Novel Functions Of Thymic Stromal Lymphopoietin In Immune Responses

Thymic stromal lymphopoietin (TSLP) is an epithelial cell derived cytokine essential for the initiation and development of Th2-mediated allergic inflammation by acting on myeloid and lymphoid populations to coordinate innate and adaptive immunity. Therefore TSLP holds promise as a novel therapeutic target for asthma and other allergic diseases. Considering pro-inflammatory and inflammatory cytokines induce TSLP expression in various tissues, it won’t be surprising to find that TSLP has a more generalized function in inflammatory diseases in addition to Th2mediated allergic diseases. In the present study, we further investigated the role for TSLP in the pathogenesis of allergic inflammation, especially in regulating the balance of airway tolerance and allergic inflammation, its function in LPS-induced Th2inflammation, and its ability to promote IL-9-secreting Th9cells in allergic airway inflammation.Induction of Ag-specific Tregs is an important mechanism to maintain mucosal tolerance against harmless Ags. In this study, we demonstrated that TSLP inhibited the generation of mouse inducible regulatory T cells (iTregs) in a dose-dependent manner. Since significantly increased serum TSLP was reported in children with atopic dermatitis, inhibition of iTreg differentiation may be an important component of the function of TSLP to suppress airway tolerance and initiate allergen sensitization leading to the development of allergic airway inflammation.Whether and how TSLP is involved in the initial priming of Th2differentiation against harmless antigen remains unclear. Using an intranasal sensitization protocol with OVA and LPS, we showed that TSLP signaling was required for low-dose LPS induced Th2inflammation, but not for high-dose LPS induced Thl immunity. We further demonstrated that low-dose LPS-activated bone marrow derived dendritic cells expressed relatively high Tslp but low Il12a, and were able to prime naive DO11.10T cells to differentiate into Th2in a TSLP dependent manner. After transfer into wild type recipient mice, the low-dose LPS-activated OVA-loaded DC induced airway eosinophilia, but primed neutrophil-dominated airway inflammation when Tslp-/-DC were used. These studies suggest that TSLP released by DC in response to a low concentration of LPS plays a role in priming Th2differentiation and thus may serve as a polarizing third signal, in addition to antigen/MHC II and costimulatory factors, from antigen presenting DC to direct Th2effector T cell differentiation.We demonstrated that TSLP enhanced IL-9production by Th9cells in a dose-dependent manner. Adoptive transfer of OVA-specific Th2or Th9cells to wild type recipient followed by OVA or OVA+TSLP intranasal challenge showed that TSLP increased IL-9production and allergic inflammation in mice injected with Th9cells, but had less effect on Th2cell-transferred mice. Anti-IL-9treatment of the Th9-transferred recipient mice dramatically decreased their lung inflammation. TSLP was still effective in enhancing Th9-mediated airway inflammation in TSLP receptor-deficient recipients, suggesting a direct action of TSLP on Th9cells in vivo. Furthermore, lung-specific transgenic expression of TSLP stimulated IL-9production in the lung, and anti-IL-9treatment attenuated TSLP-induced airway inflammation.Together, our results demonstrate that TSLP promotes Th9differentiation and function, and define a requirement for IL-9in TSLP-induced allergic inflammation.