Immunological tolerance to antigens in the intestine

Abstract

The major part of the immune system is found in the epithelium and lamina propria of the gut. The distribution of the leukocytes is structured probably because the immune system has to distinguish between harmful microorganisms, food antigens and the normal gut flora. Feeding a protein to animals results in a state of systemic hyporesponsivness of certain immune responses. This mechanism is called oral tolerance. The aim of this thesis was to investigate the events associated with the development of oral tolerance to food proteins and bacterial antigens in the gut. Rats were colonized with a transgenic Esherichia coli O6K13 strain producing ovalbumin (OVA), at birth or at adult age. Animals colonized from birth had lower delayed type hypersensitivity (DTH) reaction against OVA and lower levels of IgG antibodies against OVA, type 1 pili and O6 LPS in serum compared to uncolonized controls. The tolerance to the bacterial antigens was partly due to regulatory cells. Adult colonized animals responded with an increased DTH reaction against OVA and increased levels of IgG antibodies against OVA and O6 LPS in serum compared to uncolonized age-matched controls. Rats were fed OVA or control diet. Two hours later blood was taken and serum prepared and injected intraperitoneally into recipients. The DTH reaction to OVA and IgG anti-OVA antibody levels were suppressed in animals receiving serum from OVA fed rats compared to controls. The most prominent tolerogenic activity was found in a serum fraction containing structures larger than 100 000 Dalton. The activity in serum could also down regulate the immune response to a bystander antigen indicating the presence of regulatory cells. In vitro, CD25+ lymphocytes were shown to mediate the suppression against OVA. Further, the tolerogenic serum factor had the capacity to down regulate an established immune response in recipients. The tolergenic activity was associated with MHC class II molecules and could be recovered in the pellet from unltracentrifuged serum taken from OVA fed donors. Rats made tolerant to OVA by feeding and immunized with OVA and human serum albumin had significantly smaller draining lymph nodes than controls. The smaller size of the lymph nodes coincided with the presence of TGF-b producing cells in the nodes Conclusions: Rats develop regulatory mechanisms to antigens associated with bacteria in the intestine. Feeding an antigen induces an antigen specific tolerogenic factor in serum. This factor is larger than 100 000 Dalton and can transfer tolerance to recipients. The factor can suppress an established immune response as well as impair a new response. It is proposed that the tolerogenic serum factor consists of vesicular structure produced by the small intestinal epithelium, which is named tolerosome.