Dysregulated Intrathymic Development in the IL-2-Deficient Mouse Leads to Colitis-Inducing Thymocytes

Gene-targeted mice lacking the IL-2 gene (IL-2 -/- mice) develop various forms of autoimmunity as well as severe colitis, either spontaneously in a conventional environment or after immunization with 2,4,6-trinitrophenol (TNP)-conjugated keyhole limpet hemocyanin (KLH) in a specific pathogen-free environment. We show here that the induction of colitis with TNP-KLH induces a change in the thymocyte population characterized by decreased numbers of double positive (DP; CD4⁺CD8⁺) thymocytes (IL-2 +/+, 45.2 × 10⁶ vs IL-2 -/-, 23.6 × 10⁶) and increased numbers of single positive (SP; CD4⁺CD8^- or CD4^-CD8⁺) thymocytes (IL-2 +/+, 5.3 × 10⁶ vs IL-2 -/-, 20.9 × 10⁶). The latter also bear activation markers. In addition, thymocytes from TNP-KLH-immunized IL-2 -/- mice produce more IFN-γ and less IL-4 than similarly immunized IL-2 +/+ mice. These defects in thymocyte maturation and lymphokine production are IL-12 driven, since they are prevented when immunized IL-2 -/- mice are coadministered with anti-IL-12. Furthermore, we demonstrate that IL-2 -/- mice exhibit decreased cortical apoptosis as determined by thymocyte numbers and detection of apoptotic cells in situ. Finally, we show that colitis-inducing thymocytes are generated in the immunized IL-2 -/- thymus, since IL-2 +/+ mice develop colitis following injection of small numbers of single positive thymocytes from immunized IL-2 -/- mice but not from IL-2 +/+ mice. Taken together, these data indicate that, in the absence of IL-2, thymocyte maturation is abnormally directed by IL-12 toward the generation of mature, activated Th1-type thymocytes that are capable of mediating colitis.