Immunomorphological characteristics of human Peyer's patches.

Human follicle-associated epithelium (FAE) was found not to express the secretory component (SC) or polymeric immunoglobulin (pIg) receptor, and is therefore unable to transport pIgA to the gut lumen. However, human FAE (except the M cells) was positive for MHC class II (HLA-DR) determinants. It may therefore perform class II-restricted uptake and presentation to T cells of antigens that have been adequately processed, whereas the function of the M cells is perhaps limited to transport of particulate or undergraded luminal antigens to subjacent antigen-presenting cells (APC). A large number of APC of the inter-digitating cell phenotype (positive for HLA-DR and S-100 protein) and relatively few L1-positive reactive macrophages were found in PP between the follicles and beneath the FAE, compared with villous mucosa. Also, there were significantly more intra- and subepithelial T cells in PP than in distant villi, and the T cells were concentrated adjacent to the M cells. The proportion of the CD4+ subset (putative helper T cells) was much higher in FAE (40%) than in villous epithelium where the CD8+ subset (putative suppressor T cells) predominated strikingly (90%). This disparity might reflect differences in capacity for positive and negative immune regulation at the two sites. The relatively few B cells terminating with Ig production in PP apparently belonged to relatively mature memory clones as they showed a large proportion of IgG immunocytes and reduced J-chain expression. Conversely, both IgG and IgA immunocytes in distant mucosa showed a high percentage of J-chain positivity (80-100%); such expression was also considerable (45-60%) in mesenteric lymph nodes (MLN) in contrast to peripheral lymph nodes (PLN) and palatine tonsils. Moreover, there was a decreasing percentage of IgA2 immunocytes in the order of PP (52%), distant ileal mucosa (40%), MLN (32%), PLN (11%), tand tonsils (5%). These results support the notion that migration of relatively immature memory B-cell clones takes place from PP through MLN preferentially to distant intestinal mucosa.