The distribution of lymphoid cells in the small intestine of germ-free and conventional piglets.

Porcine ileum is populated with a high proportion of B cells but previous studies have shown that they are not developed there. While B cells prevail in the ileum even in germ-free animals, microbial colonization is a major factor that causes even a greater prevalence of B cells in the ileum and further differential representation of lymphoid cells throughout small intestine. Analysis of lymphoid subpopulations showed that the effector cells appear only after colonization. These include class-switched IgM(+)IgA(+) B cells, primed CD2(-)CD21(+) B cells, antibody-producing/memory CD2(+)CD21(-) B cells, and effector/memory CD4(+)CD8(+) αβ Th cells. While colonization resulted in a uniform distribution of effector cells throughout the gut, it caused a decrease in the frequency of cytotoxic αβ and CD2(+)CD8(+) γδ T cells. These results suggest that the ileum is a site where naive B cells expand presumably to increase antibody repertoire but the entire small intestine is immunofunctionally comparable.