Immune Regulatory Functions of Liver in Ruminants

摘要

The liver is the biggest and complex digestive gland in ruminant. The liver participates in body's digestion, absorption, excretion, detoxifcation, substance transportation and energy metabolism. The liver's function is regulated by substrate supply and by endocrine hormones. The main methods for liver function research are multiple hepatic catheters technique, in vivo sampling and in vitro cell and tissue culture. With the development of immunology, studies shows that liver plays an important role in the regulation of body's immune function. The main compositive cells of the liver have immune regulation function, which are Kupffer's cells, essential cells and epithelial cells of hepatic sinus. The immune regulatory factors secreted by liver include TNF-α, IL-1, IL-6, NO and PGE2 etc. Endotoxin lipopolysaccharide (LPS) is a kind of macromolecule substance, and it's the main component of ectoblast of Gram's negative bacteria. LPS in ruminant's body mainly is derived from rumen metabolism. When ruminant animals are fed high concentrate, lactic acid accumulated in the rumen leads to death of Gram's negative bacteria and LPS is released. When LPS is absorbed by the body, it has an effect on body's immune function. LPS has both harmful and advantageous effect on the body. The liver plays an important role in destroying and eliminating LPS. When liver is stimulated by LPS, it releases immune regulatory substance to regulate liver's and whole body's immune function. Surgical installation of multiple hepatic catheters in ruminant and infusion of LPS via portal vein can be as a model to study the effect of liver on body immune regulation. When LPS enters into liver, it combines with LBP and CD14 and activate monocyte-macrophage system to trigger production of cytokine. Toll-like receptor (TLR4) distributing in liver tissue can also recognize LPS and activate TLR4 signal pathway and promote cytokine release. LPS induces liver to produce bioactive cytokine to support immune response by providing nutrient substance expenditure. Ruminants re-partition their metabolic substrate supply in the face of immune stresses, and the available substrate supply diverted to processes vital for the animal's own life, so the performance of ruminant animal is reduced.