Triennial Growth Symposium: a review of science leading to host-targeted antibody strategies for preventing growth depression due to microbial colonization.

In this review, the science used to develop host-targeted therapies for improving animal growth and feed efficiency is presented. In contrast to targeting the microbiota of the host, endogenous host proteins are targeted to regulate an overactive inflammatory response in the host. Activation of the immune/inflammatory systems of an animal is costly in terms of growth and feed efficiency. For example, reduced rates of BW gain and poorer feed efficiency in vaccinated animals compared with nonvaccinated animals have been well documented. Also, the growth rate and feed efficiency of animals colonized by microorganisms is only 80 to 90% of their germ-free counterparts. Further evidence of a cost associated with immune activation is that strategies that enhance the immune capability of an animal can reduce animal growth and feed efficiency. Research now indicates that the growth-promoting effects of antibiotics are indirect, and more likely the result of reduced immune activation due to decreased microbial exposure. Studies of mechanisms by which immune/inflammatory activation reduces animal growth and feed efficiency have shown that cytokines of the acute inflammatory response (i.e., IL-1 and tumor necrosis factor α) are key triggers for host muscle wasting. Cytokine-induced muscle wasting is linked to PG signaling pathways, and it has been proposed that regulation of the PG signaling pathways provide host targets for preventing an overreactive or unwarranted inflammatory event. Intestinal secretory phospholipase A(2) (sPLA(2)) has been found to be a useful and accessible (i.e., found in the intestinal lumen) host target for the regulation of an overreactive inflammatory response to conventional environments. This review presents the science and strategy for the regulation of intestinal sPLA(2) using orally administered egg yolk antibody against the enzyme. Clinically healthy animals fed egg antibodies to sPLA(2) had improved growth and feed efficiency. Literature presented indicates that use of host-targeted strategies for regulating the overexpression of inflammatory processes in an animal may provide new mechanisms to improve animal growth and feed efficiency.