Intestinal attraction: CCL25 functions in effector lymphocyte recruitment to the small intestine

A large surface area and continuous exposure to the outside environment through food intake make the intestines an ideal location for the growth and/or entry of pathogenic microorganisms and parasites. It is therefore not surprising that large numbers of lymphocytes are associated with the intestines within specialized gut-associated lymphoid tissues (GALT) and in the intestines themselves (1). Within the intestines, lymphocytes are localized in two main compartments, the lamina propria (LP) and the intestinal epithelium (IE). Intestinal intraepithelial lymphocytes (IELs) consist mainly of cytotoxic T cells and have been divided into two subsets, type a and type b, based on their mode of antigen recognition (2). Responses of type a IELs are MHC restricted and directed against peptides derived from invading pathogens. Type b IELs display limited T cell receptor (TCR) diversity (both αβ and γδ TCRs are utilized), are not restricted by classical MHC molecules, and are thought to be largely autoreactive, recognizing