Symposium review: The importance of the ruminal epithelial barrier for a healthy and productive cow

Toll–IL-1–resistance (TIR) domain–containing adaptor-inducing IFN-β (TRIF)–related adaptor molecule (TRAM) is the fourth TIR domain–containing adaptor protein to be described that participates in Toll receptor signaling. Like TRIF, TRAM activates interferon regulatory factor (IRF)-3, IRF-7, and NF-κB-dependent signaling pathways. Toll-like receptor (TLR)3 and 4 activate these pathways to induce IFN-α/β, regulated on activation, normal T cell expressed and secreted (RANTES), and γ interferon–inducible protein 10 (IP-10) expression independently of the adaptor protein myeloid differentiation factor 88 (MyD88). Dominant negative and siRNA studies performed here demonstrate that TRIF functions downstream of both the TLR3 (dsRNA) and TLR4 (LPS) signaling pathways, whereas the function of TRAM is restricted to the TLR4 pathway. TRAM interacts with TRIF, MyD88 adaptor–like protein (Mal)/TIRAP, and TLR4 but not with TLR3. These studies suggest that TRIF and TRAM both function in LPS-TLR4 signaling to regulate the MyD88-independent pathway during the innate immune response to LPS.

During subacute ruminal acidosis (SARA) rumen pH is depressed for several hours per day due to accumulation of volatile fatty acids and insufficient rumen buffering. Surveys suggested an incidence of SARA of between 19% and 26% in early and mid-lactation dairy cows. Causes of SARA include feeding excessive amounts of non-structural carbohydrates and highly fermentable forages, and insufficient dietary coarse fiber. Consequences of SARA include feed intake depression, reduced fiber digestion, milk fat depression, diarrhea, laminitis, liver abscesses, increased production of bacterial endotoxin and inflammation characterized by increases in acute phase proteins. The increase in endotoxin is similar among methods for SARA induction, but depends on the diet fed before induction. Increases in acute phase proteins vary among methods of SARA induction, even when the methods result in similar rumen pH depressions. This suggests that the inflammatory response might not be solely due to bacterial endotoxin in the rumen.

The effects of a grain-based subacute ruminal acidosis (SARA) challenge on translocation of lipopolysaccharide (LPS) into the peripheral circulation, acute phase proteins in blood and milk, feed intake, milk production and composition, and blood metabolites were determined in 8 lactating Holstein cows. Between wk 1 and 5 of 2 successive 6-wk periods, cows received a total mixed ration ad libitum with a forage to concentrate (F:C) ratio of 50:50. In wk 6 of both periods, the SARA challenge was conducted by replacing 21% of the dry matter of the total mixed ration with pellets containing 50% wheat and 50% barley. Rumen pH was monitored continuously using indwelling pH probes in 4 rumen cannulated cows. Rumen fluid samples were collected 15 min before feed delivery and at 2, 4, 6, 12, 14, 16, 18, and 24 h after feed delivery for 2 d during wk 5 (control) and wk 6 (SARA). Peripheral blood samples were collected using jugular catheters 15 min before feeding and at 6 and 12 h after feeding at the same days of the rumen fluid collections. The SARA challenge significantly reduced average daily pH from 6.17 to 5.97 and increased the duration of rumen pH below pH 5.6 from 118 to 279 min/d. The challenge reduced dry matter intake (16.5 vs. 19 kg/d), milk yield (28.3 vs. 31.6 kg/d), and milk fat (2.93 vs. 3.30%, 0.85 vs. 0.97 kg/d), and tended to increase milk protein percentage (3.42 vs. 3.29%), without affecting milk protein yield (1.00 vs. 0.98 kg/d). The challenge also increased the concentration of free LPS in rumen fluid from 28,184 to 107,152 endotoxin units (EU)/mL. This was accompanied by an increase in LPS in peripheral blood plasma (0.52 vs. <0.05 EU/mL) with a peak at 12 h after feeding (0.81 EU/mL). Concentrations of the acute phase proteins serum amyloid A, haptoglobin, and LPS-binding protein (LBP) in peripheral blood as well as LBP concentration in milk increased (438.5 vs. 167.4, 475.6 vs. 0, 53.1 vs. 18.2, and 6.94 vs. 3.02 microg/mL, respectively) during SARA. The increase in LBP in combination with the increase in LPS in peripheral blood provides additional evidence of translocation of LPS. Results suggest that the grain-based SARA challenge resulted in translocation of LPS into the peripheral circulation, and that this translocation triggered a systemic inflammatory response.