須藤 明

Allergic inflammation is mediated by Th2 cell-derived cytokines, including IL-4, IL-5, and IL-13, and down-regulated by IFN-gamma and IL-12. Tyk2 is a member of the Janus family of protein tyrosine kinases and is activated by a variety of cytokines: IFN-alphabeta, IL-6, IL-10, IL-12, and IL-13. In this study, we investigated the role of Tyk2 in the regulation of Ag-induced Th cell differentiation and Ag-induced allergic inflammation in the airways using Tyk2-deficient (Tyk2(-/-)) mice. When splenocytes were stimulated with antigenic peptide, IL-12-mediated Th1 cell differentiation was decreased, but IL-4-mediated Th2 cell differentiation was increased in Tyk2(-/-) mice. In vivo, Ag-specific IgE and IgG1 production was increased, but Ag-specific IgG2a production was decreased in Tyk2(-/-) mice as compared with those in control mice. In addition, Ag-induced eosinophil and CD4+ T cell recruitment, as well as the production of Th2 cytokines in the airways, was increased in Tyk2(-/-) mice. Adoptive transfer experiments revealed that CD4+ T cells were responsible for the enhanced Ag-induced eosinophil recruitment in Tyk2(-/-) mice. In contrast, although the level of IL-13 was increased in the airways of Tyk2(-/-) mice after Ag inhalation, the number of goblet cells, as well as Muc5ac mRNA expression, was decreased in Tyk2(-/-) mice. Together, these results indicate that Tyk2 plays a bilateral role in the regulation of allergic inflammation in the airways: Tyk2 plays a role in the down-regulation of Th2 cell-mediated Ab production and eosinophil recruitment in the airways by regulating Th1/Th2 balance toward Th1-type, while Tyk2 is necessary for the induction of IL-13-mediated goblet cell hyperplasia in the airways.

Interleukin 21 (IL-21) has recently been identified as a multifunctional cytokine that induces the proliferation of T cells and B cells and differentiation of natural killer cells. To determine whether IL-21 regulates IL-4-mediated immune responses, we examined the effect of IL-21 on antigen-specific IgE production in mice. We also examined the effect of IL-21 on IL-4-induced IgE production from B cells and antigen-induced T-helper 2 (T(h)2) cell differentiation. The in vivo injection of IL-21 prevented antigen-specific IgE but not IgG2a production on immunization. IL-21 did not affect T(h)2 cell differentiation or IL-4 production from CD4(+) T cells but directly inhibited IL-4-induced IgE production from B cells at single-cell levels. Moreover, IL-21 inhibited IL-4-induced germ line Cepsilon transcription in B cells without the inhibition of signal transducer and activator of transcription 6 (Stat6) activation. Taken together, these results indicate that IL-21 down-regulates IgE production from IL-4-stimulated B cells through the inhibition of germ line Cepsilon transcription and thus suggest that IL-21 may be useful for the treatment of IgE-dependent allergic diseases.

Accumulating evidence has shown the importance of Stat6-mediated signaling in allergic diseases. In this study, we show a novel regulatory mechanism of Stat6-mediated signaling in mast cells. When Stat6 is activated by interleukin (IL)-4 and translocated to the nucleus, Stat6 is cleaved by a nucleus-associated protease in mast cells. The cleaved 65-kD Stat6 lacks the COOH-terminal transactivation domain and functions as a dominant-negative molecule to Stat6-mediated transcription. The retrovirus-mediated expression of cleavage-resistant Stat6 mutants prolongs the nuclear accumulation of Stat6 upon IL-4 stimulation and enhances IL-4-induced gene expression and growth inhibition in mast cells. These results indicate that the proteolytic processing of Stat6 functions as a lineage-specific negative regulator of Stat6-dependent signaling in mast cells, and thus suggest that it may account for the limited role of Stat6 in IL-4 signaling in mast cells.

It has recently been shown that CD4(+)CD25(+) T cells are immunoregulatory T cells that prevent CD4(+) T-cell-mediated organ-specific autoimmune diseases. In this study, the regulatory mechanism of CD4(+)CD25(+) T-cell development were investigated using T-cell receptor (TCR) transgenic, mice. It was found that CD4(+)CD25(+) T cells preferentially expressed the endogenous TCRalpha chain in DO10(+) TCR transgenic mice compared with CD4(+)CD25(-) T cells. Moreover, it was found that CD4(+)CD25(+) thymocytes were severely decreased in DO10(+) TCR-alpha(-/-) mice In positively selecting and negatively selecting backgrounds, whereas CD4(+)CD25(-) thymocytes efficiently developed by transgenic TCR in DO10(+) TCR-alpha(-/-) mice in positively selecting backgrounds, indicating that the appropriate affinity of TCR to major histocompatibility complex (MHC) for the development of CD4(+)CD25(+) thymocytes is different from that of CD4(+)CD25(-) thymocytes and that a certain TCR-MHC affinity is required for the development of CD4(+)CD25(+) thymocytes. Finally, it was found that, in contrast to thymus, CD4(+)CD25(+) T cells were readily detected in spleen of DO10(+) TCR-alpha(-/-) mice in positively selecting backgrounds and that splenic CD4(+)CD25(+) T cells, but not CD4(+)CD25(+) thymocytes, were significantly decreased in B-cell-deficient mice, suggesting that B cells may control the peripheral pool of CD4(+)CD25(+) T cells. Together, these results indicate that the development of CD4(+)CD25(+) T cells in thymus and the homeostasis of CD4(+)CD25(+) T cells in periphery are regulated by distinct mechanisms. (Blood. 2002;99:555-560) (C) 2002 by The American Society of Hematology.