In the rat, mesenteric lymphadenectomy allows collection of dendritic cells (DC) derived from the small intestine after cannulation of the thoracic duct. We prepared rats this way and administered antigens by oral feeding or intraintestinal injection. DC enriched from the thoracic duct lymph collected over the first 24 h from these animals are able to stimulate sensitized T cells in vitro and to prime popliteal lymph node CD4+ T cells after footpad injection, while B and T cells from the same thoracic duct lymph are inert in priming. 500 or less DC pulsed in vitro with antigen can prime T cells in vivo, whereas 100 times more B cells or macrophages pulsed in vitro are quite inert. 1 mg of ovalbumin administered orally is sufficient to load DC for in vivo priming of T cells. Antigen could not be detected directly in DC but was present in macrophages in the lamina propria. Direct presentation of antigen by DC to T cells was demonstrated by injecting F1 recipients with parental DC and showing restriction of T cell sensitization to the major histocompatibility complex of the injected DC. Antigen-bearing DC do not induce a detectable primary antibody response but a small secondary antibody response can be detected after a boosting injection. These results show that acquisition of antigens by DC in the intestine is very similar to what occurs in vitro or in other tissues, suggesting that there may be no special difference in antigen handling at mucosal surfaces. One implication of these results is that hypotheses designed to explain oral tolerance must take into account the presence of immunostimulatory, antigen-bearing DC in animals that have received oral antigens.

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