2 differentiation is known to be driven by IL-4 in a STAT6-dependent fashion 10,11
; in addition, IL-2 and STAT5 proteins critically regulate this process 12,26,34
. A role for IL-2 in altering chromatin accessibility at the Il4
gene in a STAT5A-dependent fashion in TH
2 cells was established12
. However, ‘priming’ must occur to allow efficient responsiveness to IL-4. This clearly requires induction of IL-4Rα, which plays a central role in mediating signals by IL-4 and IL-13.
In lymphocytes, the functional IL-4 receptor consists of IL-4Rα plus γc
(the type I IL-4 receptor), whereas in non-immune cells, the functional IL-4 receptor is IL-4Rα plus IL-13Rα1 (the type II IL-4 receptor); this latter receptor is also the functional receptor for IL-13 15-17
. Targeted disruption of the Il4ra
gene in mice prevents responsiveness to IL-4 and IL-13 and normal TH
2 cell differentiation, with abrogation of the IgE response to parasites, defective allergen sensitization, diminished airway hypersensitivity, and defective mucus secretion 18,35-38
Despite the important role played by IL-4Rα, little is known of the molecular mechanisms regulating its expression. We herein demonstrated that IL-2 up-regulates expression of the Il4ra
gene in a STAT5-dependent manner, and thereby promotes augmented IL-4Rα expression and ‘priming’ cells for responsiveness to IL-4 and TH
2 differentiation after TCR stimulation. As TH
2 differentiation proceeds, STAT5 binding to the Il4ra
gene is maintained or even further enhanced. In the absence of IL-2, TH
2 differentiation does not progress, but notably can be rescued by retroviral expression of IL-4Rα, underscoring that IL-4Rα induction is a critical IL-2— and STAT5-dependent control step in priming cells for TH
2 differentiation. IL-2 is a TCR-induced cytokine with pleiotropic actions; IL-2 influences T cell proliferation, activation-induced cell death, regulatory T cell development, and B and NK cells 39
. In this study, we highlight IL-4Rα upregulation is another critical function of IL-2.
Herein, we also showed that both STAT5A and STAT5B can bind to the Il4
locus in cells after TH
2 differentiation, consistent with an earlier report focusing on the role of STAT5A 12
; our findings differ from this earlier study in indicating a role for STAT5B as well as STAT5A, and moreover we discovered that STAT5 proteins bind to HSV, the LCR, and more broadly within the genes within the TH
2 locus, as well as to the Maf
genes. In addition, our data reveal that STAT5 proteins also bind to the Il4ra
locus relatively early in the TH
2 differentiation process, helping to increase IL-4Rα expression and cellular responsiveness to IL-4.
In summary, we have identified a critical cross-talk among γc family cytokines, including IL-2 and others that activate STAT5, that promotes IL-4Rα expression. This induction of IL-4Rα primes cells for responsiveness to IL-4 and thus is a critical early step in the initiation of TH2 responses. Moreover, our study reveals STAT protein occupancy of TH2 genes many days after the induction of TH2 differentiation. Given the very transient nature of STAT protein activation based on in vitro assays, sustained STAT occupancy of GAS motifs within chromatin was unanticipated and indicates the importance of STAT proteins in not only the induction but also in the maintenance of a differentiated state.