell-mediated immune function was measured by interferon-gamma (IFN-γ) production using flow cytometry. Twelve Angus heifers (285±28 kg) were randomly assigned to individual stalls for 11 weeks and assigned to either an endophyte-infected (E+; 10μg ergovaline + ergovalinine/kg BW) or endophyte-free (E-) diet fed at 1.8xNEm. Diets contained 12.5% fescue seed, 30.1% cottonseed hulls, 39.0% cracked corn, 12.7% supplement, 4.7% molasses, and 1.0% MGA (0.5 mg/hd/d), and met protein and mineral requirements. Weekly 45 mL blood samples were collected from each heifer. Heifers were adapted to pens for 3 weeks at thermoneutral temperatures and fed the E- diet with samples collected during this period used as covariates. Phase 1 (P1) included one week of heat stress with all animals on the E- diet. Heat stress periods consisted of cycling ambient temperature between 32°C (daytime) and 22°C (nighttime). Treatment diets were applied during Phase 2 (P2; 3 weeks) with continued heat stress exposure. All heifers were fed the E- diet and returned to thermoneutral temperatures during Phase 3 (P3). Peripheral blood mononuclear cells (PBMCs) were isolated from each sample and counted. PBMCs were then plated in 24 well plates, stimulated and stained for IFN-γ, then analyzed using flow cytometry to determine the percentage of cells producing IFN-γ and the mean fluorescence intensity (MFI), or amount of production per cell, of IFN-γ. No differences due to treatment, covariate, or interactions were observed during P1. Percent cells producing IFN-γ and IFN-γ MFI decreased across P1 (P < 0.02). E+ heifers had lower (P=0.02) percentage of cells producing IFN-γ during P2. Significant (P≤0.02) covariate effects were found for IFN-γ MFI (P2) and percentage of cells producing IFN-γ (P3), with higher baseline values associated with increased production for both. A treatment x covariate interaction occurred during P3 for IFN-γ MFI, with no treatment difference with low baseline production and increased production for E+ heifers at higher covariate values (P=0.05). Time effects (P≤0.01) were observed for IFN-γ MFI (P2) and percentage of cells producing IFN-γ (P3). IFN-γ MFI increased over the first week and decreased during the second week of P2, whereas percent cells producing IFN-γ increased across P3. These data indicate that endophyte may have immunosuppressive effects during heat stress periods with evidence of an increase in IFN-γ MFI after removal of stressors.