Taking off the brakes: T cell immunity in the liver.

In lymphatic tissue, professional antigen-presenting cells (APCs) such as dendritic cells (DCs), mature after sensing microbe-associated molecular patterns (MAMPs) by pattern recognition receptors (PRRs), and subsequently activate T cell immunity. Non-pathogenic MAMPs, derived for example from commensal bacteria, are delivered to the liver from the gastrointestinal tract via the portal vein. However, in contrast to splenic DCs, PRRs-expressing liver APCs induce T cell tolerance rather than immunity. This is explained partly by the distinct effects of PRRs on the maturation of liver APCs: these cells activate T cell immunity only when PRRs stimulation is accompanied by microbial infection through mechanisms that are not employed by DCs in lymphatic tissue. Understanding the molecular basis of T cell tolerance and immunity in the liver may help develop novel immune therapy for persistent viral infection or liver cancer.