We have taken advantage of the Cre/lox system to generate a mouse model with inducible deficiency of transforming growth factor ␤ receptor II (T␤RII). Using this approach, transforming growth factor ␤ (TGF-␤) signaling deficiency can be restricted to the hematopoietic system by bone marrow transplantation. Mice that received transplants with T␤RII ؊/؊ bone marrow develop a lethal inflammatory disorder closely resembling that of TGF-␤1-null mice. Previous in vitro studies have suggested multiple roles for TGF-␤ in T-cell development, including proliferation, apoptosis, and differentiation. We used our transplantation model to ask whether T-cell development is normal in the absence of TGF-␤ signaling. The findings show for the first time in vivo and in fetal thymus organ culture (FTOC) that TGF-␤ is not required for thymocytes to differentiate along the entire pathway of thymic T-cell development, as defined by the expression patterns of CD4, CD8, CD25, and CD44. In contrast to previous investigations, no increase of thymocyte apoptosis was observed. However, T␤RII-deficient CD8 ؉ thymocytes displayed a 2-fold increase in proliferation rate, as determined by bromodeoxyuridine ( IntroductionTransforming growth factor ␤ (TGF-␤) is a group of growth factor isoforms (TGF␤1, TGF␤2, and TGF␤3) involved in a multitude of physiologic functions including the immune system. They primarily interact with the TGF-␤ type I receptor (T␤RI) and type II receptor (T␤RII), both essential for propagation of the intracellular signal, including phosphorylation of Smad (small mother against decapentaplegic) proteins regulating transcriptional activation. The development of immature thymocytes to CD4 ϩ or CD8 ϩ singlepositive (SP) T cells is defined by their sequential expression of cell surface markers such as CD3, CD4, CD8, CD25, CD44, and the T-cell receptor (TCR). 1,2 Involvement of TGF-␤ in T-cell function has been suggested by in vitro studies demonstrating that TGF-␤ inhibits interleukin-1 (IL-1)-, IL-2-, and IL-7-dependent thymocyte proliferation. [3][4][5][6] Expansion of thymocytes may also be indirectly promoted by an inhibitory action of TGF-␤ on apoptosis. [7][8][9] Inhibitory effect of TGF-␤ on thymocyte development has been shown in fetal thymus organ culture (FTOC). 10 This study demonstrated that TGF-␤ causes a very early block in development within the CD4 Ϫ CD8 Ϫ population, affecting differentiation of the CD44 ϩ CD25 Ϫ into the CD44 ϩ CD25 ϩ subset of thymocytes. Further evidence for an inhibitory mode of action on T-cell development derives from the observation that TGF-␤ inhibits differentiation of CD4 ϩ thymocytes into the T helper 1 (Th1) and Th2 subsets of T helper cells. 11,12 A more complex immunoregulatory role of TGF-␤ emerged by the finding that TGF-␤ may also have stimulatory effects on immune functions. Thus, at certain stages of T-cell development, TGF-␤ stimulates, rather than inhibits, proliferation. 13 A critical role of TGF-␤ in T-cell homeostasis and function has been reinforced by studies using kn...

Thymic expression of insulin has been suggested to play a major role in negative selection of autoreactive T cells and tolerance induction against pancreatic β cells. Furthermore, the expression of insulin in peripheral antigen‐presenting cells (APC) has been clearly demonstrated but whether thymic negative selection and tolerance induction also depends on peripheral influx of self‐antigens (Ag) remains to be conclusively demonstrated. In this study, we wanted to test whether peripheral influx of insulin expressing cells might contribute to negative selection. In order to address this question, we used mice deficient in the Ins1 and Ins2 genes. Embryonic thymi either deficient in both insulin genes or expressing Ins2 were dissected and transplanted under the kidney capsule of athymic nude mice recipients. After indicated time points, grafted thymi were removed and analysed for insulin re‐expression and for the emergence of autoreactive T cells. The analysis revealed a re‐expression of Ins2 in grafted insulin deficient thymi suggesting that self‐Ag expression in the thymus is not only intrinsically regulated but peripheral influx of APC capable of expressing insulin might contribute to thymic selection and tolerance induction.

Ectopic expression of insulin in thymus has been suggested to be involved in tolerance induction against pancreatic beta cells and in type 1 diabetes (T1D) pathogenesis. However, it is not known whether thymic insulin expression would also influence thymocyte maturation and differentiation. To address these questions, we have used mice that are insulin deficient. Early fetal thymi were cultured in fetal thymic organ cultures (FTOCs) and the development of thymocytes was studied by flow cytometry. The results revealed no significant difference in thymocyte maturation in the absence of thymic insulin. Taken together, these data do not support a role for thymic insulin in thymocyte differentiation and growth.