Postnatal development of lymphocyte subsets in different compartments of the small intestine of piglets.

In contrast to rodents, all compartments of the porcine small intestine contain lymphoid cells at birth. During the first few days of life maternal antibodies and leukocytes are taken up by the intestinal epithelium. The number of intraepithelial lymphocytes (IEL) increases by a factor of 12 from Day 1 to Day 60, but in germfree pigs only a minor increase is observed. Immediately after birth, low numbers of T cells are present in the small intestinal lamina propria (LP), and many of these express neither CD4 nor CD8 on their surface. This type of subset composition is still present in germfree pigs at an age of 1.5 months. The appearance of IgA+ and IgM+ lymphocytes in the LP differs. Many more of these B cells are found in the LP of the crypts than of the villi. The development of the discrete jejunal Peyer's patches (PP) differs from that of the continuous PP in the terminal ileum. The lymphocyte subset composition shows the most obvious differences between conventional and germfree piglets and between the jejunal and ileal PP at an age of 1.5 months. Several markers, e.g. for antigen-presenting cells, memory T cells, M cells in the domes of PP, have recently become available for the pig. These should now be applied in experiments on pigs in the early postnatal period to study the mechanisms of the development of tolerance and protective immune reactions.