Potential for a localized immune response by the ruminal epithelium in nonpregnant heifers following a short-term subacute ruminal acidosis challenge

摘要

The aim of this study was to investigate whetherthe ruminal epithelium activates a local inflammatoryresponse following a short-term subacute ruminal acidosis(SARA) challenge. Seven ruminally cannulated,nonpregnant, nonlactating beef heifers, fed a baselinetotal mixed ration (TMR) with 50:50 forage-to-concentrateratio, were used in a crossover design with 2periods and 2 treatments: SARA and control (CON).Induction of SARA included feed restriction (25% ofdry matter intake [DMI] for 24 h) followed by a grainoverload (30% of baseline DMI) and provision of thefull TMR; whereas, the CON group received the TMRad libitum. Ruminal pH was recorded using indwellingprobes, and ruminal lipopolysaccharide (LPS)concentration was measured daily following the challengeuntil d 6. Biopsies of ruminal papillae from theventral sac were collected on d 2 and 6 after the grainoverload. Transcript abundance of genes associatedwith acute inflammation was measured by quantitativereal-time PCR, normalized to the geometric meanof 3 stable housekeeping genes. Target genes includedtoll-like receptor-2 (TLR2), TLR4, TLR9, tumor necrosisfactor-α (TNFA), prostaglandin endoperoxidesynthase-1 (PTGS1), PTGS2 transforming growthfactor β-1 (TGFB1), and 4 intermediate enzymes ofleukotriene synthesis (ALOX5, ALOX5AP, LTA4H,and LTC4S). Protein localization and expression ofTLR4 were quantified by image analysis of fluorescenceintensity. Statistical analysis was performed using as acrossover design with fixed effects of treatment, day,and the treatment × day interaction with the randomeffect of day within period. Ruminal pH was below 5.6for 4.5 h/d and below 5.8 for 6.9 h/d in the SARAgroup compared with 22 and 72 min/d, respectively,for CON. Ruminal LPS concentration peaked on d 2in SARA heifers at 51,481 endotoxin units (EU)/mLcompared with 13,331 EU/mL in CON. Following grainoverload, small but statistically significant decreases inthe transcriptional abundance of TLR2, TLR4, TNF,PTGS2, ALOX5, and ALOX5AP were seen in SARAversus CON heifers. A functionally relevant decrease inTLR4 expression in SARA heifers compared with CONwas confirmed by a decrease in fluorescence intensity ofthe corresponding protein following immunohistofluorescentstaining of papillae. The study results indicate asuppression of the inflammatory response in the ruminalepithelium and suggest that the response is tightlyregulated, allowing for tissue recovery and return tohomeostasis following SARA.