Ntigen-Specific T Cell Development during Colitisbe useful as well as information about the triggers that lead to their development. To study adaptive immune responses within colitis, the T cell transfer model of colitis is preferred [13]. In this model, na e T cells are transferred to an immune compromised host. The caveat of this model is that it relies on a genetically compromised host and an abnormal imbalance of na e and regulatory T cells that is not found in wild type animals. This model, thus, does not give insight into the immunological processes behind the development of pathological T cells in an, otherwise, healthy animal. The dextran sodium sulfate (DSS) model of colitis, in contrast to the T cell transfer model, is a robust model of colitis induced by administering dissolved DSS in the drinking water and is inducible in all backgrounds of mice [14]. It also responds to many drugs used to treat IBD, making it highly representative of IBD [15]. DSS is often considered a toxicity model as in vitro studies testing the effects of DSS on epithelial cell lines show that direct exposure causes the cell cycle arrest of epithelial cells [16]. However, these in vitro studies did not take into account the role of the mucus layer found in in vivo conditions. It is now known that DSS causes intestinal mucus to become permeable to bacteria and possibly to other luminal antigens. This would allow bacteria to come into contact with the epithelial layer below [17] and with the transepithelial dendrites of antigenseeking dendritic cells [18]. This would suggest that the DSS model, instead of being purely a toxicity model, is also modeling mucus loss and the eventual bacterial penetration found during intestinal trauma. The fact that acute DSS colitis can be induced without the help of T cells, using purely the innate immune system [19], has made it a poor candidate for T cell research. However, it is known that an adaptive immune response does develop, and T cells accumulate at the site of inflammation [20]. Furthermore, certain mouse strains (including C57Bl/6) develop long-term chronic inflammation characterized by substantial neutrophil infiltration that does not subside [21,22]. This suggests a possible role for T cells that do develop during the DSSinduced acute inflammation. Very little specific research regarding T cell development in the model exists. As it has not been previously published if Title Loaded From File antigen-specific T cells develop in mice experiencing DSS colitis, our aim was to investigate if CD4+ T cells directed against oral antigens could be found after the resolution of colitis. We found that while healthy mice developed CD4+ T cells that were reactive against the tracking antigen, ovalbumin (OVA), in only the Treg (CD4+ Foxp3+) population, DSStreated mice developed reactive CD4+ T cells in both the conventional T cell population (CD4+ Foxp3-) and the Treg population. This demonstrates that potentially proinflammatory, antigen-specific CD4+ T cells do develop during DSS colitis and that they can be tracked, making the DSS model more useful for T cell research.Materials and MethodsEthics statementAll experiments were performed in accordance with the guidelines issued by the Dutch ethics Title Loaded From File committee for animal studies. The protocol was specifically approved by the ethics committee for animal studies of Utrecht University (DEC Number: 2008.II.06.051 and 2010.II.01.013). All efforts were made to minimize suffering.AnimalsFemale C57BL/6 mic.Ntigen-Specific T Cell Development during Colitisbe useful as well as information about the triggers that lead to their development. To study adaptive immune responses within colitis, the T cell transfer model of colitis is preferred [13]. In this model, na e T cells are transferred to an immune compromised host. The caveat of this model is that it relies on a genetically compromised host and an abnormal imbalance of na e and regulatory T cells that is not found in wild type animals. This model, thus, does not give insight into the immunological processes behind the development of pathological T cells in an, otherwise, healthy animal. The dextran sodium sulfate (DSS) model of colitis, in contrast to the T cell transfer model, is a robust model of colitis induced by administering dissolved DSS in the drinking water and is inducible in all backgrounds of mice [14]. It also responds to many drugs used to treat IBD, making it highly representative of IBD [15]. DSS is often considered a toxicity model as in vitro studies testing the effects of DSS on epithelial cell lines show that direct exposure causes the cell cycle arrest of epithelial cells [16]. However, these in vitro studies did not take into account the role of the mucus layer found in in vivo conditions. It is now known that DSS causes intestinal mucus to become permeable to bacteria and possibly to other luminal antigens. This would allow bacteria to come into contact with the epithelial layer below [17] and with the transepithelial dendrites of antigenseeking dendritic cells [18]. This would suggest that the DSS model, instead of being purely a toxicity model, is also modeling mucus loss and the eventual bacterial penetration found during intestinal trauma. The fact that acute DSS colitis can be induced without the help of T cells, using purely the innate immune system [19], has made it a poor candidate for T cell research. However, it is known that an adaptive immune response does develop, and T cells accumulate at the site of inflammation [20]. Furthermore, certain mouse strains (including C57Bl/6) develop long-term chronic inflammation characterized by substantial neutrophil infiltration that does not subside [21,22]. This suggests a possible role for T cells that do develop during the DSSinduced acute inflammation. Very little specific research regarding T cell development in the model exists. As it has not been previously published if antigen-specific T cells develop in mice experiencing DSS colitis, our aim was to investigate if CD4+ T cells directed against oral antigens could be found after the resolution of colitis. We found that while healthy mice developed CD4+ T cells that were reactive against the tracking antigen, ovalbumin (OVA), in only the Treg (CD4+ Foxp3+) population, DSStreated mice developed reactive CD4+ T cells in both the conventional T cell population (CD4+ Foxp3-) and the Treg population. This demonstrates that potentially proinflammatory, antigen-specific CD4+ T cells do develop during DSS colitis and that they can be tracked, making the DSS model more useful for T cell research.Materials and MethodsEthics statementAll experiments were performed in accordance with the guidelines issued by the Dutch ethics committee for animal studies. The protocol was specifically approved by the ethics committee for animal studies of Utrecht University (DEC Number: 2008.II.06.051 and 2010.II.01.013). All efforts were made to minimize suffering.AnimalsFemale C57BL/6 mic.