The intestinal mucosa is the largest lymphoid organ in the body, and it simultaneously has to protect us from any pathogen that might enter the gastrointestinal tract as well as maintaining tolerance to most antigens, mainly in the form of dietary constituents and microbial products. A dysfunctional control of local intestinal immune responses may lead to development of Inflammatory Bowel Disease (IBD) - the common name for ulcerative colitis (UC) and Crohn´s disease (CD). These diseases are both manifested as chronic intestinal inflammation characterised by nausea, vomiting, recurrent attacks of diarrhoea and severe pain, sometimes even leading to death. Mice homozygously deficient in the G protein ai2 subunit (Gai2-/-) develop IBD with an immunopathology strikingly similar to that observed in patients with ulcerative colitis, including the development of adenocarcinomas of the colon. Previous studies performed on Gai2-/- mice have demonstrated immune changes characterised by activation of proinflammatory T helper 1 cells in late stages of the disease. This thesis focuses on immunological alterations preceding the colitic state of the Gai2-/- mice. Furthermore, a new therapeutic strategy for Th1 mediated inflammatory reactions is described.The inflammation in the large intestine of Gai2 -/- mice is preceded by immunological alterations in both the small and large intestine characterised by increased spontaneous production of proinflammatory cytokines, increased frequencies of T and B lymphocytes with an activated phenotype homing to the intestinal mucosa and antibodies specific for normal intestinal flora as well as self-tissues, present locally in the intestine. Antibodies specific for dietary antigens and the cytoplasmic neutrophil protein pANCA are however not present until after onset of disease. The precolitic changes also include the regression of Peyer´s patches (PP) of the small intestine, likely caused by excessive apoptosis due to decreased levels of the anti-apoptotic protein Bcl-2 in PP lymphocytes from Gai2 -/- mice. Gai2-/- mice have a Th1 biased response in the PP characterised by increased production of IFN-g and IL-2 following challenge with orally administered or luminal antigens. The cytokine production from the lamina propria lymphocytes of both the small and large intestine is also dominated by IFN-g, concurrently with undetectable production of IL-10. The frequency of Ig producing antigen specific plasma cells in the lamina propria of Gai2-/- mice is decreased following oral immunisations, probably at least partly due to the impaired local T cell help. Treatment of Gai2-/- mice with a three-component B. pertussis vaccine significantly increased the spontaneous production of IL-10 in the intestinal mucosa, attenuated inflammation of the colon and reduced the mortality. The attenuating effect on colitis is suggested to be due the B. pertussis antigen filamentous hemagglutinin (FHA), which selectively induced apoptosis of T helper 1 cells concomitantly with decreased production of proinflammatory cytokines
