Antigen presenting cells in the porcine gut

It has long been known that mucosal responses are most effectively stimulated by local presentation of antigen but the mechanisms whereby the gut immune system is able to distinguish between potential pathogens and harmless dietary antigens are not clear. The gut immune system is capable of mounting both primary and secondary responses to potentially harmful antigens while avoiding the expression of damaging responses to harmless food proteins. Historically, most attention has focused on Peyerʹs patches and there is evidence of their role in the induction of both primary and secondary responses. Fed antigen can also be detected in the intestinal lamina propria (LP) and it has been shown that murine LP cells can stimulate allogeneic mixed lymphocyte responses and present KLH to naive T cells. In contrast to guinea pigs, rodents and humans, pig intestinal epithelial cells do not express MHC Class II molecules, but they are present on a number of other cell types in the subepithelial LP. Amongst these are a significant proportion of non-professional APC including endothelial cells and eosinophils. Phenotypically pig LP T cells are a homogeneous population and the majority of CD4 T cells express the low molecular weight isoform of CD45. This is compatible with the suggestion that they are CD45RO-positive cells. A significant proportion of LP CD4 T cells are CD25 (IL-2R) positive, but following activation they secrete IL-4, with little or no IL-2 production. Based upon these observations, we would conclude that the lamina propria is a unique immunological microenvironment, and suggest that it may be of significance not only in surveillance and the provision of help during rapid responses to recall antigens but also in the down-regulation of local responses to food-derived peptides.